Pattern-evoked potentials and Bloch's law.

Experiments are described which show that the latencies, unlike the amplitudes, of the C1 and C2 components of the human pattern-onset VEP do not conform to the contrast equivalent of Bloch's law. Whereas each component's amplitude is linearly related to the log of the (contrast X duration) product below a critical duration of 50 msec, its latency is independent of the stimulus pattern's duration and determined by its contrast alone; increasing by 30-35 msec for a 1.4 log unit contrast decrease. The relationship between these results and corresponding single-unit findings in the cat is discussed.