Repetitively pulsed small spot light flashes. Depression of steady-state pattern visual evoked potentials

Steady-state visual evoked potentials (VEP's) were elicited from human volunteers by contrast reversal of a 43% contrast checkerboard pattern resolvable with 6/24 (20/80) Snellen acuity. Superimposed on the fixation point was a Xenon flash lamp subtending 0.358 degrees visual angle, which produced a 0.05 J/cm2 sr flash in 2 microseconds (full width, half maximum). Single pulses of light had no significant effect on the VEP. Increasingly significant decreases in VEP signal magnitude were noted with 2 and 5 pulses of 20 Hz white light. No disruption of VEP phase entrainment was noted with any of the flash exposures. Partial reciprocity was demonstrated between number of flashes at this frequency and response decrement.