The whole of a face is more than the sum of its parts: direct evidence from frequency-tagging of a composite face B Rossion1, A Norcia2, A Boremanse1 |
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1Institute of Psychology and Neuroscience, University of Louvain, Belgium |
| The face is often considered as the quintessential whole, or Gestalt. This is illustrated by the composite face illusion, in which the top and bottom halves of two faces fuse to form a perceived novel face. Objective evidence that the whole of a face is more than the sum of its parts is still lacking. Here we contrast-modulated the top and bottom halves of a composite face with different flicker frequencies (f1: 5.87 Hz; f2: 7.14 Hz) while recording scalp electroencephalogram (EEG) in 15 observers. A face was presented during 70 sec while they fixated the top face half. Thanks to this frequency-tagging approach, we distinguished objectively the responses to the simultaneously presented top and bottom face halves. Most importantly, we observed intermodulation components (IMs: f1-f2: 1.26 Hz; f1+f2= 13.01 Hz) over the right occipito-temporal hemisphere, reflecting the nonlinear interaction of the frequencies. While the fundamental frequencies response remained unchanged following inversion and spatial misalignment of face parts, the IM components decreased substantially in these conditions. These observations constitute an objective trace of a unified face representation in the human brain, demonstrating that the whole of a face is more than the sum of its parts. |
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