Chromatic relationships in metacontrast suppression

Interference across time: dissociating short from long temporal interference

Our ability to identify an object is often impaired by the presence of preceding and/or succeeding task-irrelevant items. Understanding this temporal interference is critical for any theoretical account of interference across time and for minimizing its detrimental effects. Therefore, we used the same sequences of 3 orientation items, orientation estimation task, and computational models, to examine temporal interference over both short (<150 ms; visual masking) and long (175-475 ms; temporal crowding) intervals. We further examined how inter-item similarity modifies these different instances of temporal interference. Qualitatively different results emerged for interference of different scales. Interference over long intervals mainly degraded the precision of the target encoding while interference over short intervals mainly affected the signal-to-noise ratio. Although both interference instances modulated substitution errors (reporting a wrong item) and were alleviated with dissimilar items, their characteristics were markedly disparate. These findings suggest that different mechanisms mediate temporal interference of different scales.

Neural correlates of color-selective metacontrast in human early retinotopic areas

Metacontrast is a visual illusion in which the visibility of a target stimulus is virtually lost when immediately followed by a nonoverlapping mask stimulus. For a colored target, metacontrast is color-selective, with target visibility markedly reduced when the mask and target are the same color, but only slightly reduced when the colors differ. This study investigated neural correlates of color-selective metacontrast for cone-opponent red and green stimuli in the human V1, V2, and V3 using functional magnetic resonance imaging. Neural activity was suppressed when the target was rendered less visible by the same-colored mask, and the suppression was localized in the cortical region retinotopically representing the target, correlating with the perceptual topography of visibility/invisibility rather than the physical topography of the stimulus. Retinotopy-based group analysis found that activity suppression was statistically significant for V2 and V3 and that its localization to the target region was statistically significant for V2. These results suggest that retinotopic color representations in early visual areas, especially in V2, are closely linked to the visibility of color.

Metacontrast masking is specific to luminance polarity

A 1 degrees -spot was flashed up on a screen, followed by a snugly fitting annular mask. We measured the amount of masking as a function of stimulus luminance. The surround was always mid-gray, the masking ring was either black or white, and the luminance of the spot target ranged from 0% to 100% of white in 4% steps. Observers reported the apparent lightness of the masked spot by adjusting a matching spot.

RESULTS

A black annular mask made all spots that were darker than the gray surround appear to be transparent, that is, of the same luminance as the surround (complete masking). The black ring had virtually no masking effect on spots that were lighter than the surround. Conversely, a white ring made all spots that were lighter than the gray surround look apparently the same luminance as the surround (complete masking), but had virtually no masking effect on spots that were darker than the surround. In summary, a black ring masked spatial decrements but not increments, whilst a white ring masked spatial increments but not decrements. Thus masking occurred only when the spot and the ring had the same luminance polarity. This same-polarity masking still occurred when the target spot was larger than the 'donut hole' of the masking ring, so that the target and ring partly overlapped. This ruled out simple edge-cancellation theories. Instead, masking disrupts the filling-in process that normally propagates inward from the edges of a spot [Vision Res. 31 (7-8) (1991) 1221]. We conclude that metacontrast masking occurs within, but not between, separate visual ON and OFF pathways.