Visual evoked responses (VER) to four geometric shapes (a square, circle, el and omega) were recorded from multiple scalp locations in twelve subjects. Significant differences were found between the occipital VERs to the square and el and between the VERs to the circle and omega. Consistent differences could not be demonstrated between the responses to the square and circle or to the el and omega.
The differences between the responses were quantified by three different methods, including measurement of peak latencies and amplitudes, computation of a ratio index called λ, and the performance of discriminant functions derived by Stepwise Discriminant Analysis Program (SWDA) in classifying other single trial responses. The amplitudes (but not the latencies) of a negative peak at approximately 150 msec, and a positive one at 220 msec, latency were found to be different in the pairs of shapes described. SWDA was able to separate these same pairs and classify from 63 to 68% of new trials correctly. The latency points chosen to make up the discriminant functions correlated partly with the and components that were found to be significantly different in amplitude.
It is suggested that the VER differences in the earlier part of the wave-form are related to the physical properties of the stimuli, including the contrast densities in the central 1.5° of the visual field and/'or the spatial frequencies contained in the stimuli. Differences demonstrated in later parts of the VERs may reflect differences in symbolic meaning of the stimuli.