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Benjamin Thompson, Zili Liu; Motion discrimination with psychophysically suppressed MT: an fMRI study. Journal of Vision 2006;6(6):114. doi: https://doi.org/10.1167/6.6.114.
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The MT+/V5 complex is regarded as the principal visual motion processing area in the human brain. One major mechanism MT neurons are thought to employ is opponency; excitation from one motion direction and inhibition from the opposite. Using counter-phase paired-dots it is possible to take advantage of this opponency and suppress MT activity (Qian, Andersen, & Adelson 1994). However, even for these stimuli which have balanced local motion directions, discrimination of motion-axis orientation is still well above chance (Lu, Qian, & Liu 2004). We used fMRI to investigate where in the early visual areas this discrimination might take place.
During an fMRI scanning protocol, participants performed a 2AFC motion discrimination task judging whether motion-axis orientation changed clockwise or anti-clockwise. Both counter-phase paired-dots and a control, in-phase paired-dots, were used.
MT activity was reduced by counter-phase dots relative to in-phase, whereas no other early visual areas showed differential activation by the two stimuli. Furthermore, MT activity was not significantly correlated with behavioral performance for the counter-phase condition but was the only area correlated with behavioral performance for the in-phase condition. For the counter-phase stimulus, BOLD signal in areas V2 and V3 was correlated with behavioral performance. These results support the idea that human MT has motion opponent mechanisms and also suggest that, when necessary, motion direction processing can be shifted to areas other than MT.
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