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A new perceptual illusion reveals mechanisms of sensory decoding


Perceptual illusions are usually thought to arise from the way sensory signals are encoded by the brain, and indeed are often used to infer the mechanisms of sensory encoding1. But perceptual illusions might also result from the way the brain decodes sensory information2, reflecting the strategies that optimize performance in particular tasks. In a fine discrimination task, the most accurate information comes from neurons tuned away from the discrimination boundary3,4, and observers seem to use signals from these ‘displaced’ neurons to optimize their performance5,6,7. We wondered whether using signals from these neurons might also bias perception. In a fine direction discrimination task using moving random-dot stimuli, we found that observers’ perception of the direction of motion is indeed biased away from the boundary. This misperception can be accurately described by a decoding model that preferentially weights signals from neurons whose responses best discriminate those directions. In a coarse discrimination task, to which a different decoding rule applies4, the same stimulus is not misperceived, suggesting that the illusion is a direct consequence of the decoding strategy that observers use to make fine perceptual judgments. The subjective experience of motion is therefore not mediated directly by the responses of sensory neurons, but is only developed after the responses of these neurons are decoded.

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Figure 1: The combined discrimination–estimation experiment.
Figure 2: Discrimination and estimation responses.
Figure 3: Summary data for all six subjects.
Figure 4: Subjective estimates in a coarse direction discrimination task.


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This work was supported by a research grant from the NIH. We are grateful to B. Lau, E. Simoncelli, D. Heeger, M. Landy and N. Graham for advice and discussion.

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Correspondence to Mehrdad Jazayeri.

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This file contains Supplementary Methods, Supplementary Figures 1–3 with Legends and Supplementary Discussion. The Supplementary Information includes two Supplementary Figures detailing the fit of a more complete model to the data, a Supplementary Figure showing a breakdown of the data from Fig. 4, and a discussion of the possible role of response bias in the results reported. (PDF 622 kb)

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Jazayeri, M., Movshon, J. A new perceptual illusion reveals mechanisms of sensory decoding. Nature 446, 912–915 (2007).

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