Independent control of cortical representations for expression and identity of dynamic faces

Humans can easily extract who someone is and what expression they are making from the complex interplay of invariant and changeable visual features of faces. Recent evidence suggests that cortical mechanisms to selectively extract information about these two socially critical cues are segregated. Here we asked if these systems are independently controlled by task demands. We therefore had subjects attend to either identity or expression of the same dynamic face stimuli and examined cortical representations in topographically and functionally localized visual areas using fMRI. Six human subjects performed a task that involved detecting changes in the attended cue (expression or identity) of dynamic face stimuli (8 presentations per trial of 2s movie clips depicting 1 of 2 facial identities expressing happiness or anger) in 18-20 7min scans (20 trials/scan in pseudorandom order) in 2 sessions. Dorsal areas such as hMT and STS were disassociated from more ventral areas such as FFA and OFA by their modulation with task demands and their encoding of exemplars of expression and identity. In particular, dorsal areas showed higher activity during the expression task (hMT: p< 0.05, lSTS: p< 0.01; t-test) where subjects were cued to attend to the changeable aspects of the faces whereas ventral areas showed higher activity during the identity task (lOFA: p< 0.05; lFFA: p< 0.05). Specific exemplars of identity could be reliably decoded (using linear classifiers) from responses of ventral areas (lFFA: p< 0.05; rFFA: p< 0.01; permutation-test). In contradistinction, dorsal area responses could be used to decode specific exemplars of expression (hMT: p< 0.01; rSTS: p< 0.01), but only if expression was attended by subjects. Our data support the notion that identity and expression are processed by segregated cortical areas and that the strength of the representations for particular exemplars is under independent task control