Perception

It has previously been shown that when a moving and a stationary display are superimposed, illusory self-rotation (circular vection) is induced only when the moving display appears as the background. Three experiments are reported on the extent to which illusory forward self-motion (forward vection) induced by a looming display is inhibited by a superimposed stationary display as a function of the size and location of the stationary display and of the depth between the stationary and looming displays. Results showed that forward vection was controlled by the display that was perceived as the background, and background stationary displays suppressed forward vection by about the same amount whatever their size and eccentricity. Also, the perception of foreground — background properties of competing displays determined which controlled forward vection, and this control was not tied to specific depth cues. The inhibitory effect of a stationary background on forward vection was, however, weaker than that found with circular vection. This difference makes sense because, for forward body motion, the image of a distant scene is virtually stationary whereas, when the body rotates, it is not.

The effect of line of sight on the perception of spatial configuration has been investigated in a well-known painting (The Music Lesson by Vermeer) and in two control patterns. In experiment 1, subjects indicated the perceived inclination of two major contours which defined the sidewall—floor and backwall—floor joints in (i) a projected image of the painting, (ii) a three-line representation of the major spatial elements of the painting, and (iii) a three-dimensional wire model of these same contours, when standing in each of eighteen positions on a line running parallel to the surface of the screen. Results indicated a significant change in the perception of the sidewall—floor, but not of the backwall—floor contour, as viewing angle changed, in both the painting and the three-line representation. However, the angular setting in the latter case was significantly less than when the painting was used, ie subjects underestimated the depicted inclination. Settings for the wire model did not deviate with viewing angle and reflected geometrically correct adjustments. In experiment 2, the results of experiment 1 were confirmed using enantiomorphs. These findings are discussed in the light of other view-dependent illusions in paintings.

A solid object looks larger than its outline or silhouette under many viewing conditions. This solid-superiority effect may result from the assimilation or confusion of visual contours within the projection of a three-dimensional object on the picture plane. An aspect of the Müller-Lyer illusion may also play a role.

When a flat ellipse is slowly rotated in the frontoparallel plane, two different 3-D percepts can be obtained: (i) a rigid circular disc tilting back and forth in 3-D space, and (ii) an elongated egg, slanted into 3-D space, whose end parts seem to be located at different distances from the observer and describe a circular trajectory with respect to the frontal plane. Under prolonged observation, the two 3-D percepts alternate in time, separated by brief intervals in which either the rotation of a rigid flat ellipse in the frontal plane or an amoeba-like distortion of a 2-D shape can be perceived. An experiment is reported in which the sequence of perceptual alternations was investigated. Results show that the 3-D disc is mostly preceded by impressions of elastic amoeba-like deformations, whereas the 3-D egg is mostly preceded by the percept of a rotating flat ellipse. Direct transitions from egg to disc are not as frequent as transitions from disc to egg. Results are discussed in relation to Braunstein and Andersen's hypothesis that phenomena of this sort might result from the stimulation of automatic mechanisms for perceiving size change (changing—size neural channels).

The tendency to interpret as figure, relative to background, those regions that are lighter, smaller, and, especially, more convex is well known. Wherever convex opaque objects abut or partially occlude one another in an image, the points of contact between the silhouettes form concave cusps, each indicating the local assignment of figure versus ground across the contour segments. It is proposed that this local geometric feature is a preattentive determiner of figure—ground perception and that it contributes to the previously observed tendency for convexity preference. Evidence is presented that figure—ground assignment can be determined solely on the basis of the concave cusp feature, and that the salience of the cusp derives from local geometry and not from adjacent contour convexity.

The encoding and relative importance of first-order (discrete) and second-order (configural) features in mental representations of unfamiliar faces have been investigated. Nonmetric multidimensional scaling (KYST) was carried out on similarity judgments of forty-one photographs of faces (homogeneous with respect to sex, race, facial expression, and, to a lesser extent, age). A large set of ratings, measurements, and ratios of measurements of the faces was regressed against the three-dimensional KYST solution in order to determine the first-order and second-order features used to judge similarity. Parameters characterizing both first-order and second-order features emerged as important determinants of facial similarity. First-order feature parameters characterizing the appearance of the eyes, eyebrows, and mouth, and second-order feature parameters characterizing the position of the eyes, spatial relations between the internal features, and chin shape correlated with the dimensions of the KYST solution. There was little difference in the extent to which first-order and second-order features were encoded. Two higher-level parameters, age and weight, were also used to judge similarity. The implications of these results for mental representations of faces are discussed.

With the classic Poggendorff illusion a set of parallel ‘induction lines’ will cause a set of oblique line segments to look misaligned even though they are collinear. A different kind of misalignment can be produced by placing the induction lines so that they form a corner. Under these conditions the obliques will appear to be angled slightly, one relative to the other. The effects are small, but can be seen and reliably reported by a group of naive subjects. The influence of the induction lines drops sharply as their relative position is moved from parallel to orthogonal, but there is a small residual influence which may be called the corner Poggendorff effect.

When a limb is used for locomotion, patterns of afferent and efferent activity related to its own motion are present as well as visual, vestibular, and other proprioceptive information about motion of the whole body. A study is reported in which it was asked whether visual stimulation present during whole-body motion can influence the perception of the leg movements propelling the body. Subjects were tested in conditions in which the stepping movements they made were identical but the amount of body displacement relative to inertial space and to the visual surround varied. These test conditions were created by getting the subjects to walk on a rotatable platform centered inside a large, independently rotatable, optokinetic drum. In each test condition, subjects, without looking at their legs, compared, against a standard condition in which the floor and drum were both stationary, their speed of body motion, their stride length and stepping rate, the direction of their steps, and the perceived force they exerted during stepping. When visual surround motion was incompatible with the motion normally associated with the stepping movements being made, changes in apparent body motion and in the awareness of the frequency, extent, and direction of the voluntary stepping movements resulted.

A stationary vertical test grating appears to drift to the left after adaptation to an inducing grating drifting to the right, this being known as the motion aftereffect (MAE). Pattern-specific motion aftereffects (PSMAEs) induced by superimposed pairs of gratings in which the component gratings drift up and down but the observer sees a single coherent plaid drifting to the right have been investigated. Two experiments are reported in which it is demonstrated that the PSMAE is tuned more to the motion of the pattern than to the orientation and direction of motion of the component gratings. However, when subjects adapt to the component gratings in alternation, aftereffect magnitude is dependent upon the individual grating orientations and motion directions. These results can be interpreted in terms of extrastriate contributions to the PSMAE, possibly arising from the middle temporal area, where some cells, unlike those in striate cortex (V1), are tuned to pattern motion rather than to component motion.

A study is reported on the perception of empty time intervals marked by auditory signals. Nakajima's supplement hypothesis, which states that the subjective duration of a subjectively empty time interval is proportional to its physical duration plus a constant of ~80 ms, was examined quantitatively. Although this hypothesis has been used to explain various general aspects of time perception, from a global viewpoint, it has lacked the quantitative data necessary to describe the shape of the psychophysical functions mathematically. In the present study, subjects used two positive numbers to estimate the subjective ratio (m: n) between the durations of two serial or separate empty intervals. The psychophysical functions for empty durations 50–600 ms long could be approximated by a straight line with a positive y-intercept, as predicted by the hypothesis. The effective range of the hypothesis could be extended to ~1200 ms. A power function (without any modifications) also gave good approximations. The reliability and validity of the supplement hypothesis are discussed.

The relationship between the behavior of single-celled organisms and cognition in higher animals is explored. Recent research and theory in bacterial chemotaxis are presented, together with a discussion of the implications of chemotaxis for perceptual theory. A number of parallels between chemotaxis and perception in higher organisms are drawn. It is suggested that Koshland's model of the chemical processes controlling chemotaxis is an example of a mechanism for direct perception of change and can help elucidate Runeson's work on ‘smart mechanisms’ of perception. It is argued, more generally, that the growing body of knowledge about the perceptual activities of lower organisms should be used to broaden the factual base on which theories of perception are constructed: eg explication of perceptual parallels between humans and lower organisms should help clarify the nature of these phenomena in humans and, perhaps, help in the development of theories of greater generality. Also, the debate between direct and indirect theories of perception may be advanced by analysis of the specific mechanisms used by lower organisms. Contrasts to mediated perception are pointed out and arguments for the relative simplicity and explanatory power of theories of direct perception are provided.

A problem in experiments on adaptation to moving stimuli is the tendency of subjects to track the stimuli visually, which can complicate results. A simple optical method is described which eliminates this tendency by presenting two identical stimuli moving in opposite directions.