DYNAMIC CONTRAST PERCEPTION ASSESSED BY PATTERN MASKING

The perceived contrast of a pulsed grating varies markedly with the ex posure duration and spatial frequency of the grating. We studied dynam ic changes in perceived grating contrast with a pattern-masking paradi gm. We measured masking of a brief, localized test pattern (a D6 stimu lus, 30 ms in duration) by fixed-contrast cosine grating patterns of v arying duration (50-500 ms). The cosine mask pattern had spatial frequ ency of either 1 or 6 cycles per degree (cpd) at a contrast of 0.3. Th e D6 test pattern was centered on a light bar of the mask and was eith er positive peak contrast (same-polarity test and mask) or negative pe ak contrast (opposite-polarity test and mask). In Experiment I, the te st and mask had simultaneous onset. With a 6-cpd mask, the same-polari ty test-threshold elevation versus mask-duration function increases mo notonically. For a 1-cpd mask, the same-polarity threshold-mask-durati on function is nonmonotonic, with peak masking effect produced by a gr ating pulse of 80-100 ms. These masking effects are closely congruent with known dynamic contrast effects. With negative tests, masking-dura tion functions are elevated from same-polarity functions and are essen tially similar in shape for 1- and 6-cpd masks. The elevated threshold s suggest inhibitory interaction between ON and OFF pathways, with a s imilar time course across spatial frequency. In Experiment 2, the D6 t est was delayed from mask onset by 33 ms. Positive contrasts only were employed. For 1-cpd stimuli, the delay of test greatly reduced maskin g at all mask durations and eliminated the nonmonotonic function. This suggests that for low-spatial-frequency patterns, perceived contrast is determined by an early peak component of the neural response. But f or 6-cpd stimuli, masking of the delayed test was somewhat greater at all mask durations, consistent with a gradually increasing underlying neural response to the grating. Finally, in Experiment 3, same-polarit y masking effects at both spatial frequencies were replicated with neg ative-contrast test and mask (OFF pathway mediation). This indicates t hat the ON and OFF pathways have similar response dynamics. (C) 1998 O ptical Society of America.