Effect of number of elements and size of stimulus field on recordability of pattern reversal visual evoked response.

The interrelationships among amplitude of the pattern reversal visual evoked response (VER), check size (spatial frequency) of the stimulus pattern elements, size of the stimulus field, and number of elements in the stimulus field were investigated. When the stimulus field was smaller than 4.0 degrees X 4.0 degrees, the VER amplitude-check size curve had a high-pass filter shape. When the stimulus field was larger than 5.0 degrees X 5.0 degrees, the curve showed the inverted-U shape. At the largest stimulus field, 12.0 degrees X 12.0 degrees, the curve retained the inverted-U shape, but its peak moved to a larger check size. With smaller check sizes (10 and 20 min), the VER amplitude increased up to a certain stimulus field size and then showed either saturation or a decrease. When the number of elements in the stimulus field was less than 40, the VER amplitude did not increase significantly in all check sizes. Above this number of elements in the field, the amplitude increased linearly until saturation was observed at an element number of 300-400. We concluded that the amplitude of the pattern reversal VER depended strongly on the interrelationship among check size, stimulus field size, and number of elements in the field.