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Samantha L Strong, Edward H Silson, André D Gouws, Antony B Morland, Declan J McKeefry; Motion-Defined Form Discrimination in Human V5/MT+. Journal of Vision 2019;19(10):287. doi: https://doi.org/10.1167/19.10.287.
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© ARVO (1962-2015); The Authors (2016-present)
Human V5/MT+ is a motion-sensitive cortical region that can be parcellated into at least two smaller sub-divisions: MT/TO-1 and MST/TO-2 (Amano et al., 2009). Previous work using functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) has shown that these areas possess distinct functional preferences. For example, both MT/TO-1 and MST/TO-2 appear to process low-level local motion directions (translational), whilst processing of more complex global motion directions (radial/rotational) appears to be restricted to MST/TO-2 (Smith et al., 2006; Strong et al., 2017). However, it is rare to experience a natural visual scenario in which stimuli constitute only one direction of motion, so the next step would be to work towards determining whether these areas also contribute to ‘higher-level’ motion processing. One example of this type of processing is motion-defined form, i.e. a shape possessing borders defined by moving stimuli as opposed to a static outline. This project utilised fMRI-guided repetitive TMS (25Hz; 70%; 200ms) in order to determine the roles of MT/TO-1 and MST/TO-2 in a two-interval forced choice (2IFC) shape discrimination task. This involved identifying which interval contained the more circular of a camouflaged ellipse. This ellipse was defined by a region of supra-threshold coherently moving (translational) dots immersed within an aperture of moving noise dots. The relative size difference in semi-minor axis of the ellipse was set to each individual’s pre-determined threshold level. Results (percent correct responses) revealed that application of TMS to MST/TO-2 can disrupt ability to perceive these motion-defined shapes (MST/TO-2 versus MT/TO-1, p< 0.05; MST/TO-2 versus Baseline, p< 0.05; MST/TO-2 versus Control, p< 0.05). Overall, although both MT/TO-1 and MST/TO-2 maintain some responsibility for processing translational motion directions, discrimination of shapes defined by supra-threshold translational motion appears to be restricted to MST/TO-2. This suggests that processing of ‘higher-level’ natural stimuli may begin in MST/TO-2.
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