Stereo and motion parallax cues in human 3D vision: can they vanish without a trace?

In an immersive virtual reality environment, subjects fail to notice when a scene expands or contracts around them, despite correct and consistent information from binocular stereopsis and motion parallax, resulting in gross failures of size constancy (A. Glennerster, L. Tcheang, S. J. Gilson, A. W. Fitzgibbon, & A. J. Parker, 2006). We determined whether the integration of stereopsis/motion parallax cues with texture-based cues could be modified through feedback. Subjects compared the size of two objects, each visible when the room was of a different size. As the subject walked, the room expanded or contracted, although subjects failed to notice any change. Subjects were given feedback about the accuracy of their size judgments, where the “correct” size setting was defined either by texture-based cues or (in a separate experiment) by stereo/motion parallax cues. Because of feedback, observers were able to adjust responses such that fewer errors were made. For texture-based feedback, the pattern of responses was consistent with observers weighting texture cues more heavily. However, for stereo/motion parallax feedback, performance in many conditions became worse such that, paradoxically, biases moved away from the point reinforced by the feedback. This can be explained by assuming that subjects remap the relationship between stereo/motion parallax cues and perceived size or that they develop strategies to change their criterion for a size match on different trials. In either case, subjects appear not to have direct access to stereo/motion parallax cues

Latitude and longitude vertical disparities

The literature on vertical disparity is complicated by the fact that several different definitions of the term “vertical disparity” are in common use, often without a clear statement about which is intended or a widespread appreciation of the properties of the different definitions. Here, we examine two definitions of retinal vertical disparity: elevation-latitude and elevation-longitude disparities. Near the fixation point, these definitions become equivalent, but in general, they have quite different dependences on object distance and binocular eye posture, which have not previously been spelt out. We present analytical approximations for each type of vertical disparity, valid for more general conditions than previous derivations in the literature: we do not restrict ourselves to objects near the fixation point or near the plane of regard, and we allow for non-zero torsion, cyclovergence, and vertical misalignments of the eyes. We use these expressions to derive estimates of the latitude and longitude vertical disparities expected at each point in the visual field, averaged over all natural viewing. Finally, we present analytical expressions showing how binocular eye position—gaze direction, convergence, torsion, cyclovergence, and vertical misalignment—can be derived from the vertical disparity field and its derivatives at the fovea

Perception and action without 3D coordinate frames

Neuroscientists commonly assume that the brain generates representations of a scene in various non-retinotopic 3D coordinate frames, for example in 'egocentric' and 'allocentric' frames. Although neurons in early visual cortex might be described as representing a scene in an eye-centred frame, using 2 dimensions of visual direction and one of binocular disparity, there is no convincing evidence of similarly organized cortical areas using non-retinotopic 3D coordinate frames nor of any systematic transfer of information from one frame to another. We propose that perception and action in a 3D world could be achieved without generating ego- or allocentric 3D coordinate frames. Instead, we suggest that the fundamental operation the brain carries out is to compare a long state vector with a matrix of weights (essentially, a long look-up table) to choose an output (often, but not necessarily, a motor output). The processes involved in perception of a 3D scene and action within it depend, we suggest, on successive iterations of this basic operation. Advantages of this proposal include the fact that it relies on computationally well-defined operations corresponding to well-established neural processes. Also, we argue that from a philosophical perspective it is at least as plausible as theories postulating 3D coordinate frames. Finally, we suggest a variety of experiments that would falsify our claim

Perception and action without 3D coordinate frames

Neuroscientists commonly assume that the brain generates representations of a scene in various non-retinotopic 3D coordinate frames, for example in 'egocentric' and 'allocentric' frames. Although neurons in early visual cortex might be described as representing a scene in an eye-centred frame, using 2 dimensions of visual direction and one of binocular disparity, there is no convincing evidence of similarly organized cortical areas using non-retinotopic 3D coordinate frames nor of any systematic transfer of information from one frame to another. We propose that perception and action in a 3D world could be achieved without generating ego- or allocentric 3D coordinate frames. Instead, we suggest that the fundamental operation the brain carries out is to compare a long state vector with a matrix of weights (essentially, a long look-up table) to choose an output (often, but not necessarily, a motor output). The processes involved in perception of a 3D scene and action within it depend, we suggest, on successive iterations of this basic operation. Advantages of this proposal include the fact that it relies on computationally well-defined operations corresponding to well-established neural processes. Also, we argue that from a philosophical perspective it is at least as plausible as theories postulating 3D coordinate frames. Finally, we suggest a variety of experiments that would falsify our claim

A demonstration of 'broken' visual space

It has long been assumed that there is a distorted mapping between real and ‘perceived’ space, based on demonstrations of systematic errors in judgements of slant, curvature, direction and separation. Here, we have applied a direct test to the notion of a coherent visual space. In an immersive virtual environment, participants judged the relative distance of two squares displayed in separate intervals. On some trials, the virtual scene expanded by a factor of four between intervals although, in line with recent results, participants did not report any noticeable change in the scene. We found that there was no consistent depth ordering of objects that can explain the distance matches participants made in this environment (e.g. A > B > D yet also A < C < D) and hence no single one-to-one mapping between participants’ perceived space and any real 3D environment. Instead, factors that affect pairwise comparisons of distances dictate participants’ performance. These data contradict, more directly than previous experiments, the idea that the visual system builds and uses a coherent 3D internal representation of a scene

Size and shape constancy in consumer virtual reality

With the increase in popularity of consumer virtual reality headsets, for research and other applications, it is important to understand the accuracy of 3D perception in VR. We investigated the perceptual accuracy of near-field virtual distances using a size and shape constancy task, in two commercially available devices. Participants wore either the HTC Vive or the Oculus Rift and adjusted the size of a virtual stimulus to match the geometric qualities (size and depth) of a physical stimulus they were able to refer to haptically. The judgments participants made allowed for an indirect measure of their perception of the egocentric, virtual distance to the stimuli. The data show under-constancy and are consistent with research from carefully calibrated psychophysical techniques. There was no difference in the degree of constancy found in the two headsets. We conclude that consumer virtual reality headsets provide a sufficiently high degree of accuracy in distance perception, to allow them to be used confidently in future experimental vision science, and other research applications in psychology

How to Pay for Public Education

For years now, public education, and especially public higher education has been under attack. Funding has been drastically reduced, fees increased, and the seemingly irresistible political force of ever-tightening austerity budgets threatens to cut it even more. But I am not going to take the standard line that government financial support for public higher education should be increased. I view that battle as already lost. What I am going to propose is that we stop arguing about the allocation or reallocation of ever more scarce public resources and think of another way to fund public higher education. It's time for a new approach, one that satisfies the left's claim that higher education should be affordable for all, yet one that does not involve increasing expenditure of public funds or commit the government to entitlement programs that it cannot now or at least cannot long afford. What we need is a new proposal that is acceptable to both sides if we are to bring public education into the twenty-first century. And this is what this paper is devoted to providin

School performance in Australia: is there a role for quasi-markets?

Recent changes to the organisation of Australia's education system have raised the possibility of implementing wide-ranging market reforms. In this article we discuss the scope for introducing reforms similar to the United Kingdom's 'quasi-market' model. We discuss the role of school league tables in providing signals and incentives in a quasi-market. Specifically, we compare a range of unadjusted and model-based league tables of primary school performance in Queensland's public education system. These comparisons indicate that model-based tables which account for socio-economic status and student intake quality vary significantly from the unadjusted tables

Borrowing against the future: the response to the public consultation on the NHS bursary

This paper discusses the UK Government's public consultation into the NHS bursary and the response from the Nursing and Midwifery Council. A public consultation stipulated that the current arrangements for funding, by the State, were not to be considered for discussion. Instead, the consultation only appraised views that would lead to the successful introduction of student finance loans for NHS professional education. Testimonies from nurses, midwives and nursing students expressed concern that the new funding arrangements were unaffordable, dis-incentivising and biased towards the marketisation of student loans in the UK, yet the changes went ahead. The changes to NHS bursary funding resulted from the UK Government's desire for growth in student numbers (and ostensibly not growth in financial figures), and the fact that nursing (and other healthcare) students will become more fiscally indebted, despite society's moral obligation to them