High density event related potentials or ERPs allow for tracking of the rapid temporal dynamics of the functioning human brain. To acquire high density ERP data, the electrode cap is first placed on the participant's head, and the electrode locations are digitized to accommodate variability and head shape across subjects. Electrode impedances are then reduced by injecting gel in each electrode.
Data is acquired while visual stimuli are presented to the participant. When recording is finished, the electrode cap is thoroughly cleaned. Acquired data is used to examine the spatial and temporal dynamics of brain activity associated with a given cognitive process.
Hi, I'm Scott Slotnik from the Memory and Perception Lab in the Department of Psychology at Boston College. Today we're going to show you a protocol for high density event related potential data acquisition. We use this procedure in our laboratory to study the spatiotemporal dynamics of brain activity during memory for visual items.
So let's get started. Event related potentials or ERPs provide a method to capture the spatial temporal dynamics of human brain function with excellent temporal resolution. Data are typically measured in a shielded room, minimizing interference with the neural signal that is picked up by the electrodes.
Each electrode connects to an amplifier, which in turn connects to a data acquisition computer sitting outside the shielded room. A separate computer is used for stimulus presentation because of possible interference with the neural signal. Only essential powered components should be housed inside the shielded chamber, such as the stimulus display and the response keyboard.
During experiments, participants are seated in a comfortable chair with armrests and a back height that provides shoulder support to minimize neck muscle artifact. Let's begin by seeing how to place the electrode cap on. A participant electrodes are embedded in a spandex cap, which significantly reduces application time to position the cap place electrode oz according to the ten five electrode system while applying the cap, ensure that it has left right symmetry with midline electrodes placed over the midline of the head.
Also, make sure that the most posterior inferior electrodes are superior to the skull neck boundary to avoid neck muscle artifact. Once the cap is in place, attach the side straps with a chin strap or a custom made belt at waist level to improve lateral electrode contact with the head. Apply electrodes adjacent to the eyes to allow for subsequent removal of eye movement or blink artifacts to address variability in electrode placement and head shape across subjects electrode locations are measured for each participant with a polyus fast track digitizer.
The digitizer includes a transmitter, three receivers, which mount on the cap and a stylus for recording each electrode location with the cap in place and electrode digitization complete. Let's move on to reducing the electrode impedances seat, the participant comfortably in the recording chair. Some participants find it more comfortable when a folded hand towel is lightly tucked between their shoulders and the chair back.
Now, plug the multi electrode cap into the amplifiers. Next, the impedance of each electrode must be reduced such that it is below approximately five kilo ohmes. This is accomplished in our lab by measuring real-time impedances with neuros scan scan software while using a sterile syringe and blunt tip needle to inject a conducting gel into each electrode opening.
There are a number of techniques that can speed up the impedance reduction process, which is the most time consuming aspect of multichannel recording. Beginning with the ground or reference electrode, use your dominant hand to make a few circles with the syringe while it rests against the head. To move the intervening hair, be sure not to press the syringe against the participant's head.
The gel application process should never cause the participant discomfort. Next, while lightly pressing the electrode down with the non-dominant hand, inject a small amount of gel at the scalp and then pull the syringe away while continuing to inject. Creating a gel bridge between the scalp and the electrode gel that protrudes from the electrode opening should be wiped off with a tissue and discarded at this point.
If the first set of electrode impedances all remain high, gel should be reapplied to the ground and reference electrodes as gel usually becomes more conductive over time. One strategy is to inject gel in electrodes within a scalp quadrant, such as the right posterior scalp until all impedances begin to decrease. Then inject gel in electrodes within the next quadrant until all impedances begin to decrease and then cycle through the quadrants.
Re-injecting gel into the highest impedance electrodes. In some cases, conducting gel will connect to adjacent electrodes such that the impedances will be linked. This reduces spatial resolution but is usually of minor concern as there are a high number of electrodes with the impedances reduced.
Let's see how to record data before beginning a recording session. Encourage the participant to get into a comfortable, relaxed position and to remain still. This will minimize muscle and movement artifacts.
Once the participant is comfortable, hand them a response keyboard and remove or shut off all non-essential equipment near the participant such as lights to allow for subsequent event related analysis. The onset of each stimulus event must be signaled or triggered by the stimulus computer and stored along with the electrophysiological data. We send trigger pulses at each stimulus onset through the parallel port via ePrime programs that include custom inline port initialization, and trigger scripts that are freely available.
These triggers are received and stored by the scan software. When recording electrophysiological data, use a standard high pass filter to remove very low frequency components such as DC that are irrelevant to the transient neural response. In environments with very little interference, a low pass filter with a very high frequency cutoff, such as 200 hertz can be used in environments with higher levels of ambient electromagnetic interference.
A lowpass filter with a cutoff around 80 hertz and a 60 hertz notch filter can be used, so the response is dominated by the neural signal. Now let's see how to clean the cap. After a session is finished, the multi electrode cap must be thoroughly cleaned and disinfected immediately after recording is complete.
Begin by soaking the cap in warm water for five to 10 minutes and then rinsing each electrode with a running water stream. To thoroughly remove all of the conducting gel, use the blunt end of a wooden stick cotton swab to clear the holes in the electrodes. Next, soak the cap in a soapy warm water bath consisting of four quarts of water and one to two ounces of dial soap for 30 minutes to ensure all the gel has been removed, then thoroughly rinse the cap with water.
To avoid cross participant contamination, the cap must be soaked a final time for 1530 minutes in a water disinfectant mixture such as four parts, water to one part and ide, and then rinse thoroughly again with water when hanging up the cap to dry. It should be placed symmetrically and without tension as it may retain some degree of its drying position, which can reduce the ease of subsequent impedance reduction. Let's wrap up by taking a look at some representative results.
Now we'll show some representative results following ERP data acquisition and analysis. The scalp voltage topography to the left and right reflect rapid rettino topic reactivation of visual cortical regions during accurate memory for items previously presented in the right and left visual field respectively. I've just shown you how to acquire high density ERP data when using this procedure.
It's important to remember that participant comfort always comes first. So that's it. Thanks for watching and good luck with your experiments.
