Is it always so? Unexpected visions.

If we consider perceptions as arising from predictive processes, we must consider the manner in which the underlying expectations are formed and how they are applied to the sensory data. We provide examples of cases where expectations give rise to unexpected and unlikely perceptions of the world. These examples may help define bounds for the notion that perceptual hypotheses are direct derivatives of experience and are used to furnish sensible interpretations of sensory data.

In the Eye of the Illusion: Variation in Perceptual Phenomena Within an Inverted Percept.

Monocular depth cues can lead not only to illusory depth in two-dimensional patterns but also to perspective reversals in three-dimensional objects. When a viewer perceptually inverts (reverses) a three-dimensional object, stimuli on the inner surfaces of that object also invert. However, the perceptual fate of anything occurring within the space that is enclosed by the walls of a perceptually reversible object is unknown. In the present study, perceptions of the relative vertical heights of stimuli within a truncated pyramidal chute were compared for stimuli placed laterally, on the inner surface of the chute, or centrally, suspended within the volume enclosed by the chute. The typical inversion was obtained for lateral stimuli, but central stimuli did not invert. While central stimuli maintained their veridical vertical order, participants experienced a considerable compression of perceptual depth. These results imply a dilution of the illusion within the centre of the volume of space that it encloses.

Illusions within an Illusion.

Effect of perceptual inversion on illusion figures seen monocularly was explored using Ponzo and Muller-Lyer stimuli. It was found that inversion nullified the Ponzo effect but not the Muller-Lyer effect. This suggests that the determinants of the two illusory effects differ in their origin.

Bilateral symmetry and perceptual reversals.

Imposition of patterns varying in symmetry about the observer's median plane on a reversible figure affects duration for which each of the fluctuating percepts is experienced. Symmetry of the imposed patterns is associated with lesser difference between the two durations. This effect is particularly pronounced when the patterns are enantiomorphic and therefore jointly form a symmetrical pattern, but is also present when the patterns are symmetrical but unrelated.

Perspectival reversals of a solid: a phenomenal description.

The paper presents a phenomenal investigation of monocular perception of perspectival reversal of a conical shell. The inverted shell is seen as shorter than the stimulus, and both relative size of elements within it and movement within it appear to be affected by reversal. Similarity of the observed effects to those recorded in studies of afterimages are noted, as is similarity to reversals of Mach's Book.

On the Müller-Lyer illusion in the Carpentered World.

Implications of the little noticed fact that projective geometry restricts the range of interfin angles of the Müller-Lyer illusion, which are congruent with retinal projections cast by carpentered objects, are presented and tested. Experimental results support the theoretical restrictions. This implies that only Müller-Lyer figures with interfin angles within the 90 degrees to 180 degrees range are appropriate for investigation of the Carpentered World hypothesis.

Eyespots in a reversible setting.

When eyespots were presented on a reversible figure, the total duration for which elements bearing the eyespots were seen as closer to the observer was found to be greater than the total for counterpart elements. It is speculated that the tendency to see eyespots as nearer than they really are is related to the manner in which they are responded to in nature.

On representation of solids: a note.

Current theories of representation of 3-D objects assume that they are geometrical facsimiles of represented objects and, therefore, imply that various aspects of the representations are as concordant as are those of the objects themselves. Empirical data presented here question this as they show that subjects' choices of distinct views of objects are not mutually concordant. This being so, it seems unlikely that a facsimile representation of a solid yields a full description of the process of representation.

Perception and the art of depiction of cylindrical objects.

Many works of art, notably those in the Byzantine tradition, contain depictions of upright cylindrical objects such that the outline of their top surfaces is incompatible with that of their bottom surfaces. We endeavour to elucidate whether this is a consequence of a painterly usage or a perceptual effect and conclude, on the basis of empirical evidence, that the latter cause is more likely.

Oppel-Kundt illusion in three-dimensional space.

A three-dimensional form of the Oppel-Kundt illusion was examined. Subjects viewed arrays consisting of two parallel rows of lights. For one group the rows consisted of equal numbers of lights (2, 3, or 4), while for a second group the row nearest the subject always had the greater number of lights. Subjects viewed these arrays from two vantage points, one directly in front of the array and the other displaced laterally. For each array subjects adjusted the extent of the far array until they felt the two rows were the same length. Both the nature of the array and the viewpoint had a significant influence on the perceived length of the far row. The size of the near row was overestimated significantly more when the array was viewed from the central position and also when the number of lights in the near row exceeded that of the far row. These results confirm that a lateral viewing position decreases the perspective effect and indicate that the Oppel-Kundt illusion can occur with three-dimensional stimuli.

The eyes have it: a perceptual investigation of eyespots.

Previous work with non-human species has shown that eyespots function as sign stimuli for defensive behaviour. But it is unknown to what extent eyespots are a dominant feature of objects for human perceptual responses. We examined whether young children perceive eyespots as a dominant object feature. One hundred and nineteen children from the first, second, and third grades were presented with small manikins, called "Joopes" that varied in the number, type, and arrangement of facial features. Four kinds of Joope heads were used: single element (eyes, mouth, or beak only), monovalent (spatially concordant eyes and mouth, or eyes and beak), ambivalent (asymmetrical eyes and beak, or eyes and mouth), and cyclopean (single eye with orthogonal beak). Two task groups (Peepers and Gobs) 'helped' the Joopes to either 'see' their food or 'eat' it, by placing food in one of 24 feeding dishes. Results indicated that responses made to the ambivalent Joopes differed, with greater 'drift' shown by the 'eating' group towards the 'seeing' responses than by the 'seeing' group towards the 'eating' responses. The dominant role of eyespots was thus confirmed for children in the second and third grades, but response inconsistencies in the youngest group suggested difficulties in handling incongruent stimuli. The implications of these results for understanding basic perceptual processes are discussed.