Perception

Lower motion thresholds for rotational and radial flow have been measured for stimuli consisting of four closely packed circular apertures, each containing patches of drifting grating or plaid. Detection and direction thresholds were measured for gratings and plaids as a function of the relative orientation of the pattern components. There was a similarity between both types of threshold, supporting the existence of specialised rotation and radial-flow detectors. Further, thresholds increased with the relative component orientation for both gratings and plaids. This suggests that component information from a first stage, tuned spatiotemporally and to orientation, is being used directly to compute the optic flow in a two-stage process. A model based on this architecture is described by means of simple template receptive-field arrays with separable temporal and spatial tuning at the first stage.

When an observer judges the orientation in depth of a trapezoidal surface, the pictorial information of the surface is often more influential than motion information. Motion information might be more effective if pictorial information is simplified: this prompts the present study. Surfaces were triangular and pictorial information resided only in the visual lengths of the surfaces. In experiment 1, monocular observers viewed during head motions of 0 to 30 cm extent. Static judgments were somewhat dependent on visual length and tended to be frontal. Contrary to predictions, moving judgments were similarly affected: only 30 cm motion elicited near-veridical perception, as in previous studies with trapezoidal surfaces, although visual length had a residual effect. Experiment 2 involved investigation of whether visual length requires prior exposure to triangular surfaces to be effective; this was found not to be the case, which argues that observers rely on internal models of triangular surfaces. Depth perception appears to balance rapidity of processing against accuracy, in a way suggesting that ‘direct’ approaches are incomplete. Finally, it is argued that depth-from-motion simulations—influential in assertions that motion information is fully effective—depend on pictorial information.

A face is surprisingly difficult to recognise when presented in photographic negative, and negation has also been shown to affect simple perceptual judgments about a face. Two possible explanations for this effect are examined. In the shape-from-shading explanation it is argued that negating an image results in an impossible pattern of shading, and that this disrupts the formation of a three-dimensional representation of the surface geometry of the face. In an alternative account for this effect it is suggested that identification errors occur as a consequence of changes to the apparent pigmentation of the face caused by negating the image. Three experiments are reported which are designed to test these explanations by using novel colour-image transformations in which the hue and luminance components of images are independently manipulated. The results of these studies suggest that although changes to the apparent pigmentation of a face might result in identification errors in some situations, the loss of shape-from-shading cues is a more important cause of the negation effect. The role of these two sources of information in the recognition of normal faces is also discussed.

The method of constant stimuli was used to examine the accuracy with which two-dimensional spatial information can be represented in mental images. In experiment 1, subjects had to decide which of two successively presented two-dot separations was wider. Over the range of interstimulus intervals employed (0 to 30 s), there was a linear relationship between interstimulus interval and spatial interval thresholds.

In experiment 2 subjects' abilities to represent accurately more than one spatial interval at a time was investigated. Three dot pairs were presented, but only two pairs were to be compared, the third being completely irrelevant to the task. This manipulation doubled thresholds (relative to a two-dot-pair control condition), whether or not subjects were obliged to attend to the irrelevant dots. Overall, the results suggest that mental representations of spatial information may be temporally durable, but only in the absence of extraneous stimuli. The latter not only disrupt memory for spatial information, but appear to have obligatory access to it.

Human psychophysical evidence congruent with neurophysiological findings of a sustained input to directionally selective motion sensors in cat visual cortex is reported. Apparent motion was produced by displaying a group of dots in two frames (F1 and F2), where F2 was a translated version of F1. All stimulus sequences included a period during which F1 and F2 were displayed concurrently (combined images) and a period during which only F1 or F2 was on display (single images). There were three stimulus sequences: a display beginning with combined and ending with single image, a display beginning with single and ending with combined image, and a display beginning with F1, continuing with combined image, and ending with F2. Six durations of single and of combined images (10, 20, 40, 80, 160, and 320 ms) were crossed factorially in each stimulus sequence. Directional motion was seen easily at long durations of the single image in all stimulus sequences, as would be expected on the basis of a sustained input to the directional-motion-sensing mechanisms. Perception of directional motion improved with the duration of single images, but declined as the duration of combined images was increased. Baker and Cynader's model could account for the effect of duration of single images, but not for the effect of duration of combined images. An elaborated version of the model provides a good qualitative match to all empirical findings.

The Zöllner figure contains stacks of short parallel segments oriented obliquely to the direction of the stack. Adjacent parallel stacks of opposite polarity seem to diverge where their top segments form an arrowhead. To probe whether or not the opposite polarities are necessary to the illusion, three ‘half-Zöllner’ configurations were designed, containing stacks of a single polarity. The ‘orientation profile’ of these configurations was studied, that is, the way the strength of the perceived illusion varies with the orientation of the stacks. The subjects had to align two stacks or align stacks with target segments situated at a slight distance from them. All three half-Zöllner configurations produced errors that could be assimilated to global-orientation misjudgments. These errors were of opposite sign for the two types of stacks and varied with the orientation of the stacks as in the standard Zöllner illusion.

A further study was conducted in which the effect of several configurational parameters was explored for a single observer. The standard Zöllner illusion increases with the separation of the stacks. The illusion is also increased when the orientations of the segments in different stacks are orthogonal, independently of the particular longitudinal orientations of the stacks.

When the ends of the short segments are curved so that at their endpoints they become precisely perpendicular to the axis of the stacks, the standard and half-Zöllner illusions are reduced, but not abolished. Therefore, they cannot be entirely accounted for by a mechanism of alignment of illusory contours generated at these endpoints.

The results are consistent with the existence of a single common mechanism at work in both the standard and the half-Zöllner illusion. It is suggested that the illusion itself is not a rotation of the stacks but either a shear deformation in which the segments of a stack slide with respect to one another, or an expansion of the stacks orthogonally to the segments.

How well do observers perceive the local shape of an object from its shaded image? This problem was addressed by first deriving a potential representation of local solid shape. The descriptor of local shape, called shape characteristic, provides a viewpoint-independent continuum between hyperbolic (saddle-shaped) and elliptic (egg-shaped) points. The ability of human observers to make categorical judgments of local solid shape was then studied. This question was investigated by using a smooth ‘croissant’, a simple object made of two connected regions of elliptic and hyperbolic points. Observers decided whether the surface was locally elliptic or hyperbolic at various points on the object. The task was natural, and the observers could reliably partition the shaded image of the object into two regions, one elliptic and one hyperbolic. The ability of observers to perform this partition shows that they can, at least implicitly, localize the parabolic curves on a surface. This ability to locate the parabolic curve could in turn be exploited for other purposes, for instance to segment an object into its parts.

A haptic aftereffect of curved surfaces is demonstrated. Two spherical surfaces were presented sequentially to human subjects. They rested one hand on the first (conditioning) surface. After a fixed conditioning period they transferred their hand to the second (test) surface and judged whether the test surface was convex or concave. In experiment 1 the curvature of the conditioning surface was varied; the subject's judgment of convexity or concavity of the test surface was strongly shifted in the direction opposite to the curvature of the conditioning surface (negative aftereffect). Therefore, subjects judged a flat surface to be concave after being exposed to a convex surface. After a conditioning period of 5 s the shift was about 20% of the curvature of the conditioning surface. In experiment 2 the duration of the conditioning period was varied; the magnitude of the aftereffect could be described by a first-order integrator with a time constant of 2 s. In experiment 3 the time interval between the conditioning period and the touching of the second surface was varied; the magnitude of the aftereffect could be described by an exponential decay with a time constant of 40 s. It is concluded that the haptic aftereffect of curved surfaces is an important effect that occurs almost instantaneously and lasts for an appreciable period.