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Duration thresholds were obtained for a single test stripe succeeded by a masking pattern of alternating dark and bright bars delivered to the same retinal region. The effect of the angle (alpha) between test stripe and masking contours was studied in conjunction with both vertical and horizontal test stripes. With either of the test-stripe orientations duration threshold decreased as alpha increased from 0 ° to 90°. In another experiment both alpha and the interstimulus interval (ISI) were varied. The effectiveness of alpha as a determinant of masking seemed to be restricted to ISI below 60 msec. When two brief flashes are presented in rapid succession to the same retinal area the detectability of the first is reduced. This phenomenon has been termed visual backward masking. The effect of the spatial relationship between the first (or test) stimulus and the second (or masking) stimulus has yet to receive adequate systematic analysis though several lines of evidence testify to its importance. Battersby and Wagman (1962, 1964) have shown the important role played by the spatial relationship between the perimeters of otherwise contourless test and masking flashes. Kolers (1962; Kolers Rosner, 1960) has extended the analysis using a concentric disk and ring as test and masking stimuli. In order to understand the contribution of contours within the mask-1 This paper is based on a portion of a dissertation
Robert Sekuler (Fri,) studied this question.