Perception

There is a strong tendency to complete a partly occluded shape. Two types of pattern completion, global and local, are frequently reported. By means of the primed-matching paradigm, it has previously been shown that global completions are prevalent for stimuli in which regularity is abundantly present. In our study the primed-matching paradigm is applied to such stimuli in order to find out whether the rival local completion is generated as well. Therefore anomalous completions are added to the experimental design. Priming effects both on global and on local completions are compared with priming effects on those anomalous completions. The data indeed suggest that the occlusion patterns evoked not only a global but also a local completion.

The strength of visual priming of briefly presented gray-scale pictures of real-world objects, measured by reaction times and errors in naming, was independent of whether the primed picture of the object was presented in the same size as or different size from the original picture. These findings replicate results on size invariance in shape recognition, which were obtained with line drawings, and extend them to the domain of gray-level images. Entry-level shape identification is based predominantly on scale-invariant representations incorporating orientation and depth discontinuities which are well captured by line drawings.

Naïve observers of random-dot stereograms depicting complex surfaces often find that they require several tens of seconds before the impression of depth emerges. With practice, however, perception times often decrease markedly: perceptual learning occurs. Current explanations of these effects were assessed in two experiments. In the first experiment the perception times of naïve observers for random-dot stereograms which depicted the same complex shape but contained different ranges of disparity were measured. In the second experiment the minimum times required by experienced observers to perceive a given complex shape in stereograms that contained different ranges of disparity were determined. Perception times for the naïve observers were all very fast (<3 s) and showed no evidence of perceptual learning. There was no effect of disparity range on perception times in either experiment. It was found that very large-disparity (80 min arc) complex stereograms could be perceived quickly, even by naïve observers. It is concluded that the long initial latencies previously reported are not due to surface complexity nor to the range of disparities present. Other factors, such as dot size, dot density, and the correlation of the stereo images, appear to be important determinants of efficient stereoscopic performance when viewing complex random-dot stereograms.

A single stimulus determined the alternative perceptions of an illusory transparent grey disk or of an internally illuminated circular hole. A square on a far background was visible through this disk or hole. Subjects rated the grey colour of the transparent disk or the phenomenal illumination inside the hole. The luminance difference relative to the transparent disk and the square and that relative to this disk and its background determined the probability of perceiving the transparent disk or the hole. Rated colour and illumination substantially depended only on this second difference. These results have implications for models of phenomenal transparency and illumination based on the idea that proximal contours activate neural representations of phenomenal attributes.

One of the open questions within the study of texture discrimination is whether the underlying neural mechanisms are located within the two monocular pathways, or whether they are located at more central areas that process binocular information. This question is considered here in psychophysical experiments of texture discrimination involving stereograms. The results show that texture discrimination for differences in form and size occur after binocular fusion. Moreover, the suitability of random-dot stereograms versus line-figure stereograms for this research has been studied. It was found that discrimination for differences in form was better with line-figure stereograms whereas discrimination for differences in size was better with random-dot stereograms.

The accuracy with which observers could judge whether two visual stimuli were the same or different was measured with the rating method of detection theory. For judgments of whether two pictures referred to natural or manufactured things, the shape of the obtained receiver operating characteristic (ROC) was consistent with the observers adopting an optimal decision strategy. A similar result was found for judgments of complex but meaningless visual patterns. For judgments of whether two colours that differed along a simple sensory dimension were the same or different, however, the resulting ROC was consistent with the observers adopting a suboptimal differencing strategy. The accuracy of the judgments did not depend on the visual field to which the stimuli were presented.

An overview is presented in which visual awareness is regarded as the epitome of the general mind—body problem. An experimental solution of the problem is considered first, followed by a philosophical outlook. It is argued that the scientific approach may eventually discover the neural correlate of visual awareness, but visual perceptions, even simple qualia like ‘brightness’ or ‘color’, occur in the mind of a conscious observer and are not reducible to observable activity of a specific set of neurons in the brain.

In monocular viewing there is a region in the peripheral visual field that is blind owing to the absence of photoreceptors at the site where the optic nerve exits the eye. This region, like certain other blind spots, nonetheless appears filled in. Several novel demonstrations of filling in at the blind spot have recently been reported. Here the implications of many of these effects are critically reevaluated. Specifically, it is argued that many blind-spot phenomena taken to support early filling in (eg pop out and alteration in apparent motion) are actually consistent with the thesis that the visual blind spot is treated by early perceptual processing as a region of reduced or absent information. In support of this, it is shown that many perceptual effects observed in blind-spot completion are similar in detail to the amodally perceived completion of partly occluded objects viewed somewhat peripherally. The goals were to point out striking similarities between blind-spot completion and the amodal completion of occluded parts of surfaces, and to provide a common theoretical framework for understanding these phenomena in the context of surface segregation and perceptual interpolation.