The overall goal of the present oculogram is to observe the abnormality of saccade performance in patients especially in those with neurological disorders and to look at the pathophysiology underlying the disorder providing useful information about the diagnosis. This method can help answer key questions in the field such as diagnosis of patients with neurological disorders and to elucidate the pathophysiology underlying the disease. The main advantage of this technique is that it allows stable recording of eye movements noninvasively in a relatively short time and is widely applicable to neurological patients.
Demonstrating the procedure will be Dr.Hideki Fukuda, an oculomotor researcher at Segawa Neurologic Clinic for Children and Dr.Yuseke Sugiyama, a graduate student from the Department of Neurology, the University of Tokyo. Begin by escorting the subject into a room with low ambient illumination. Have the subject sit in front of a black concave dome-shaped screen measuring 90 centimeters in diameter that contains Light-Emitting Diodes, LEDs, embedded in pinholes which serve as the fixation points and saccade targets used for the oculomotor paradigms.
Then provide the subject with a micro switch button connected to a microcomputer which allows them to initiate and terminate the test trial by pressing and releasing the button. Stabilize the subject's head position by chin and forehead rests as well as by a headband. Next, acquire a silver/sulfur chloride cup electrode for recording the Electrooculogram or EOG.
Wipe the skin with an alcohol swab. Then fill the cup with electrode paste. Stably fixate the electrode on the skin by placing double stick adhesive tape beneath the plastic and attach the fringe to the skin.
For recording horizontal saccades by EOG, place the electrodes at the bilateral canthi of the eyes. Whereas for recording vertical saccades, place the electrodes above and below one eye. Finally, wait 10 to 20 minutes after placing the EOG electrode on the skin until sufficient light adaptation takes place.
Begin by using a Direct Current, DC, amplifier for recording the EOG with the signal digitized at 500 hertz. Record Video Oculography, VOG, simultaneously using the video-based eye tracking system which records ocular fixation position data at a sampling rate of 500 to 1, 000 hertz. Then feed the analog output of the horizontal and vertical eye positions and set the filter of the data acquisition system with the signal low-pass filter at 20 hertz.
Perform eye movement calibration before each test session by having the subject look at five pre-specified locations such that they view visual targets in the center and those that appear 20 degrees to the left, right, upwards and downwards of the fixation point both for EOG and VOG. Then adjust the gain of EOG as the subject fixates on these spots so that using the custom-built data acquisition system for monitoring the current eye position displayed on the computer screen matches the target position displayed on the screen. Next, instruct the subject about the oculomotor paradigms.
Then have the subject press the button and begin the trials. During the session, adjust the gain of EOG during the task performance so that the current eye position displayed on the monitor is always aligned with the target positions simultaneously displayed on the same screen. After the experiment is complete, analyze the filtered and digitized EOG signals from the DC amplifier and VOG by a custom-built computer program and show the EOG and VOG signals together in the same trace in order to compare the performances of the two methodologies.
The records obtained by the two methods largely overlap with each other except that the EOG traces are slightly displaced towards the right compared to the VOG traces. These results show a comparison of EOG and VOG traces in the visually-guided saccade task. There is again substantial overlap between the traces for EOG and VOG, but the latency is slightly longer for EOG and the velocity curve of EOG shows a slightly lower peak velocity than that of VOG.
Once mastered, this technique can be done in 30 minutes including preparation if it is performed properly. While attempting this procedure, it is important to remember to select proper electrodes and stably attach them as well as to take sufficient time for light adaptation to occur. Following this procedure, other methods like eye movement recording while the eyes are closed can be performed in order to answer additional questions like how the eyes move during sleep.
After its development, this technique paved the way for researchers in the field of neurology and psychiatry to explore the pathophysiology of neurological disorders in the clinical setting. After watching this video, you should have a good understanding of how to implement stable recording of electrooculography which can be reliably used in the clinical setting.
