Perception

The human visual system is adept at detecting and encoding statistical regularities in its spatiotemporal environment. Here, we report an unexpected failure of this ability in the context of perceiving inconsistencies in illumination distributions across a scene. Prior work with arrays of objects all having uniform reflectance has shown that one inconsistently illuminated target can ‘pop out’ among a field of consistently illuminated objects (eg Enns and Rensink, 1990 Science 247 721–723; Sun and Perona, 1997 Perception 26 519–529). In these studies, the luminance pattern of the odd target could be interpreted as arising from either an inconsistent illumination or inconsistent pigmentation of the target. Either cue might explain the rapid detection. In contrast, we find that once the geometrical regularity of the previous displays is removed, the visual system is remarkably insensitive to illumination inconsistencies, both in experimental stimuli and in altered images of real scenes. Whether the target is interpreted as oddly illuminated or oddly pigmented, it is very difficult to find if the only cue is deviation from the regularity of illumination or reflectance. Our results allow us to draw inferences about how the visual system encodes illumination distributions across scenes. Specifically, they suggest that the visual system does not verify the global consistency of locally derived estimates of illumination direction.

The ability of observers to perceive distances and spatial relationships in outdoor environments was investigated in two experiments. In experiment 1, the observers adjusted triangular configurations to appear equilateral, while in experiment 2, they adjusted the depth of triangles to match their base width. The results of both experiments revealed that there are large individual differences in how observers perceive distances in outdoor settings. The observers' judgments were greatly affected by the particular task they were asked to perform. The observers who had shown no evidence of perceptual distortions in experiment 1 (with binocular vision) demonstrated large perceptual distortions in experiment 2 when the task was changed to match distances in depth to frontal distances perpendicular to the observers' line of sight. Considered as a whole, the results indicate that there is no single relationship between physical and perceived space that is consistent with observers' judgments of distances in ordinary outdoor contexts.

It is well established that under a wide range of conditions when a sparse collection of texture elements varies smoothly in depth, the spaces between the elements are assigned depth values. This disparity interpolation process has been studied in an effort to define some of its fundamental spatial and temporal constraints. To assess disparity interpolation we employed two tasks: a novel task that relies on the bisection of illusory boundaries created when subjective stereoscopic surfaces intersect, and one that relies on a 3-D shape discrimination. The results of both experiments show that there is no improvement in performance when texture density is increased from near 0.20 to 0.85 or when exposure duration is increased from 50–100 to 1000 ms. This lack of dependence on the addition of features that define the interpolated surface, along with the abrupt decline in performance below a critical value, is consistent with the view that surface interpolation is an important function of human stereoscopic vision.

In previous studies, we have found that the accuracy in judging collinearity of lines or dots varies considerably from one subject to another as a function of the relative angle of the stimulus elements. A model of errors generally shows large excursions across several subranges of angular position. These do not appear to be motor errors, at least not ones that are well separated from perceptual mechanisms. The errors are most likely generated at primary visual cortex, or beyond. We examined and modeled accuracy in judging collinearity of dot pairs, varying the angular position of the dots through 360°, the distance between the dots (stimulus span), and the distance at which the subject was required to respond (response span). Subjects manifested idiosyncratic profiles of error across angular positions, as reported previously. But across the tested range of spans, from 4 to 8 deg, the errors tended to be the same, irrespective of stimulus or response span. This suggests that the judgments are based on a radial (angular) measure of spatial position. We discuss these results in the context of proposals that the brain maps spatial position using rotation coordinates. These new data are consistent with the hypothesis that subjects use the z-axis coordinates as a mental protractor for judging angular position and collinearity.

In three experiments, we explored how pigeons use edges, corresponding to orientation and depth discontinuities, in visual recognition tasks. In experiment 1, we compared the pigeon's ability to recognize line drawings of four different geons when trained with shaded images. The birds were trained with either a single view or five different views of each object. Because the five training views had markedly different appearances and locations of shaded surfaces, reflectance edges, etc, the pigeons might have been expected to rely more on the orientation and depth discontinuities that were preserved over rotation and in the line drawings. In neither condition, however, was there any transfer from the rendered images to the outline drawings. In experiment 2, some pigeons were trained with line drawings and shaded images of the same objects associated with the same response (consistent condition), whereas other pigeons were trained with a line drawing and a shaded image of two different objects associated with the same response (inconsistent condition). If the pigeons perceived any correspondence between the stimulus types, then birds in the consistent condition should have learned the discrimination more quickly than birds in the inconsistent condition. But, there was no difference in performance between birds in the consistent and inconsistent conditions. In experiment 3, we explored pigeons' processing of edges by comparing their discrimination of shaded images or line drawings of four objects. Once trained, the pigeons were tested with planar rotations of those objects. The pigeons exhibited different patterns of generalization depending on whether they were trained with line drawings or shaded images. The results of these three experiments suggest that pigeons may place greater importance on surface features indicating materials, such as food or water. Such substances do not have definite boundaries—cued by edges—which are thought to be central to human recognition.

The Hermann grid illusion consists of smudges perceived at the intersections of a white grid presented on a black background. In 1960 the effect was first explained by a theory advanced by Baumgartner suggesting the illusory effect is due to differences in the discharge characteristics of retinal ganglion cells when their receptive fields fall along the intersections versus when they fall along non-intersecting regions of the grid. Since then, others have claimed that this theory might not be adequate, suggesting that a model based on cortical mechanisms is necessary [Lingelbach et al, 1985 Perception 14(1) A7; Spillmann, 1994 Perception 23 691–708; Geier et al, 2004 Perception 33 Supplement, 53; Westheimer, 2004 Vision Research 44 2457–2465]. We present in this paper the following evidence to show that the retinal ganglion cell theory is untenable: (i) varying the makeup of the grid in a manner that does not materially affect the putative differential responses of the ganglion cells can reduce or eliminate the illusory effect; (ii) varying the grid such as to affect the putative differential responses of the ganglion cells does not eliminate the illusory effect; and (iii) the actual spatial layout of the retinal ganglion cell receptive fields is other than that assumed by the theory. To account for the Hermann grid illusion we propose an alternative theory according to which the illusory effect is brought about by the manner in which S1 type simple cells (as defined by Schiller et al, 1976 Journal of Neurophysiology 39 1320–1333) in primary visual cortex respond to the grid. This theory adequately handles many of the facts delineated in this paper.

Walking without vision to previously viewed targets was compared with visual perception of allocentric distance in two experiments. Experimental evidence had shown that physically equal distances in a sagittal plane on the ground were perceptually underestimated as compared with those in a frontoparallel plane, even under full-cue conditions. In spite of this perceptual anisotropy of space, Loomis et al (1992 Journal of Experimental Psychology: Human Perception and Performance 18 906–921) found that subjects could match both types of distances in a blind-walking task. In experiment 1 of the present study, subjects were required to reproduce the extent of allocentric distance between two targets by either walking towards the targets, or by walking in a direction incompatible with the locations of the targets. The latter condition required subjects to derive an accurate allocentric distance from information based on the perceived locations of the two targets. The walked distance in the two conditions was almost identical whether the two targets were presented in depth (depth-presentation condition) or in the frontoparallel plane (width-presentation condition). The results of a perceptual-matching task showed that the depth distances had to be much greater than the width distances in order to be judged to be equal in length (depth compression). In experiment 2, subjects were required to reproduce the extent of allocentric distance from the viewing point by blindly walking in a direction other than toward the targets. The walked distance in the depth-presentation condition was shorter than that in the width-presentation condition. This anisotropy in motor responses, however, was mainly caused by apparent overestimation of length oriented in width, not by depth compression. In addition, the walked distances were much better scaled than those in experiment 1. These results suggest that the perceptual and motor systems share a common representation of the location of targets, whereas a dissociation in allocentric distance exists between the two systems in full-cue conditions.