Different neural representations for detection of symmetry in dot-patterns and in faces: A state-dependent TMS study

The occipital face area (OFA) has been shown to code the presence of symmetry in faces and in vertically symmetric dot patterns. However, it is not clear whether symmetry processing of face and non-face stimuli involve overlapping neural mechanisms in OFA. This was assessed using state-dependent TMS by employing a priming paradigm. Specifically, we examined whether prior presentation of low-level symmetry affects the impact of TMS on discrimination of symmetry in subsequently presented faces - indicating that the same neural mechanisms encode symmetry in both face and non-face stimuli. Participants performed a symmetry discrimination task on a series of faces, each of which was preceded by either a vertically symmetric, a horizontally symmetric or a non-symmetric dot configuration (prime) while receiving stimulation over either the right OFA, the right Lateral Occipital Cortex (rLO) or over a control site (Vertex). Vertically symmetric dot patterns primed symmetry discrimination in faces. The key finding was that the priming effect was not affected by TMS applied over OFA; stimulation of this site (but not of rLO) impaired the discrimination of facial symmetry regardless of prime type. Overall, these results suggest that distinct neural representations in OFA are involved in symmetry detection in face and non-face stimuli.