The role of the human extrastriate visual cortex in mirror symmetry discrimination: A TMS-adaptation study

The human visual system is able to efficiently extract symmetry information from the visual environment. Prior neuroimaging evidence has revealed symmetry-preferring neuronal representations in the dorsolateral extrastriate visual cortex; the objective of the present study was to investigate the necessity of these representations in symmetry discrimination. This was accomplished by the use of state-dependent transcranial magnetic stimulation, which combines the fine resolution of adaptation paradigms with the assessment of causality. Subjects were presented with adapters and targets consisting of dot configurations that could be symmetric along either the vertical or horizontal axis (or they could be non-symmetric), and they were asked to perform a symmetry discrimination task on the targets while fixating the center of the screen. TMS was applied during the delay between the adapter and the test stimulus over one of four different sites: Left or Right V1/V2, or left or right dorsolateral extrastriate cortex (DLO). TMS over both Left and Right DLO reduced the adaptation effect in detecting vertical and horizontal symmetry, although the Left DLO effect on horizontal symmetry and the Right DLO effect on both vertical and horizontal symmetry were present only when considering subjects who showed a behavioral adaptation effect in the baseline No-TMS condition. Application of TMS over the Left or Right V1/V2 did not modulate the adaptation effect. Overall, these data suggest that both the Left and Right DLO contain neuronal representations tuned to mirror symmetry which play a causal role in symmetry discrimination.

On each trial, participants were adapted to a visual stimulus consisting of either a horizontally or a vertically symmetric configuration or a random dot configuration prior to the presentation of a target stimulus. The behavioral task required the participants to discriminate the axis of symmetry of the target. Transcranial magnetic stimulation (TMS) was applied during the delay between the adapter and the target stimulus. The application of TMS over the Left and Right dorsolateral occipital cortex reduced the behavioral impact of adaptation, whereas no effect was found with V1/V2 TMS.Figure optionsDownload high-quality image (93 K)Download as PowerPoint slideHighlights
► The dorsolateral extrastriate cortex (DLO) is involved in processing visual symmetry.
► TMS was used to investigate the causal role of DLO and V1/V2 in symmetry perception.
► TMS over both DLOs was found to affect vertical and horizontal symmetry detection.
► TMS over V1/V2 did not affect symmetry detection.