This video demonstrates a procedure for monitoring eye movements while participants view images. An eye tracker helmet is first placed on the participant, and then cameras are adjusted to sit just below each eye. The cameras are focused to obtain pupil and corneal reflections.
By having the participant fixate on a series of targets, eye movements are calibrated and validated Through inspection of changes in the eye movements, scanning patterns, it can be shown that details regarding previously viewed images have been stored in memory. Hi, I am Dr.Jennifer Ryan, senior scientist at the Rotman Research Institute at Baycrest in Toronto. Today we would like to show you our protocol, eye movement monitoring of memory.
This method can help answer key questions in the field of memory, such as when memories are accessed and what details are maintained in those representations. To monitor eye movement during memory integrity experiments, an iLink two eye tracker system is used. This head mounted video based eye tracker records eye position in the XY coordinate frame at a sampling rate of either 500 or 250 hertz with a spatial resolution of less than 0.1 degree.
One camera is used to monitor head position by sending infrared markers to sensors placed on the four corners of the display monitor that is viewed by the participants. Two additional cameras mounted on the headband situated below each of the eyes and infrared illuminators are used to note that pupil and corneal reflections eye position may be based on pupil and corneal reflections, or based on the pupil, only the padded headband of the eye tracker can be adjusted in two planes to comfortably fit the head size of an adult participant, most eyeglasses and contact lenses can be accommodated by the eye tracker. Two PCs are used for eye movement recording one computer serves as the display computer, which presents the calibration screens necessary task instructions and the images used in the experimental paradigm to the participants.
The host computer calculates real-time gaze position and records the eye movement data, as well as any button press or keyboard responses made by the participants. The software program Experiment builder is used both to present the experimental stimuli to the participants and to program the manner in which I position is collected by the host pc. The software program data viewer is used to characterize eye movements during recording.
Now let's see how to set up and conduct an eye tracking experiment. Select the appropriate experimental paradigm on the display computer and designate a file name for the ensuing eye movement recordings. In preparation for an experiment, see the participant a fixed distance from the monitor so that the same visual angle is maintained across.
Participants adjust the eye tracker helmet so it is snug and unlikely to move, but not uncomfortable around the head. Further, adjust the helmet so that the head camera can send infrared illumination to the external markers on the display monitor. Each of the eye cameras is situated on individual rods that extend from the helmet, which allow for adjustment in all axes.
Position the eye cameras just under and slightly away from each eye without obstructing the participant's view of the display monitor. Focus the cameras to get a clear and stable image of the pupil and corneal reflections. Status panels on the host computer indicate where pupil and corneal reflections are being acquired.
Adjust the illumination threshold to obtain the most stable recording of the pupil and corneal reflections. Once the cameras are set up to obtain accurate recordings of the eye position on the display monitor, initiate a calibration procedure. Instruct the participant to look at a series of targets that appear at various locations on the display.
Typically, nine target locations are used with the center location repeated twice, but calibration can be performed with as few as three target locations. From the recorded locations. Eye position at any point on the screen can be interpolated.
Following calibration. A validation procedure is used to check the accuracy of the eye movement recording. The same nine target locations are provided to the participant to fixate, and the difference is computed between the current fixation position and the previously recorded fixation position.
If the average accuracy for recording of position for one or both eyes exceeds an error of 0.5 degrees, calibration and validation procedures are repeated. Okay, so now we're gonna do the calibration and the validation, and I just want to tell you that it's okay to move your head slightly, but we prefer for you to stay as still as possible for the remainder of the experiment. Is that all right?
Yep.Perfect.All right, let's get started then. Conversion of the eye movement data to a series of fixation and SEC card events that a timelock, a stimulus presentation is achieved through the host computer and can be interrogated with the software program data viewer. Here, the detection of fixations and SEC cards are dependent on an online passer, which separates raw eye movement samples into meaningful states.
If the velocity of two successive eye movement samples exceeds 22 degrees per second over a distance of 0.1 degree, the samples are labeled as a SEC card. If the pupil is missing for three or more samples, the eye activity is marked as a blink within the data stream. non-SEC card and non blink activity are considered fixations Viewings to specific regions of a stimulus can be characterized by analyzing eye movements with respect to experimental drawn regions of interest that are created for each image, either at the paradigm programming phase or after data collection.
Now we'll show some representative eye tracking results to probe memory for a single image. The degree to which the eyes move around the image with each viewing is measured representative results for the number of eye fixations made to faces. As shown here, faces were viewed once in each of five blocks.
Over time, there is a decrease in the number of fixations made to the faces that were repeated across the experiment to probe memory for details within an image. Viewing between changed images and images that have not been altered are contrasted in these representative results. A greater proportion of eye fixations are directed to a region that has been changed from manipulated scenes compared to the unchanged region for novel scenes and scenes that have been repeated.
This increase in viewing reflects memory for what had originally occupied the changed region. This increase of viewing to a changed region is observed only in younger adults and is typically not present in older adults and amnesiac patients who report memory problems After its development. This technique paved the way for researchers in psychology and cognitive neuroscience to explore cognitive function such as memory in multiple participant populations, including people who have brain lesions or other neurodegenerative diseases.
