Speaker: Prof Semir Zeki, Wellcome Laboratory of Neurobiology, University College London, UK
Results from a variety of sources, some many years old, lead ineluctably to a re-appraisal of the twin strategies of hierarchical and parallel processing used by the brain to construct an image of the visual world. Contrary to common supposition, there are at least three “feed-forward” anatomical hierarchies that reach the primary visual cortex (V1) and the specialized visual areas outside it, in parallel. These anatomical hierarchies do not conform to the temporal order with which visual signals reach and activate V1 and the specialized visual areas, which obey a different (temporal) hierarchy. Furthermore, neither the anatomical hierarchies nor the temporal order predict the perceptual hierarchy. The latter shows that we see (and become aware of) different visual attributes at different times, with colour leading form (orientation) and visual motion, even though it is signals from fast moving stimuli that are the earliest to reach the visual cortex (of area V5). Parallel processing, on the other hand, is much more ubiquitous than commonly supposed, but is subject to a barely noticed but fundamental aspect of brain operations, namely that different parallel systems operate asynchronously with respect to each other and reach perceptual end-points at different times.
This re-assessment leads to the conclusion that the visual brain is constituted of multiple, asynchronously operating, task- and stimulus–dependent parallel hierarchies even within systems sub-serving a single attribute such as form or visual motion; which of these parallel anatomical hierarchies has temporal and perceptual precedence at any given moment is stimulus- and task-related and critically dependent upon the visual brain’s ability to undertake multiple operations asynchronously.