Spatiotemporal characteristics of cortical responses to biological motion

Abstract

Previous fMRI work has indicated that both biological form and biological kinematics information have overlapping representations in the human brain; yet, it is unclear as to whether there is a temporal distinction in terms of their relative engagement that is stimulus-dependent.We presented observers (N=21) with upright and inverted biological motion walkers that contained solely biological form (global stimulus), solely biological kinematics (local natural stimulus), or neither natural form nor kinematics information (modified stimulus) and asked them to discriminate the facing direction of the stimuli while concurrently imaging neuromagnetic responses using magnetoencephalography (Elekta Neuromag 360). For all three stimulus classes, we found early (100 ms) responses in lateraloccipital regions that preceded responses in inferiortemporal and fusiform (150-200 ms), and superiortemporal regions (350-500 ms), with response amplitudes differing among the three stimulus classes in extrastriate regions only.Specifically, amplitudes were larger for the inverted global stimulus than for the upright counterpart in fusiform cortex, in addition to surrounding inferior- and superior-temporal regions.In these same regions, amplitudes were higher for the local natural stimulus than for the modified stimulus, but only when stimuli were presented upright. Moreover, amplitudes were higher for the global stimulus than the local natural and modified stimuli, but only when stimuli were presented upside-down. We then compared the representational dissimilarity of MEG sensor patterns with ROI-multivariate response patterns acquired in a second group of observers (N=19) using fMRI (3T) and identical stimuli.Interestingly, we found a marked distinction between the onset of MEG-fMRI representational correspondence, occurring much earlier in early visual cortex (V1-V3) than in higher-order extrastriate body areas.These data suggest that biological motion perception proceeds with temporal systematicity in cortex, engaging early visual cortex prior to inferior-temporal cortex, and finally the oft-implicated superior temporal regions, with stimulus-specificity emerging in later stages.