Perception

Phenomenal transparency was studied in a stimulus geometry that differs markedly from the conventional Metelli configuration, namely four squares that abut at a common vertex. In case of subjective transparency one perceives either a bipartite square ground overlaid with a uniform transparent rectangle, or a uniform square background overlaid with a pair of mutually orthogonal, uniform, transparent rectangular regions. Thus, the generic interpretations are limited to “left”, “right”, “lower”, or “upper” transparent rectangles or (when no subjective transparency occurs) “mosaic”. All transparent cases are congruent, whereas the Metelli configuration allows distinct Gestalt interpretations. This avoids interactions between Gestalt factors and subjective transparency per se. Formal analysis reveals that—as in Metelli's case—a number of ambiguous cases (eg “left” or “lower”) are to be expected. In the experiment we included these ambiguous cases as additional response categories. Observers who differ markedly on the Metelli configuration are virtually indistinguishable under the quad-square configuration. Moreover, observers reliably categorise the ambiguous instances as such; thus, “multiple transparency” has to be reckoned with as a bona fide percept.

Images with excessive energy at medium spatial frequencies (Fernandez and Wilkins, 2008 Perception 37 1098–1113), or that have high color contrast and little or no luminance contrast (Wilkins et al, 2008 Perception 37 Supplement, 144–145) appear uncomfortable or aversive and can induce headaches in hypersensitive observers. Such stimuli are uncharacteristic of natural images, and we examined whether visual discomfort more generally increases with deviations from the spatial and chromatic properties of natural scenes. Observers rated the level of discomfort or artistic merit in color images generated from noise or random overlapping rectangles (Mondrians). In one set, the slopes of the amplitude spectra for luminance or chromatic contrast were varied independently to create images ranging from strongly blurred to sharpened relative to a ‘natural’ 1/f spectrum. Perceived blur was dominated by the luminance slopes, with discomfort rated lowest for ∼1/f spectra. In a second set of focused Mondrians, color was varied along different axes in the L–M versus S–LM chromatic plane. Discomfort ratings were lowest around bluish–yellowish axes that are again typical of natural outdoor scenes. A final set varied in the levels of both luminance and chromatic contrast. Discomfort increased with increasing color contrast or decreasing luminance contrast, and tended toward lower ratings for images with balanced levels of luminance and color contrast. Notably, these ratings of discomfort were not related to judgments of artistic merit. Thus, for both spatial and chromatic content, the least aversive images corresponded to characteristic properties of the natural visual environment, and may reflect a normalization of visual coding to the natural world.

Temporal processing is of fundamental importance to the understanding of orders and durations of events in daily life. While recent research found that the perception of event durations is modulated by the visibility of perceived stimuli, it is still not known whether consciously and unconsciously perceived stimuli modulate temporal perception of a following stimulus in a similar vein. We investigated this using a temporal-comparison task that requires duration judgments of a standard stimulus and a probe. A prime prior to the standard stimulus reduced the subjective duration of the standard stimulus when observers were aware of the prime, and this effect changed with the temporal distance between the prime and standard. In contrast, a prime increased the subjective duration of the following standard stimulus when observers were unaware of the presence of the prime. Our findings indicate that a temporally neighbouring transient stimulus produces essentially different effects on temporal perception of following events when it is consciously and unconsciously perceived, leading to compression and expansion of subjective time, respectively.

The oblique effect is a decreased sensitivity for oblique orientations compared with horizontal/vertical orientations. We investigated whether orientation-matching acuity for binocularly presented stimuli is dependent on orientations of retinal projections or on the perceived orientation of the stimulus. Using a stereoscope, we constructed a stimulus, such that retinal orientations were oblique, while the perceived orientation was vertical. We then used this stimulus to test observers in an orientation-matching task. If orientation matching depends on the orientations of retinal projections, low matching performance would be expected, since retinal projections are oblique. However, if orientation matching depends on perceived orientation (vertical), high matching performance would be expected. We found that observers' performance can only be predicted from the perceived orientation, not the orientations of retinal projections, implying that in binocular viewing the oblique effect occurs after binocular fusion.

Elements that move with velocity gradients have been shown to give the impression of depth. In this study, it was found that dots in circular motion around a line of sight give a depth impression corresponding to the gradients of the angular velocities of circular motion on a screen. The results of two experiments show that depth perception through circular motion is as effective as that through expansion or spiral motion, but less effective than that through lateral motion parallax when the local speed distributions on the screen are matched. The present depth effect shows that expansion – contraction, spiral, and circular motions are a continuum in terms of producing depth effects; they differ from lateral motion parallax.

People rapidly and automatically evaluate faces along many social dimensions. Here, we focus on judgments of trustworthiness, which approximate basic valence evaluation of faces, and test whether these judgments are an overgeneralization of the perception of emotional expressions. We used a behavioral adaptation paradigm to investigate whether the previously noted perceptual similarities between trustworthiness and emotional expressions of anger and happiness extend to their underlying neural representations. We found that adapting to angry or happy facial expressions causes trustworthiness evaluations of subsequently rated neutral faces to increase or decrease, respectively. Further, we found no such modulation of trustworthiness evaluations after participants were adapted to fearful expressions, suggesting that this effect is specific to angry and happy expressions. We conclude that, in line with the overgeneralization hypothesis, a common neural system is engaged during the evaluation of facial trustworthiness and expressions of anger and happiness.

The midpoint of the axis of bisection in a triangle appears to be displaced toward the apex so that the apical extent seems to be shorter than the basal extent, an effect originally reported in 1895 by Thiéry and later in 1898 by Wundt and, therefore, referred to here as the Thiéry–Wundt illusion. Following a demonstration strongly suggesting that the illusion is yet another version of the Müller-Lyer effect in a stimulus figure with inward-directed angles, four exploratory experiments designed to throw more light on this group of illusions are reported. The first showed that the effect occurs in an open angle, between converging lines that do not meet to form an apex, between converging stepped lines, and when one of the converging lines is removed, leaving a single oblique line. The other three experiments showed that the illusion also occurs in an open semicircle and a rectangular bracket, is weakened by the addition of a complete or partial baseline to form a triangle, and weakly but reliably when the angle is minimally formed from dots marking the ends of oblique lines. On the basis of these data, Judd's version of the conventional Müller-Lyer figure, and informal but easily repeatable observations, it is concluded that the Thiéry–Wundt illusion is, like other variants of the Müller-Lyer group of illusions, due to a process of directional biasing—an extension of the concept of biasing proposed originally by Morgan et al (1990, Vision Research 30 1793–1810).

In this research we compare blind and normally sighted people in mental scanning of spatial maps using locomotor or visual/locomotor exploration of a real 3-D environment. Different types of visual experience were tested: early (congenital) and late (adventitious) onset of blindness, short-term deprivation (blindfolded-sighted), and full vision (sighted). Participants were asked to learn six positions in a large parking area with movement alone (congenital, adventitious, blindfolded-sighted) or with vision plus movement (sighted), and then to mentally scan between positions in the path. Finally, they had to describe how they imagined scanning the learned pathway. We found a significant linear relation between space and time, ie the classic mental scanning effect, in all tested groups. However, the linear component was lower in blind participants, especially congenital. Instead, short-term visual deprivation had minimal impact on mental scanning. Overall, blind participants had shorter scanning times than both sighted groups, and the effect was particularly evident with farther distances. These results suggest that there is a quantitative rather than a qualitative difference between the blind and the sighted. In addition, the mental scanning strategies reported by participants also affected mental scanning times. The theoretical implications of these findings are discussed.

Strong claims have been made about the importance of orientation in visual art. Although there have been a few studies whether left or right oriented pictures are more aesthetically pleasing, there have been no empirical studies whether the meaning and expressiveness of pictures depend on orientation. Thomas Bewick (1753–1828) made explicit decisions about whether the main protagonist in his pictures should face left or right and did so to express particular meaning. In three experiments we examined whether orientation changes the expressiveness of an image. In experiment 1 participants viewed eight of Bewick's animal wood engravings facing either in their original orientation or reversed, in a between-subjects design. They rated each print on ten characteristics, for example: docile–wild, clumsy–agile, and weak–strong. The original received more extreme ratings, across characteristics, than the reversal. Experiment 2 confirmed this result with participants from Italy. In experiment 3, using a within-subjects design, participants viewed ten wood engravings of dogs and rated them on characteristics specifically identified by Bewick. Again, the ratings of the original orientation were more extreme. Thus, in agreement with Bewick, we conclude that orientation affects expressiveness.

A variety of short time delays inserted between pairs of subjects were found to affect their ability to synchronize a musical task. The subjects performed a clapping rhythm together from separate sound-isolated rooms via headphones and without visual contact. One-way time delays between pairs were manipulated electronically in the range of 3 to 78 ms. We are interested in quantifying the envelope of time delay within which two individuals produce synchronous performances. The results indicate that there are distinct regimes of mutually coupled behavior, and that ‘natural time delay’—delay within the narrow range associated with travel times across spatial arrangements of groups and ensembles—supports the most stable performance. Conditions outside of this envelope, with time delays both below and above it, create characteristic interaction dynamics in the mutually coupled actions of the duo. Trials at extremely short delays (corresponding to unnaturally close proximity) had a tendency to accelerate from anticipation. Synchronization lagged at longer delays (larger than usual physical distances) and produced an increasingly severe deceleration and then deterioration of performed rhythms. The study has implications for music collaboration over the Internet and suggests that stable rhythmic performance can be achieved by ‘wired ensembles’ across distances of thousands of kilometers.

The characteristics of raised-line drawing discrimination can be defined as the sum of the discriminability of the length, curvature, and angles of the edges. The size of the angle between two edges constitutes an important feature of these tactile stimuli. In the first experiment, five standard angles (30°, 60°, 90°, 120°, and 150°) and twenty comparison angles for each standard angle were used to investigate the human capacity for tactile discrimination of raised angles by passive finger movement. The subjects in this study were asked to identify the larger angle of each pair by passive finger movement. We found that the threshold doubled when the standard angle was increased from 30° to 90°; however, the threshold remained unchanged when the standard angle was greater than 90°. In the second experiment, to investigate the influence of the endpoints on angle discriminability, we used one standard angle (60°) and seven comparison angles that changed in four bisector orientations. The results indicate that cutaneous feedback from the local apex and endpoints of the angle contributed to the discrimination of acute angles. Taken together, these results suggest that, when an acute angle is presented, both local apex and endpoint informations are used, while cutaneous mechanoreceptors rely more on apex information to discriminate the angle size when an obtuse angle is presented.