Visual signals used in time-interval discrimination

Thresholds for the detection of differences in the duration of visual stimu li were determined for a variety of programs of stimulus onset and offset. Performance suffers when a time interval begins with an ON step and ends wi th another ON stimulus, compared to the standard ON-OFF stimulation, but th e decrement is reversed when the light is ramped down to background during the interval. Neither the magnocellular nor the parvocellular streams can b e excluded because there is relatively little impairment of duration discri mination when the stimulus has low contrast or is heterochromatic at isolum inance. Performance at a variety of intensity levels suggests that sustaine d neural firing in an early state of visual processing provides a backgroun d activity, which prevents good temporal precision of signals.