EOG Protocol Summer 2023(1)

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Drexel University *

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302

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Electrical Engineering

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Feb 20, 2024

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Lab Module 3: EOG Overview For this lab you will be measuring eye movement via the rotations of the natural dipole always present in the eye. You will need:  3 electrodes  Electrode lead set  Alcohol pads (as usual)  Tape measure  3 Stickers or something to mark visual reference points Introduction This series of experiments using EOG will first calibrate voltage to eye movements, then explore features of the EOG measurement during different tasks including head rotations and reading. You will also explore how the calibration methodology influences EOG measurements. Prepare the electrodes:  Abrade the skin with alcohol on either side of the eyes and the back of the neck.  Place a disposable electrode on the outside of each eye (the canthus) – these will be your positive and negative leads. Place a third electrode on the back of the neck on the bony protuberance. This will be your ground.  Make sure electrodes are set up ~ 5 min before the experiment actually starts To set up Biopac:  Open Biopac  Sample rate: 200Hz  Channel setup: select EOG from the presets Set up a calibration as seen in Figure 1.  First select the distance your subject is comfortable reading the computer screen – usually 18” but some people do 2’. Protocol is written with 2’ but you can use 18” if that is more comfortable for your subject.  Next, mark your targets . Using your measuring tape and a sticker/post it note, mark 2 ft straight ahead, 2 ft to the right of the straight ahead mark, and 2 ft to the left. Be sure to also mark where you subject is! You are creating a straight ahead, 45° right, and 45° left target. Keep in mind that all your experiments should be done the same distance apart from one another (so your subject should be positioned 2 ft from the computer screen during the reading experiment, for example*)

Figure 1: Top down view of calibration setup. Select a distance x that is comfortable for your subject. You will use that distance for all your base experiments. Note: You will perform a calibration by having the subject look straight ahead to the 0° target, to one 45° target, back to straight ahead, and then to the 45° target in the other direction – all without moving the head (at all). Make sure your subject is JUST moving their eyes. You do this calibration to convert voltages to eye angles . It is incredibly important that you get a good calibration. If your calibration data doesn’t look good you are going to struggle with your data analysis. Also, make sure your subject performs the movements rapidly to and from the extreme angle s to reduce signal drift. HINT: Make sure that your calibration setup and all your data are saved for future analysis.  Perform the calibration. Be sure to start the recording and ensure that the baseline is stable before starting any experiments. While recording, the subject will look straight ahead to the 0° mark for at least 2 seconds; then quickly look at mark to the left (only find the mark then go back); then center again, repeating this (straight, direction 1, straight, direction 2) several times. NOTE: ONLY THE EYES SHOULD MOVE DURING THE CALIBRATION – NOT THE HEAD. Check your calibration data. DO NOT PROCEED if the data are not good. Minimize the time the eyes are focused on the extreme points . Longer durations of time spent at +45° and -45° will cause drift in your signal.  Your data should look something like Figure 2. Keep in mind that how you set up your electrodes determines whether looking left is positive or negative. Make sure you are clear in your lab report about where you positioned your positive and negative electrodes.

Figure 2: Ideal calibration data Note: When you analyze your data (later), the first step in doing so will be to apply the calibration. This will let you convert your data from voltage to degrees of eye movement. You will find the average voltages for looking 45° right and 45° left. These will be V1 and V2. They should be very consistent. For example, if every 45° right is not similar, then you will need to redo the calibration data collection . In your data analysis (later), to calibrate, you will identify your average V1 and V2 (see Figure 3) Figure 3: Finding V1 and V2 Once you have found V1 and V2, to convert your data to angles, simply use the following equation: where ϕ (t) is the eye angle, V(t) is the voltage reading from your data [throughout all of your experiments, the data you are actually collecting is voltage over time V(t)], V1 is the more positive voltage V2 is the more negative voltage. Once you are confident that you have a good set of calibration data, you will perform a series of additional experiments. Body moves, eyes locked with head (Tunnel Vision)  Instruct the subject to keep their head as still as possible and their eyes focused in line with their body (looking wherever the body is pointed – tunnel vision). Using the same targets from the calibration, have the chair pointed straight ahead, rotate it to point the subject 45° to the right for ~1 sec, straight ahead (~1 sec), to the left (~1 sec). Repeat moving between the points several times.

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Body moves, eyes locked on (focused on) center point.  Repeat the experiment but this time instruct the subject to lock their gaze on the center target regardless of the direction the chair is pointed. The head must move with the body, so ONLY the eyes remain fixed on the center target. Text Reading Experiments  The next set of experiments will look at saccades during reading. Download the EOG reading file from BBlearn. You will want to keep notes on when the subject begins and finishes each section, and have the subject return their eyes to the 0 ° marker prior to starting each section. This will help you to correct for any drift later. You can zoom in on the screen before the subject starts reading, but NO scrolling WHILE the subject is actively reading text. Confirm all your reading data looks good and that you can count saccades on at least 6 lines for each text alignment segment. If you cannot, then re-do that segment, ensuring the subject is not moving their head at all. You may also consider zooming in on the text a little to make it easier to read, just make sure you don’t overdo, the subject should still feel like they are reading normally, and you should NOT have to scroll to complete the segment. Complete your own EOG experiment on the effects of electrode alignment. From our prior work, you already know that the more our underlying signal is in line with our electrodes, the more of the signal we will capture. For your experiment this week, I want you to construct a clear hypothesis that you can test about this concept. The challenge here is to ensure that you have something quantifiable and testable, then to create an experiment that will give you the data to test it. Keep in mind you will need to do a statistical analysis of some sort. You also MUST INCLUDE AT LEAST 3 CONDITIONS TO TEST YOUR HYPOTHESIS FOR THE STATISTICAL ANALYSIS. For your lab reports:  Don’t forget you have the choice here – positive and negative leads being right vs left, so MAKE SURE YOU INCLUDE WHAT YOU DID.  Show your calibration data and the equations you use to convert from voltage to eye angle.  Convert data to degrees. If you had a lot of drift over time, you will be expected to note the drift in the results, correct for it (mentioning this in the methods) and address it in the discussion.  Compare the calibration data set with the chair swivel gaze locked with head and with the chair swivel gaze locked at 0° – tunnel vision (qualitatively, what about these data sets is similar or different? DESCRIBE what you see)  Present the results of the reading experiment. Identify saccades and when the eyes changed to the next line. Address the differences in the data when the subject was reading text that was justified or aligned to the right or left. Don’t forget you have multiple measurements that will need statistical analysis: N saccades, time/line, start angle, end angle.  Present the results (and of course discuss) your own experiments. Consider….. 1) What was happening during the tunnel vision condition? Why do the data look the way they do? 2) How and why do the eyes move during reading? What does this tell you about the eyes vs the brain? What do the changes you observed when the subject was reading out loud relate to the physiology of the eye and the mental task of reading out loud?

Required Comparisons and Things you Probably (definitely) want to include 1) Methods for calibration figure (1 part figure? 2 part figure?) 2) Methods figure for reading data showing for a line of data where your measurements are coming from. 3) Results comparing a cycle of the calibration/eyes on target/tunnel vision 4) Reading Data:  Effect of text alignment (silent reading data) on: -N Saccades, reading time/line, angle measurements (start angle and end angle). Note that for all reading data, you should look to do measurements in 6 lines of text, but it can be any 6 lines of text with that alignment. 5) Your experimental results including statistical analysis.