Binding, spatial attention and perceptual awareness

Abstract

The binding problem is a real problem in human vision. It can be observed under special laboratory conditions in normal healthy populations and under less special conditions in patients with lesions that affect areas of the parietal lobes or areas of the thalamus that are strongly connected to the parietal cortex. Proper binding of certain features of an object in vision (for example, motion, shape, colour and orientation) requires spatial and attentional contributions that are associated with parietal function. This conclusion has received converging support from imaging, electrophysiological and neuropsychological studies in humans. The possible roles of spatial attention and temporal synchrony in binding are discussed. Features that are believed to be encoded in separate cortical feature maps do not require spatial attention for detection, and their detection does not activate parietal areas. Perceptual awareness of a feature does not necessarily require information about its location, but awareness of the co-localization of two different features (such as shape and colour) does. Again, evidence for this idea comes from imaging, electrophysiological and neuropsychological studies. Synaesthesia is a relatively rare condition in which a feature that is not present in the stimulus is bound to a feature that is, and is perceived together with the features that are present. Imaging data have suggested that the synaesthetic phenomenon arises as an interaction between the parietal cortex and areas of the ventral cortex that encode different visual features, similar to evidence for feature binding in non-synaesthetes. This suggests that at least certain forms of synaesthesia require attentional input, and that binding of a feature that is absent involves similar architecture to binding of features that are present. Behavioural evidence supports the conclusion that most (and perhaps all) synaesthetic experience requires attention to and awareness of the stimulus that induces the experience. The visual system does not appear to bind synaesthetic features preattentively in most synaesthetes, which is consistent with results in normal visual binding.