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Hierarchy of direction-tuned motion adaptation in human visual cortex.

Prolonged exposure to a single direction of motion alters perception of subsequent static or dynamic stimuli and induces substantial changes in behaviors of motion-sensitive neurons, but the origin of neural adaptation and neural correlates of perceptual consequences of motion adaptation in human brain remain unclear. Using functional magnetic resonance imaging, we measured motion adaptation tuning curves in a fine scale by probing changes in cortical activity after adaptation for a range of directions relative to the adapted direction. We found a clear dichotomy in tuning curve shape: cortical responses in early-tier visual areas reduced at around both the adapted and opposite direction, resulting in a bidirectional tuning curve, whereas response reduction in high-tier areas occurred only at around the adapted direction, resulting in a unidirectional tuning curve. We also found that the psychophysically measured adaptation tuning curves were unidirectional and best matched the cortical adaptation tuning curves in the middle temporal area (MT) and the medial superior temporal area (MST). Our findings are compatible with, but not limited to, an interpretation in which direct impacts of motion adaptation occur in both unidirectional and bidirectional units in early visual areas, but the perceptual consequences of motion adaptation are manifested in the population activity in MT and MST, which may inherit those direct impacts of adaptation from the directionally selective units.