Differential Processing of the Direction and Focus of Expansion of Optic Flow Stimuli in areas MST and V3A of the Human Visual Cortex

Human neuropsychological and neuroimaging studies have raised the possibility that different attributes of optic flow stimuli, namely radial direction and the position of the focus of expansion (FOE), are processed within separate cortical areas. In the human brain, visual areas V5/MT+ and V3A have been proposed as integral to the analysis of these different attributes of optic flow stimuli. In order to establish direct causal relationships between neural activity in V5/MT+ and V3A and the perception of radial motion direction and FOE position, we used Transcranial Magnetic Stimulation (TMS) to disrupt cortical activity in these areas whilst participants performed behavioural tasks dependent on these different aspects of optic flow stimuli. The cortical regions of interest were identified in seven human participants using standard fMRI retinotopic mapping techniques and functional localisers. TMS to area V3A was found to disrupt FOE positional judgements, but not radial direction discrimination, whilst the application of TMS to an anterior sub-division of hV5/MT+, MST/TO-2, produced the reverse effects, disrupting radial direction discrimination but eliciting no effect on the FOE positional judgement task. This double dissociation demonstrates that FOE position and radial direction of optic flow stimuli are signalled independently by neural activity in areas hV5/MT+ and V3A.