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. 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.