Decoupling Stimulus Duration From Brightness in Metacontrast Masking: Data and Models.

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Abstract
A brief target that is visible when displayed alone can be rendered invisible by a trailing stimulus (metacontrast masking). It has been difficult to determine the temporal dynamics of masking to date because increments in stimulus duration have been invariably confounded with apparent brightness (Bloch's law). In the research reported here, stimulus luminance was adjusted to maintain constant brightness across all durations. Increasing target duration yielded classical U-shaped masking functions, whereas increasing mask duration yielded monotonic decreasing functions. These results are compared with predictions from 6 theoretical models, with the lateral inhibition model providing the best overall fit. It is tentatively suggested that different underlying mechanisms may mediate the U-shaped and monotonic functions obtained with increasing durations of target and mask, respectively. A briefly presented target shape that is clearly visible when shown alone becomes nearly invisible when it is followed by a second (masking) shape consisting of closely adjacent but non- overlapping contours. If perception were true to the temporal sequence of events, the display would be seen as consisting of a leading target and a trailing mask. Instead, at optimal target-mask intervals, the target is seen dimly or not at all, and only the mask is seen clearly. This indicates that the neural code of the target remains vulnerable to interference from the activity elicited by the trailing mask over delays of up to 100 ms. This form of masking, called metacontrast masking, has provided important insights into the temporal and spatial dynamics of visual processing for almost 100 years (Stigler, 1910; see reviews by Breitmeyer 1984; Breit- meyer & Ogmen, 2000). Given the rich legacy of research on metacontrast masking, it is surprising that there is still uncertainty regarding its critical tem- poral factors. Three hypotheses have been proposed. A commonly advanced hypothesis specifies that the critical factor is the time that elapses between the onset of the target and the onset of the mask (stimulus onset asynchrony (SOA)). This hypothesis has received considerable support in the literature, with some research- ers even referring to the onset- onset law (Kahneman, 1967) or SOA law (Breitmeyer, 1984). A second hypothesis identifies the critical factor as the time that elapses between the offset of the target and the onset of the mask (interstimulus interval (ISI); Francis, 1997). A third hypothesis singles out the interval that elapses between the offset of the target and the offset of the mask (stimulus termination asynchrony(STA); Macknik & Livingstone, 1998). The temporal relationships among SOA, ISI, and STA, as well as those between target and mask durations, are illustrated in Figure 1A.