The underlying strategy for much of our work is to understand how the responses of single cortical neurons can account for the performance of specific perceptual tasks.  A review paper with Bill Newsome (Parker and Newsome 1998) sets out this general strategy for making comparisons between neuronal activity and perceptual decisions, in particular, by proposing a set of benchmark criteria against which experimental data should be evaluated.

In early work with Mike Hawken (now at New York University), I discovered, to the surprise of many in the field, that individual neurons in the primary visual cortex can perform at least as reliably as human observers in a variety of tasks.  Most notably, we developed a plausible, physiologically-based account, at the level of single cortical neurons, of fine-grain spatial tasks, such as vernier acuity, in which observers can make judgements of spatial position with accuracy better than that of the photoreceptor matrix in the retina.

Current Research Programme

Our group is currently engaged in understanding work in the processing of binocular stereoscopic depth and 3-D shape by neurons in the visual cortex.  The long-term aim has been to make a systematic comparison between the properties of single neurons at various stages in the visual pathways and the perceptual performance of the binocular visual system.

The responses of neurons in the primary visual cortex to binocular depth differ in characteristic ways from how depth is actually perceived.  A crucial, novel step in this work was to show that anti-correlated random-dot stereograms (patterns of opposite contrast in the two eyes) produce ‘tuned’ responses from neurons in the primary visual cortex (albeit inverted in sign, compared with those for correlated patterns), whereas such patterns do not support normal stereoscopic depth perception.  We have also determined at the physiological level a a significant distinction between absolute and relative disparity detection, with the important discovery that neurons in the primary visual cortex respond only to absolute disparity, whilst the secondary visual area contains some neurons that are specialized for encoding relative disparity.

Although the primary visual cortex is a crucial stage in processing of stereoscopic information, it is only a preliminary stage in the generation of a stereoscopic depth percept. This insight alters our view of clinical conditions, such as amblyopia, in which there is a loss of binocular function.  Hitherto, the focus has been on the loss of binocularity in V1:  the present work makes clear that loss or dysfunction of neurons outside V1 may be crucial. To search for those sites in human visual cortex, we are continuing our neurophysiological studies and have initiated the use of functional magnetic resonance imaging (fMRI) to examine the role of human extrastriate cortex in stereoscopic depth perception

We are also interested in what stereoscopic depth perception can tell us about spatial cognition in general. Our group has a fully-equipped virtual reality laboratory and has developed collaborations with colleagues in Psychiatry to examine whether simple visual tasks may be associated with psychological morbidity.

Andrew Parker