Perception

This paper presents the results of some recent experiments in neuroscience and perceptual science that reveal the role of virtual reality in normal visual perception, and the use of television technology by the visual brain. This involves particularly the cholinergic system in the forebrain. This research throws new light on the nature of perception and the relation of phenomenal consciousness and its brain. It is directly relevant to criticisms by certain analytical philosophers of aspects of neuroscience relating to these matters. Particular attention is paid to their support for Naive Realism.

Virtual-reality technology is being increasingly used to understand how humans perceive and act in the moving world around them. What is currently not clear is how virtual reality technology is perceived by human participants and what virtual scenes are effective in evoking movement responses to visual stimuli. We investigated the effect of virtual-scene context on human responses to a virtual visual perturbation. We hypothesised that exposure to a natural scene that matched the visual expectancies of the natural world would create a perceptual set towards presence, and thus visual guidance of body movement in a subsequently presented virtual scene. Results supported this hypothesis; responses to a virtual visual perturbation presented in an ambiguous virtual scene were increased when participants first viewed a scene that consisted of natural landmarks which provided ‘real-world’ visual motion cues. Further research in this area will provide a basis of knowledge for the effective use of this technology in the study of human movement responses.

In this series of studies we examine the importance of component features (line segments and corners) of a distractor in producing distractor interference. We postulated that corners are non-accidental features and therefore crucial components in the processing of distractors (Kritikos and Pavlis 2007, Experimental Brain Research 183 159–170). We presented non-degraded or degraded (line segments or corners removed) distractors (line drawings of tools or musical instruments) simultaneously with a target, either at fixation or peripherally. Non-degraded as well as degraded distractors caused higher levels of interference (increased reaction times and a greater number of errors), particularly when presented at fixation, while non-degraded distractors caused greater interference than degraded. Contrary to expectation, however, there was no difference in the level of interference caused by lines-missing and corners-missing distractors, either at fixation or at periphery. Thus, we conclude that, where response to a complex object is required, both lines and corners are equally important components in the visual processing of distractor stimuli.

The distinction between different spatial representations in the peripersonal space was examined in two experiments by requiring sighted blindfolded and blind participants to remember the locations of objects haptically explored. In experiment 1, object relocation took place from either the same position as learning—with the same (centred egocentric condition) or 90°-rotated (rotated egocentric condition) object array—or from a position different from the learning position (allocentric condition). Results revealed that, in both sighted and blind people, distance errors were higher in the allocentric and rotated conditions than in the centred egocentric condition, and that blind participants made more distance errors than sighted subjects only in the allocentric condition. Experiment 2 repeated rotated egocentric and allocentric conditions, while the centred egocentric condition was replaced by a decentred egocentric condition in which object relocation took place from the same position as learning (egocentric) but started from a decentred point. The decentred egocentric condition was found to remain significantly different from the rotated condition, but not from the allocentric condition. Moreover, blind participants performed less well in the allocentric condition, but were specifically impaired. Overall, our results confirm that different types of spatial constraints and representations, including the decentred egocentric one, can be distinguished in the peripersonal space and that blind people are as efficient as sighted in the egocentric and rotated conditions, but they encounter difficulties in recalling locations also in the peripersonal space, especially when an allocentric condition is required.

Williams syndrome (WS) is a genetic disorder associated with impairments of spatial cognition. This has primarily been studied in small-scale space, and rarely in large-scale environments. In order to fully characterise the spatial deficits in WS, and also to address claims that the deletion of LIM-kinase 1 (LIMK1) on chromosome 7 is responsible for those deficits, we report an automated large-scale search task for humans that places the participant egocentrically within the search space. Search locations were defined as lights and switches embedded in the floor, and participants attempted to locate a hidden target by pressing the switch at potential locations. We compared individuals with WS to patients with smaller deletions (including LIMK1) in the critical region on chromosome 7. Whilst partial-deletion participants performed efficiently on the task, participants with WS demonstrated inefficient search profiles: their search slopes were steeper and they made significantly more erroneous revisits to previously inspected locations. Our findings indicate that spatial deficits associated with WS also affect large-scale spatial processing and suggest that hemizygous deletion of LIMK1 is not sufficient to account for any of the spatial deficits associated with WS.

The face-processing skills of people with schizophrenia were compared with those of a group of unimpaired individuals. Participants were asked to make speeded face-classification decisions to faces previously rated as being typical or distinctive. The schizophrenic group responded more slowly than the unimpaired group; however, both groups demonstrated the customary sensitivity to the distinctiveness of the face stimuli. Face-classification latencies made to typical faces were shorter than those made to distinctive faces. The implication of this finding with the schizophrenic group is discussed with reference to accounts of face-processing deficits attributed to these individuals.

Response times (RTs) in visual search were measured with either a single target specified by colour, motion, spatial frequency, or orientation alone, or specified by pairwise conjunctions of these features, or by presenting double targets, each specified by a separate feature. First, for all feature combinations, except for motion–colour, RTs were faster when double features were used to specify a single target than when they specified separate targets, implying location-specific redundancy gains predicted by coactivation on a common location-specific map. Second, coactivation, as revealed by race-model violations, occurred for all double-feature single-target conditions except the motion–colour and colour–orientation combinations. No violations occurred in double-target conditions. Taken together, these results are accounted for by well-known feature-specific sensitivities of cortical V1 cells and provide further evidence for a V1 locus of redundancy gain in visual search.

Observers prefer Mondrian's paintings in their original orientation compared to when rotated—“the oblique effect” (Latto et al, 2000 Perception 29 981–987). We tested whether eye movements could provide any insight into this aesthetic bias. While recording fixation duration and saccade length, we presented eight Mondrian paintings dated 1921–1944 on a CRT in either their original or seven rotated positions to ten observers who used a Likert scale to report how (dis)pleasing they found each image. We report on eye-movement patterns from nine pairs of images that had a significant orientation effect. During the 20 s scans, fixation durations increased linearly, more so for pleasing images than for non-pleasing images. Moreover, saccade distances oscillated over the viewing interval, with larger saccade-distance oscillations for the pleasing images than the non-pleasing images. Both of these findings agree with earlier work by Nodine et al (1993 Leonardo 26 219–227), and confirm that as an abstract painting becomes more aesthetically pleasing, it shows both a greater amount of diversive/specific types of image exploration and balance. Thus, any increase in visual fluency in localizing vertical and horizontal versus oblique lines can lead to an increase in the aesthetic pleasure of viewing Mondrian's work.

It is widely accepted that illusory contours have been first displayed and discussed by Schumann (1900, Zeitschrift für Psychologie und Physiologie der Sinnesorgane 23 1–32). Here we show that, before him, Jastrow (1899, Popular Science Monthly 54 299–312) produced illusory contours consisting of a shadow word. A brief history of shadow writing in psychological literature from Jastrow to Brunswik is presented, in which the contributions of Pillsbury, Warren, Koffka, and Benussi are examined.

Terrestrial-passage theory proposes that the ‘moon’ and ‘sky’ illusions occur because observers learn to expect an elevation-dependent transformation of visual angle. The transformation accompanies daily movement through ordinary environments of fixed-altitude objects.

Celestial objects display the same visual angle at all elevations, and hence are necessarily non-conforming with the ordinary transformation. On hypothesis, observers should target angular sizes to appear greater at elevation than at horizon. However, in a sample of forty-eight observers there was no significant difference between the perceived angular size of a constellation of stars at horizon and that predicted for a specific elevation. Occurrence of the illusion was not restricted to those observers who expected angular expansion. These findings fail to support the terrestrial-passage theory of the illusion.

We examined the influence of spatial factors in temporal processing. Participants categorised as short or long empty intervals marked by two brief flashes delivered from locations differing in height and depth (experiment 1), or from two of three locations on a vertical plane (experiment 2). The perceived duration of intervals, as determined by the point of subjective equality, was affected by the height and depth of the signals (experiment 1). Experiment 2 showed that the point of fixation plays a critical role in perceived duration. The duration of an interval located in the upper visual field is perceived as longer when participants fixate the higher visual source and shorter when the fixation point is set in the middle; this latter result also generally applies when the fixation point is in the lower source. Finally, for the sensitivity level, there was a significant segment (upper versus lower) × direction (descending versus ascending) interaction in experiment 1; a similar interaction effect varied according to the fixation point in experiment 2. In experiment 2, the Weber fractions were around 0.22. Most results can be explained in terms of the need to shift attention from one visual source—for marking time intervals—to another.

We investigated whether recognition memory benefits when stimuli are organised in a visuo-spatial array. Three different memory tasks were compared: (a) verbal task (what); (b) spatial task (where); (c) combination task, where both verbal and spatial informations were combined together (what/where). We hypothesised that when visual stimuli are organised in a spatial array the recognition is better. Recognition memory was assessed by quantifying the speed of correct responses and the total number of correct responses in a group of male volunteers (N = 20). Subjects' task was to recognise a stimulus probe from sequences (one versus three) of stimuli. All stimuli appeared randomly in one of nine locations on a 3 × 3 grid. The results show that subjects were faster and more accurate when they had to recognise only spatial information (where) than when they had to recognise verbal information (what) or the combination of both verbal and spatial information (what/where). Additionally, when the stimuli were presented in a spatial array there were benefits for the spatial task, whilst verbal and combination tasks were unaffected. Additionally, asymmetry between left and right hemifields has been found. These findings are discussed with regard to different memory processes that operate independently.

Apotemnophilia straddles the boundary between neurology and psychiatry. It is a condition in which individuals experience the strong and specific desire for amputation of a healthy limb. Research suggests this disorder may be of neurological origin, specifically that the body image centers of the brain lack a cortical representation for a particular limb. A curious aspect of this condition is that sufferers often report an attraction to amputees in addition to desiring their own limb be removed. We postulate that sexual ‘aesthetic preference’ for certain body morphology is dictated in all individuals in part by the cortical representation of one's body image.

We describe how illusory apparent motion of one's own fingers and other body parts can easily be experienced in spite of grossly conflicting proprioceptive signals. This simple illusion may be used to shed light on the crossmodal integration of visual and proprioceptive signals. Our preliminary observations suggest that the visual and proprioceptive signals do not merge into a common crossmodal percept. Instead, mutually inconsistent visual and bodily sensations are experienced simultaneously.

In certain real-life situations, moiré patterns can enhance the observer's ability to resolve fine spatial detail of solid structures, theoretically to unlimited degrees. An example of such a situation can be seen in traffic signs on the M25 motorway around London. Moreover, owing to the interferential nature of the moiré pattern, its angular physical size is independent of observer distance, and thus creates an apparent size discrepancy: the closer you get, the smaller the size of the pattern appears to be—an illusory situation resembling Michael Ende's tale of the pseudo-giant Mr Tur Tur.