Preventing Local Clustering in Random-Dot Patterns: A New Method to Avoid Perceptual Artifacts

Patterns consisting of random dots are frequently used in visual science. One disadvantage of using random-dot patterns is the possible clustering of dots. These clusters have a lower spatial frequency than the one derived from dot size. The cluster formation is not present in patterns in which dots are homogeneously distributed, but with these patterns other problems, eg ambiguity in stereograms, occur. A good balance between homogeneous and random distribution has to be found. This problem has often been addressed in half-toning techniques. One of these techniques is force-field random dithering. In this technique each dot has a force field that repels newly placed dots. For generating random patterns, we used a modification of this dithering technique. This technique is empirically compared with the traditional random-dot patterns. Subjects viewed for 70 ms a 6 deg × 6 deg square consisting of 100 × 100 dots placed randomly or with a force field. Each dot subtended 0.6 min arc. The task was to detect a vertical band of 30 × 100 dots with increased luminance, which could appear either left or right of the midline. The density of the dots was varied between 5% and 25%. The results indicate a significantly larger error rate when using the force-field generated pattern. We conclude that subjects are using clusters as local cues. These results should warn investigators using random-dot patterns that local clusters could act as serious artifacts.

Figure and Ground in Space and Time: 2. Frequency, Velocity, and Perceptual Organization

The figure – ground organization of an ambiguous bipartite pattern in which the two regions of the pattern contained sine-wave gratings which differed in spatial frequency was examined for two pairs of spatial frequencies: 1 and 4 cycles deg−1, and 1 and 8 cycles deg−1. The region of higher spatial frequency underwent contrast reversal at one of four rates: 0, 3.75, 7.5, or 15 Hz. The region of lower spatial frequency was equated with either the temporal frequency or the velocity of the grating of higher spatial frequency in three sets of conditions: one stationary condition, three in which temporal frequency was equated, and three in which velocity was equated. For the 1 and 4 cycles deg−1 pair, the region of lower spatial frequency tended to be seen as the background a higher percentage of the time. There were significant linear trends for the appearance as background of the region of lower spatial frequency with respect to the magnitude of the velocity difference between the two regions of the pattern. The faster the 1 cycle deg−1 grating moved with respect to the 4 cycles deg−1 grating, the higher the percentage of the time it was seen as the ground. The results for the 1 and 8 cycles deg−1 pair were in some cases unexpected in that the 8 cycles deg−1 grating was seen as the ground behind the 1 cycle deg−1 grating even though it was of a higher spatial frequency and moved at a slower velocity. The spatiotemporal tuning of the visual system is discussed.

A Test of the Spatial-Frequency Explanation of the Müller-Lyer Illusion

Previous investigations have shown that the response of spatial-frequency-specific channels in the human visual system is differentially affected by adaptation to gratings of distinct spatial frequencies and/or orientations. A study is reported of the effects of adaptation to vertical or horizontal gratings of a high or a low spatial frequency on the extent of the Brentano form of the Müller-Lyer illusion in human observers. It is shown that the illusion decreases after adaptation to vertical gratings of low spatial frequency, but seems unaffected otherwise. These results are consistent with the notion of visual channels that are spatial-frequency and orientation specific, and support the argument that the Müller-Lyer illusion may be due primarily to lower-spatial-frequency components in the Fourier spectra of the image.