Can processing of face trustworthiness bypass early visual cortex? A transcranial magnetic stimulation masking study
As a highly social species, we constantly evaluate human faces to decide whether we can trust someone. Previous studies suggest that face trustworthiness can be processed unconsciously, but the underlying neural pathways remain unclear. Specifically, the question remains whether processing of face trustworthiness relies on early visual cortex (EVC), required for conscious perception. If processing of trustworthiness can bypass EVC, then disrupting EVC should impair conscious trustworthiness perception while leaving forced-choice trustworthiness judgment intact. We applied double-pulse transcranial magnetic stimulation (TMS) to right EVC, at different stimulus onset asynchronies (SOAs) from presentation of a face in either the left or right hemifield. Faces were slightly rotated clockwise or counterclockwise, and were either trustworthy or untrustworthy. On each trial, participants discriminated 1) trustworthiness, 2) stimulus rotation, and 3) subjective visibility of trustworthiness. At early SOAs and specifically in the left hemifield, orientation processing (captured by the rotation task) was impaired by TMS. Crucially, though TMS also impaired subjective visibility of trustworthiness, no effects on trustworthiness discrimination were obtained. Conscious perception of face trustworthiness (captured by visibility ratings) relies on intact EVC, while forced-choice trustworthiness judgments may not. These results are consistent with the hypothesis that trustworthiness processing can bypass EVC. For basic visual features, extrastriate pathways are well-established; but face trustworthiness depends on a complex configuration of features. Its processing without EVC and outside of awareness is therefore of particular interest, further highlighting its ecological relevance.