Dynamic coupling of bistable stimuli reveals long-range connectivity

Abstract

Are the dynamics of bistable stimuli distributed in the visual field independent? We address this issue by characterising the endogenous bistable dynamics of a motion stimulus in presence of a similar, exogenously driven, contextual stimulus whose perception periodically alternates between a motion-bound and a motion-unbound state. In four experiments, observers reported their perceptual alternations during 3-4 min runs for different relative positions -within or across hemifields-, direction of motion and mirror symmetry between the two displays. Results showed that the dynamics of endogenous perceptual transitions of an ambiguous stimulus are coupled depending on the inductive context's characteristics: coupling was stronger for symmetrical stimuli presented in both hemifields and decreased for other spatial configurations. Coupling was weakest when stimuli with opposite motion direction were presented within the same -left or right- hemifield. Overall, mirror symmetry and common fate reliably influence the dynamic coupling of bistable stimuli. The distribution of the relative coupling strengths across experimental conditions indicates that it does not result from a decision bias or from ‘high-level’ Bayesian inferences. Our findings rather suggest that bistable neural attractors underlying the processing of each stimulus are coupled. The effect of common fate could reflect activity of neurons with large receptive fields encompassing the two stimuli, e.g. in area MT/MST. On the other hand, the strong effect of mirror symmetry could reflect the contribution of long-range connections through the corpus callosum. We tested this hypothesis by changing the relative locations of the inductive and target stimuli in each hemifield, thus altering mirror symmetry while keeping stimulus separation identical. Results showed decreased coupling whenever vertical symmetry was broken. Overall, we suggest that coupled dynamics of bistable stimuli reflect long-range connectivity, thus allowing a behavioural mapping of its functional properties.