Perception

We propose an explanatory approach to Café Wall type illusions that is simple yet fairly comprehensive. These illusions are constructed out of basic elementary units in a jigsaw-like manner. Each unit, in general, contains both a solid body and a thin tail: the contrast polarity between the two determines the direction of the contributory illusory tilt produced by that element—according to a heuristic rule illustrated in figure 1. Ensembles of these elements exhibit illusory tilts only when the tails of the elements align along a common line in an additive manner. When elements of opposing polarity alternate, the illusion is cancelled. This approach extends and supersedes those presented in Pinna's illusion of angularity and Kitaoka's ‘acute’ corner effect. Furthermore, it appears to be, in part, compatible with existing mechanisms proposed to account for the emergence of local tilt cues, and it suggests several novel variations on the Café Wall theme.

In a recent study, Pelli (1999 Science 285 844–846) performed a set of perceptual experiments using portrait paintings by Chuck Close. Close's work is similar to the ‘Lincoln’ portraits of Harmon and Julesz (1973 Science 180 1194–1197) in that they are composite images consisting of coarsely sampled, individually painted, mostly homogeneous cells. Pelli showed that perceived shape was dependent on size, refuting findings that perception of this type is scale-invariant. In an attempt to broaden this finding we designed a series of experiments to investigate the interaction of 2-D scale and 3-D structure on our perception of 3-D shape. We present a series of experiments where field of view, 3-D object complexity, 2-D image resolution, viewing orientation, and subject matter of the stimulus are manipulated. On each trial, observers indicated if the depicted objects appeared to be 2-D or 3-D. Results for face stimuli are similar to Pelli's, while more geometrically complex stimuli show a further interaction of the 3-D information with distance and image information. Complex objects need more image information to be seen as 3-D when close; however, as they are moved further away from the observer, there is a bias for seeing them as 3-D objects rather than 2-D images. Finally, image orientation, relative to the observer, shows little effect, suggesting the participation of higher-level processes in the determination of the ‘solidness’ of the depicted object. Thus, we show that the critical image resolution depends systematically on the geometric complexity of the object depicted.

We used holes to study unilateral border ownership and in particular the information carried by the sign of the curvature along the contour (ie the difference between convex and concave regions). When people perceive a hole, its shape has a reversed curvature polarity (ie a changed sign of curvature) compared to the same region perceived as an object. Bertamini (2001 Perception 30 1295–1310), and Bertamini and Croucher (2003 Cognition 87 33–54) suggested and found evidence to support the hypothesis that, because convex regions are perceived as parts, positional information is more readily available for convex regions. Therefore a change is predicted when a given region is perceived as either a hole or a figure. We confirm that finding in this study, using holes defined by binocular disparity. We conclude that a change from figure to hole always reverses the encoding of curvature polarity. In turn, polarity obligatorily affects perceived part structure and the processing of position.

By systematically varying cue availability in the stimulus and response phases of a series of same-modality and cross-modality distance matching tasks, we examined the contributions of static visual information, idiothetic information, and optic flow information. The experiment was conducted in a large-scale, open, outdoor environment. Subjects were presented with information about a distance and were then required to turn 180° before producing a distance estimate. Distance encoding and responding occurred via: (i) visually perceived target distance, or (ii) traversed distance through either blindfolded locomotion or during sighted locomotion. The results demonstrated that subjects performed with similar accuracy across all conditions. In conditions in which the stimulus and the response were delivered in the same mode, when visual information was absent, constant error was minimal; whereas, when visual information was present, overestimation was observed. In conditions in which the stimulus and response modes differed, a consistent error pattern was observed. By systematically comparing complementary conditions, we found that the availability of visual information during locomotion (particularly optic flow) led to an ‘under-perception’ of movement relative to conditions in which visual information was absent during locomotion.

The purpose of our study was to estimate the perceptual span for facial information: how many faces can be processed during a single eye fixation. We used a visual-search task, in which the targets and distractors were facial photographs. The task of the observer was to search for and identify a target face in an array of faces. We measured the time needed for one search—threshold search time—by using a multiple-alternative staircase method. The threshold represents the duration of stimulus presentation at which the probability of correct responses was 79%. The array size was varied from 2 × 2 to 8 × 8 faces. Simultaneously with the performance measurements we measured eye movements with a video eye tracker. We found that threshold search time increased with increasing set size nearly linearly. The number of fixations also increased linearly from unity at the smallest set size to about fifteen at the largest set size. The result of 2 × 2 faces during a single fixation gave an estimate of 4 faces for the perceptual span. If, on average, only half of the elements had to be scanned for finding the target, 15 fixations at the largest set size (8 × 8) gave another estimate of 2.13 faces. The mean fixation duration was around 200 ms. Thus, the results suggest that 2–4 faces can be processed during one fixation of about 200 ms.

Context sensitivity of size perception has previously been used to study individual differences related to the distinction between local, analytic, or field-independent and global, holistic, or field-dependent perceptual styles. For example, it has been used in several recent studies of autistic spectrum disorders, which may involve an excessive bias toward local processing. Autism is much more common in males, and there is evidence that this may be in part because males in general tend to be less context-sensitive than females, and thus are more affected by conditions that further reduce context sensitivity. There is also evidence that a bias to local processing is more common in professions that require attention to detail. Context sensitivity of size perception was therefore studied as a function of sex and academic discipline in sixty-four university staff and students by a simple, sensitive, and specific psychophysical measure based on the Ebbinghaus illusion. The results show that in this task males are on average less context-sensitive than females, that the overlap is large, and that subjects with very high or very low context sensitivity tend to have the sex and profession predicted by the above hypotheses.

Can potentially antisocial or criminal behaviour be predicted? Our study aimed to ascertain (a) whether observers can successfully predict the onset of such behaviour when viewing real recordings from CCTV; (b) where, in the sequence of events, it is possible to make this prediction; and (c) whether there may be a difference between naïve and professional observers. We used 100 sample scenes from UK urban locations. Of these, 18 led to criminal behaviour (fights or vandalism). A further 18 scenes were matched as closely as possible to the crime examples, but did not lead to any crime, and 64 were neutral scenes chosen from a wide variety of noncriminal situations. A signal-detection paradigm was used in conjunction with a 6-point rating scale. Data from fifty naïve and fifty professional observers suggest that (a) observers can distinguish crime sequences from neutral sequences and from matches; (b) there are key types of behaviour (particularly gestures and body position) that allow predictions to be made; (c) the performance of naïve observers is comparable to that of experts. However, because the experts were predominantly male, the absence of an effect of experience may have been due to gender differences, which were investigated in a subsidiary experiment. The results of experiment 2 leave open the possibility that females perform better than males at such tasks.

We investigated the consistency between tactually and visually designated empty time intervals. In a forced-choice discrimination task, participants judged whether the second of two intervals was shorter or longer than the first interval. Two pulses defined the intervals. The pulse was either a vibro-tactile burst presented to the fingertip, or a foveally presented white square. The comparisons were made for uni-modal and cross-modal intervals. We used four levels of standard interval durations in the range of 100–800 ms.

The results showed that tactile empty intervals must be 8.5% shorter to be perceived as long as visual intervals. This cross-modal bias is larger for small intervals and decreases with increasing standard intervals. The Weber fractions (the threshold divided by the standard interval) are 20% and are constant over the standard intervals. This indicates that the Weber law holds for the range of interval lengths tested. Furthermore, the Weber fractions are consistent over uni-modal and cross-modal comparisons, which indicates that there is no additional noise involved in the cross-modal comparisons.

In this experiment, the effects of prior experience on odour perception and discrimination were explored. Participants repeatedly sniffed a mixture composed of two odours, AX, as well as smelling two further odours alone, B and Y. After this training phase, participants were asked to rate the similarity of the odours A and X, B and Y, and a non-exposed pair C and Z. A and X were judged as significantly more alike than the other pairs. Exactly the same pattern emerged on a second test, in which participants were asked to select the odd odour out of sets of three. It was consistently harder for participants to pick the odd odour when the stimuli were drawn from the AX pair (eg A versus AX versus AX). Not only do these findings demonstrate that prior experience can affect odour perception, a finding not predicted by theories of odour perception based solely upon the physiochemical properties of odours, they also suggest that experience can act to selectively decrease discriminability.