STEADY-STATE VISUAL EVOKED CORTICAL POTENTIALS FROM STIMULATION OF VISUAL-FIELD QUADRANTS - OPTIMIZING PATTERN VARIABLES FOR THE SIZE OF THE FIELD TO BE INVESTIGATED

The effects of check size and stimulus size were investigated to optimize the steady-state visual evoked cortical potentials from pattern-reversal stimulation of the visual field quadrants. Check size of 15', 30', 60', 90', 120' and 180' were investigated at a pattern reversal rate of 11.6 per second for field sizes varying from 2-degrees x 2-degrees to 24-degrees x 24-degrees. The visual evoked cortical potentials were recorded from mid occipital, right occipital and left occipital positions. In the inferonasal quadrant. the largest amplitudes were obtained with 30' and 60' check sizes, however. for these check sizes, the visual evoked cortical potential yielded limited additional information for field sizes greater than 4-degrees x 4-degrees and 6-degrees x 6-degrees, respectively. When a field size of 12-degrees x 12-degrees was investigated, a 90' check size was optimal. The results indicated that, with the above recording positions and check sizes of 15' to 120', there is an optimal number of pattern elements, 40 to 100, for stimulation of the inferonasal quadrant. This should be taken into account when a check size is selected to investigate a field quadrant of a particular size. Digital signal processing techniques were applied to analyze the visual evoked cortical potential, and the system shows promise for objective examination of the visual field