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Effect of the geometric form and meaning of the stimulus on the configuration of the visual evoked response Purves, Sherrill Jean Swift

Abstract

In the visual system it has been established that certain physical parameters of the stimulus including intensity, focus, and size of the elements of a pattern have significant effects on the configuration of the evoked response as recorded from the occipital region. While claims have been made in a few publications, to this date the question of whether any part of the evoked response is affected by higher perceptual processes rather than ones more directly related to stimulus input has not been resolved. Evoked responses to four geometric shapes (a square, circle, el and omega) were recorded from multiple scalp locations under two experimental conditions; in the first the shapes were presented at random intervals and in random order with no meaning assigned to them. In the second they were presented in a fixed rhythmic sequence, and two of the shapes occasionally failed to appear in their usual time interval in the sequence. The subject was required to rapidly signal such omissions by a button press. The cerebral electrical activity during this time interval of the expected, but omitted stimulus was recorded and separately averaged, and these responses were called emitted potentials. There were significant, differences between the evoked responses (in the occipital region) to the square and el shapes and between those to the circle and omega shapes. These differences were demonstrated by three measurement techniques: performance of the discriminant functions computed by SWDA in classifying single trial responses, a ratio statistic called X as described by John, Herrington & Sutton (1967), and amplitude differences in the N₂ and P₂ components. The emitted potentials recorded from the vertex, contained a small and variable early negative component and an obvious late positive component similar to the one seen in the responses to the present square and el during the second experimental condition. Manipulation of the waveforms from the single trials prior to averaging showed that this negative component was not time-locked to the P₃ peak. The evoked potential differences found were believed to be due to two classes of variables; the physical characteristics of the stimulus (the contours in the central 1.5° of the visual field) and task-related changes in the meaning of the stimulus; and these affected earlier and later parts of the waveform respectively.

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