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Gorana Pobric, Johan Hulleman; The role of V5/MT in visual search amongst moving items: new evidence from transcranial magnetic stimulation. Journal of Vision 2011;11(11):1341. doi: 10.1167/11.11.1341.
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Mechanisms for selective attention are vital for perception of motion. It has long been known that the right posterior parietal cortex (PPC) plays a role in a certain visual search tasks, while a human motion area V5 is involved in tasks processing attention to motion. Several theories have been proposed for the role of V5 in relation to visual search. The influential “motion filter” hypothesis (McLeod et al., 1988) states that area V5 acts as a motion filter, where moving items are represented strongly, and static items are represented only weakly. We explored the role of V5 as a motion filter by investigating visual search of constantly moving items with transcranial magnetic stimulation (rTMS). Specifically, we compared the effect of stimulating the right V5 and right PPC in normal participants while searching for a moving T either amongst moving L's (feature search) or amongst moving O's and static T's (conjunction search). The task was to indicate the pointing direction of the moving T. If the V5 acts as a motion filter, as proposed, then rTMS should generate impairment on a conjunction search. If the PPC is crucially implicated in directing visual spatial attention (Mesulam, 1981; Bisley and Goldberg, 2003) then stimulation should impact visual search performance on both tasks. We found that stimulation of V5/MT decreased search performance in the feature search. Search slopes in the T amongst O's task were not affected. PPC stimulation did not disrupt processing of either conjunction, or feature search. We demonstrate novel and striking evidence that area V5 is causally involved in the processing of search displays where all items are moving, arguing against its role as a motion filter. Our findings challenge the conventional interpretation of the role of V5 in conjunction search and spatial attention.
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