random dot stereograms but if the contrast in one eye is reversed so that all the black dots become white and vice versa, they can't. The pattern just looks like a shimmery mess. To neurophysiologists, this type of pattern is interesting because neurons that are responsive to the disparity in an image continue to respond to the disparity in an anti-correlated pattern, despite the fact that the observer's perception is so different in the two cases. This lack of correspondence between neural response and perception might seem perplexing at first. In terms of the idea explored in these pages, however, the fact that neurons might contribute to perception in one case (the correlated stereogram) but not in another (anti-correlated), is understandable. As in previous examples (a single neuron and vernier acuity), the extent to which a particular stimulus parameter or a particular neuron influences a perception/action/interpretation depends on the context. In the case of a stereogram, the rest of the stimulus seems to act as a 'gate' that allows perceptual access to the firing of a particular disparity-tuned neuron in one instance (the correlated stereogram) but not the other (anti-correlated). If the current sensory+motivational state travels along a familiar path, e.g. from coarse to medium and then fine scale match (as discussed earlier in relation to recognising a face or detecting a predator), then the current sensory+motivational state can land up in a Voronoi cell where a small number of neurons can determine the perceived depth of a part of the stimulus. The same is not true of an anti-correlated pattern. Although disparity-tuned neurons are stimulated, there is no orderly binocular structure that can be described at multiple scales (i.e. the hierarchical scale structure is quite different for the left and right eyes, so there is no way to compare the structures sensibly between the left and right eyes. A similar point was raised in relation to the behaviour of an animal being determined by the firing rate of a single neuron in some cases but not others.
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- Parker, A. J. (2007). Binocular depth perception and the cerebral cortex. Nature Reviews Neuroscience, 8(5), 379-391.
- Cumming, B. G., & Parker, A. J. (1997). Responses of primary visual cortical neurons to binocular disparity without depth perception. Nature, 389(6648), 280-283