Dynamics of contrast decrement and increment responses in human visual cortex
The goal of the present experiments was to determine whether electrophysiological response properties of the ON and OFF visual pathways observed in animal experimental models can be observed in human. Visual Evoked Potentials (VEPs) were recorded in response to equivalent magnitude contrast increments and decrements presented using sawtooth temporal waveforms at a temporal frequency of 2.73 Hz. VEP response waveforms and response spectra for incremental and decremental stimuli were analyzed as a function of stimulus size and visual field location in 68 healthy adult participants. VEP response were larger in amplitude and shorter in latency for contrast decrements than for contrast increments. The spatial tuning was narrower for contrast decrements than for contrast increments and responses were larger for displays that were scaled for cortical magnification. VEPs recorded at the scalp differ between contrast decrements and increments of equal Weber contrast in a fashion that parallels results from the early visual system of cats and monkeys. Because our assay allows differential detection of ON and OFF pathway activity in human, the approach may be useful in future work on disease detection and treatment monitoring.