Effects of stimulus luminance and duration on responses to onset and offset.

The visual responses to stimulus onset and offset can be isolated psychophysically from the response to steady-state stimulation. Six experiments investigated the perception of onset and offset of dot bigrams within random-dot patterns. Onset and offset persistence were studied at two levels of stimulus luminance (8 and 45ft-L). Similar performance was obtained for targets defined by onset and offset; the lower luminance produced a decrement in response at offset. Onset and offset perceptibility were studied at six stimulus durations between 20 and 1260 msec. Effects of stimulus duration were distinguished from masking effects due to pattern offset. Decreasing stimulus duration had little or no effect on recognition of bigrams defined by onset but produced a large decrement in the perceptibility of bigrams defined by offset. A brief stimulus therefore seems to give a relatively strong on-response and a weak off-response. These psychophysical results are consistent with electrophysiological findings concerning on- and off-responses and contrast with the idea that persistence consists of a decaying trace of the steady-state properties of the stimulus.