Understanding how the human brain discriminates complex visual patterns, such as individual faces, is an important issue in Vision Science. Here we tested sensitivity to individual faces using steady-state visual-evoked potentials (SSVEPs). Twelve participants were presented with 90-s sequences of faces appearing at a constant rate (3.5 faces/s) while high-density electroencephalogram (EEG) was recorded. Fast Fourier Transform (FFT) of EEG showed a large response at the fundamental stimulation frequency (3.5 Hz) over posterior electrode sites. This response was much larger when the face identity changed at that rate (different faces) than when an identical face was repeated. The reduction of signal in the identical face condition was not due to low-level feature adaptation, since it was observed despite changes of stimulus size, and was localized specifically over the right lateral occipital cortex. Moreover, the difference between conditions disappeared when faces were inverted. This first observation of habituation of the SSVEP to repeated face identity in the human brain provides further evidence for face individualization in the right occipito-temporal cortex by means of a simple, fast, and high signal-to-noise approach. Most importantly, it offers a promising tool to study the sensitivity to visual features of individual faces and objects in the human brain.

No abstract

Whether and how the parts of a visual object are grouped together to form an integrated ("holistic") representation is a central question in cognitive neuroscience. Although the face is considered to be the quintessential example of holistic representation, this issue has been the subject of much debate in face perception research. The implication of holistic processing is that the response to the whole cannot be predicted from the sum of responses to the parts. Here we apply techniques from nonlinear systems analysis to provide an objective measure of the nonlinear integration of parts into a whole, using the left and right halves of a face stimulus as the parts. High-density electroencephalogram (EEG) was recorded in 15 human participants presented with two halves of a face stimulus, flickering at different frequencies (5.88 vs. 7.14 Hz). Besides specific responses at these fundamental frequencies, reflecting part-based responses, we found intermodulation components (e.g., 7.14 - 5.88 = 1.26 Hz) over the right occipito-temporal hemisphere, reflecting nonlinear integration of the face halves. Part-based responses did not depend on the relative alignment of the two face halves, their spatial separation, or whether the face was presented upright or inverted. By contrast, intermodulations were virtually absent when the two halves were spatially misaligned and separated. Inversion of the whole face configuration also reduced specifically the intermodulation components over the right occipito-temporal cortex. These observations indicate that the intermodulation components constitute an objective, configuration-specific signature of an emergent neural representation of the whole face that is distinct from that generated by the parts themselves.

It is well known that the integration of facial features into a holistic representation is dramatically disrupted by picture-plane inversion. To investigate the nature of this observation, we tested for the first time the so-called face composite effect at various angles of rotation (0 degrees to 180 degrees , 7 angles). During an individual face matching task, subjects perceived two identical top halves of the same face as being slightly different (increase of error rates and RTs) when they were aligned with different bottom parts. This face composite illusion was equally strong for stimuli presented at 0 degrees until 60 degrees rotation, then fell off dramatically at 90 degrees and remained stable until complete inversion of the stimulus. The non-linear relationship between orientation and holistic processing supports the view that inversion affects face processing qualitatively. Most importantly, it rules out the hypothesis that misoriented faces are perceptually realigned by means of linear rotation mechanisms independent of internal representations derived from experience. Altogether, these observations suggest that a substantial part of the face inversion effect is accounted for by the inability to apply an experience-derived holistic representation to an incoming visual face stimulus that it is rotated horizontally or beyond that orientation.

In order to isolate the repetition suppression effects for each part of a whole-face stimulus, the left and right halves of face stimuli were flickered at different frequency rates (5.88 or 7.14 Hz), changing or not changing identity at every stimulation cycle. The human electrophysiological (electroencephalographic) responses to each face half increased in amplitude when different rather than repeated face half identities were presented at every stimulation cycle. Contrary to the repetition suppression effects for whole faces, which are usually found over the right occipito-temporal cortex, these part-based repetition suppression effects were found on all posterior electrode sites and were unchanged when the two face halves were manipulated by separation, lateral misalignment, or inversion. In contrast, intermodulation components (e.g. 7.14-5.88 = 1.26 Hz) were found mainly over the right occipito-temporal cortex and were significantly reduced following the aforementioned manipulations. In addition, the intermodulation components decreased substantially for face halves belonging to different identities, which form a less coherent face than when they belong to the same face identity. These observations provide objective evidence for dissociation between part-based and integrated (i.e. holistic/configural) responses to faces in the human brain, suggesting that only responses to integrated face parts reflect high-level, possibly face-specific, representations.