Nonmonotone backward masking functions and brightness reversals

Abstract

Increasing the target-field luminance aids detection for a simultaneously presented black target disc and a black masking annulus. At an intermediate interval separating the onset of the target from the mask, increasing the target-field luminance reduces target detection. This decrease in performance occurs with both temporal and spatial forced choice tasks. With a spatial forced choice, an observer's performance can fall below chance. We associate below-chance performance with a brightness reversal of the black target disc, such that the target disc appears brighter than its surround. The occurrence of brightness reversals follows from our model of the Broca-Sulzer effect, and nonmonotone masking functions result from a generalization of luminance summation.Keywords Visual backward masking . U-shaped functions . Broca-Sulzer effect . Brightness reversal . Metacontrast maskingThe detection of a briefly flashed target, such as a black disc, is sometimes suppressed by the onset of a second stimulus, the masking stimulus. When the target is followed by a surrounding figure, such as a black annulus, the interplay of target and mask is called backward masking. Under conditions of backward masking, both target detection and letter identification can decrease as the delay between the onset of the stimulus and the onset of the mask increases. The resulting nonmonotone performance curves are addressed by most accounts of visual masking . Under some conditions, however, target detection is worse than chance. No current explanation of masking accounts for below-chance performance.Our account of below-chance performance rests on three propositions: (i) A spatial two-alternative forced choice (2AFC) task can produce nonmonotone performance curves. For pronounced nonmonotone curves, detection can be worse than chance. (ii) These nonmonotone backward-masking curves represent a change in an observer's sensitivity for certain delays between target and mask onset, even for targets that fall on the fovea. A dark target, followed by an appropriate masking stimulus, sometimes appears to be brighter than its background, a phenomenon we have called a brightness reversal . Pronounced brightness reversals occur with black-on-white figures and can lead to below-chance target detection. (iii) Our conjecture is that brightness reversals result from the same combination of excitation, inhibition, and luminance summation that produces the Broca-Sulzer effect (Berman & Stewart, 1978a, 1978b, 1979Marks, 1974). These brightness reversals result in below-chance performance with a spatial 2AFC.

Visual experience forms a multidimensional pattern that is not reducible to a single measure: Evidence from metacontrast masking

“…The percept Bright Target refers to the metacontrast literature in which several authors reported not only the phenomenon of a suppression of target contrast but also a phenomenon of brightness reversal (e.g., Stewart et al, 2011;Werner, 1935). Our results contribute to and extend these findings by showing a low but reliable frequency of yes reports regarding the percept Bright Target, which declines with increasing SOA.…”

“…This finding adds to previous evidence that a brightness reversal seems to be a rather weak or instable phenomenon that may occur only under certain conditions. Consistent with this, Stewart et al (2011) found brightness reversal only in a spatial forced-choice task, not in a temporal forced-choice task.…”

“…At intermediate SOAs of 50 to 100 ms, this metacontrast suppression can reduce the perceived contrast to the background level so that the target is not experienced at all. This has been shown with simple light flashes as target and mask (Alpern, 1953;Fehrer & Raab, 1962;Kahneman, 1967;Schiller & Smith, 1966;Weisstein & Growney, 1969) and with contour-defined stimuli of either polarity (Breitmeyer, Tapia, Kafaligonul, & Ögmen, 2008;Stewart, Purcell, & Pinkham, 2011).…”

“…In addition, several studies have reported not only a reduction but also a reversal of perceived contrast under certain conditions (Heckenmueller & Dember, 1965;Purcell & Dember, 1968;Stewart et al, 2011;Werner, 1935). Werner (1935) used a black target disc and a black annulus as a metacontrast mask and reported that on 7% of all cases the inner field of the annulus appeared to be whiter than the gray background.…”

“…Werner (1935) used a black target disc and a black annulus as a metacontrast mask and reported that on 7% of all cases the inner field of the annulus appeared to be whiter than the gray background. More recently, Stewart et al (2011) presented a small black target disk to the left or to the right of the fixation cross followed by two ring-shaped metacontrast masks on either side of the fixation cross. Participants indicated the side on which the target disc had been presented.…”