Changes in potential are known to occur in the orbital area during saccades. The sign of these changes depends on the position of the electrode and the direction of eye rotation, while their amplitude depends on the rotation angle. The patterns of potentials can be used to resolve the reverse task, i.e., that of describing the gaze trajectory, such that the eye can be used to control a computer in an on-line regime. This requires a screen cursor to be placed at the calculated gaze fixation point, i.e., the point at which the observer is looking. Electrodes beneath the eyes were used to assess the vertical component of gaze displacement, while side electrodes at the corners of the orbit were used to assess the horizontal component. Sharp unipolar changes in potential occurring on saccades were apparent as steps which could be detected and measured. The signal was digitally filtered using a complex filter constructed by ourselves. The ratio of the amplitudes at the four points was then used to calculate the direction and angle of eye rotation. Characteristic changes in potential during spontaneous blinking were identified and ignored. Voluntary blinks of one eye were used to simulate mouse clicks. Subjects were initially told to make changes in gaze through specified angles in eight directions. This allowed calibration of standard saccades (in microV). Calibration curves were used to resolve the reverse task - changes in potential (in microV) were used to calculate the point on the screen (the pixel) to which the cursor was to be moved. Subjects were then trained to control the cursor using their eyes, and control of the computer was then completely handed over to the subject. The apparatus described here provides a brain-computer interface. Some interesting data on eye coordination were obtained during these studies: saccades were preceded by short negative electrooculogram (EOG) potentials lasting 10-15 msec. With age, the amplitude of saccade-related EOG potentials decreased. When gaze was shifted to the left, deviation of the eyes was more significant than when gaze was shifted to the right, while on shifting of gaze to the right, the lateral deviations of the eyes were similar. On diagonal right-down and left-up movements, right eye skew was greater than left eye skew, while on right-up and left-down movements, left eye skew was greater than right eye skew. Differences in eye coordination between genders were minor.
A total of 22 healthy subjects in the EEG laboratory and 62 patients in the clinical functional diagnostic unit were studied. Spontaneous EEG recordings were made using the 10-20 scheme relative to combined ear electrodes in the state of rest with the eyes closed and open and during various functional loads. Traces were analyzed by computer animation of the EEG phase structure. The main concept of the method for extracting phase structure was based on not using a single reference lead. Time shifts were measured only between neighboring electrodes, with the result that the oscillations being compared were highly coherent. Time discordance was assessed in terms of the shift in the peak of the cross-correlation function. The results showed that from the point of view of phase structure, the differences between the high-and low-frequency EEG rhythms were purely quantitative. Qualitatively, the properties of the rhythms were identical and were reduced to slow (in the seconds range) oscillations of phase shifts. Low-frequency activity was characterized by large (in absolute terms, msec) phase shifts from electrode to electrode as compared with high-frequency activity. The phase shifts of potentials formed a structure which was overall very similar in different subjects and was reproduced in different leads. The initial appearance of EEG waves was statistically linked with the main sensory projections--the visual (occipital areas), auditory (temporal areas), and somatic (parietal areas), with addition of the frontal areas. Rearrangement of phase leadership in favor of the occipital pole at the expense of both temporal areas was observed on opening the eyes. This appears to depend on the level of sensory influx to this cortical area from the thalamus. It is suggested that the direction of the phase gradient reflects a gradient of cortical current density parallel to the surface. This can be used to locate compact sources lying close to the surface.
Baseline EEG traces were recorded from the right occipital area with the eyes closed in 15 subjects. Rhythmic stimulation with bright, diffuse flashes of light was applied through the closed eyelids at specific points in the alpha-wave phase (trigger photic stimulation). Ten versions of stimulation were used, with application at different phases for 1 min at each phase. Responses occurring in conditions of stimulation in different phases were compared using the mean alpha wave amplitude in each version and the wave shape in terms of the level of asymmetry of the leading and trailing fronts. When flashes were delivered at the middle of the descending front of the potential (positivization), changes in wave shape were most marked and were very different from those seen in the other nine stimulation phases. This effect was most marked in subjects with low-amplitude alpha rhythms. These results suggest that the individual level of the alpha rhythm in a given subject and the dynamics of the wave shape can be explained by the characteristics of the structure of the system of recurrent-lateral inhibition.
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