Rapid contrast sweeping of steady-state visual evoked potentials: a new method for investigation of the visual system.

Rapid sweeping of contrast modulates the steady-state visual evoked potentials (VEP) obtained by sinusoidal gratings reversing at 16 Hz. The modulation results in a null interval of which the width measures the contrast threshold and of which the delay measures the apparent latency of the steady-state VEP, provided that the dynamic contrast sensitivity is symmetric. Data from 25 normal subjects and 27 patients with head injury show that this requirement is met in normal subjects but is often not verified in pathologic conditions. By measuring three aspects of the response to contrast changes (latency and static and dynamic contrast sensitivity) at five different spatial frequencies, one obtains a very sensitive method for investigation of the visual system.