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Expectations facilitate perception of expected stimuli but may hinder perception of unexpected alternatives. Here, we consider how prior expectations about others’ intentions are integrated with visual kinematics over time in detecting the intention of an observed motor act (grasp-to-pour vs. grasp-to-drink). Using rigorous psychophysics methods, we find that the processes of ascribing intentions to others are well described by drift diffusion models in which evidence from observed movements is accumulated over time until a decision threshold is reached. Testing of competing models revealed that when kinematics contained no discriminative intention information, prior expectations predicted the intention choice of the observer. When kinematics contained intention information, kinematics predicted the intention choice. These findings provide evidence for a diffusion process in which the influence of expectations is modulated by movement informativeness and informative kinematics can override initial expectations.

The bistable motion quartet, an ambiguous motion stimulus, leads to either vertical or horizontal motion perception, the latter requiring integration across hemispheres of the visual motion areas (V5). We causally tested for the impact of V5 interhemispheric connectivity on horizontal motion perception using a novel cortico-cortical Paired Associative Stimulation (ccPAS) dual coil TMS paradigm aimed at inducing Hebbian plastic potentiation of the relevant neural circuit. In exp 1 ( n = 16) we activated the V5 interhemispheric neural pathway across 4 ccPAS conditions. Participants received Left-to-Right (L–R) and Right-to-Left (R–L) ccPAS (testing for the impact of directionality on horizontal motion perception). The inter-stimulus-interval (ISI) of 25 ms corresponded to the optimal timing for the activation of V5 interhemispheric projections. Control conditions consisted of simultaneous V5 stimulation (0 ms ISI, controlling for timing) and sham stimulation (controlling for unspecific TMS effects). Participants performed the task before (baseline) and after the ccPAS phase. Results showed that both experimental (but not control) conditions enhanced horizontal motion perception but asymmetrically, with L–R being more effective than R–L – thus suggesting an interhemispheric asymmetry. In exp 2 ( n = 56), we tested the potential impact of such asymmetry on movement perceptual bias. Participants were presented with a counterphase grating stimulus, which contained no net motion in any direction but induced a bistable perception of motion to the left or the right. Over 73% of the participants reported a bias towards leftward motion. In exp 3 ( n = 16) we directly tested whether this leftward bias depends on asymmetric interhemispheric connectivity, by applying R-L V5 ccPAS and found that the leftward apparent motion was reversed. In sum, ccPAS can causally test functional models of brain connectivity. Applied over 2 functionally connected visual regions, ccPAS biases visual processing depending on the stimulation parameters. Directionality of stimulation highlighted for the first time an asymmetry in the way V5 areas causally interact during motion perception. This demonstrates the functional relevance and plasticity of interhemispheric interactions in the integration and perception of motion stimuli.