Laboratory 2

Human electrooculargrams

Electrooculography (EOG) Angular Displacement

Recording eye movements: an introduction to acquisition of biological signals

We move our eyes constantly during our daily activities to keep our line of sight point-ed at a target of interest. In order to generate eye movements we have three antagonistic pairs of muscles that are attached to the globe of the eye. These three sets of muscles function to move the eyes horizontally (left versus right), vertically (up versus down) and torsionally (clockwise versus counter clockwise).

Electrooculography (EOG) is the electric transduction of combined eye movement. Without moving the head, a variety of horizontal and vertical eye movements encompass a visual field in which you can explore an image, utilizing spontaneous tendencies of gaze during conversations or reading. Sometimes, analysis of spontaneous gaze tendencies can indicate the attitude of a subject. For example, spontaneous movements towards the left are usually under control of the right hemisphere, which is generally associated with negative emotions.
Among motor activities that are easily observed and strongly reliant on cognitive function, saccades are of great interest. Saccades are rapid eye movements whose speed is proportional to the amplitude of angular displacement and can reach 800°/sec. They compose two of the three major stages in the ocular fixation process:
• initiation of movement, voluntary or not
• saccade corresponding to gaze displacement
• maintenance (micro-saccades) or suspension of fixation
For a given angular displacement, saccade duration is constant (see graph and table below). Its latency time, for visual stimulations, is around 200 ms. Despite any attempts by subjects, they cannot influence the speed of the saccade, but that speed can often vary as a function of attention level.

The Technique: Electrooculography (EOG)

In the 1920's it was discovered that by placing electrodes on the skin in the region of the eyes, one could record electrical activity that changed in synchrony with movements of the eyes in the head. It was initially believed that these potentials reflected the action potentials in the muscles that are responsible for moving the eyes in the orbit.
However, it is now generally agreed that these electrical potentials are generated by the permanent potential difference that exists between the cornea and the retina (cornea-retinal potential, 10-30 mV: the cornea being more positive). This potential difference sets up an electrical field in the tissues surrounding the eye. As the eye rotates, the field vector rotates correspondingly. Therefore, eye movements can be detected by placing electrodes on the skin in the area of the head around the eyes. Vertical movements of the eyes are best measured by placing the electrodes on the lids, while horizontal eye movements can be best measured by placing the electrodes on the external canthi (the bone on the side of the eye). Limitations of the technique.

Underlying assumptions in this technique:

The assumption made when using this method of recording eye movements is that the movement of the electric field in the conducting tissues surrounding the eye is related, in a simple (usually assumed to be linear) way to movements of the eye itself. Due to the non-uniformity of these tissues and the shapes of the tissues surrounding them, this can only be an approximation to the biological reality. However, for horizontal eye movements within the range of 30 degrees, the potential measured is assumed to be linear to the actual movement of the eye in the orbit (in degrees of arc, see Fig. 8). The resolution of EOG is considered to be about 1 degree. Because it is a relatively simple technique, EOG is still commonly used clinically for testing eye movements in patients.

For a fixed eye position, the EOG is far from being constant in magnitude, but can be influenced by a number of external factors. These factors include 1) the noise generated between the electrodes' contacts and the skin, 2) the metabolic state of the tissues (pO2, pCO2, and temperature), 3) visual stimulation, and 4) contraction of the facial muscles. In addition, recorded EOG, particularly for vertical eye movements, is quite sensitive to movements of the eye lids. In summary, there are a number of external factors that can complicate the interpretation of the EOG, and for that reason EOG is considered to be highly sensitive to artifacts. Artifacts, which can be induced through the contact between the electrode contacts and the skin, can be minimized by reducing the resistance between the electrodes and the skin. For this reason, the subject should be sure to have thoroughly scrubbed his/her skin with the alcohol prior to attaching the EOG electrodes.

Subject and Equipment Preparation for Horizontal EOG

Your experiments will be recorded with silver/silver chloride surface electrodes. These electrodes have been specially designed for recording biological potentials.

• Safety issue! Once hooked to the electrode cable, the subject should stay seated and refrain from touching the electronics and computer.

Surface electrodes: Skin Preparation and Placement

Both skin preparation and electrode are important to the production of a clean signal

• Thoroughly clean the skin on the side of each eye (the area between the eye and the hairline) and behind one ear with alcohol and a small square of abrasive pad. Dry the cleaned surfaces with a Chem wipe.
• Place an electrode between the hairline and corner of each eye with the contact between the eyebrow and the purple.
• Place the third electrode behind one ear, as shown. The correct location for the placement of each electrode is illustrated to the right.
• Have the subject firmly press the electrodes on his/her skin, once they have been correctly positioned.

• Wait at least 5 minutes once the preceding exercises have been completed, and then connect the EOG electrodes to the pinch lead set. Attach the red shielded cable to the right electrode, the black shielded cable to the left electrode and the unshielded ground to the reference electrode in the center of the head.

 

 




• Plug the other end of the cables into the electrode extension cable (BSCBL8).  The red wire into Vin+ socket (white)and its shield into the shield socket (green), The black cable into the Vin- socket (black) and its shield to the shield socket (dark red) and the unshielded reference wire into the ground socket (brown).  There is no shield for the reference electrode.
• Plug the other end of the extension cable into the MP35 electrode check.  Clip the extension cable to the collar of the experimental animal and tape the electrode cables to together at the hair line and at the back of the head.  Add a pointer hat (there are several flattering colors to choose from) to hold the cables in place with out tension on the electrodes.
• Turn on the MP35 and start BPSLpro. Verify that the MP35 is communicating by clicking the start button. The electrode impedance should be at the low end of the scale.

Note: the subject should not wear eyeglasses during eye movement recordings. Can't see the screen with out your glasses?  Use another set of eyes. Contacts are ok; its the metal frames acting as anteing that are the problem.

Recording Your Eye Movements

Eye movements fall into two main categories: 1) Eye movements that function to stabilize the position of the eye in space during head movements (Reflex eye movements) and 2) Eye movements that function to redirect the line of sight to follow a moving target or to attend to a new target of interest (Voluntary eye movements). We will explore both. 

Voluntary Eye Movements - SACCADES:  Saccadic eye movements (saccades) move the eye rapidly to a specific target of interest in visual space. The word saccade originates from the French term for "jerk", named so to reflect the great speed at which these eye movements occur. Eye movements are generally described in terms of the angle (in degrees) that the eye has rotated. Saccades last for only a fraction of a second and can reach speeds up to 900 deg/s.

Angle of Eye Movement

Saccades are under voluntary control and can be made in the dark, or with the eyes closed. During our daily activities, we constantly generate saccadic eye movements to scan our visual surroundings. When viewing an object of interest, we make saccadic eye movements to specific features of the image in order to "analyze" it; in the case of the human face, saccades are made primarily to areas containing the eyes. Likewise, while reading we generate small saccadic eye movements to move from word to word, phrase to phrase, and line to line. As in the previous experiment we need to first configure the computer to acquire this signal.

Software setup

• The template containing the setup information for this experiment is EOG saccade_temp and it is in the Biol436 folder on Stushares.
• Again you can reload the template more rapidly if you download it to your desk top. A summary of the setup can be found here.

System check

• Have the subject first look straight ahead, then repeatedly look as far to the left, and then as far to the right as they can. If the signal is very small, increase the gain until it fits appropriately with in the "green guidelines"
• Your plot should be as tall as possible with out clipping. If the signal is small, increase the gain until it fits appropriately with in the "green clipping guidelines" (click the figure to enlarge)

Construction of a calibration curve:

Now that you know that your maximum and minimum values will not exceed the input range for the ADC, you are configured to record your EOG but The units for EOG are degrees and the input to the MP35  is volts. In order to determine the relationship between the amplitude of the measured voltage and the actual eye movement generated by the subject, it is necessary to construct a calibration curve. There is no fundamental difference between this calibration curve and those you constructed in POB.

• The technologically advanced dual purpose pointer hat should be carefully centered so that when a distant object absolutely centered in the subjects field of vision, it is also aligned with the pointer the pointer.
• A dry run using a short calibration movie should be performed so you know what to expect. 
  • Double click on the EOGcalibration.mov file to launch Quick time
  • open the movie drop down menu and select present movie. You will see a slowed down version of the image below. (click to play)
    

• Position the subject directly in front of the monitor such that the calibrated pointer gently touches the screen and points exactly at the vertical blue line while the subjects eyes are focused on the intersection of the vertical and horizontal blue lines.  This will provide an eye to target distance of 20cm and permit the display of targets at precise angular deviation from center.
• The subject focus directly on the red ball ball when it appears and follow its movement with their eyes with out having the pointer deviate from the vertical blue line.
• It takes far longer to describe than to master but a practice run or two will help.
• Calibration run
  • Stop the calibration and leave Quick time ready to start it up again when you simply press play.
  • The easiest way to choreograph the experiment is to move the Quick time window to the upper left and the BPSLpro graph window to the lower right so you can see them both.  Then activate the BSLpro window (click any where in the window) and click start.
  • As soon as the recording starts click on the quick time window and click start.
  • Then just follow the bouncing ball as in the practice runs.
• Again, when the subject has successfully completed a full trial, save the data to disk. Your data should look like this. Click on the image for a full scale calibration run.

Processing the data

You will need to determine the mean input voltage and its standard deviation that resulted from each of the calibrated angular deviation of the eyes. The distance the target moves along the horizontal axis was calculated from the sine of the required calibration angle, and the length of the adjacent side of the triangle (the 20cm pointer). The targets are placed at 0 degrees (intersection of blue lines and 10, 20,30, and 40 degrees on either side of 0 movement to the left is assigned a positive angle while movement to the right was assigned a negative value to correspond the voltages produced when the eye with a positive cornea is rotated.

• Set your first output value (top of the graph pane) to "mean" using the drop down menu; set the second output value to standard deviation "St de"
• Select the I beam tool from the lower right tool pallet. Click on the beginning of a segment representing 0 and drag it out to the end of that segment. The value in value boxes represent the average voltage and its standard deviation.  The average of each + and - angular deviation. can be determined the same way.
• Enter the all of means, standard deviations and corresponding angle into the Excel table found of stushares. Be certain that you use the page that corresponds to your workstation.

What is the measured value for the center target? This value represents the offset reading for the electrodes, at this subject at this time.

• Plot the data (angle vs EOG potential delivered to the MP35 in mV ) The graph template is here or on stushares
• Over what range (in degrees) is a linear (Eye position directly proportional to EOG amplitude) approximation valid ?
• Select these values and fit a trend line through the data. Display the regression coefficient and the equation along with the regression line.
• From the excel sheet determine the the potential for ± 20 degrees of rotation from the regression equation.
• Open the the set up channels box and select setup to access the scaling dialog box
• Enter ± 20 degrees in the "scale value boxes for cal 1 and cal2. It doesn't matter if the - or positive value is first only that it is paired with its calibrated value.
• Now enter the voltage you calculated for ± 20 degrees in to the input value box.  That's it!  Your set up will now graph actual angular deviation of the subjects eyes.
• Please think about this until the principle becomes clear. You are simply assigning (mapping) an angular deviation to an input voltage. This is exactly the same as assigning a stage micrometer value to an ocular micrometer and you are all expert at that.
• Finally copy the calibration graph into the word document that will become your lab report.

The experiments

Smooth pursuit

Humans and other primates can follow a moving visual target of interest by generating voluntary smooth pursuit eye movements. The smooth eye movements which are generated are designed to keep a small moving target on the center of the receptive surface of the eye so that it is viewed with the greatest accuracy. In the absence of a moving visual target, it is not possible to generate smooth pursuit eye movements. Therefore, it is impossible to generate smooth pursuit eye movements with the eyes closed, in the dark, or when viewing a stationary visual scene.

Horizontal pursuit

• Open the setup acquisitions box and change the acquisition period to 15 seconds for this experiment.
• Have the subject sit with their marvelous pointer just touching the screen on the vertical blue line and their eyes focused on the intersection of vertical and horizontal blue lines.
• Click on "horizpersuit.mov" .and ask the subject to fixate at the intersection of the blue lines and then follow the moving target once it appears. Again a a dry run or two will help.
• Again, it is important that the subject does not move his/her head. and the pointer remain on the blue line.
• As before activate the BPSLpro window and click start.  As soon as acquisition starts activate the Quick time window and start the movie.
• When the subject has successfully completed a full trial, save it data to your desk top with your workstation number and experiment in the file name (MacxxHorizPersuit.acq) then drag a copy to the network drive (Biol435_student_data.
• Copy the biopack graph into the MSWord file that you are using for your lab report (snap and drag is the easiest way) and
  • Describe the resultant eye movements during horizontl persuit.
  • Do they look as you would expect? Why or Why not?
  • What is the amplitude (in degrees) of the peak to peak eye position attained during horizontal pursuit.
  • Was their a frequency component?
  • What was the frequency of the pursuit target stimulus? Explain.

Pursuit with out target

• Click on the "woTarget.mov" file.
• Ask the subject to generate smooth eye movements between the two targets (take about 5 seconds to move from a to b) as in the pursuit paradigms above.
• Again, it is important that the subject does not move his/her head.  
• After a few dry runs, start the acquisition and movie as in the experiments above
• When the subject has successfully completed a full trial, save it data to your desk top with your workstation number and experiment in the file name (MacxxPersuitWOtarget) then drag a copy to the network drive (Biol435_student_data.
• Copy the biopack graph into the MSWord file that you are using for your lab report (snap and drag is the easiest way) describe
  • What was different between the eye movments with and without targets?
  • Why?
    

Pursuit of knowledge

• Increase the acquisition time to 60 seconds
• Click on the "read.mov" file and get it ready to run.
• Activate the BPSLpro window and click start. As soon acquisition starts, start the reading movie file.
• Ask the subject to read the paragraph that appears.
• When the subject has successfully completed a full trial, save it data to your desk top with your workstation number and experiment in the file name (MacxxReading) then drag a copy to the network drive (Biol435_student_data.
• Copy the biopack graph into the MSWord file that you are using for your lab report (snap and drag is the easiest way) and
  • What types of eye movement are generated during this task?
  • Explain.

Reflex Eye Movements

Vestibule- ocular reflex
Un blurred vision is only possible if the eye is stationary (fixed) with respect to a viewed object. The vestibulo-ocular reflex (VOR) is an important mechanism by which un blurred vision is made possible during head movements that are generated during every day activities such as walking and running. For example, if the head is turned to the left, this reflex causes the eyes to move to the right (i.e. in the opposite direction of the head movement). The oppositely directed eye movement occurs at the same velocity as the head movement, and therefore generates an eye movement which keeps our line of sight fixed on the same point in visual space both during and following the movement.

During short head movements, these compensatory eye movements remain well within the mechanical limits of eye rotation. However during large amplitude head rotation, the eye can reach its limit of excursion long before the head movement is completed. Consequently, during this condition, an additional feature is added to the VOR: when the eye reaches an extreme position, it is rapidly flicked back to a new starting position. From this new starting position, the eye then continues a new cycle of compensatory movement during continuing head movement. The resulting "saw tooth" pattern of slow compensatory/ rapid resetting eye movements (slow phases and quick phases respectively) are referred to as vestibular nystagmus.

To see the VOR in action

• Reset the acquisition time to 10 sec.
  • Ask the subject to stare at a fixed object across the room. The pointer should point to it
  • Then ask him/her to rotate their head back and forth quickly while still looking at the object
  • When the subject has successfully completed a full trial, save it data to your desk top with your workstation number and experiment in the file name (MacxxVORwTarget) then drag a copy to the network drive (Biol435_student_data).
  • Copy the biopack graph into the MSWord file that you are using for your lab report (snap and drag is the easiest way)
    • Describe the eye movement during voluntary head movement?
      
  • Repeat the above experiment, but this time, ask the subject to close their eyes and imagine the same target. While the subject rotates their head.
  • When the subject has successfully completed a full trial, save it data to your desk top with your workstation number and experiment in the file name (MacxxVORwithoutTarget) then drag a copy to the network drive (Biol435_student_data).
  • Copy the biopack graph into the MSWord file that you are using for your lab report (snap and drag is the easiest way)
    • Describe what was different about the eye movments during the imaginging of a target?
    • Do the eye movements that are generated depend on whether the subject had the visual target to "look at"?
      
      
• Reset the acquisition time to 60 seconds'
  • Rotate the subject (slowly!) a full 360 degrees to the left while they look at their surroundings.
  • Be careful of the EOG leads!
  • Save the data to disk with rotateL in the file name.
  • Repeat this experiment but rotate the subject to the right.
  • When the subject has successfully completed a full trial, save it data to your desk top with your workstation number and experiment in the file name (MacxxVORrotateR and MacxxVORrotateL) then drag a copy to the network drive (Biol435_student_data.
  • Copy the biopack graph into the MSWord file that you are using for your lab report (snap and drag is the easiest way) describe what was different?
    • How does the direction of the head motion affect the eye movements generated by the subject?

Saccade speed sorry the led's went missing and we can not perform this experiment They can be replaced if you want to incorporate them into a project

But,

You can still determine the rotational velocity of your eyes (rpm) from data on your calibration run. Reopen the calibration run and using that data calculate the rotational velocity. I am not going to provide the formula so think about it for a min and ask yourself what you want to know (RPM = revolutions per min) and write out the formula.

• What data does your graph contain?
• How many degrees in an R?
• What does per mean
• How many seceond in a min.
• What is your formula for RPM calculated from the data at hand.

Remember that the X axis is in seconds not min so yo need to adjut your formula

If you can time the appearance of a target you can determine how long it took to move your eyes to acquire it.

• Load the speed movie.
• Set up the subject
  • Start the movie
  • Have the subject fix on the intersection of the blue lines and then move their gaze as quickly as possible to the target as it appears.
• When the subject has successfully completed a full trial, save it data to your desk top with your workstation number and experiment in the file name (Mac) then drag a copy to the network drive (Biol435_student_data.
• Copy the biopack graph into the MSWord file that you are using for your lab report (snap and drag is the easiest way) describe what was different?
  • You should no be able to compute the rotational velocity of the subjects eyes in degrees/second. Does it differ from right to left?

Ocular fixations while viewing an image (Optional)

Adding a second coupled electrode set will permit tracking the exact path of a visual examination of an image. While this does not provide much additional physiological information, it does provide psychological information like attitudes or areas of specific interest. This is optional but it is cool.

Subject and Equipment Preparation for image fixation EOG

Remember both skin preparation and electrode are important to the production of a clean signal. Prepare electrode sites as above.

• leave the electrodes to the side of each eye untouched. Leave the electrode being the ear. It will replace one of the forehead electrodes in the image below.
• Place an additional electrode below one eye (same side as the ear electrode)
• Place an additional electrode above the same eye.
• Place an additional electrode in the center of the for head.
• The correct location for the placement of each electrode is illustrated to the right.
• Have the subject firmly press the electrodes on his/her skin, once they have been correctly positioned.

• Move the unshielded (black) cable from the ear electrode to the center forehead electrode
  
• Connect an addition BSCBL8 to channel 2 of the MP35.
  • White Below the eye
  • Red above the eye
  • Black behind the ear

  Calibration

  • Have the subject sit facing the computer monitor.
  • Instruct the subject to relax and close his/her eyes, and tell the subject that he/she must not move his/her head during the recording.
  • load fixation.mov from stuserver
  • Instruct the subject to open his/her eyes and focus on the center of the image (intersection of blue lines) and to vissually track the dot with out moving his/her head
  • Click "Start" and then start the movie. Click "Stop after no more than 5 seconds.
  • Review the data.
  • If the data resembles the following screen shot, select File Save as... and proceed to the Image Fixation Experiment.
  • If it does not resemble the following screen shot, repeat Calibration.
    

  Running the Experiment

  • Hints for minimizing data error
    • The subject should not talk or laugh during any of the recording segments.
    • The subject should be relaxed in the position noted for each recording segment.
    • The subject should not move his head during the experiment.
    • Active electrodes should be vertically and horizontally aligned.
  • load your image movie but do not start it
  • Have the subject relax and look at the center of the quicktime movie frame
  • Click "Start" to begin recording and then start the movie.

  • After 5 seconds, click "Stop." and save the file Data Analysis

    • Recall the calibration data by choosing File > Open... and selecting the appropriate file.
    • Select “Display > Autoscale Horizontal” to show the entire series on the screen.
    • Click on the “XY” button in the tool bar.
    • Adjust the amplitude by clicking on the vertical scale.
    • Adjust the time scale by clicking on the horizontal scale (smaller values amplify the plot).
    • Select File > Print graph.
    • Proceed as above for each image.
    • Use the calibration plot as a reference for the image plots.

    Report

    No formal report is required for this lab but you should present your recording for each experiment (copy and past it in to MSWord) and answer the questions about that experiment.

    Walter I. Hatch
    wihatch@smcm.edu

    September 10, 2012