Patterns consisting of random dots are frequently used in visual science. One disadvantage of usi... more 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.
An illusory display is presented in which a luminance gradient moves through the visual field. In... more An illusory display is presented in which a luminance gradient moves through the visual field. In front of this moving gradient, a grid of outlined, filled, squares is positioned. The movement of the gradient induces an illusory 'wavelike' motion of the superimposed squares, depending on the grey values of the squares and their borders. This effect of the moving gradient was decomposed into a dynamic part and a static part. The dynamic part was investigated by means of an experiment in which a temporal gradient induced a forward/backward pulsing of a single outlined square. The static distortions were investigated by means of two experiments. In the first one, an outlined square was positioned on a (stationary) spatial gradient, which caused the squares to look like trapezoid shapes. In the second one, the outlined squares were positioned on different luminance fields, which affected the apparent size of the squares. In all experiments the same ranges of luminance values for both the inner surface of the square and its borders were orthogonally combined. In addition, displays were also created in which the gradient was inside the square and the luminances of the outside surface and the contour were orthogonally combined. For each of the displays, observers were asked to indicate the direction and strength of the induced distortions. The overall results show large agreements between the dynamic distortion (forward/backward pulsing) and the aforementioned static distortion (trapezoid-like appearances). In contrast, different tendencies emerged between these distortions and the second static distortion dealing with the apparent size. The results are discussed and related to various phenomena reported in the literature.
It is known that monocular depth cues become much less effective under isoluminance. One of these... more It is known that monocular depth cues become much less effective under isoluminance. One of these depth cues, occlusion, gives rise to surface completion. A study is reported in which the loss of completion under isoluminance was tested. A pair of horizontally aligned bars of different lengths is detected automatically in a display filled with pairs of bars of the same length. The pair is detected serially, when vertical bars are placed over the gaps between the pairs. Because the vertical bars are occluders, and the pairs of horizontal bars are aligned, completion behind the vertical bars takes place and the two parts together behave perceptually as a single bar. We used this knowledge to measure completion under isoluminance. When occlusion is lost under isoluminance, we expect that an occluding surface, isoluminant with the background, will not lead to object completion and as a consequence, the pair with unequal lengths of the parts will pop out. Using this procedure we have demonstrated that completion is lost under isoluminance.
Patterns consisting of random dots are frequently used in visual science. One disadvantage of usi... more 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.
An illusory display is presented in which a luminance gradient moves through the visual field. In... more An illusory display is presented in which a luminance gradient moves through the visual field. In front of this moving gradient, a grid of outlined, filled, squares is positioned. The movement of the gradient induces an illusory 'wavelike' motion of the superimposed squares, depending on the grey values of the squares and their borders. This effect of the moving gradient was decomposed into a dynamic part and a static part. The dynamic part was investigated by means of an experiment in which a temporal gradient induced a forward/backward pulsing of a single outlined square. The static distortions were investigated by means of two experiments. In the first one, an outlined square was positioned on a (stationary) spatial gradient, which caused the squares to look like trapezoid shapes. In the second one, the outlined squares were positioned on different luminance fields, which affected the apparent size of the squares. In all experiments the same ranges of luminance values for both the inner surface of the square and its borders were orthogonally combined. In addition, displays were also created in which the gradient was inside the square and the luminances of the outside surface and the contour were orthogonally combined. For each of the displays, observers were asked to indicate the direction and strength of the induced distortions. The overall results show large agreements between the dynamic distortion (forward/backward pulsing) and the aforementioned static distortion (trapezoid-like appearances). In contrast, different tendencies emerged between these distortions and the second static distortion dealing with the apparent size. The results are discussed and related to various phenomena reported in the literature.
It is known that monocular depth cues become much less effective under isoluminance. One of these... more It is known that monocular depth cues become much less effective under isoluminance. One of these depth cues, occlusion, gives rise to surface completion. A study is reported in which the loss of completion under isoluminance was tested. A pair of horizontally aligned bars of different lengths is detected automatically in a display filled with pairs of bars of the same length. The pair is detected serially, when vertical bars are placed over the gaps between the pairs. Because the vertical bars are occluders, and the pairs of horizontal bars are aligned, completion behind the vertical bars takes place and the two parts together behave perceptually as a single bar. We used this knowledge to measure completion under isoluminance. When occlusion is lost under isoluminance, we expect that an occluding surface, isoluminant with the background, will not lead to object completion and as a consequence, the pair with unequal lengths of the parts will pop out. Using this procedure we have demonstrated that completion is lost under isoluminance.
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