The authors would like to thank Dr. Harvey Hung for his advice&#160;on the manuscript.

This research project was approved by the Joint Chinese University of Hong Kong-New Territories East Cluster Clinical Research Ethics Committee (CREC Ref. No.: 2015.263).
1. Participants Recruitment and Baseline Assessment
<ol>
	<li>Recruit Parkinson&#39;s disease patients aged less than or equal to 70 from a neurology specialist clinic with the diagnosis made based on the United Kingdom Parkinson&#39;s Disease Society (UKPDS) Brain Bank Diagnostic Criteria12.
	<ol>
		<li>Exclude subjects with psychiatric illnesses, ophthalmological diseases that would impair eye movement, or other neurological disorders. Also, exclude cases using anticholinergics as they are known to affect cognitive performance and eye movement.</li>
	</ol>
	</li>
	<li>Recruit healthy controls on a 1:1 basis matched by sex, age, and education.</li>
	<li>Obtain informed consent from the subject.</li>
	<li>Conduct a clinical diagnostic interview with the subject and, if available, their relatives, to exclude dementia and screen for cognitive impairment with Mini-Mental State Examination (MMSE)13 and Montreal Cognitive Assessment (MoCA)14. Exclude dementia cases from the study or if the subject&#8217;s scores of either MMSE or MoCA is &#60;22/30.</li>
	<li>Assess the visual acuity with a Snellen chart. Exclude the subject if the visual acuity is less than 20/40.</li>
	<li>Assess motor severity and staging of Parkinson&#39;s disease using the Unified Parkinson&#39;s Disease Rating Scale (UPDRS) Part II &#38; III15 and Modified Hoehn and Yahr (H&#38;Y) Staging16, respectively. Also, obtain information about the current medications taken by the subject.</li>
	<li>Assess the depressive mood state by the Beck Depression Inventory-II (BDI-II)17.</li>
</ol>
2. Experimental Setup
<ol>
	<li>Conduct the experiment in a quiet room with an adequate light source.</li>
	<li>Conduct the experiment for Parkinson&#39;s disease subjects when they are on medication with optimal motor function.</li>
	<li>Prepare the setup that consists of a screen-based eye tracker, a computer, a mouse, a standard keyboard, a chin rest, and cognitive assessment tools (Table of Materials).</li>
	<li>Use an eye tracker with a sampling rate of at least 300 Hz.</li>
	<li>Place the chin rest 60 cm in front of the eye tracker screen.</li>
</ol>
3. The Flow of the Cognitive Assessment and the Visual Search Task
<ol>
	<li>Carry out the Chinese Categorical Verbal Fluency Test18. Instruct the subject to name as many animals as possible in a minute. Record the number of answers and perseverative error. Then repeat the same in the category of fruits and vegetables.</li>
	<li>Conduct the registration part (Trial 1, 2 and 3) of the Hong Kong List Learning Test (HKLLT)19 by reading out a pre-defined 16-vocabulary word list and instruct the subject to remember them. Afterwards ask the subject to do free recall of the word list and record the answer (Trial 1).
	<ol>
		<li>Repeat step 3.2 twice for Trial 2 and Trial 3.</li>
	</ol>
	</li>
	<li>Wait 10 min and 30 min after the registration part of the HKLLT for the 10&#160;and 30 min delay recall.</li>
	<li>Before the 10 min delayed recall of the HKLLT, perform the Pattern Recognition Memory (PRM) from Cambridge Neuropsychological Test Automated Battery (CANTAB)20 (Table of Material).
	<ol>
		<li>Using the tablet computer, present 24 visual patterns, one at a time, at the center of the screen. Instruct the subject to remember the pattern.</li>
		<li>After the presentation, in a 2-choice force discrimination paradigm, instruct the subject to choose the pattern that he/she can recognize.</li>
	</ol>
	</li>
	<li>Perform the 10 min delay recall of HKLLT by asking the subject to do free recall of the 16-vocabulary word list.</li>
	<li>Before the 30 min delayed recall of the HKLLT, perform Spatial Span (SSP) from CANTAB20.
	<ol>
		<li>Use the tablet computer to show a pattern of white boxes which change in color, one by one, in variable sequences.</li>
		<li>Afterwards instruct the subject to touch the boxes in the same sequence they were presented and record the spatial span length that the subject could attain as the difficulty (number of boxes change in color) of the task increases.</li>
	</ol>
	</li>
	<li>Carry out the 30 min delay recall by asking the subject to do free recall of the 16-vocabulary word list.
	<ol>
		<li>Conduct the recognition and discrimination part of the HKLLT by reading out another pre-defined 32-vocabulary word list, of which half of the vocabularies are from the original word list in 3.2. Instruct the subject to determine whether each vocabulary read out is from the original word list or not.</li>
	</ol>
	</li>
	<li>Allow the subject to rest quietly if they finish the tasks in 3.4 and 3.6 before the 10- and 30-min delay recall, respectively.</li>
	<li>Perform the Stocking of Cambridge (SOC) from CANTAB20.
	<ol>
		<li>Using the tablet computer, present 20 scenarios of two parallel displays of 3 balls held in 3 vertical stockings, of which the arrangement of the balls in the displays varies in each scenario.</li>
		<li>Instruct the subject to determine, in each scenario, the least number of moves required to rearrange the balls in the lower display in order to copy the pattern shown in the upper display. Record the mean number of choices to correct answer.</li>
	</ol>
	</li>
	<li>Perform the Stroop Test21.
	<ol>
		<li>Give the subject 3 cards consecutively; the first card contains dots printed in different colors, the second card contains Chinese characters printed in different colors while the last card has Chinese characters denoting different colors (e.g., Chinese words of &#34;blue&#34;, &#34;yellow&#34;, &#34;green&#34;, or &#34;red&#34;) but printed in a color not denoted by the name (e.g., the word &#34;red&#34; printed in blue ink).</li>
		<li>Ask the subject to read out the printed color of the dots/Chinese characters as quickly as possible and record the time required for each card (T1, T2, and T3).</li>
		<li>Calculate the interference index with the formula (T3-T1)/T1.</li>
	</ol>
	</li>
	<li>Proceed to the visual search task after completing the cognitive tests. 
	NOTE: Do not carry out any verbal cognitive task after the registration part of HKLLT until the end of the whole HKLLT (3.7) to prevent interference effect on verbal memory performance.</li>
</ol>
4. Visual Search Task
<ol>
	<li>Position the subject on a chair and place their chin on the chin rest with their forehead against a bar to minimize head movement. Align the subject&#39;s eyes to approximately the center of the computer screen. Begin by clicking the Start Recording button in the computer program.</li>
	<li>Calibration
	<ol>
		<li>Calibrate the eye tracker with the in-built calibration program by clicking the Start button in the calibration interface.</li>
		<li>Ask the subject to gaze at a red dot moving across the screen with 9 fixation points, while keeping the head still.</li>
		<li>Check for the quality of the calibration by viewing the calibration plot (Figure 1). Make sure that the length of the green lines, which represent the error vectors, fall within the grey circles for an acceptable quality of the calibration. Redo the calibration if there is any missing point or the green lines fall outside the grey circles. Click Accept to proceed to the visual search task.</li>
	</ol>
	</li>
	<li>Instruction
	<ol>
		<li>Provide verbal instruction to the subject and start with 5 practice runs to familiarize the subject with the task.</li>
		<li>Instruct the subject to fixate their gaze on the central fixation cross at the beginning of each trial. Then, press Enter on the keyboard to begin a trial, at which the computer screen will display a single number and 79 distracter alphabets scattered randomly (Figure 2).</li>
		<li>Instruct the subject to look as quickly as possible for the number and then simultaneously click on the mouse and state the number aloud as soon as the number is located.</li>
		<li>Cross check if the number stated is correct or not.</li>
		<li>Administer a total of 40 trials after the 5 practice runs.</li>
	</ol>
	</li>
	<li>Design of the trial images in the visual search task 
	NOTE: The program code, written in PHP, for this section can be found in Supplement File 1.
	<ol>
		<li>Use the numbers 4, 6, 7 and 9 exclusively (Supplementary File 1 - Line 5). 
		NOTE:&#160;The pilot study11 showed that these numbers are most easily discriminated from the alphabets.</li>
		<li>Ensure that the location of the target number is randomized from trial to trial with the rule that it could not be in the same visual quadrant for more than three successive trials (Supplementary File 1 - Line 48-52).</li>
		<li>Do not use ambiguous alphabets such as &#34;I&#34;&#160;and &#34;O&#34;&#160;(Supplementary File 1 - Line 76-78).</li>
		<li>Set the size of the fixation cross, alphabets, and numbers at 0.85&#176; visual angle (equivalent to around 0.9 cm on a 23 inches computer screen). 
		NOTE: Numbers and alphabets are used because these are easily recognizable visual stimuli yet require foveation for identification.</li>
		<li>Allow a time lapse of 1.5 s after the investigator has pressed Enter in 4.3.2 and before the display of the central fixation cross is switched to a trial image to begin a trial (Supplementary File 2 - Line 71; 156-158).</li>
		<li>Ensure that the screen will go blank with the fixation cross reappearing as the mouse is clicked or after 10 s have elapsed since the beginning of a trial, whichever that is earlier (Supplementary File 2 - Line 72; 162-180).</li>
		<li>As the task is finished, generate a .csv file that contains the timestamps of the beginning and the end of each trial (Supplementary File 2 &#8211; line 48-59; 199-208). Use this file in the data analysis in section 5.</li>
	</ol>
	</li>
</ol>
5. Eye Tracking Data Processing and Analysis
<ol>
	<li>In the Replay section of the computer program, check the Samples Percentage of the eyes during the visual search task (Figure 3). Discard the subject&#8217;s data if more than 20% missing data are observed. 
	NOTE: Samples Percentage denotes the percentage of time the eyes are successfully located by the eye tracker during the visual search task.</li>
	<li>Click on the Play button for the recording to check the quality of the data by eyeballing the visualized scan path video generated (Figure 4). Discard the whole subject&#39;s data if it is grossly erroneous (Figure 5).</li>
	<li>Discard any trial(s) in which the subject pressed the mouse accidentally and prematurely.</li>
	<li>In the Data Export section of the program, select GazePointX (ADCSpx) and GazePointY (ADCSpx) and the subject of interest (Figure 6). Click Export Data to export each subject&#8217;s data and save as a .csv file. The file contains the x and y coordinates of the subject&#39;s eyes position on the computer screen, in pixels, at every time point.</li>
	<li>Use the Visual Search Analyzer and in the interface (Figure 7), select the data exported in 5.4 as the input of Eye Data and the .csv file generated in 4.4.7 as the input of Action data. Select ST DBScan as the classification algorithm and click on Run. Then, click on Summary to generate a spreadsheet file containing the mean saccade amplitude and the mean fixation duration of the subject.</li>
	<li>Design of the visual search analyzer 
	NOTE: The coding for the design of the analyzer could be found at https://github.com/lab-viso-limited/visual-search-analyzer. Its program code can be found in Supplementary File 3.
	<ol>
		<li>Program the analyzer such that it extracts and analyzes only the data from the beginning to the end of the trial (i.e., from the display of the number and alphabets until the mouse is clicked or 10 s have elapsed), using the .csv file generated in 4.4.7 (Supplementary File 3 - Line 6-173).</li>
		<li>Program the analyzer such that it fills in the data loss due to eye-blinking by averaging the x and y coordinates of the gaze point immediately before and after the blinking (Supplementary File 3 - Line 176-260).</li>
		<li>Program the analyzer such that it classifies the raw data into either saccade or fixation by using the algorithm developed based on ST-DBSCAN22 (Program code in Supplementary File 4).&#160;</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Hely, M. A., Reid, W. G. J., Adena, M. A., Halliday, G. M., Morris, J. G. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Sydney+Multicenter+Study+of+Parkinson&#8217;s+disease:+The+inevitability+of+dementia+at+20+years.">The Sydney Multicenter Study of Parkinson&#8217;s disease: The inevitability of dementia at 20 years.</a> Movement Disorders. 23 (6), 837-844 (2008).</li><li>Braak, H., Del Tredici, K., Bratzke, H., Hamm-Clement, J., Sandmann-Keil, D., R&#252;b, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Staging+of+the+intracerebral+inclusion+body+pathology+associated+with+idiopathic+Parkinson's+disease+(preclinical+and+clinical+stages).">Staging of the intracerebral inclusion body pathology associated with idiopathic Parkinson's disease (preclinical and clinical stages).</a> Journal of Neurology. 249, 1-5 (2002).</li><li>Williams-Gray, C. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+distinct+cognitive+syndromes+of+Parkinson&#8217;s+disease:+5+year+follow-up+of+the+CamPaIGN+cohort.">The distinct cognitive syndromes of Parkinson&#8217;s disease: 5 year follow-up of the CamPaIGN cohort.</a> Brain. 132 (11), 2958-2969 (2009).</li><li>Buter, T. C., van den Hout, A., Matthews, F. E., Larsen, J. P., Brayne, C., Aarsland, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dementia+and+survival+in+parkinson+disease:+A+12-year+population+study.">Dementia and survival in parkinson disease: A 12-year population study.</a> Neurology. 70 (13), 1017-1022 (2008).</li><li>Aarsland, D., Larsen, J. P., Tandberg, E., Laake, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Predictors+of+nursing+home+placement+in+Parkinson's+disease:+A+population-based,+prospective+study.">Predictors of nursing home placement in Parkinson's disease: A population-based, prospective study.</a> Journal of the American Geriatrics Society. 48 (8), 938-942 (2000).</li><li>Rascol, O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Abnormal+ocular+movements+in+parkinson's+disease:+Evidence+for+involvement+of+dopaminergic+systems.">Abnormal ocular movements in parkinson's disease: Evidence for involvement of dopaminergic systems.</a> Brain. 112 (5), 1193-1214 (1989).</li><li>Orban De Xivry, J. J., Lef&#232;vre, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Saccades+and+pursuit:+Two+outcomes+of+a+single+sensorimotor+process.">Saccades and pursuit: Two outcomes of a single sensorimotor process.</a> Journal of Physiology. 584 (1), 11-23 (2007).</li><li>Crawford, T. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inhibitory+control+of+saccadic+eye+movements+and+cognitive+impairment+in+Alzheimer's+disease.">Inhibitory control of saccadic eye movements and cognitive impairment in Alzheimer's disease.</a> Biological Psychiatry. 57 (9), 1052-1060 (2005).</li><li>Archibald, N. K., Hutton, S. B., Clarke, M. P., Mosimann, U. P., Burn, D. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Visual+exploration+in+Parkinson's+disease+and+Parkinson's+disease+dementia.">Visual exploration in Parkinson's disease and Parkinson's disease dementia.</a> Brain. 136 (3), 739-750 (2013).</li><li>Litvan, I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diagnostic+criteria+for+mild+cognitive+impairment+in+Parkinson's+disease:+Movement+Disorder+Society+Task+Force+guidelines.">Diagnostic criteria for mild cognitive impairment in Parkinson's disease: Movement Disorder Society Task Force guidelines.</a> Movement Disorders. 27 (3), 349-356 (2012).</li><li>R&#246;sler, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Alterations+of+visual+search+strategy+in+Alzheimer's+disease+and+aging.">Alterations of visual search strategy in Alzheimer's disease and aging.</a> Neuropsychology. 14 (3), 398-408 (2000).</li><li>Hughes, A. J., Daniel, S. E., Kilford, L., Lees, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Accuracy+of+clinical+diagnosis+of+idiopathic+Parkinson's+disease:+A+clinico-pathological+study+of+100+cases.">Accuracy of clinical diagnosis of idiopathic Parkinson's disease: A clinico-pathological study of 100 cases.</a> Journal of Neurology Neurosurgery and Psychiatry. 55 (3), 181-184 (1992).</li><li>Chiu, H. F. K., Lee, H. C., Chung, W. S., Kwong, P. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reliability+and+Validity+of+the+Cantonese+Version+of+Mini-Mental+State+Examination-A+Preliminary+Study.">Reliability and Validity of the Cantonese Version of Mini-Mental State Examination-A Preliminary Study.</a> Hong Kong Journal of Psychiatry. 4 (2), 25 (1994).</li><li>Wong, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+validity,+reliability+and+clinical+utility+of+the+Hong+Kong+Montreal+Cognitive+Assessment+(HK-MoCA)+in+patients+with+cerebral+small+vessel+disease.">The validity, reliability and clinical utility of the Hong Kong Montreal Cognitive Assessment (HK-MoCA) in patients with cerebral small vessel disease.</a> Dementia and Geriatric Cognitive Disorders. 28 (1), 81-87 (2009).</li><li>Fahn, S., Elton, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Members+of+the+UPDRS+Development+Committee.+Unified+Parkinson's+disease+rating+scale.">Members of the UPDRS Development Committee. Unified Parkinson's disease rating scale.</a> Recent Development in Parkinson's Disease. 2, 293-304 (1987).</li><li>Hoehn, M. M., Yahr, M. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Parkinsonism:+onset,+progression,+and+mortality.">Parkinsonism: onset, progression, and mortality.</a> Neurology. 17 (5), 427-427 (1967).</li><li>Wu, P. C., Chang, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Psychometric+properties+of+the+Chinese+version+of+the+Beck+Depression+Inventory-II+using+the+Rasch+model.">Psychometric properties of the Chinese version of the Beck Depression Inventory-II using the Rasch model.</a> Measurement and Evaluation in Counseling and Development. 41 (1), 13-31 (2008).</li><li>Chiu, H. F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+modified+Fuld+Verbal+Fluency+Test:+a+validation+study+in+Hong+Kong.">The modified Fuld Verbal Fluency Test: a validation study in Hong Kong.</a> The journals of gerontology. Series B, Psychological sciences and social sciences. 52 (5), 247-250 (1997).</li><li>Chan, A. S., Kwok, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hong+Kong+list+learning+test:+manual+and+preliminary+norm.">Hong Kong list learning test: manual and preliminary norm.</a> Hong Kong: Department of Psychological and Clinical Psychology Center. , (1999).</li><li>Robbins, T. W., James, M., Owen, A. M., Sahakian, B. J., McInnes, L., Rabbitt, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cambridge+Neuropsychological+Test+Automated+Battery+(CANTAB):+A+Factor+Analytic+Study+of+a+Large+Sample+of+Normal+Elderly+Volunteers.">Cambridge Neuropsychological Test Automated Battery (CANTAB): A Factor Analytic Study of a Large Sample of Normal Elderly Volunteers.</a> Dementia and Geriatric Cognitive Disorders. 5 (5), 266-281 (1994).</li><li>Lee, T. M. C., Wang, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Neuropsychological Measures: Normative Data for Chinese. , (2010).</li><li>Birant, D., Kut, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=ST-DBSCAN:+An+algorithm+for+clustering+spatial-temporal+data.">ST-DBSCAN: An algorithm for clustering spatial-temporal data.</a> Data and Knowledge Engineering. 60 (1), 208-221 (2007).</li><li>Wong, O. W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Eye+movement+parameters+and+cognitive+functions+in+Parkinson's+disease+patients+without+dementia.">Eye movement parameters and cognitive functions in Parkinson's disease patients without dementia.</a> Parkinsonism and Related Disorders. 52, 43-48 (2018).</li><li>Muslimovic, D., Post, B., Speelman, J. D., Schmand, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cognitive+profile+of+patients+with+newly+diagnosed+Parkinson+disease.">Cognitive profile of patients with newly diagnosed Parkinson disease.</a> Neurology. 65 (8), 1239-1245 (2005).</li><li>Winograd-Gurvich, C., Georgiou-Karistianis, N., Fitzgerald, P. B., Millist, L., White, O. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Self-paced+saccades+and+saccades+to+oddball+targets+in+Parkinson's+disease.">Self-paced saccades and saccades to oddball targets in Parkinson's disease.</a> Brain Research. 1106 (1), 134-141 (2006).</li><li>Briand, K. A., Strallow, D., Hening, W., Poizner, H., Sereno, A. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Control+of+voluntary+and+reflexive+saccades+in+Parkinson's+disease.">Control of voluntary and reflexive saccades in Parkinson's disease.</a> Experimental Brain Research. 129 (1), 38-48 (1999).</li><li>Rivaud-P&#233;choux, S., Vidailhet, M., Brandel, J. P., Gaymard, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mixing+pro-+and+antisaccades+in+patients+with+parkinsonian+syndromes.">Mixing pro- and antisaccades in patients with parkinsonian syndromes.</a> Brain. 130 (1), 256-264 (2007).</li><li>Perneczky, R., Ghosh, B. C. P., Hughes, L., Carpenter, R. H. S., Barker, R. A., Rowe, J. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Saccadic+latency+in+Parkinson's+disease+correlates+with+executive+function+and+brain+atrophy,+but+not+motor+severity.">Saccadic latency in Parkinson's disease correlates with executive function and brain atrophy, but not motor severity.</a> Neurobiology of Disease. 43 (1), 79-85 (2011).</li><li>Matsumoto, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Small+saccades+restrict+visual+scanning+area+in+Parkinson's+disease.">Small saccades restrict visual scanning area in Parkinson's disease.</a> Movement Disorders. 26 (9), 1619-1626 (2011).</li><li>MacAskill, M. R., Anderson, T. J., Jones, R. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Adaptive+modification+of+saccade+amplitude+in+Parkinson's+disease.">Adaptive modification of saccade amplitude in Parkinson's disease.</a> Brain. 125 (7), 1570-1582 (2002).</li><li>Salvucci, D. D., Goldberg, J. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identifying+fixations+and+saccades+in+eye-tracking+protocols.">Identifying fixations and saccades in eye-tracking protocols.</a> Proceedings of the 2000 symposium on Eye tracking research &amp; applications. , 71-78 (2000).</li><li>Rana, A. Q., Kabir, A., Dogu, O., Patel, A., Khondker, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Prevalence+of+blepharospasm+and+apraxia+of+eyelid+opening+in+patients+with+parkinsonism,+cervical+dystonia+and+essential+tremor.">Prevalence of blepharospasm and apraxia of eyelid opening in patients with parkinsonism, cervical dystonia and essential tremor.</a> European Neurology. 68 (5), 318-321 (2012).</li></ol>

The authors have nothing to disclose.

The protocol presented above was designed as the first part of a longitudinal study in exploring the potential clinical utility of eye movement parameters as surrogate markers for cognitive functions in Parkinson&#39;s disease. While there are studies that examine more classical eye tracking paradigms such as self-paced saccade, reflexive saccade, and anti-saccade25,26,27, a visual search task was used in this study to measure eye movement parameters. As discussed, the design of this visual search task is of paramount importance as it must minimize the known confounding effect of a cognitive ability on the performance of the eye tracking task, as it may affect the eye movement parameters recorded. An example of which would be the effect of frontal executive functions on the saccadic latency28. The critical issue in the design would be the random scattering of the number and alphabets and varying quadrants of the number&#39;s location, making it more difficult to use cognitive strategies to enhance the performance of the task. Together with an average of roughly 650 saccades measured in 40 trials per subject, the average saccade amplitude calculated represents more of a physiological ability of the eye to generate saccade. In accordance with previous literature, it was found that the saccade amplitude is smaller in Parkinson&#39;s disease patients29,30. The choice of parameters extracted from the eye tracking task also needs to be taken care of with respect to the issue of the potential confounding effect by cognition. For example, parameters such as the speed of finding the number, error rate, and accuracy, which are a direct measurement of attention and processing speed, were not used.
Another critical step for this study is to ascertain the validity of the algorithm that was used in the classification of eye movement parameter. There exist numerous ways of classifying eye tracking data into saccade and fixation: velocity-based, dispersion-based algorithm and so on31. Each of these algorithms has its own pros and cons and there is no gold standard for doing so such that one has to also take into consideration the specifications of the eye tracker used and the design of eye tracking task to determine the best way of classifying the data. For this study, an in-house, density-based clustering algorithm, developed based on ST-DBSCAN22, was used. The research team has cross-validated the validity of this classification algorithm against manual classification in a pilot study before applying the algorithm to the data of this study. The computer program incorporating the algorithm would automatically splice out and classify the data within the trials, from the moment the trial starts (with the alphabets and number appearing on the screen) to the end (that the subject clicks on the mouse or 10 s has lapsed) so that no non-trial data recorded (e.g., during the display of the fixation cross) will be analyzed to contaminate the results.
The use of domain-specific cognitive tests in this study allow correlations of the eye movement parameters with individual cognitive function performance. As discussed, this has significance over using general overall cognitive measures as the neural circuitry and biochemical basis for each cognitive function are different. The contemporary knowledge on the neural mechanisms of eye movement control and individual cognitive functions allow us to make inference and interpretation of the results found. For example, the significant negative correlations of fixation duration with temporal-, parietal-, and cholinergic-based cognitive functions are of particular interest as impairment of these functions may predict the development of dementia3. Detailed discussions of the scientific basis that explain the correlations can be found in the original paper published23.
The battery of cognitive examination and the visual search task were highly tolerable to the subjects of this study. Requiring roughly 1.5 h to complete the entire battery, none of the subjects were unable to finish because of fatigue or physical discomfort. The visual search task consisted of 40 trials and took only around 5-10 min to complete. The noninvasive, simple and quick nature of the task makes it suitable as a screening tool if supported by more robust data. This paradigm could also be applied transdiagnostically in other neurocognitive disorders to answer similar research questions. One major practical limitation encountered in this protocol is the incompatibility of the eye tracker in subjects wearing certain progressive lens, as presbyopia is not an uncommon condition in the elderly. Eyelid apraxia and blepharospasm are also seen in Parkinson&#39;s disease32 and sufferers of these conditions may not be able to complete the task.
As an explorative and cross-sectional study, the design of the study does not allow us to infer any definite neuroanatomical and biochemical basis that explains the results found. The interpretations of the results were mostly based on independent knowledge on the physiologies of cognitive functions and eye movement control and, therefore, remained as postulations. The longitudinal data on how these parameters may change over time during the neurodegenerative process is unknown. Yet, it is worthwhile to have a follow-up study to investigate the predictive values of the baseline eye movement parameters on cognitive impairment development. Future studies should incorporate neuroimaging to address the neurostructural underpinnings for more solid support of any postulation, without which further development of eye tracking as a proxy marker of cognitive function will not be possible.

Parkinson&#8217;s disease is classically a motor disorder; yet, the disease is also associated with cognitive deficits, and progression into dementia is common1. The pathophysiology of cognitive impairment in Parkinson&#39;s disease is not well understood. It is thought to be related to alpha-synuclein deposition in the cortical area based on Braak&#39;s staging2. It was also proposed that a dual syndrome of degeneration of the dopaminergic and the cholinergic system leads to different cognitive deficits with prognostic implication3. More research is needed to further elucidate the exact mechanisms involved in cognitive impairment in Parkinson&#39;s disease. On the clinical aspect, the presence of cognitive impairment has a significant impact on prognosis4,5. Assessment of cognitive function in clinical practice is, therefore, essential. However, a lengthy cognitive assessment is limited by patients&#8217; mental and motor conditions. Therefore, a noninvasive and simple measurement that can reflect the disease&#39;s burden on cognitive function is needed.
The eye movement abnormalities are widely described detectable signs of Parkinson&#39;s disease from its early stages6, yet the pathophysiology is even less well-characterized than that of cognitive impairment. The generation of eye movement is through a transformation of the visual sensory input, subserved by an intertwined cortical and subcortical network, into signals to the oculomotor nuclei in the brainstem for effect7. Involvement of Parkinson&#39;s disease pathologies in these networks may lead to observable eye movement abnormalities. There is, perhaps overlapping of neuroanatomical structures that govern the control of eye movement and cognitive function. Furthermore, there have been studies examining the relationship between saccadic eye movement and cognitive function in other neurodegenerative disorders8. On such grounds, it is worthwhile to explore the use of eye movement parameters as a proxy marker of cognitive functions in Parkinson&#39;s disease. One cross-sectional study9 showed that reduced saccadic amplitude and longer fixation duration was associated with the severity of global cognitive impairment in Parkinson&#39;s disease. However, there is a lack of data on the correlation between eye movement parameters and specific cognitive domains. The significance and need of measurement of specific cognitive domains, rather than a general cognitive state, is that individual cognitive domain informs differential prognostic information in Parkinson&#39;s disease3 and they are subserved by different neural networks. The aim of this study is to explore the specific relationship between eye movement metrics and different cognitive functions. This is the first step to establish a foundation on which the development of biomarkers of cognitive decline in Parkinson&#39;s disease using eye tracking technology could be built.
The experimental paradigm presented is composed of 2 major parts: the cognitive assessment and the eye tracking task. The cognitive assessment battery encompassed a range of cognitive functions, including attention and working memory, executive function, language, verbal memory and visuospatial function. The choice of these 5 cognitive domains is based on the Movement Disorder Society Task Force Guidelines for the mild cognitive impairment in Parkinson&#39;s disease10, and a set of locally available cognitive tests were selected to build the assessment battery. In a previous similar eye tracking study on Parkinson&#39;s disease cognition mentioned9, the author extracted the eye movement parameters while the subjects were engaged in visual cognitive tasks, where the parameters may potentially be influenced by the subject&#39;s cognitive ability. As this study aimed to assess the correlation between the eye movement parameters and different cognitive domains, the potential confounding effect of cognitive abilities on the eye parameters must be addressed. In this regards, a visual search task, adapted from another eye tracking study on Alzheimer&#8217;s disease11, was employed to capture the eye movement parameters of the subjects. During the task, subjects had to search for a single number on a computer screen among multiple alphabet distracters. This task would elicit the alternate use of saccadic eye movement and visual fixation, the abnormalities of which are described widely in Parkinson&#39;s disease. The identification and differentiation of number and alphabet is an overlearned task where the demand for cognitive functions is only minimal and would, therefore, be suitable to answer the research question of this study. A computer program was developed based on the specifications and design as stated by R&#246;sler et al.11. in their original study to be run within the in-built software of our eye tracker. An in-house algorithm for classification and analysis of the eye tracking data was also developed for this study.

<table><tbody><tr><td>Computer&amp;nbsp;</td><td>Intel</td><td></td><td></td></tr><tr><td>Computerized cognitive assessment tool</td><td>CANTAB</td><td>CANTAB Research Suite</td><td>Contains Pattern Recognition Memory, Spatial Span, and Stockings of Cambridge</td></tr><tr><td>Eye Movement Analyzer</td><td>Lab Viso Limited</td><td></td><td>https://github.com/lab-viso-limited/visual-search-analyzer</td></tr><tr><td>Eye tracker</td><td>Tobii</td><td>Tx300</td><td>23 inch computer screen with resolution of 1920 x 1080, Sampling rate at 300 Hz</td></tr><tr><td>Hong Kong List Leanrning Test</td><td>Department of Psychology, The Chinese University of Hong Kong</td><td>The Hong Kong List Learning Test (HKLLT) 2nd Edition</td><td></td></tr><tr><td>Stroop test</td><td>Laboratory of Neuropsychology, The University of Hong Kong</td><td>Neuropsychological Measures: Normative Data for Chinese, Second Edition (Revised)</td><td></td></tr><tr><td>Tobii Studio</td><td>Tobii</td><td>Tobii Studio version 3.2.2</td><td>Computer programme for running the visual search task</td></tr><tr><td>Visual Search Task</td><td>Lab Viso Limited</td><td></td><td>https://www.labviso.com/#products</td></tr></tbody></table>

characterizing the relationship between eye movement parameters and cognitive functions in non-demented parkinson's disease patients with eye tracking

Cognitive impairment is a common phenomenon in Parkinson&#8217;s disease that has implications on the prognosis. A simple, noninvasive and objective proxy measurement of cognitive function in Parkinson&#8217;s disease will be helpful in detecting early cognitive decline. As a physiological metric, eye movement parameter is not confounded by the subject&#39;s attributes and intelligence and can function as a proxy marker if it correlates with cognitive functions. To this end, this study explored the relationship between the eye movement parameters and performance in cognitive tests in multiple domains. In the experiment, a visual search task with eye tracking was set up, where subjects were asked to look for a number embedded in an array of alphabets scattered randomly on a computer screen. The differentiation between the number and the alphabet is an overlearned task such that the confounding effect of cognitive ability on the eye movement parameters is minimized. The average saccadic amplitude and fixation duration were captured and calculated during the visual search task. The cognitive assessment battery covered domains of frontal-executive functions, attention, verbal and visual memory. It was found that prolonged fixation duration was associated with poorer performance in verbal fluency, visual and verbal memory, allowing further exploration on the use of eye movement parameters as proxy markers for cognitive function in Parkinson&#8217;s disease patients. The experimental paradigm has been found to be highly tolerable in our group of Parkinson&#39;s disease patients and could be applied transdiagnostically to other disease entities for similar research questions.

The full result of this study is available in the original paper published23. Parkinson&#8217;s disease subjects (n = 67) were recruited and completed the assessment. However, 5 cases failed to complete the visual search task as they wore progressive lens incompatible with the eye tracker and their data was discarded. The mean age of the subjects was 58.9 years (SD = 7.5 years) with a male to female ratio of 1.7:1. 62 healthy age-, sex-, and education-matched controls were recruited for comparison.
Cognitive and eye movement parameters 
Consistent with other previous studies24, the Parkinson&#8217;s disease group showed poorer performance in multiple cognitive tasks as compared to the control group (Table 1). Using the in-house algorithm for classification of the visual search task data, fixations and saccades are identified and extracted for calculation and analysis. It was found that the disease group had a smaller mean saccadic amplitude (16.36&#176; &#177; 2.36) as compared to controls (17.27&#176; &#177; 2.49; p = 0.037). The mean fixation duration was not significantly different between the groups (216.58 ms &#177; 31.64 vs, 211.59 ms &#177; 24.90; p = 0.331) (Table 2).
Correlation between eye movement parameters and cognitive function 
After adjusting for covariates, there were negative correlations found between the mean fixation duration and the performance in verbal recognition memory score (Recognition and Discrimination scores; F = 5.843, t = -2.417, p = 0.017 and F = 12.771, t = -3.574, p&#160; = 0.001, respectively), pattern recognition memory (F = 5.505, t = -2.346, p = 0.021) and categorical verbal fluency test in the categories of fruit (F = 5.647, t = -2.376, p = 0.009) and vegetable (F = 9.744, t = -3.122, p = 0.002). (Table 3). However, there was no significant interaction found in these correlations between the disease and control group, suggesting that the correlations are not specific to the disease group. It is speculated that as the control of visual fixation and the correlated cognitive functions commonly involve temporal and parietal regions of the brain with a predominantly cholinergic basis, pathological changes to these neuroanatomical and biochemical mechanisms may explain the findings.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60052/60052fig1.jpg" /> 
Figure 1: A calibration plot of the eye tracker. The plot shows the result of the calibration. The length of each green line indicates the difference between the gaze point calculated by the eye tracker and the actual dot position. As all the green lines fall within the grey circles and there is no missing point, the quality of this calibration is acceptable. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60052/60052fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/60052/60052fig2.jpg" /> 
Figure 2: An example of a trial of the visual search task. Display of a non-linear array of 80 stimulus items, of which there is 1 number among 79 distracter alphabets. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60052/60052fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60052/60052fig3.jpg" /> 
Figure 3: The interface to check the overall sampling percentage. In the Replay section of the computer program, the Samples Percentage, which denotes the percentage of time that the eyes are successfully located by the eye tracker during the visual search task, could be checked for each subject. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60052/60052fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/60052/60052fig4.jpg" /> 
Figure 4: An example of a visualized scan path from the visual search task. The scan path during this trial was visualized, with the red straight lines representing the saccadic eye movement and the red dots for visual fixations. Note that the end of each visual fixation is followed by a saccade and vice versa in a normal scan path. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60052/60052fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/60052/60052fig5.jpg" /> 
Figure 5: An example of a grossly erroneous visualized scan path. This example of a grossly erroneous scan path is taken from a subject wearing a pair of incompatible progressive lens. In contrast to the normal scan path in Figure 4, the red lines (saccade) run in zigzag and fall out of the computer screen. The fixation points are not on either the alphabets or the number. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60052/60052fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/60052/60052fig6.jpg" /> 
Figure 6: The data export interface in the computer program. This shows the interface where the subject and the kind of the eye tracking data captured can be selected for data export. In our experimental paradigm, the x and y coordinate, in pixels, of the eyes position on the screen at every time point will be used for data analysis. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60052/60052fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/60052/60052fig7.jpg" /> 
Figure 7: The interface of the Visual Search Analyzer. This shows the interface of the in-house analysis program for eye tracking data. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60052/60052fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td></td>
			<td>Control group</td>
			<td>Parkinson&#39;s group</td>
			<td>p-value</td>
		</tr>
		<tr>
			<td colspan="4">Global Cognitive scales</td>
		</tr>
		<tr>
			<td>MMSE</td>
			<td>28.53 (1.63)</td>
			<td>28 (1.84)</td>
			<td>0.09</td>
		</tr>
		<tr>
			<td>MoCA</td>
			<td>27.10 (2.25)</td>
			<td>26 (2.34)</td>
			<td>0.009*</td>
		</tr>
		<tr>
			<td colspan="4">Specific Cognitive tests &#8211; Frontal executive &#38; Frontal-temporal</td>
		</tr>
		<tr>
			<td>Stocking of Cambridgea</td>
			<td>1.16 (0.14)</td>
			<td>1.24 (0.19)</td>
			<td>0.018*</td>
		</tr>
		<tr>
			<td>Stroop testb</td>
			<td>1.24 (1.77)</td>
			<td>1.36 (1.65)</td>
			<td>0.697</td>
		</tr>
		<tr>
			<td>Verbal fluency - animalb</td>
			<td>0.92 (1.47)</td>
			<td>0.26 (1.31)</td>
			<td>0.01*</td>
		</tr>
		<tr>
			<td>Verbal fluency - fruitb</td>
			<td>-0.71 (0.74)</td>
			<td>-1.01 (0.79)</td>
			<td>0.028*</td>
		</tr>
		<tr>
			<td>Verbal fluency - vegetableb</td>
			<td>-0.66 (1.04)</td>
			<td>-1.11 (0.90)</td>
			<td>0.011*</td>
		</tr>
		<tr>
			<td colspan="4">Specific Cognitive tests &#8211; Verbal Memory (Hong Kong List Learning Test)</td>
		</tr>
		<tr>
			<td>Total learningb</td>
			<td>0.03 (0.90)</td>
			<td>-0.30 (0.87)</td>
			<td>0.037*</td>
		</tr>
		<tr>
			<td>10 minutes delay free recallb</td>
			<td>-0.17 (0.90)</td>
			<td>-0.44 (1.10)</td>
			<td>0.131</td>
		</tr>
		<tr>
			<td>30 minutes delay free recallb</td>
			<td>-0.19 (0.90)</td>
			<td>-0.39 (1.04)</td>
			<td>0.206</td>
		</tr>
		<tr>
			<td>Recognition scoreb</td>
			<td>0.10 (1.00)</td>
			<td>0.15 (0.73)</td>
			<td>0.722</td>
		</tr>
		<tr>
			<td>Discrimination scoreb</td>
			<td>-0.05 (1.02)</td>
			<td>-0.13 (0.97)</td>
			<td>0.636</td>
		</tr>
		<tr>
			<td colspan="4">Specific Cognitive tests &#8211; Visual spatial memory</td>
		</tr>
		<tr>
			<td>Pattern recognition memoryc</td>
			<td>91.33 (9.40)</td>
			<td>87.77 (10.20)</td>
			<td>0.045*</td>
		</tr>
		<tr>
			<td colspan="4">Specific Cognitive tests &#8211; Attention/Working memory</td>
		</tr>
		<tr>
			<td>Spatial spand</td>
			<td>6.15 (1.10)</td>
			<td>5.65 (1.17)</td>
			<td>0.016*</td>
		</tr>
	</tbody>
</table>
Table 1: Comparison of cognitive scores between two groups using independent sample t-test. MMSE, Mini-Mental State Examination; MoCA, Montreal Cognitive Assessment; *&#8211; p&#60;0.05 a &#8211; mean choices to correct; b &#8211; scores transformed into z-score; c &#8211; percentage correct; d &#8211; span length. This table has been reproduced from23.
<table border="1" fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td></td>
			<td>Control group</td>
			<td>Parkinson&#39;s disease group</td>
			<td>p-value</td>
		</tr>
		<tr>
			<td>Mean fixation duration, in miliseconds (SD) [Range]</td>
			<td>211.59 (24.90) [165.77 - 264.63]</td>
			<td>216.58 (31.64) [145.43-312.68]</td>
			<td>0.331</td>
		</tr>
		<tr>
			<td>Mean saccadic amplitude, in degrees (SD) [Range]</td>
			<td>17.27 (2.49) [13.34 - 22.99]</td>
			<td>16.36 (2.36) [11.66-23.20]</td>
			<td>0.037*</td>
		</tr>
	</tbody>
</table>
Table 2: Comparison of eye tracking parameters between two groups using independent sample t-test. * - p &#60; 0.05. This table has been modified from23.
<table border="1" fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td>Source</td>
			<td>Dependent Variable</td>
			<td>df</td>
			<td>F</td>
			<td>B</td>
			<td>Beta</td>
			<td>Std. Error</td>
			<td>t</td>
			<td>p-value</td>
		</tr>
		<tr>
			<td>Mean fixation duration</td>
			<td>Verbal fluency - fuit</td>
			<td>1</td>
			<td>5.647</td>
			<td>-0.006</td>
			<td>-0.227</td>
			<td>0.002</td>
			<td>-2.376</td>
			<td>0.009*</td>
		</tr>
		<tr>
			<td></td>
			<td>Verbal fluency - vegetable</td>
			<td>1</td>
			<td>9.744</td>
			<td>-0.009</td>
			<td>-0.288</td>
			<td>0.003</td>
			<td>-3.122</td>
			<td>0.002*</td>
		</tr>
		<tr>
			<td></td>
			<td>Recognition score</td>
			<td>1</td>
			<td>5.843</td>
			<td>-0.007</td>
			<td>-0.215</td>
			<td>0.003</td>
			<td>-2.417</td>
			<td>0.017*</td>
		</tr>
		<tr>
			<td></td>
			<td>Discrimination score</td>
			<td>1</td>
			<td>12.771</td>
			<td>-0.011</td>
			<td>-0.314</td>
			<td>0.003</td>
			<td>-3.574</td>
			<td>0.001*</td>
		</tr>
		<tr>
			<td></td>
			<td>Pattern recognition memory</td>
			<td>1</td>
			<td>5.505</td>
			<td>-0.071</td>
			<td>-0.215</td>
			<td>0.03</td>
			<td>-2.346</td>
			<td>0.021*</td>
		</tr>
	</tbody>
</table>
Table 3: Correlations between cognitive scores and eye-tracking parameters using General Linear Model: Significant findings only. * - p &#60; 0.05. This table has been reproduced from23.
<img alt="Supplemental File 1" src="/files/ftp_upload/60052/60052supfig1.jpg" /> 
Supplemental File 1: Codes related to the trial image design. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60052/Supplementary_File_1.pdf" target="_blank">Please click here to download this file.</a>
<img alt="Supplemental File 2" src="/files/ftp_upload/60052/60052supfig2.jpg" /> 
Supplemental File 2: Codes related to the actual run of the visual search task. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60052/Supplementary_File_2.pdf" target="_blank">Please click here to download this file.</a>
<img alt="Supplemental File 3" src="/files/ftp_upload/60052/60052supfig3.jpg" /> 
Supplemental File 3: Codes related to the software (e.g., analyzer program). <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60052/Supplementary_File_3.pdf" target="_blank">Please click here to download this file.</a>
<img alt="Supplemental File 4" src="/files/ftp_upload/60052/60052supfig4.jpg" /> 
Supplemental File 4: Codes related to the ST-DBSCAN algorithm used for classifying eye movement metrics. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/60052/Supplementary_File_4.pdf" target="_blank">Please click here to download this file.</a>

Watch this Scientific Journal Video about Characterizing the Relationship Between Eye Movement Parameters and Cognitive Functions in Non-demented Parkinson's Disease Patients with Eye Tracking at JoVE

Characterizing the Relationship Between Eye Movement Parameters and Cognitive Functions in Non-demented Parkinson's Disease Patients with Eye Tracking

Here, we present a protocol to study the relationship between the eye movement parameters and cognitive functions in non-demented Parkinson&#39;s disease patients. The experiment used an eye tracker to measure the saccadic amplitude and fixation duration in a visual search task. The correlation with performance in multi-domain cognitive tasks was subsequently measured.

Cognitive impairment is a common phenomenon in Parkinson&#8217;s disease that has implications on the prognosis. A simple, noninvasive and objective proxy ...

characterizing-relationship-between-eye-movement-parameters-cognitive

Universidad San Sebastian

Research

JoVE Journal

Neuroscience

7.7K Views.  The Chinese University of Hong Kong. Here, we present a protocol to study the relationship between the eye movement parameters and cognitive functions in non-demented Parkinson's disease patients. The experiment used an eye tracker to measure the saccadic amplitude and fixation duration in a visual search task. The correlation with performance in multi-domain cognitive tasks was subsequently measured.

Video: Characterizing the Relationship Between Eye Movement Parameters and Cognitive Functions in Non-demented Parkinson's Disease Patients with Eye Tracking

Eye Movement Monitoring of Memory

Explicit (often verbal) reports are typically used to investigate memory (e.g. "Tell me what you remember about the person you saw at the bank yesterday."), however such reports can often be unreliable or sensitive to response bias 1, and may be unobtainable in some participant populations. Furthermore, explicit reports only reveal when information has reached consciousness and cannot comment on when memories were accessed during processing, regardless of whether the information is subsequently accessed in a conscious manner. Eye movement monitoring (eye tracking) provides a tool by which memory can be probed without asking participants to comment on the contents of their memories, and access of such memories can be revealed on-line 2,3. Video-based eye trackers (either head-mounted or remote) use a system of cameras and infrared markers to examine the pupil and corneal reflection in each eye as the participant views a display monitor. For head-mounted eye trackers, infrared markers are also used to determine head position to allow for head movement and more precise localization of eye position. Here, we demonstrate the use of a head-mounted eye tracking system to investigate memory performance in neurologically-intact and neurologically-impaired adults. Eye movement monitoring procedures begin with the placement of the eye tracker on the participant, and setup of the head and eye cameras. Calibration and validation procedures are conducted to ensure accuracy of eye position recording. Real-time recordings of X,Y-coordinate positions on the display monitor are then converted and used to describe periods of time in which the eye is static (i.e. fixations) versus in motion (i.e., saccades). Fixations and saccades are time-locked with respect to the onset/offset of a visual display or another external event (e.g. button press). Experimental manipulations are constructed to examine how and when patterns of fixations and saccades are altered through different types of prior experience. The influence of memory is revealed in the extent to which scanning patterns to new images differ from scanning patterns to images that have been previously studied 2, 4-5. Memory can also be interrogated for its specificity; for instance, eye movement patterns that differ between an identical and an altered version of a previously studied image reveal the storage of the altered detail in memory 2-3, 6-8. These indices of memory can be compared across participant populations, thereby providing a powerful tool by which to examine the organization of memory in healthy individuals, and the specific changes that occur to memory with neurological insult or decline 2-3, 8-10.

Eye movement monitoring (or eye tracking) reveals where in space the eyes linger, when and for how long. Here, we demonstrate how eye tracking can be used to investigate the integrity of memory in multiple participant populations, without requiring verbal, or otherwise explicit, reports.

Explicit (often verbal) reports are typically used to investigate memory (e.g. "Tell me what you remember about the person you saw at the bank ...

VisualEyes: A Modular Software System for Oculomotor Experimentation

Eye movement studies have provided a strong foundation forming an understanding of how the brain acquires visual information in both the normal and dysfunctional brain.1 However, development of a platform to stimulate and store eye movements can require substantial programming, time and costs. Many systems do not offer the flexibility to program numerous stimuli for a variety of experimental needs. However, the VisualEyes System has a flexible architecture, allowing the operator to choose any background and foreground stimulus, program one or two screens for tandem or opposing eye movements and stimulate the left and right eye independently. This system can significantly reduce the programming development time needed to conduct an oculomotor study. The VisualEyes System will be discussed in three parts: 1) the oculomotor recording device to acquire eye movement responses, 2) the VisualEyes software written in LabView, to generate an array of stimuli and store responses as text files and 3) offline data analysis. Eye movements can be recorded by several types of instrumentation such as: a limbus tracking system, a sclera search coil, or a video image system. Typical eye movement stimuli such as saccadic steps, vergent ramps and vergent steps with the corresponding responses will be shown. In this video report, we demonstrate the flexibility of a system to create numerous visual stimuli and record eye movements that can be utilized by basic scientists and clinicians to study healthy as well as clinical populations.

Neural control and cognitive processes can be studied through eye movements. The VisualEyes software allows an operator to program stimuli on two computer screens independently using a simple, custom scripting language. The system can stimulate tandem eye movements (saccades and smooth pursuit) or opposing eye movements (vergence) or any combination.

Eye movement studies have provided a strong foundation forming an understanding of how the brain acquires visual information in both the normal and ...

Eye-Tracking Control to Assess Cognitive Functions in Patients with Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder with pathological involvement of upper and lower motoneurons, subsequently leading to progressive loss of motor and speech abilities. In addition, cognitive functions are impaired in a subset of patients. To evaluate these potential deficits in severely physically impaired ALS patients, eye-tracking is a promising means to conduct cognitive tests. The present article focuses on how eye movements, an indirect means of communication for physically disabled patients, can be utilized to allow for detailed neuropsychological assessment. The requirements, in terms of oculomotor parameters that have to be met for sufficient eye-tracking in ALS patients are presented. The properties of stimuli, including type of neuropsychological tests and style of presentation, best suited to successfully assess cognitive functioning, are also described. Furthermore, recommendations regarding procedural requirements are provided. Overall, this methodology provides a reliable, easy to administer and fast approach for assessing cognitive deficits in patients who are unable to speak or write such as patients with severe ALS. The only confounding factor might be deficits in voluntary eye movement control in a subset of ALS patients.

Cognitive deficits are common in about one third of patients with amyotrophic lateral sclerosis, a neurological condition leading to progressive impairments in speech and movement abilities. To conduct cognitive tests in patients unable to speak or write a reliable and easy to administer eye-tracking paradigm was developed.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder with pathological involvement of upper and lower motoneurons, subsequently leading to ...

Behavior

Driving Simulation in the Clinic: Testing Visual Exploratory Behavior in Daily Life Activities in Patients with Visual Field Defects

Patients suffering from homonymous hemianopia after infarction of the posterior cerebral artery (PCA) report different degrees of constraint in daily life, despite similar visual deficits. We assume this could be due to variable development of compensatory strategies such as altered visual scanning behavior. Scanning compensatory therapy (SCT) is studied as part of the visual training after infarction next to vision restoration therapy. SCT consists of learning to make larger eye movements into the blind field enlarging the visual field of search, which has been proven to be the most useful strategy1, not only in natural search tasks but also in mastering daily life activities2. Nevertheless, in clinical routine it is difficult to identify individual levels and training effects of compensatory behavior, since it requires measurement of eye movements in a head unrestrained condition. Studies demonstrated that unrestrained head movements alter the visual exploratory behavior compared to a head-restrained laboratory condition3. Martin et al.4 and Hayhoe et al.5 showed that behavior demonstrated in a laboratory setting cannot be assigned easily to a natural condition. Hence, our goal was to develop a study set-up which uncovers different compensatory oculomotor strategies quickly in a realistic testing situation: Patients are tested in the clinical environment in a driving simulator. SILAB software (Wuerzburg Institute for Traffic Sciences GmbH (WIVW)) was used to program driving scenarios of varying complexity and recording the driver's performance. The software was combined with a head mounted infrared video pupil tracker, recording head- and eye-movements (EyeSeeCam, University of Munich Hospital, Clinical Neurosciences).
The positioning of the patient in the driving simulator and the positioning, adjustment and calibration of the camera is demonstrated. Typical performances of a patient with and without compensatory strategy and a healthy control are illustrated in this pilot study. Different oculomotor behaviors (frequency and amplitude of eye- and head-movements) are evaluated very quickly during the drive itself by dynamic overlay pictures indicating where the subjects gaze is located on the screen, and by analyzing the data. Compensatory gaze behavior in a patient leads to a driving performance comparable to a healthy control, while the performance of a patient without compensatory behavior is significantly worse. The data of eye- and head-movement-behavior as well as driving performance are discussed with respect to different oculomotor strategies and in a broader context with respect to possible training effects throughout the testing session and implications on rehabilitation potential.

Patients with visual deficits after stroke report about different constraints in daily life most likely due to variable compensatory strategies, which are difficult to differentiate in clinical routine. We present a clinical set-up which allows measurement of different compensatory head- and eye-movement-strategies and evaluating their effects on driving performance.

Patients suffering from homonymous hemianopia after infarction of the posterior cerebral artery (PCA) report different degrees of constraint in daily ...

Medicine

Eye-tracking to Distinguish Comprehension-based and Oculomotor-based Regressive Eye Movements During Reading

Regressive eye movements are eye movements that move backwards through the text and comprise approximately 10-25% of eye movements during reading. As such, understanding the causes and mechanisms of regressions plays an important role in understanding eye movement behavior. Inhibition of return (IOR) is an oculomotor effect that results in increased latency to return attention to a previously attended target versus a target that was not previously attended. Thus, IOR may affect regressions. This paper describes how to design materials to distinguish between regressions caused by comprehension-related and oculomotor processes; the latter is subject to IOR. The method allows researchers to identify IOR and control the causes of regressions. While the method requires tightly controlled materials and large numbers of participants and materials, it allows researchers to distinguish and control the types of regressions that occur in their reading studies.

The method was designed to investigate the role of inhibition of return (IOR) in regressive eye movements during reading. The focus is on differentiating between regressions triggered as a result of comprehension difficulty versus those triggered from oculomotor error, including the role of IOR in the two types of regressions.

Regressive eye movements are eye movements that move backwards through the text and comprise approximately 10-25% of eye movements during reading. As ...

Efficiently Recording the Eye-Hand Coordination to Incoordination Spectrum

The objective analysis of eye movements has a significant history and has been long proven to be an important research tool in the setting of brain injury. Quantitative recordings have a strong capacity to screen diagnostically. Concurrent examinations of the eye and upper limb movements directed toward shared functional goals (e.g., eye-hand coordination) serve as an additional robust biomarker-laden path to capture and interrogate neural injury, including acquired brain injury (ABI). While quantitative dual-effector recordings in 3-D afford ample opportunities within ocular-manual motor investigations in the setting of ABI, the feasibility of such dual recordings for both eye and hand is challenging in pathological settings, particularly when approached with research-grade rigor. Here we describe the integration of an eye tracking system with a motion tracking system intended primarily for limb control research to study a natural behavior. The protocol enables the investigation of unrestricted, three-dimensional (3D) eye-hand coordination tasks. More specifically, we review a method to assess eye-hand coordination in visually guided saccade-to-reach tasks in subjects with chronic middle cerebral artery (MCA) stroke and compare them to healthy controls. Special attention is paid to the specific eye- and limb-tracking system properties in order to obtain high fidelity data from participants post-injury. Sampling rate, accuracy, permissible head movement range given anticipated tolerance and the feasibility of use were several of the critical properties considered when selecting an eye tracker and an approach. The limb tracker was selected based on a similar rubric but included the need for 3-D recording, dynamic interaction and a miniaturized physical footprint. The quantitative data provided by this method and the overall approach when executed correctly has tremendous potential to further refine our mechanistic understanding of eye-hand control and help inform feasible diagnostic and pragmatic interventions within the neurological and rehabilitative practice.&#160;

Cerebral injury can damage both ocular and somatic motor systems. Characterization of motor control post-injury affords biomarkers that assist in disease detection, monitoring, and prognosis. We review a method to measure eye-hand movement control in health and in pathologic incoordination, with look-and-reach paradigms to assess coordination between eye and hand.

The objective analysis of eye movements has a significant history and has been long proven to be an important research tool in the setting of brain ...

Dynamic Digital Biomarkers of Motor and Cognitive Function in Parkinson's Disease

As Parkinson&#8217;s disease (PD) is a heterogeneous disorder, personalized medicine is truly required to optimize care. In their current form, standard scores from paper and pencil symptom- measures traditionally used to track disease progression are too coarse (discrete) to capture the granularity of the clinical phenomena under consideration, in the face of tremendous symptom diversity. For this reason, sensors, wearables, and mobile devices are increasingly incorporated into PD research and routine care. These digital measures, while more precise, yield data that are less standardized and interpretable than traditional measures, and consequently, the two types of data remain largely siloed. Both of these issues present barriers to the broad clinical application of the field&#8217;s most precise assessment tools. This protocol addresses both problems. Using traditional tasks to measure cognition and motor control, we test the participant, while co-registering biophysical signals unobtrusively using wearables. We then integrate the scores from traditional paper-and-pencil methods with the digital data that we continuously register. We offer a new standardized data type and unifying statistical platform that enables the dynamic tracking of change in the person&#8217;s stochastic signatures under different conditions that probe different functional levels of neuromotor control, ranging from voluntary to autonomic. The protocol and standardized statistical framework offer dynamic digital biomarkers of physical and cognitive function in PD that correspond to validated clinical scales, while significantly improving their precision.&#160;

This protocol offers a digitization of portions of traditional clinical tasks commonly used to measure cognition and motor control in Parkinson&#8217;s disease. Clinical tasks are digitized while biophysical rhythms are co-registered from different functional levels of the nervous systems, ranging from voluntary, spontaneous, automatic to autonomic.

As Parkinson&#8217;s disease (PD) is a heterogeneous disorder, personalized medicine is truly required to optimize care. In their current form, standard ...

Assessing Binocular Central Visual Field and Binocular Eye Movements in a Dichoptic Viewing Condition

Macular degeneration typically results in heterogeneous binocular central visual defects. Currently available approaches to assess central visual field, like the microperimetry, can test only one eye at a time. Therefore, they cannot explain how the defects in each eye affect the binocular interaction and real-world function. Dichoptic stimulus presentation with a gaze-controlled system could provide a reliable measure of monocular/binocular visual fields. However, dichoptic stimulus presentation and simultaneous eye-tracking are challenging because optical devices of instruments that present stimulus dichoptically (e.g., haploscope) always interfere with eye-trackers (e.g., infrared video-based eye-trackers). Therefore, the goals were 1) to develop a method for dichoptic stimulus presentation with simultaneous eye-tracking, using 3D-shutter glasses and 3D-ready monitors, that is not affected by interference and 2) to use this method to develop a protocol for assessing central visual field in subjects with central vision loss. The results showed that this setup provides a practical solution for reliably measuring eye-movements in dichoptic viewing condition. In addition, it was also demonstrated that this method can assess gaze-controlled binocular central visual field in subjects with central vision loss.

Presented here is a protocol for assessing binocular eye movements and gaze-controlled central visual field screening in participants with central vision loss.

Macular degeneration typically results in heterogeneous binocular central visual defects. Currently available approaches to assess central visual field, ...

Eye Tracking During A Complex Aviation Task For Insights Into Information Processing

Eye tracking has been used extensively as a proxy to gain insight into the cognitive, perceptual, and sensorimotor processes that underlie skill performance. Previous work has shown that traditional and advanced gaze metrics reliably demonstrate robust differences in pilot expertise, cognitive load, fatigue, and even situation awareness (SA).
This study describes the methodology for using a wearable eye tracker and gaze mapping algorithm that captures naturalistic head and eye movements (i.e., gaze) in a high-fidelity flight motionless simulator. The method outlined in this paper describes the area of interest (AOI)-based gaze analyses, which provides more context related to where participants are looking, and dwell time duration, which indicates how efficiently they are processing the fixated information. The protocol illustrates the utility of a wearable eye tracker and computer vision algorithm to assess changes in gaze behavior in response to an unexpected in-flight emergency.
Representative results demonstrated that gaze was significantly impacted when the emergency event was introduced. Specifically, attention allocation, gaze dispersion, and gaze sequence complexity significantly decreased and became highly concentrated on looking outside the front window and at the airspeed gauge during the emergency scenario (all p values &#60; 0.05). The utility and limitations of employing a wearable eye tracker in a high-fidelity motionless flight simulation environment to understand the spatiotemporal characteristics of gaze behavior and its relation to information processing in the aviation domain are discussed.

Eye tracking is a non-invasive method to probe information processing. This article describes how eye tracking can be used to study gaze behavior during a flight simulation emergency task in low-time pilots (i.e., &lt;350 flight hours).

Eye tracking has been used extensively as a proxy to gain insight into the cognitive, perceptual, and sensorimotor processes that underlie skill ...

A Visuospatial Planning Task Coupled with Eye-Tracker and Electroencephalogram Systems

The planning process, characterized by the capability to formulate an organized plan to reach a goal, is essential for human goal-directed behavior. Since planning is compromised in several neuropsychiatric disorders, the implementation of proper clinical and experimental tests to examine planning is critical. Due to the nature of the deployment of planning, in which several cognitive domains participate, the assessment of planning and the design of behavioral paradigms coupled with neuroimaging methods are current challenges in cognitive neuroscience. A planning task was evaluated in combination with an electroencephalogram (EEG) system and eye movement recordings in 27 healthy adult participants. Planning can be separated into two stages: a mental planning stage in which a sequence of steps is internally represented and an execution stage in which motor action is used to achieve a previously planned goal. Our protocol included a planning task and a control task. The planning task involved solving 36 maze trials, each representing a zoo map. The task had four periods: i) planning, where the subjects were instructed to plan a path to visit the locations of four animals according to a set of rules; ii) maintenance, where the subjects had to retain the planned path in their working memory; iii) execution, where the subjects used eye movements to trace the previously planned path as indicated by the eye-tracker system; and iv) response, where the subjects reported the order of the visited animals. The control task had a similar structure, but the cognitive planning component was removed by modifying the task goal. The spatial and temporal patterns of the EEG revealed that planning induces a gradual and lasting rise in frontal-midline theta activity (FM&#952;) over time. The source of this activity was identified within the prefrontal cortex by source analyses. Our results suggested that the experimental paradigm combining EEG and eye-tracker systems was optimal for evaluating cognitive planning.

The study of cognitive planning combining EEG and eye-tracking systems provides a multimodal approach to investigate the neural mechanisms that mediate cognitive control and goal-directed behavior in humans. Here, we describe a protocol for investigating the role of brain oscillations and eye movements in planning performance.

The planning process, characterized by the capability to formulate an organized plan to reach a goal, is essential for human goal-directed behavior. Since ...