The effect of luminance on metacontrast with internally contoured targets

Abstract

Recent studies have provided evidence which contradicts the previous finding that target detectability decreases with increases in the number of internal contours in the target. The present study attempted to reconcile these discrepant findings by examining the effect of luminance level on detectability of internally contoured targets under mask and no-mask conditions. The results showed that the different luminance levels used in these studies may have indeed contributed to the discrepant results.Visual perception does not occur instantaneously. When an image of the world strikes the retina of the eye, it activates a series of intricate processes which take time to transform the retinal image into a percept. Normally, an image is completely processed in the time it takes to shift our gaze and fIxate another object. However, in the laboratory it is possible to present two stimuli in such rapid succession that the second stimulus occurs before the fIrst stimulus is completely processed. As a result, the second stimulus (the mask) may decrease the detectability of the first stimulus (the target). This inhibitory effect is generally referred to as backward masking; the term "metacontrast" is used when the fIrst and second stimuli do not spatially overlap.Since its discovery in 1910 by Stigler, backward masking has provided a means for "stopping" the visual image at various points during processing in order to delineate the various stages of processing as well as to study the variables which affect those stages. In particular, many vision researchers have used metacontrast as a device to help reveal the operation of visual inhibitory processes.Of particular interest to the present study is the work of Dember and his co-workers, who have been engaged in a series of experiments stemming from Werner's pioneering work dealing with the effect of confIgurational variables on metacontrast. In his classic experiment, Werner (1935) employed a target disk followed by a surrounding black ring and reported that target disks containing internal contours were harder to mask than a homogeneous black disk.Werner's fIndings relied on the phenomenal reports of his subjects; a study was undertaken by Sherrick and Dember (1970) to replicate Werner's results using an objective psychophysical task. As expected, the results demonstrated that a half-black half-white target disk was harder to mask than a homogeneous black disk. In an Requests for reprints should be sent to William N. Dember,

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“…As a result of this manipulation, display durations were idiosyncratic between participants. However, nonmonotonic metacontrast has been found over a range of target and mask durations-for example, 100 msec (Merikle, 1977), 60 msec (Bernstein, Proctor, Belcher, & Schurman, 1974), and 50 msec (Proctor, Nunn, & Pallos, 1983)-whereas Arand and Dember (1978) varied target duration, keeping the mask duration constant at 105 msec. Therefore, it was not anticipated that the differences in target and mask durations would influence the nature of the metacontrast function.…”