New illusion

The regularities of the environment make it possible to predict future information and to adapt in the event of irregularity or prediction error. However, little is known about how these errors influence conscious perception, especially when the predictions concern elementary visual features (orientation, contrast, edges: the assembly of these features enables object recognition). We have therefore developed a new experimental approach using moving objects to study the perceptual consequences of prediction errors on elementary visual features.

Two squares move towards each other. The participants indicated whether or not they were in contact before they disappeared. When two objects move towards each other, we expect them to meet. However, this is not what the participants in our study reported. A strong illusion of a large gap between the squares occurs when the edges of the squares briefly touch.

We have carried out several experimental studies with healthy volunteers, and the results consistently suggest that the origin of the effects lies in the initial visual processing of the basic features of the squares. The results rule out an explanation in terms of decision bias, attentional effects or masking. We propose an explanation in the context of 'predictive coding'; violations of the extrapolation of the figure-ground contrast have strong perceptual consequences, as if the subject saw his or her prediction (although not consciously) about the figure-ground contrast, rather than the disappearance of the contrast on contact.

This new illusion constitutes a tool for exploring the mechanisms of elementary visual prediction in autism and schizophrenia in our laboratory.