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  • Efficiency of stereopsis in random-dot stereograms.

    24 October 2018

    The statistical efficiency of stereopsis was investigated by measurement of the discriminability d' of a step edge in depth in a random-dot stereogram and its comparison with d' calculated for an ideal observer model. Efficiencies of approximately 20% were found for stimuli with very few (less than 30) dots. As the number of dots was increased, efficiency fell markedly to 2% and below. Over the ranges explored in this study, there was little variation in efficiency with changes in dot density (with dot number constant) or stimulus size. Efficiency loss was also found to be almost entirely due to incomplete use of the available information rather than to intrinsic observer variance. Results were compared with two simple models that implemented possible detection strategies.

  • Disparity channels in early vision.

    24 October 2018

    The past decade has seen a dramatic increase in our knowledge of the neural basis of stereopsis. New cortical areas have been found to represent binocular disparities, new representations of disparity information (e.g., relative disparity signals) have been uncovered, the first topographic maps of disparity have been measured, and the first causal links between neural activity and depth perception have been established. Equally exciting is the finding that training and experience affects how signals are channeled through different brain areas, a flexibility that may be crucial for learning, plasticity, and recovery of function. The collective efforts of several laboratories have established stereo vision as one of the most productive model systems for elucidating the neural basis of perception. Much remains to be learned about how the disparity signals that are initially encoded in primary visual cortex are routed to and processed by extrastriate areas to mediate the diverse capacities of three-dimensional vision that enhance our daily experience of the world.

  • Vertical disparities and perception of three-dimensional shape.

    24 October 2018

    The information about depth and three-dimensional shape available from the horizontal component of the stereo disparity field requires interpretation in conjunction with information about egocentric viewing distance (D). A novel computational approach for estimating D was proposed by Mayhew and Longuet-Higgins, who demonstrated that the horizontal gradient of vertical disparities uniquely specifies the viewing distance. We have now used random dot stereograms in a shape judgement task to show that changes in vertical disparities have no effect on perceived three-dimensional shape. Changes in ocular convergence do alter perceived shape, suggesting substantial changes in the subjects' scaling of horizontal disparities. We conclude that vertical disparities are not used to scale disparities for viewing distance, and that extraretinal signals must be considered when analysing human three-dimensional shape perception.

  • Refinement of the use of food and fluid control as motivational tools for macaques used in behavioural neuroscience research: report of a Working Group of the NC3Rs.

    24 October 2018

    This report provides practical guidance on refinement of the use of food and fluid control as motivational tools for macaques used in behavioural neuroscience research. The guidance is based on consideration of the scientific literature and, where data are lacking, expert opinion and professional experience, including that of the members of a Working Group convened by the United Kingdom National Centre for the Replacement, Refinement and Reduction of Animals in Research (NC3Rs). The report should be useful to researchers, veterinarians and animal care staff responsible for the welfare of macaques used in food and fluid control protocols, as well as those involved with designing, performing and analysing studies that use these protocols. It should also assist regulatory authorities and members of local ethical review processes or institutional animal care and use committees concerned with evaluating such protocols. The report provides a framework for refinement that can be tailored to meet local requirements. It also identifies data gaps and areas for future research and sets out the Working Group's recommendations on contemporary best practice.

  • Misaligned viewpoints

    24 October 2018

  • Neuronal activity and its links with the perception of multi-stable figures.

    24 October 2018

    In order to isolate the neuronal activity that relates to the making of perceptual decisions, we have made use of a perceptually ambiguous motion stimulus. This stimulus lies on the boundary between two perceptual categories that correspond to clockwise and counter-clockwise rotation of a three-dimensional figure. It consists of a two-dimensional pattern of moving dots that are capable of generating these two, distinct, three-dimensional percepts. We have studied the responses of neurons in cortical area V5/MT whilst macaque monkeys report judgements about the perceptual configuration of this stimulus. We extract a quantitative statistic called 'choice probability' that expresses the covariation of neuronal activity and perceptual choice. An analysis of choice probabilities shows that the pool of neurons involved in the perceptual decisions is a tightly constrained subset of the population of sensory neurons relevant to the perceptual task.

  • Human cortical activity evoked by the assignment of authenticity when viewing works of art.

    24 October 2018

    The expertise of others is a major social influence on our everyday decisions and actions. Many viewers of art, whether expert or naïve, are convinced that the full esthetic appreciation of an artwork depends upon the assurance that the work is genuine rather than fake. Rembrandt portraits provide an interesting image set for testing this idea, as there is a large number of them and recent scholarship has determined that quite a few fakes and copies exist. Use of this image set allowed us to separate the brain's response to images of genuine and fake pictures from the brain's response to external advice about the authenticity of the paintings. Using functional magnetic resonance imaging, viewing of artworks assigned as "copy," rather than "authentic," evoked stronger responses in frontopolar cortex (FPC), and right precuneus, regardless of whether the portrait was actually genuine. Advice about authenticity had no direct effect on the cortical visual areas responsive to the paintings, but there was a significant psycho-physiological interaction between the FPC and the lateral occipital area, which suggests that these visual areas may be modulated by FPC. We propose that the activation of brain networks rather than a single cortical area in this paradigm supports the art scholars' view that esthetic judgments are multi-faceted and multi-dimensional in nature.

  • Independent neural mechanisms for bright and dark information in binocular stereopsis.

    24 October 2018

    Early visual processing is organized into a number of independent channels. In the retina, increments and decrements of brightness are processed independently by different groups of neurons. For psychophysical measurements of human vision, independence can be tested statistically. Using this criterion in a depth judgement task, we show here that, for binocular stereo vision, increments and decrements are treated independently, at least as far as the level at which information from the left and right eyes is first combined. At later stages of stereo processing, the information from the two channels is no longer independent. Because the signals for stereo vision are first combined at the visual cortex, these results suggest that the neural 'on' and 'off' channels remain independent right up to early cortical stages. Theoretical studies of stereo vision have proposed that visual features in the views of the two eyes are matched on the basis of 'similarity'. Our results show that stereo matching treats features as statistically independent (and therefore dissimilar) if they appear perceptually bright and dark relative to the background. If features differ perceptually but only in the degree of brightness or darkness, human stereo vision treats them as similar.