Perception

We examined the stage of vertical-disparity processing that produces a global stereoscopic slant. In two psychophysical experiments, we measured perceived slant about a vertical axis for two-dimensional stereoscopic patterns consisting of random dots, concentric lines, and radial lines. Binocular image differences were introduced into each pattern by vertically magnifying either the entire image for the right eye or that for the left eye. Because the continuous lines were geometrically ambiguous in local stereo correspondence, the three patterns differed from each other in the local horizontal disparity measured in retinal coordinates. The two experiments revealed that, despite the differences in the retinal horizontal disparity, the slant settings were generally similar for the three patterns, in both short and long viewing distances (25 cm and 120 cm, respectively). These results are consistent with the idea that the visual system uses vertical disparity at least when establishing local stereo correspondence. A Bayesian model is proposed to account for the results.

Visual search impairment can occur following stroke. The utility of optimal spectral filters on visual search in stroke patients has not been considered to date. The present study measured the effect of optimal spectral filters on visual search response time and accuracy, using a task requiring serial processing. A stroke and control cohort undertook the task three times: (i) using an optimally selected spectral filter; (ii) the subjects were randomly assigned to two groups with group 1 using an optimal filter for two weeks, whereas group 2 used a grey filter for two weeks; (iii) the groups were crossed over with group 1 using a grey filter for a further two weeks and group 2 given an optimal filter, before undertaking the task for the final time. Initial use of an optimal spectral filter improved visual search response time but not error scores in the stroke cohort. Prolonged use of neither an optimal nor a grey filter improved response time or reduced error scores. In fact, response times increased with the filter, regardless of its type, for stroke and control subjects; this outcome may be due to contrast reduction or a reflection of task design, given that significant practice effects were noted.

In a disk – ring pattern composed of horizontally oriented checks in the centre and vertically oriented checks in the surround, the centre appears to slide relative to the surround when the pattern is slowly moved about. This phenomenon has been called the Ōuchi illusion. Slow sliding movements may represent involuntary ocular drifts, while occasional jerks suggest a contribution by microsaccades. The conditions under which the illusion occurs both with free viewing and in the absence of involuntary eye movements are reviewed. Illusory sliding is most pronounced with diagonal movement of the stimulus pattern, resulting in apparent motion orthogonal to the orientation of the disk. The illusion requires checks of low spatial frequency; it persists with low luminance contrast and blur of the inducing pattern, but is absent at equiluminance. These results suggest a magnocellular neuronal mechanism. Sliding continues to be seen with an empty annular zone separating centre and surround, and is seen even when there is no concentric surround at all, just two abutting flanks. Nine hypotheses proposed to account for the illusory motion of the centre relative to the surround are reviewed. Among these the models based on an integration bias of local motion vectors (Mather 2000 Perception 29 721–727) and Type II plaid motion (Ashida 2002 Vision Research 42 1413 – 1420) come closest to explaining illusory sliding in the Ōuchi figure, but open questions remain. Specifically, more research is warranted to correlate the predicted with the perceived direction of sliding motion in both check and grating patterns.

When viewing reverspective stimuli, data-driven signals such as disparity, motion parallax, etc, help to recover veridical three-dimensional (3-D) shape. They compete against schema-driven influences such as experience with perspective, foreshortening, and other pictorial cues that favor the perception of an illusory depth inversion. We used three scaled-size versions of a reverspective to study the roles of retinal size, binocular disparity, and viewing distance—that influences both vergence and accommodation—in recovering the true 3-D shape. Experiment 1 used three conditions, in each of which a parameter was kept fixed across the three stimulus sizes: (a) fixed retinal size, (b) fixed viewing distance, (c) fixed disparity. The predominance of the veridical percept was recorded. Generally, the illusion strength was the same when the viewing distance was fixed, despite significantly different disparities and retinal sizes; conversely, illusion strength changed significantly in fixed-disparity and fixed-retinal-size conditions. Experiment 2 confirmed the results of experiment 1b (roughly equal performances for fixed viewing distance, independent of size) for two additional distances. Viewing distance and “scaled disparity” (disparity divided by retinal size) are good predictors of the data trends. We propose that disparity scaling is supported by both mathematical and 3-D shape considerations.

Some experiments have stablished that certain kinematic parameters can influence the subjective aesthetic perception of the dance audience. Neave, McCarty, Freynik, Caplan, Hönekopp, and Fink (2010, Biology Letters 7 221–224) reported eleven movement parameters in non-expert male dancers, showing a significant positive correlation with perceived dance quality. We aim to identify some of the kinematic parameters of expert dancers' movements that influence the subjective aesthetic perception of observers in relation to specific skills of contemporary dance. Four experienced contemporary dancers performed three repetitions of four dance-related motor skills. Motion was captured by a VICON-MX system. The resulting 48 animations were viewed by 108 observers. The observers judged beauty using a semantic differential. The data were then subjected to multiple factor analysis. The results suggested that there were strong associations between higher beauty scores and certain kinematic parameters, especially those related to amplitude of movement.

The apparent slope of a hill or staircase, termed geographical slant perception, is exaggerated in explicit awareness. Across two experiments this paper tests the use of a laboratory environment to study geographical slant perception. First, using a student-aged sample (N = 166), we examine the similarity of slant estimates in the field with those made in the laboratory using life-sized images of the built environment as stimuli. Results reveal no differences in slant estimates between the two test environments. Furthermore, three traditional measures of perceived geographical slant (verbal, visual, and haptic) appear sensitive to a difference in slant of only 3.4° in both the field and laboratory environments. In a follow-up experiment we test the effect of fatigue on slant estimates in the laboratory. In line with previous research with outdoor stimuli, fatigued participants provided more exaggerated explicit reports of slant relative to those in a control group, and females gave more exaggerated slant estimates than males across both experiments. The current set of findings open the door to future studies of geographical slant perception that may be more suited to laboratory conditions.

Certain aspects of a visual search task, such as the location where a target often appears, are learned over time and can serve as a source of attentional guidance. An example of this implicit learning, spatial probability cuing speeds detection of targets that appear in probable locations. The current study investigated target feature probability learning and its relation to implicit spatial probability learning. Results reveal that individuals are sensitive to small changes in the spatial probability of targets, even incidentally learning moderate probabilities. However, feature-based probability differences impacted visual search times much less. Implications for these findings are discussed.

Patient NG is the first reported case of lower-limb ‘self-touch enhancement’ following stroke. Mobility problems prevented NG from reaching to touch her foot, thus we used a self-touch rubber-hand paradigm to mimic the conditions of self-administered touch. With vision precluded, NG administered stimulation to a prosthetic limb while the Examiner administered synchronous stimulation to NG's affected left foot. NG detected all stimulation administered with our self-touch paradigm, whereas in the control condition (with NG not involved in administering stimulation), NG had failed to detect one-third of Examiner-administered stimulation. When mobility problems are a barrier to investigating self-touch enhancement, the self-touch paradigm can be used to demonstrate residual tactile sensation following stroke.