Perception

In a previous moving-window study it was found that scene exploration benefits more from peripheral information of high spatial frequency than of low spatial frequency. In the present study, degraded versions of realistic scenes were presented peripherally during the initial 150 ms of fixations, while the undegraded scene was presented foveally. The undegraded version of the scene was visible both foveally and peripherally during the later part of fixations. During the initial 150 ms, the peripheral part of scenes was low-pass, bandpass, or high-pass filtered, blanked, or decreased in luminance. In a no-change condition, the undegraded scene was presented throughout the whole fixation. Participants freely explored the scenes in the context of an object-decision task. It was found that degrading peripheral information during the initial part of fixations had minimal effect on scene exploration. No reliable differences were found among the three filter types. The results indicate that, in the context of an object-search task, peripheral information is of minor importance during the initial part of fixations.

The visual system perceptually decomposes retinal image motion into three basic components that are ecologically significant for the human observer: object depth, object motion, and self motion. Using this conceptual framework, we explored the relationship between them by examining perception of objects’ depth order and relative motion during self motion. We found that the visual system obeyed what we call the parallax-sign constraint, but in different ways depending on whether the retinal image motion contained velocity discontinuity or not. When velocity discontinuity existed (eg in dynamic occlusion, transparent motion), the subject perceptually interpreted image motion as relative motion between surfaces with stable depth order. When velocity discontinuity did not exist, he/she perceived depth-order reversal but no relative motion. The results suggest that the existence of surface discontinuity or of multiple surfaces indexed by velocity discontinuity inhibits the reversal of global depth order.

As reported by Neumann and Klotz [1994, in Attention and Performance XV: Conscious and Nonconscious Information Processing Eds C Umiltà, M Moscovitch (Cambridge, MA: MIT Press) pp 123–150], a geometric shape masked by metacontrast can affect response latency (RT) even if it is not visible, ie if it yields a d' value of zero in a signal-detection (SD) task (metacontrast dissociation). In the initial study as well as in most subsequent experiments, the RT task was manual and the SD task was verbal. Hence tasks and output modes were confounded. In the present study, two experiments were conducted to find out which of these factors is responsible for the metacontrast dissociation. In experiment 1, participants performed an RT task in either a manual or a verbal output mode. In experiment 2, these output modes were compared in an SD task. Independently of output modes, the masked primes affected RT but could not be detected in the SD task. It is concluded that tasks, but not output modes, are crucial for the metacontrast dissociation. Implications for the mechanisms underlying the metacontrast dissociation and for the functional difference between judgments and responses are discussed.

The cognitive structure of a shape space—the space of rectangles—is explored by a nonmetric scaling technique. Our experiment was designed to extract the major transformational paths or ‘modes’ that characterize the mental shape space. Earlier studies of rectangle similarities using multidimensional scaling have provided conflicting evidence about whether the coordinate system of the mental rectangle space is based on height and width or on area and shape (ie aspect ratio). Our study reveals shape to be the single dominant factor. We suspected that earlier evidence for a height – width parameterization might have been due to the presentation of rectangles upright in a pseudo-gravitational coordinate system (whereas our rectangles are randomly rotated). In a control experiment with upright (vertical or horizontal) rectangles, the heavy bias towards shape preservation was still the dominant mode. In addition, however, a secondary bias towards change of height or width emerged, exactly following the pattern expected from the biasing change in context. This finding established a concrete path by which context and frame can influence the way shape is represented. The relevance of these findings to the cognitive organization of more complex shape spaces is discussed.

A new motion – depth illusion is reported. When a curved aperture translates vertically and stationary horizontal lines can be seen through it, the line lengths on the retina change continuously because of the occlusion. Instead of seeing the aperture translate, subjects sometimes see the lines rotate in depth around a vertical axis. This is a rare kind of illusion: an ambiguous motion which can be seen as either stationary in two dimensions or rotating in three dimensions. Three-dimensional rotation was more often observed when the luminance difference between the horizontal lines and the background was larger than that between the aperture and the background. This illusion demonstrates that motion detection and the structure-from-motion process correlate with figure – ground segregation, depth stratification, and figural-completion processes based on luminance contrast.

The corner effect, the Münsterberg illusion, and the Cafe Wall illusion are explained by a model postulating that the corner effect is an orientation illusion specific to corner edges and that the perceived orientations of these edges are shifted toward angle contraction. It is also assumed that the effect is greatest when the corner edges show the same or similar edge contrast at the corner. This model yields three new types of illusions: the ‘checkered illusion’, the ‘illusion of shifted gradations’, and the ‘illusion of striped cords’. Each of them gives many variations making a three-dimensional impression.

Naito and Cole [1994, in Contributions to Mathematical Psychology: Psychometrics and Methodology Eds G H Fischer and D Laming (New York: Springer)] provide a configuration which they describe as the Gravity Lens illusion. In this configuration, four small dots are presented in proximity to four large disks, and one is asked to compare the slope of an imaginary line which connects one pair of dots with the slope of a line which connects the other pair. In fact the slopes are the same, ie their axes are parallel, but because of the positioning of the large disks they appear to be at different orientations. Naito and Cole propose that the perceptual bias is analogous to the effects of gravity on the metrics of physical space, such that mental projections in the vicinity of a disk (or an open circle) are distorted just as the path of light is bent as it passes a massive body such as a star. Here we provide a simple test of this concept by having subjects judge alignments of dots which lie near tangents to a circle. Subjects were asked to project straight lines through pairs of stimulus dots, selecting and marking points in open space which were collinear with each pair. As would be predicted by the Gravity Lens theory, the locations selected by subjects were displaced from straight lines. However, the error magnitudes were substantially larger for judgments of dot pairs which had an oblique alignment, as compared with dot pairs which were aligned with a cardinal axis. This differential of effect as a function of stimulus orientation is not predicted by the gravity concept.

Besides the familiar moon illusion [eg Hershenson, 1989 The Moon Illusion (Hillsdale, NJ: Lawrence Erlbaum Associates)], wherein the moon appears bigger when it is close to the horizon, there is a less known illusion which causes the moon's illuminated side to appear turned away from the direction of the sun. An experiment documenting the effect is described, and a possible explanation is put forward.

A standard facial caricature algorithm has been applied to a three-dimensional (3-D) representation of human heads, those of Caucasian male and female young adults. Observers viewed unfamiliar faces at four levels of caricature—anticaricature, veridical, moderate caricature, and extreme caricature—and made ratings of attractiveness and distinctiveness (experiment 1) or learned to identify them (experiment 2). There were linear increases in perceived distinctiveness and linear decreases in perceived attractiveness as the degree of facial caricature (Euclidean distance from the average face in 3-D-grounded face space) increased. Observers learned to identify faces presented at either level of positive caricature more efficiently than they did with either uncaricatured or anticaricatured faces. Using the same faces, 3-D representation, and caricature levels, O'Toole, Vetter, Volz, and Salter (1997, Perception 26 719–732) had shown a linear increase in judgments of face age as a function of degree of caricature. Here it is concluded that older-appearing faces are less attractive, but more distinctive and memorable than younger-appearing faces, those closer to the average face.

Perceptual learning in colour/orientation visual conjunction search was examined in five experiments. Good transfer occurred to other conjunction arrays when only one element of the conjunction (either colour or orientation) was changed. When both elements (colour and orientation) were changed, but the same feature spaces were used (ie other colours and orientations) or when a new dimension was introduced to the transfer task (shapes instead of orientation), transfer was poor. The results suggest that perceptual learning of visual conjunction search is constrained mainly by stimulus parameters rather than by changes in cognitive strategies which are common to all search tasks. Contrary to other reports we found little evidence of long-term retention of learning.