Dynamic Contrast Perception Assessed by Pattern Masking

The perceived contrast of a pulsed grating of about 100 ms duration can exceed that of shorter or longer exposures. We investigated this contrast enhancement effect with pattern masking. The pulsed mask patterns were extended cosines (5 deg square, 35 cd m−2 mean luminance, 0.3 contrast) of 50 to 500 ms duration. Mask spatial frequency was 1 or 6 cycles deg−1. The test patterns (of equivalent spatial frequency) were sixth derivative Gaussians, either positive (ON pathway mediation) or negative (OFF pathway mediation) and of 30 ms duration. The test pattern could be centred on a light bar of the cosine (positive contrast) or a dark bar (negative contrast). Test and mask had simultaneous onset. For a 1 cycle deg−1 test and mask of the same polarity, the test threshold/mask duration function is nonmonotonic (peak at 83 – 100 ms). The function was similar for either positive or negative stimuli. Thus, we measured an analogue to the contrast enhancement effect, and found enhancement for negative as well as positive contrast components. For same-polarity 6 cycles deg−1 test and mask, threshold increased monotonically to 500 ms (no enhancement). For both 1 and 6 cycles deg−1 stimuli of opposite polarity, the threshold/mask duration function is sharply elevated and constant for masks of 83 ms or more. The same-polarity masking functions imply activation of either transient (1 cycle deg−1 stimuli) or sustained (6 cycles deg−1 stimuli) ON or OFF pathways. The opposite-polarity functions suggest that the time course of ON — OFF pathway interaction is similar for sustained and transient pathways.