Supra-additive contribution of shape and surface information to individual face discrimination as revealed by fast periodic visual stimulation.

Face perception depends on two main sources of information--shape and surface cues. Behavioral studies suggest that both of them contribute roughly equally to discrimination of individual faces, with only a small advantage provided by their combination. However, it is difficult to quantify the respective contribution of each source of information to the visual representation of individual faces with explicit behavioral measures. To address this issue, facial morphs were created that varied in shape only, surface only, or both. Electrocephalogram (EEG) were recorded from 10 participants during visual stimulation at a fast periodic rate, in which the same face was presented four times consecutively and the fifth face (the oddball) varied along one of the morphed dimensions. Individual face discrimination was indexed by the periodic EEG response at the oddball rate (e.g., 5.88 Hz/5 = 1.18 Hz). While shape information was discriminated mainly at right occipitotemporal electrode sites, surface information was coded more bilaterally and provided a larger response overall. Most importantly, shape and surface changes alone were associated with much weaker responses than when both sources of information were combined in the stimulus, revealing a supra-additive effect. These observations suggest that the two kinds of information combine nonlinearly to provide a full individual face representation, face identity being more than the sum of the contribution of shape and surface cues.