Dynamic shifts of the contrast-response function.
We recorded visual evoked potentials in response to square-wave contrast-reversal checkerboards undergoing a transition in the mean contrast level. Checkerboards were modulated at 4.22 Hz (8.45-Hz reversal rate). After each set of 16 cycles of reversals, stimulus contrast abruptly switched between a "high" contrast level (0.06 to 1.0) to a "low" contrast level (0.03 to 0.5). Higher contrasts attenuated responses to lower contrasts by up to a factor of 2 during the period immediately following the contrast change. Contrast-response functions derived from the initial second following a conditioning contrast shifted by a factor of 2-4 along the contrast axis. For low-contrast stimuli, response phase was an advancing function of the contrast level in the immediately preceding second. For high-contrast stimuli, response phase was independent of the prior contrast history. Steady stimulation for periods as long as 1 min produced only minor effects on response amplitude, and no detectable effects on response phase. These observations delineate the dynamics of a contrast gain control in human vision.
Victor
J D
JD
Department of Neurology and Neuroscience, Cornell University Medical College, New York, NY 10021, USA.
Conte
M M
MM
Purpura
K P
KP
D000222:Adaptation, Physiological; D015350:Contrast Sensitivity; D005074:Evoked Potentials, Visual / Q000502:physiology*; D006801:Humans; D010775:Photic Stimulation