Selective visual responses to expansion and rotation in the human MT complex revealed by functional magnetic resonance imaging adaptation.

Many neurons in the macaque visual area MSTd are sensitive to the global structure of a pattern of moving dots, responding to optic flow components such as expansion and rotation. Direct evidence for neurons with similar properties in humans has been lacking. We have explored sensitivity to optic flow in the human occipital cortex using an event-related functional magnetic resonance imaging adaptation paradigm. On each trial, two brief random-dot kinematograms were presented sequentially. Attention was controlled with a demanding task at fixation. In human MST, the compound response was smaller (indicating adaptation) when the two had the same flow structure than when they were different, suggesting the presence of separate neural populations sensitive to rotation and expansion. Surprisingly, the middle-temporal (MT) gyrus visual area also showed signs of flow specificity, and even V3A showed weak specificity. In V1, which is expected to respond only to local dot motions, no evidence of flow-specific neurons was found. The same was true in V2, V3, V3B and V4. Control experiments showed that the results cannot be attributed to adaptation to local translation within the flow pattern, or to attentional effects. Our results clearly demonstrate selective responses to specific optic flow structures in MST, and we tentatively suggest that the human MT and even V3A may show similar properties.