Perception

The position of a black – white border is perceived somewhat towards the black side of its physical location. New measurements were performed of the magnitude of this effect for foveal vision in the normal observer and it was found to be about 0.4 min of arc at medium photopic luminances. The border shift can be accounted for by postulating that the retinal light spread caused by the optics of the eye is subjected to a compressive nonlinearity of light intensity (Naka — Rushton equation). Such irradiation-induced apparent enlargement of the white elements does not fully account for the shifted-chessboard illusion. When an additional stage of center — surround (DoG) transformation is introduced, a hypothetical excitation distribution can be generated, whose contours, at the appropriate scale, can outline quite well the seen shape deviations from rectangularity of the illusion. Contributing to the final percept, in addition to the light-spread, compressive nonlinearity, and center — surround reorganization, which would be retinal in origin, there are some cortical stages: the generation of the experience of sharp, straight borders and pointed corners, the emergence of a monotonic slope of the demarcation lines made of ‘saw-tooth’ position and angle shifts, and, finally, when all the retinal effects have been nulled out, a Zöllner kind of orientation deviation, due to offset pattern elements acting as tilted virtual contours.

A case study of a rare form of synesthesia is presented, where specific words act as the inducer, and gustatory sensations function as the concurrent. The female participant (TD) was twice presented with a list of 806 English words and 222 grammatically correct non-words (with 3 months between presentations). For each presentation, TD was asked to provide: a subjective description of the gustatory experience (if any) associated with each stimulus; a rating (from 0 to 10) of the intensity of the gustatory experience; and an indication (yes/no) if the experience was aversive. TD's responses across the two presentations were quantified, and comparisons are provided. In addition, TD's ability to create and recall novel word – taste associations was compared to that demonstrated by a sample of ten age-matched non-synesthete females. TD's synesthetic experiences were found to be highly consistent, more common in English words than non-words, and rarely aversive. Although TD was superior to control participants in remembering novel word — taste associations, her reported experiences cannot be wholly explained by either an exceptional non-synesthetic learning of taste — word associations over time, or a clear response to phonemes, rather than whole words. These conclusions are compared to the five other published reports of word — gustatory synesthesia, and directions for future research are discussed.

A cross-modal conflict over location was resolved in an unexpected way. When vision and proprioception provide conflicting information, which modality should dominate is ambiguous. A visual – proprioceptive conflict was created with a prism and, to logically disambiguate the problem, auditory information was added that either agreed with vision (group 1), agreed with proprioception (group 2), or was absent (group 3). While a scarcity of research addresses the interaction of three modalities, I predicted error should be attributed to the modality in the minority. Instead, the opposite was found: adaptation consisted of a large change in arm proprioception and a small change affecting vision in group 2, and the reverse in group 1. Group 1 was not different than group 3. Findings suggested adaptation to separate two-way conflicts, possibly influenced by direction of attention, rather than a direct solution to a three-way modality problem.

The role of stimulus structure in multisensory and unisensory interactions was examined. When a flash (17 ms) was accompanied by multiple tones (each 7 ms, SOA < 100 ms) multiple flashes were reported, and this effect has been suggested to reflect the role of stimulus continuity in multisensory interactions. In experiments 1 and 2 we examined if stimulus continuity would affect concurrently presented stimuli. When a relatively longer flash (317 ms) was accompanied by multiple tones (each 7 ms), observers reported perceiving multiple flashes. In experiment 3 we tested whether a flash presented near fixation would induce an illusory flash further in the periphery. One flash (17 ms) presented 5° below fixation was reported as multiple flashes if presented with two flashes (each 17 ms, SOA = 100 ms) 2° above fixation. The extent to which these data support a phenomenological continuity principle and whether this principle applies to unisensory perception is discussed.

The ability to navigate without vision towards a previously seen target has been extensively studied, but its reliability over time has yet to be established. Our aims were to determine distance and direction errors made during blind navigation across four different directions involving three different gait patterns (stepping forward, stepping sideway, and stepping backward), and to establish the test – retest reproducibility of these errors. Twenty young healthy adults participated in two testing sessions separated by 7 days. They were shown targets located, respectively, 8 m ahead, 8 m behind, and 8 m to their right and left. With vision occluded by opaque goggles, they walked forward (target ahead), backward (target behind), and sideway (right and left targets) until they perceived to be on the target. Subjects were not provided with feedback about their performance. Walked distance, angular deviation, and body rotation were measured. The mean estimated distance error was similar across the four walking directions and ranged from 16 to 80 cm with respect to the 8 m target. In contrast, direction errors were significantly larger during sideway navigation (walking in the frontal plane: leftward, 10° ± 15° deviation; rightward, 18° ± 13°) than during forward and backward navigation (walking in the sagittal plane). In general, distance and direction errors were only moderately reproducible between the two sessions [intraclass correlation coefficients (ICCs) ranging from 0.682 to 0.705]. Among the four directions, rightward navigation showed the best reproducibility with ICCs ranging from 0.607 to 0.726, and backward navigation had the worst reliability with ICCs ranging from 0.094 to 0.554. These findings indicate that errors associated with blind navigation across different walking directions and involving different gait patterns are only moderately to poorly reproducible on repeated testing, especially for walking backward. The biomechanical constraints and increased cognitive loading imposed by changing the walking pattern to backward stepping may underlie the poor performance in this direction.

Faces are difficult to recognise when presented upside down. This effect of face inversion was effectively demonstrated with the ‘Thatcher illusion’ by Thompson (1980 Perception 9 483–484). It has been tacitly assumed that this effect is due to inversion relative to retinal coordinates. Here we tested whether it is due to egocentric (ie retinal) inversion or whether the orientation of the body with respect to gravity also influences the face-inversion effect. A 3-D human turntable was used to test subjects in 5 different body-tilt (roll) orientations: 0°, 45°, 90°, 135°, and 180°. The stimuli consisted of 4 ‘normal’ and 4 ‘thatcherised’ faces and were presented in 8 different orientations in the picture plane. The subjects had to decide in a yes – no task whether the faces were ‘normal’ or ‘thatcherised’. Analysis of the d' values revealed a significant effect of stimulus orientation and body tilt. The significant effect of body tilt was due to a drop in d' values in the 135° orientation. This result is compared to findings of studies on the subjective visual vertical, where larger errors occurred in body-tilt orientations between 90° and 180°. The present findings suggest that the face-inversion effect relies mainly on retinal coordinates, but that in head-down body-tilt orientations around 135° the gravitational reference frame has a major influence on the perception of faces.

The magnitude of perceived roughness was haptically estimated as subjects freely explored linear gratings with either the bare finger or a rigid stylus-shaped probe. A considerably expanded range of ridge and groove width was investigated, relative to the extant literature. The four experiments collectively indicate that, for both finger and probe-end effectors, the variance in the estimates of perceived roughness was predominantly predicted by a single parameter: groove width. The functions relating perceived roughness to groove width increased over a narrow band relative to the full range of values, then flattened. These data have archival values for models of roughness perception involving both direct and indirect touch.

When a figure is only partially visible and its contours represent a small fraction of total image contours (as when there is much background clutter), a fast contour classification mechanism may filter non-figure contours in order to restrict the size of the input to subsequent contour grouping mechanisms. The results of two psychophysical experiments suggest that the human visual system can classify figure from non-figure contours on the basis of a difference in some contour property (eg length, orientation, curvature, etc). While certain contour properties (eg orientation, curvature) require only local analysis for classification, other contour properties (eg length) may require more global analysis of the retinal image. We constructed a pyramid-based computational model based on these observations and performed two simulations of experiment 1: one simulation with classification enabled and the other simulation with classification disabled. The classification-based simulation gave the superior account of human performance in experiment 1. When a figure is partially visible, with few contours relative to the number of non-figure contours, contour classification followed by contour grouping can be more efficient than contour grouping alone, owing to smaller input to grouping mechanisms.

Four studies are reported which demonstrate that indirectly, loosely related information, otherwise known as conceptual set, modulates object identification. Studies 1A and 1B demonstrate the impact of indirect, nonspecific, non-perceptual, conceptual primes on the interpretation of ambiguous visual figures. Study 2 demonstrates that indirect, conceptual information (category of farm animals) biases identification without requiring the activation of direct perceptual information (here the image of a horse). Study 3 uses a non-linguistic dependent measure to address the alternative explanation that language and not perception mediates the relationship between incidental conceptual prime and biased object identification. These results suggest that conceptual set constrains object identification.

Stereoscopic slant perception around a vertical axis (horizontal slant) is often found to be strongly attenuated relative to geometric prediction. Stereo slant is much greater, however, when an adjacent surface, stereoscopically in the frontal plane, is added. This slant enhancement is often attributed to the presence of a ‘reference surface’ or to a spatial change in the disparity gradient (introducing second and higher derivatives of disparity). Gillam, Chambers, and Russo (1988 Journal of Experimental Psychology: Human Perception and Performance 14 163–175) questioned the role of these factors in that placement of the frontal-plane surface in a direction collinear with the slant axis (twist configuration) sharply reduced latency for perceiving slant whereas placing the same surface in a direction orthogonal to the slant axis (hinge configuration) had little effect. We here confirm these findings for slant magnitude, showing a striking advantage for twist over hinge configurations. We also examined contrast slant measured on the frontal-plane surface in the hinge and twist configurations. Under conditions where test and inducer surfaces have centres at the same depth for twist and hinge, we found that twist configurations produced strong negative slant contrast, while hinge configurations produced significant positive contrast or slant assimilation. We conclude that stereo slant and contrast effects for neighbouring surfaces can only be understood from the patterns and gradients of step disparities present. It is not adequate to consider the second surface merely as a reference slant for the first or as having its effect via a spatial change in the disparity gradient.

Extending the work of Corballis et al (1985, Cortex 21 225–236), we investigated mental rotation of letters (experiment 1), and of letters and shapes (experiment 2) in normal readers and developmental dyslexics. Whereas the overall response times were equal for shapes in both groups, for letters they were slower in dyslexics. For letters as well as for shapes, however, the same mental-rotation effects were obtained between the groups. The results are interpreted as support for the notion of developmental dyslexia as a deficit in functional coordination between graphemic and phonological letter representations.

Here we outline a simple method of using two mirrors which allows one to stand out-side oneself. This method demonstrates that registration of vision with touch and proprioception is crucial for the perception of the corporeal self. Our method may also allow the disassociation of taste from touch, proprioception, and movement.