Perception

It is well documented that orientation discrimination is poorer for stimuli oriented obliquely than for those that are vertical or horizontal. Buchanan-Smith and Heeley recently reported that in the absence of a spatial reference this anisotropy follows gravitational rather than retinal coordinates, suggesting a high-level basis for the anisotropy in unreferenced orientation discrimination tasks. In the present study, unlike the previous one, the effects of body tilt on orientation discrimination have been examined in the presence of explicit simultaneous spatial references. The thresholds for discrimination of two parallel or two perpendicular lines were estimated for the retinally principal and oblique orientations, with the body either erect or tilted 45° with respect to gravity. In agreement with previous studies, meridional anisotropy for both parallelism and perpendicularity discrimination was found when observers were seated upright. When the observer's body was tilted, the anisotropy for the parallelism task was mapped to retinal and not to gravitational coordinates after compensating for countertorsion. Initially, the anisotropy for the perpendicularity task was not mapped to retinal coordinates, but after extensive practice for both the erect and the tilted body conditions it eventually followed retinal coordinates. The results reported here suggest that contrary to orientation discrimination without a spatial reference, the ultimate limits for both parallelism and perpendicularity discriminations are located at orientation-sensitive cortical neurons. However, the effect of perceptual learning in the perpendicularity task suggests that the internal frame of reference (gravity cues and body axis) also plays an important role.

The motion aftereffect is strongest after viewing a moving field embedded in a patterned stationary surround, which suggests that relative motion is an important signal for its generation. The contribution of relative motion to binocular aspects of the motion aftereffect was assessed. Subjects viewed uniformly moving random dots surrounded by a stationary random-dot annulus. These displays could be presented in a variety of combinations to each eye separately or to both eyes, during adaptation and test. It was found that, although the presence of relative motion during adaptation significantly extended the duration of the monocular motion aftereffect, it did not augment interocular transfer. The presence of stationary surround contours in the nonadapting eye did not influence the aftereffect in the adapting eye. The enhancement provided by stationary surround contours is largely dependent on their presence during adaptation. The presence or absence of surround contours during the test phase did not influence the duration of the aftereffect. These findings are consistent with previous suggestions that the motion aftereffect is, in part, the result of adaptation to relative motion that occurs relatively early in the visual pathway—before binocular integration.

Masking is known to depend upon the relationship between the spatial-frequency content of target and mask. This relationship has been held constant in three experiments in order to investigate the separate contribution of the spatial parameters of the mask, in this case a grating with square-wave luminance profile. Thresholds for the detection of a probe target were highest when the background grating upon which the probe was superimposed had a spatial frequency of about 4 cycles deg⁻¹ (experiment 1) and a duty cycle of 50% (experiment 3). In experiment 2, the thresholds were strongly affected by the size of the background grating even though the size of the target was small in proportion to the grating and remained constant. The increase in threshold was linearly related to the area of visual cortex to which the grating projected. The spatial parameters of gratings that maximise masking are therefore the same as those that have been shown to be optimal for the induction of perceptual distortions, suggesting a possible physiological mechanism for both the masking and the distortions.

It is well-known that people recognize faces of their own race more accurately than faces of other races—a phenomenon often referred to as the ‘other-race effect’. Using brief presentations of faces, we show a similar effect for the task of discriminating the sex of a face. Specifically, Caucasian observers discriminated male and female Caucasian faces more accurately/efficiently than did Oriental⁽¹⁾ observers, and Oriental observers discriminated male and female Japanese faces more accurately/efficiently than did Caucasian observers. This result indicates that, under suboptimal viewing conditions, the identification of even the most salient of facial characteristics—face sex—is impaired for other-race faces. This finding suggests, also, that the nature and diversity of our experience with faces may affect not only the quality of the face representation for later access by recognition processes, but also the efficiency of a perceptual discrimination process. Intriguingly, too, we found that female observers, for both races tested, were considerably more accurate at the sex classification task than were male observers.

In past research evidence has been found for both mental imagery and propositional hierarchies in subjects' map representations. How the visual and semantic factors associated with maps influence the use of one form of representation or another was the subject of the study reported here. Subjects were required to make relational judgments about city pairs. The results of experiment 1 indicated that superordinate relationships (the relationship of the counties of which the cities were members) affected only the most difficult perceptual judgments, but affected all judgments made from memory. Experiment 2 was done to determine the extent to which these findings were attributable to perceptual distortions rather than propositionally stored rules. Results of experiment 3 showed the degree of perceptual distinction necessary for propositional rules to have no significant influence on relational judgments.

Sophisticated computer graphics were used to generate images of three-dimensional blocks-world scenes to investigate the perception of surface attitude. Three types of surface primitive (planar blocks, cylinders, and ellipsoids) were combined to form structured settings. The experiments were designed to investigate whether surface-based information such as gradients in shading and texture provide any significant advantage in attitude judgments over information derived from object contours. Images of shaded, textured, and line-drawn surfaces formed the stimulus set. The subjects' task consisted of setting an attitude probe on different parts of the scene so that the probe appeared to be locally coplanar with the perceived surfaces.

Analysis of settings according to attitude components, slant and tilt, shows remarkable agreement in slant settings for the shaded and line-drawn scenes but poor correlation between shaded and textured scenes. Similarly, tilt was also easily judged in shaded and line-drawn scenes and the experiments indicate that explicit surface boundaries are important for stable tilt perception. In general, the results suggest that, for the simple surfaces employed here, surface cues provide little extra information beyond that which is derived from contours.

Complex visual hallucinations are known to occur in individuals with impaired vision yet whose emotional and intellectual functions are within the normal range. These hallucinations, which were first reported by Charles Bonnet in 1760, have been described in many case studies, but have not been analyzed empirically to determine their major properties. In the present study, sixty complex hallucinators labeled as Charles Bonnet hallucinators were administered a questionnaire to determine the properties of their hallucinations. Combined use of multiple-correspondence analysis and hierarchical cluster analysis reveals a set of features that characterize the ‘typical’ Charles Bonnet hallucinatory experience: the hallucinators' experience occurs while they are alert and with the eyelids open; a sharply focused image appears suddenly, without any apparent trigger or voluntary control; the hallucination is present for seconds, does not move during this time, then suddenly vanishes. These features are discussed in terms of a ‘dimension’ of hallucinatory/perceptual experience, which ranges from discrete perceptual experiences to multiple, changing experiences. Possible mechanisms that underlie the Charles Bonnet hallucinations are discussed.

It is well known that biological motion, as produced by point-light displays on a human body, gives a good representation of the represented body—eg its gender and the nature of the task which it is engaged in. The question is whether it is possible to judge the emotional state of a human body from motion information alone. An ability to make this kind of judgment may imply that people are able to perceive emotion from patterns of movement without having to compute the detailed shape first. Subjects were shown brief video clips of two trained dancers (one male, one female). The dancers were aiming to convey the following emotions: fear, anger, grief, joy, surprise, and disgust. The video clips protrayed fully lit scenes and point-light scenes, with thirteen small points of light attached to the body of each dancer. Half the stimuli were presented the right way up, while half were inverted. The subjects' task was to judge which emotion was being portrayed. Full-body clips gave good recognition of emotionality (88% correct), but the results for upright biological-motion displays were also significantly above chance (63% correct). Inversion of the display reduced biological-motion (but not full-body) performance to close to chance but still significantly above chance. A space–time analysis of the motion of the points of light was carried out, and was related to the discriminability of the different emotions. Biological-motion displays, which convey no information while static, are able to give a rich description of the subject matter, including the ability to judge emotional state. This ability is disrupted when the image is inverted.

Active haptic discrimination of cylindrically curved hand-sized surfaces was investigated. Unilateral discrimination (that is, with one hand, either the left or the right one) was compared with bilateral discrimination. In this latter condition, the right hand had to touch the right surface, and the left hand the left one. The importance of which surface of the pair is placed to the right of the observer and which to the left (the so-called placement order) was also investigated.

In comparison with existing studies, performance in the present experiments was better owing to the combined effects of a larger surface area and active instead of passive discrimination. The results show clearly that curvature discrimination does not follow a Weber law; performance is relatively better with the larger curvatures. For all subjects, unilateral discrimination was much better than bilateral discrimination. A partial cause of this difference is the influence of the placement order, which is very apparent in the results for the bilateral condition. These results cannot be fully explained but the findings suggest an influence of the object–observer relation on the perception of the object.