A new perceptual illusion reveals mechanisms of sensory decoding

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Abstract
Perceptual illusions are usually thought to arise from the way sensory signals are encoded by the brain. But there is an alternative explanation. Illusions might also result from a more 'downstream' part of the neural network, at the point where the brain decodes sensory information according to the strategies suited to particular tasks. Experiments in normal subjects estimating the direction of motion of a random dot pattern relative to a fixed boundary reveal a consistent misjudgement, suggesting that the subjective experience of motion does not arise directly from the responses of sensory neurons, but develops after those responses have been decoded. Perceptual illusions are thought to arise from the way sensory signals are encoded, but this details how one could result from the way the brain decodes sensory information. Different pools of neurons contribute the most information in different motion discrimination tasks, and human observers display perceptual biases in the tasks that could correspond to the different neural decoding strategies. 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.