Compression of visual information in redundancy masking follows grouping and segmentation

In typical scenes, the available visual information exceeds the processing limits of the visual system. To reduce information, the visual system organizes visual input according to various principles. For example, in visual grouping local elements are grouped into larger wholes. Recently, it was shown how the visual system compresses redundant information by 'redundancy masking' which reduces the number of perceived items in repeating patterns. For example, when three identical items are presented in the periphery, observers often report perceiving only two items. In the current study, we examined the role of grouping and segmentation in redundancy masking. In particular, we asked whether visual grouping determines-and precedes-the units in which information is compressed in redundancy masking. Stimuli consisted of arrays of 3-5 vertical lines, briefly presented in the left or right visual field. We varied the contrast polarity of the lines to create the following patterns: uniform arrays, alternating arrays, and subgroups of identical lines with a different line at the stimulus edge. Observers reported the number of lines and then indicated the perceived feature values for each reported line. Our results revealed redundancy masking in all conditions. Importantly, redundancy masking always came after segmentation and grouping: lines were masked predominantly within segmented subgroups of identical lines. The reported patterns were highly systematic, even when redundancy masking occurred: The presence/absence of the two contrast polarities (Contrast), the stimulus edge (Edge) and their ordinal frequencies (Ratio) were largely accurately reported. Hence, important stimulus information remained largely intact. We suggest that redundancy masking comes after grouping and segmentation, and that the visual system compresses redundant information in the visual periphery by reducing the number of perceived identical items while preserving key stimulus features.