Perception

There is a distinct visual process that triggers the perception of illusory surfaces and contours along the intersections of aligned, zigzag line patterns. Such illusory contours and surfaces are qualitatively different from illusory contours of the Kanizsa type. The illusory contours and surfaces in this case are not the product of occlusion and do not imply occlusion of one surface by another. Rather, the aligned angles in the patterns are combined by the visual system into the perception of a fold or a 3-D corner, as of stairs on a staircase or a wall ending on a floor. The depth impression is ambiguous and reversible like the Necker cube. Such patterns were used by American Indian artists of the Akimel O'odham (Pima) tribe in basketry, and also by modern European and American artists like Josef Albers, Bridget Riley, Victor Vasarely, and Frank Stella. Our research aims to find out what manipulations of the visual image affect perceived depth in such patterns in order to learn about the perceptual mechanisms. Using paired comparisons, we find that human observers perceive depth in such patterns if, and only if, lines in adjacent regions of the patterns join to form angles, and also if, and only if, the angles are aligned precisely to be consistent with a fold or 3-D corner. The amount of perceived depth is graded, depending on the steepness and the density of angles in the aligned-angle pattern. The required precision of the alignment implies that early retinotopic visual cortical areas may be involved in this perceptual behavior, but the linkage of form with perceived depth suggests involvement of higher cortical areas as well.

Owing to the prevalence of fractal patterns in natural scenery and their growing impact on cultures around the world, fractals constitute a common feature of our daily visual experiences, raising an important question: what responses do fractals induce in the observer? We monitored subjects' EEG while they were viewing fractals with different fractal dimensions, and the results show that significant effects could be found in the EEG even by employing relatively simple silhouette images. Patterns with a fractal dimension of 1.3 elicited the most interesting EEG, with the highest alpha in the frontal lobes but also the highest beta in the parietal area, pointing to a complicated interplay between different parts of the brain when experiencing this pattern.

In five experiments some edgelines on different polyhedrons (house models in 3-D) were, from a certain vantage point, optically confluent, ie in optical prolongation of each other in 2-D on the retinal image and on photos. Other edgelines on the same polyhedrons were non-confluent, ie optically separate in 2-D. These conditions were found to lead to five different illusory shapes in 3-D. Five spatiofigural illusions were perceived. From these findings an edgeline principle is formulated that: “a straight edgeline in 2-D, whether confluent or separate, is perceived as a unitary and continuously straight edgeline in 3-D”. To this is added a supplementary perceptual principle, an amodal completion principle. In the experiments reported here, the illusory perception of shapes in 3-D with confluent edgelines as well as the veridical perception of other shapes in 3-D with only separate or non-confluent edgelines could all be explained by the edgeline principle and the amodal completion principle. By applying the concepts of edgeline confluence and the edgeline principle, a new explanation of the Kopfermann (1930 Psychologische Forschung 13 293–364) cube phenomena is proposed together with one example of how to test this explanation experimentally.

Visual exposure to an object can modulate an observer's degree of preference for it, initially enhancing preference (a ‘familiarity preference’ regime), and eventually lowering it again (a ‘novelty preference’ regime). Here we investigate whether there may be a functional advantage to modulating preference in this way. We put forth the simple hypothesis that degree of preference for an object of type X is the brain's estimate of the expected utility of acting to obtain X. In the light of this view of what preferences fundamentally represent, we are able to explain the ‘exposure effect’ and many of the connected phenomena.

It has recently been suggested that visual spatial attention can only affect consciously perceived events. We measured the effects of sustained spatial attention on orientation-selective adaptation to gratings, rendered invisible by prolonged interocular suppression. Spatial attention augmented the orientation-selective adaptation to invisible adaptor orientation. The effect of attention was clearest for test stimuli at peri-threshold, intermediate contrast levels, suggesting that previous negative results were due to assessing orientation discrimination at maximum contrast. On the basis of these findings we propose a constrained hypothesis for the difference between neuronal mechanisms of spatial attention in the presence versus absence of awareness.

There are more than forty million blind individuals in the world whose plight would be greatly ameliorated by creating a visual prosthesis. We begin by outlining the basic operational characteristics of the visual system, as this knowledge is essential for producing a prosthetic device based on electrical stimulation through arrays of implanted electrodes. We then list a series of tenets that we believe need to be followed in this effort. Central among these is our belief that the initial research in this area, which is in its infancy, should first be carried out on animals. We suggest that implantation of area V1 holds high promise as the area is of a large volume and can therefore accommodate extensive electrode arrays. We then proceed to consider coding operations that can effectively convert visual images viewed by a camera to stimulate electrode arrays to yield visual impressions that can provide shape, motion, and depth information. We advocate experimental work that mimics electrical stimulation effects non-invasively in sighted human subjects with a camera from which visual images are converted into displays on a monitor akin to those created by electrical stimulation.

We examined the contribution of motion information in perceiving facial expressions using point-light displays of faces. First, we established the minimum number of feature points necessary for the perception of facial expression from a single image. Next, we examined the effects of motion with a stimulus using an insufficient number of dots. We used two conditions. In the motion condition, the apparent motion was induced by a preceding neutral face image followed by an emotional face image. In the repetition condition, the same emotional face image was presented twice. The performance was higher in the motion condition than in the repetition condition. This advantage was reduced by inserting a white blank field between the neutral and emotional faces thus confirming that the improvement was due to the motion.

The circumstances under which the shapes of figure-versus-ground regions are perceived and remembered were investigated in three experiments that replicate, extend, and clarify Rubin's [1921 Visuell wahrgenommene Figuren (Copenhagen: Gyldendals)] classic study on this topic. In experiment 1, observers reported which of two regions they perceived as figure within ambiguous, bipartite, 2-D displays. In a later shape-recognition test, the shapes of regions previously seen as figures were remembered well, but the shapes of regions previously seen as grounds were remembered no better than novel distractor regions. In experiment 2 we examined the same question about memory for the shape of figure-versus-ground regions in nested displays in which the central region could be perceived either as a closer figure surrounded by a farther ground (ie as a solid object) or as a farther ground surrounded by a closer figure (ie as an empty hole). Unlike experiment 1, the shapes of regions initially perceived as grounds (holes) were remembered as well as those of regions initially perceived as figures (solid objects), and much better than those of novel distractor regions. In experiment 3 we further demonstrated that this outcome did not depend on the figure – ground instructions employed in experiment 2, because the same result was obtained with unambiguous 3-D cardboard displays of objects versus holes with no figure – ground instructions at all. The present findings support an account of hole perception in which the shape of an intrinsic hole is encoded as a shaped, immaterial (or virtual) surface where the absence of matter is coded by a functional ‘missing’ symbol (analogous to a minus sign in mathematics) to represent its non-material status.

We estimated the location of the auditory egocentre in congenitally blind, late-onset blind, and normally sighted participants, using a revised auditory version of Howard and Templeton's method. The mean location of the auditory egocentre of the congenitally blind participants was found to be close to the midpoint of the interaural axis or the centre of head rotation. It was significantly different from those in the normally sighted and late-onset blind participants, whose mean location of the auditory egocentre was close to the midpoint of the interocular axis or visual egocentre.

Four components of the haptic system were investigated, in isolation and in various combinations, during passive-guided exploration of raised-line drawings. The components were kinaesthesis, cutaneous input from the presence of a raised line, shear forces from relative movement between the skin and a textured surface, and attenuated distortions at the fingertip resulting from relative movement between the fingertip and a surface. Although the presence of kinaesthetic information was found to be positively correlated with performance in a task of identifying raised-line letters, conditions involving touch alone yielded performance equivalent to that when kinaesthesis was involved. Together, these results suggest that tactile information could be as effective as kinaesthetic information. The results are discussed in terms of applications to the design of human – machine interfaces.

Does semantic information in the form of stereotypical names influence participants' perceptions of the appearance of multiracial faces? Asian-Australian and European-Australian participants were asked to rate the appearance of Asian-Australian faces given typically Asian names, European-Australian faces given typically European names, multiracial faces given Asian names, and multiracial faces given European names. Participants rated the multiracial faces given European names as looking significantly ‘more European’ than the same multiracial faces given Asian names. This study demonstrates how socially derived expectations and stereotypes can influence face perception.

Judgments of speed and animacy from monolingual English readers were compared with those of bilingual readers of both English and a language read from right to left. Participants viewed a pair of dots moving horizontally across a screen at the same speed. Using a two-alternative forced-choice task, participants judged which dot in a pair moved faster (a direct measure of speed perception) or appeared to be alive (an indirect and correlated judgment of speed perception). In two experiments monolingual participants judged dots moving left to right to be faster and alive more often than dots moving right to left. In contrast, bilingual participants exhibited no directional bias for speed or animacy. These results suggest that the highly practiced eye movements involved in reading are associated with the presence or absence of a directional anisotropy for speed and animacy.