Visual search for simple volumetric shapes.

Five experiments measured reaction time (RT) to detect the presence or absence of a simple volumetric shape (target) dependent on the number of display items (distractors) and their depicted three-dimensional (3-D) orientation. Experiments 1-4 examined every pairwise combination of two different simple volumetric shapes in two 3-D orientations. Conditions exhibiting "pop-out" could be predicted by differences in their two-dimensional (2-D) features. Conditions in which search was slower support previously found search asymmetries for particular 2-D features. When the distractors were a mixture of the other shapes in the same 3-D orientation, search was serial, except when the target had a curved principal axis (Experiment 5). The results suggest that these simple volumetric shapes are not processed preattentively.

Conflicting figure--ground and depth information reduces moving phantom visibility.

Moving phantom visibility was measured in two experiments where the global figure-ground and depth relations within phantom-inducing patterns were manipulated. The local inducing environment where the illusion occurred was identical for all patterns. Phantom visibility was significantly reduced when occlusion cues specified the phantom-inducing parts of a pattern as ground. These results suggest conflicting figure-ground and depth information interferes with the representation and perception of phantoms as figure regions.

Spatial and temporal frequency in figure-ground organization.

Figure-ground organization of an ambiguous pattern can be manipulated by the spatial and temporal frequency content of the two regions of the pattern. Controlling for space-averaged luminance and perceived contrast, we tested patterns in which the two regions of the ambiguous pattern contained sine-wave gratings of 8, 4, 1, or 0.5 cycles per degree (cpd) undergoing on:off flicker at the rates of 0, 3.75, 7.5, or 15 Hz. For a full set of combinations of temporal frequency differences, with each spatial frequency the higher temporal frequency was seen as background for more of the viewing time. For two spatial frequency combinations, 1 and 4 cpd, and 1 and 8 cpd, tested under each of the four temporal frequencies, the lower spatial frequency region was seen as the background for more of the viewing time. When the effects of spatial and temporal frequency were set in opposition, neither was predominant in determining perceptual organization. It is suggested that figure-ground organization may parallel the sustained-transient response characteristics of the visual system.

Figure and ground in space and time: 1. Temporal response surfaces of perceptual organization.

The figure-ground organization of an ambiguous bipartite pattern can be manipulated by altering the temporal-frequency content of the two regions of the pattern. Ambiguous patterns in which the two regions of each pattern contained sine-wave gratings of either 8, 4, 1, or 0.5 cycles deg-1 undergoing contrast reversal at rates of 0, 3.75, 7.5, or 15 Hz were tested for figure-ground organization under conditions of equated space-averaged and time-averaged luminance and perceived contrast. All combinations of temporal-frequency differences between the two regions were tested at each spatial frequency. The data are reported for two levels of temporal resolution (15 and 30 s). The pattern region with the relatively higher temporal frequency tended to be seen as the background a higher percentage of the viewing time. There were significant linear trends for the appearance as background of the region of higher temporal frequency with respect to the magnitude of the temporal-frequency difference between the two regions of each pattern for all spatial frequencies and data intervals except the final 15 s interval of the lowest (0.5 cycle deg-1) spatial-frequency condition.

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 new visual illusion: flickering fields are localized in a depth plane behind nonflickering fields.

Flickering regions of the visual field are perceived to lie well behind regions which are not flickered. The depth segregation is not due to luminance differences since the average temporal luminance across all the regions was equal. This depth effect produced by flicker is not dependent on the texture of the visual field; nor does it depend on a specific configuration of the flickering and nonflickering areas. It is optimal at a temporal frequency around 6 Hz, which suggests that visual channels responding maximally to high temporal frequencies are involved in the segregation of perceptual regions in depth.

The razor's edge: a dichotomy between monohedral and dihedral edge perception.

The "motion-induced contour" is seen in an image where a motion sequence specifies a dihedral edge. Five experiments examined the role of dihedral angle size in the perception of this illusory contour. Observers used free magnitude estimation of contour perceptibility; the data were analyzed by a repeated measures ANOVA and Newman-Keuls procedure. It was found that planar motion is the motion-induced contour in the "dihedral edges" where the dihedral angles are 45, 90, or 135 deg. However, the "monohedral edge," where the dihedral angle is zero, does not show a superiority of three-dimensional motion over planar motion. The difference between these two classes of edges is described in terms of ecological optics.

The effect of perceived depth and connectedness on metacontrast functions.

When a barely visible target line is briefly flashed within a context of other lines, it is identified more accurately if the lines form a pattern that is perceived as a unified, three-dimensional object. This finding has been called the object-superiority effect [Weisstein and Harris, Science, 186, 752-755 (1974)]. The present study concerns what happens when different context patterns are presented at a variable delay after the target line. The resulting temporal functions of identification accuracy against delay of context pattern differ systematically with two perceptual variables: three-dimensionality and connectedness. The results suggest that differences in the temporal functions correspond to differences in visual response properties elicited by different perceptions of three-dimensionality and connectedness.

Sharp targets are detected better against a figure, and blurred targets are detected better against a background.

There is growing evidence that the performance of perceptual tasks is often facilitated by perceived "figureness." Accuracy in detection and discrimination of targets is higher when the targets are presented in figural regions than when they are presented in ground regions of an image. This "figure superiority" might be a result of a functional specialization in the visual analysis of figure; recent theories have also assumed a functional specialization in the visual analysis of ground. If so, we might expect "ground superiority" in situations where task performance requires information available primarily through analysis of ground. We manipulated the spatial frequency of a small line segment and found that when it was sharp (i.e., the high-spatial-frequency components were present), it was detected better in figural regions, but when we blurred it (only the low-to-medium spatial frequencies were present) it was detected better in ground regions. These findings support the view that figure and ground analyses involve different specialized functions.

A new perceptual context-superiority effect: line segments are more visible against a figure than against a ground.

Context, specifically the perceived figure or ground of an ambiguous form that surrounds a diagonal line segment, can influence the discrimination of that line segment even though the physical attributes of the context remain the same during figure-ground reversals. When the line segment was flashed on a region of the form seen as figure, discrimination was twice as accurate as when the line segment was flashed in isolation, and it was at least three times as accurate as when the line segment was flashed on that same region seen as ground.

The motion-induced contour.

An outline projection of a rectangular solid object, rotating in depth, produces a moving rotating in depth illusory contour across the empty region where an edge would normally be located even though it is not physically present. Observers see a contour demarcating the junction of the two frontal surfaces of the object when it undergoes the figural transformation of rotation-in-depth. Two experiments, in which several types of temporal change were compared, demonstrated that translation, flicker, or no motion are ineffective in producing the contour compared to rotation-in-depth. Also the four lines constituting the two inducing vertices of the object were sufficient to produce the motion-induced contour during rotation-in-depth. Three demonstrations further clarifying the nature of the motion-induced contour are also reported.

Depth, connectedness, and structural relevance in the object-superiority effect: line segments are harder to see in flatter patterns.

A briefly flashed line can be identified more accurately when it is part of certain types of pattern than in others (the 'object-superiority effects'). Three experiments were designed to investigate what aspects of these patterns determine the facilitatory effect of context. Subjects identified which of four line segments was present in various briefly flashed figures. Other subjects rated the figures for three-dimensionality, connectedness, and 'structural relevance' of the target line. Little relationship was found between connectedness ratings and accuracy in the identification task, but accuracy was highly correlated with mean depth rating (accounting for 95% of variance) and with mean structural-relevance rating (88%). Because of the high correlation (r = 0.98) between these two judgments in the present experiments, and confounding with other stimulus variables in previously published studies, the relative importance of these two global attributes cannot yet be determined definitively (though there was some evidence that for these patterns depth judgments were primary and structural-relevance judgments derivative). A reexamination of pertinent research suggests that comparisons between well-matched stimuli (as in the object-superiority effect) are likely to be more robust and informative than comparisons between lines alone and in context (the 'object-line effect').