Stéphanie Caharel

ERP evidence for the speed of face categorization in the human brain : disentangling the contribution of low-level visual cues from face perception

How fast are visual stimuli categorized as faces by the human brain? Because of their high temporal resolution and the possibility to record simultaneously from the whole brain, electromagnetic scalp measurements should be the ideal method to clarify this issue. However, this question remains debated, with studies reporting face-sensitive responses varying from 50 ms to 200 ms following stimulus onset. Here we disentangle the contribution of the information associated with the phenomenological experience of a face (phase) from low-level visual cues (amplitude spectrum, color) in accounting for early face-sensitivity in the human brain. Pictures of faces and of a category of familiar objects (cars), as well as their phase-scrambled versions, were presented to fifteen human participants tested with high-density (128 channels) EEG. We replicated an early face-sensitivity - larger response to pictures of faces than cars - at the level of the occipital event-related potential (ERP) P1 (80- ). However, a similar larger P1 to phase-scrambled faces than phase-scrambled cars was also found. In contrast, the occipito-temporal N170 was much larger in amplitude for pictures of intact faces than cars, especially in the right hemisphere, while the small N170 elicited by phase-scrambled stimuli did not differ for faces and cars. These findings show that sensitivity to faces on the visual evoked potentials P1 and N1 (N170) is functionally dissociated: the P1 face-sensitivity is driven by low-level visual cues while the N1 (or N170) face-sensitivity reflects the perception of a face. Altogether, these observations indicate that the earliest access to a high-level face representation, that is, a face percept, does not precede the N170 onset in the human brain. Furthermore, they allow resolving apparent discrepancies between the timing of rapid human saccades towards faces and the early activation of high-level facial representations as shown by electrophysiological studies in the primate brain. More generally, they put strong constraints on the interpretation of early (before 100 ms) face-sensitive effects in the human brain.

Familiarity and Emotional Expression Influence an Early Stage of Face Processing: An Electrophysiological Study.

Recent data indicate that the familiarity and the emotional expression of faces occur at an early stage of information processing. The goal of the present study was to determine whether these two aspects interact at the structural encoding stage as reflected by the N170 component of event-related potentials in tasks requiring the subjects either to identify whether the faces were familiar or the nature of the emotional expression. The results indicate that the neural responses to level of familiarity and emotional expression were observable at this early processing stage but without interacting. In particular, faces of personal importance to the subjects differed from those of less personal importance. Because familiarity did not interact with emotional expression at behavioral and electrophysiologic levels, our results support the contention of parallel and independent processing of faces.

ERPs associated with familiarity and degree of familiarity during face recognition.

Event-related potentials (ERPs) triggered by three different faces (unfamiliar, famous, and the subjects own) were analyzed during passive viewing. A familiarity effect was defined as a significant difference between the two familiar faces as opposed to the unfamiliar face. A degree of familiarity effect was defined as a significant difference between all three conditions. The results show a familiarity effect 170 ms after stimulus onset (NI70), with larger amplitudes seen for both familiar faces. Conversely, a degree of familiarity effect arose approximately 250 ms after stimulus onset (P2) in the form of progressively smaller amplitudes as a function of familiarity (subject s face < famous face < unfamiliar). These results demonstrate that the structural encoding of faces, as reflected by N170 activities, can be modulated by familiarity and that facial representations acquire specific properties as a result of experience. Moreover, these results confirm the hypothesis that N170 is sensitive to face versus. object discriminations and to the discrimination among faces.

Early adaptation to repeated unfamiliar faces across viewpoint changes in the right hemisphere : evidence from the N170 ERP component

Event-related potential (ERP) studies have shown that sensitivity to individual faces emerges as early as approximately 160ms in the human occipitotemporal cortex (N170). Here we tested whether this effect generalizes across changes in viewpoint. We recorded ERPs during an unfamiliar individual face adaptation paradigm. Participants were presented first with an adapting face ( approximately 3000ms) rotated 30 degrees in depth, followed by a second face (200ms) in a frontal view of either the same or a different identity. The N170 amplitude at right occipitotemporal sites to the second stimulus was reduced for repeated as compared to different faces. A bilateral adaptation effect emerged after 250ms following stimulus onset. These observations indicate that individual face representations activated as early as 160ms after stimulus onset in the right hemisphere show a substantial degree of generalization across viewpoints.

The effects of familiarity and emotional expression on face processing examined by ERPs in patients with schizophrenia

BACKGROUND The main objective of the study was to determine whether patients with schizophrenia are deficient relative to controls in the processing of faces at different levels of familiarity and types of emotion and the stage where such differences may occur. METHODS ERPs based on 18 patients with schizophrenia and 18 controls were compared in a face identification task at three levels of familiarity (unknown, familiar, subjects own) and for three types of emotion (disgust, smiling, neutral). RESULTS The schizophrenic group was less accurate than controls in the face processing, especially for unknown faces and those expressing negative emotions such as disgust. P1 and N170 amplitudes were lower and P1, N170, P250 amplitudes were of slower onset in patients with schizophrenia. N170 and P250 amplitudes were modulated by familiarity and face expression in a different manner in patients than controls. CONCLUSIONS Schizophrenia is associated with a genelarized defect of face processing, both in terms of familiarity and emotional expression, attributable to deficient processing at sensory (P1) and perceptual (N170) stages. These patients appear to have difficulty in encoding the structure of a face and thereby do not evaluate correctly familiarity and emotion.

Other-race and inversion effects during the structural encoding stage of face processing in a race categorization task: An event-related brain potential study

To investigate the mechanisms underlying the other-race effect, in particular at what stage of face processing differences between same-race (SR) and other-race (OR) stimuli occur, electrophysiological and behavioral data were obtained on Caucasian participants viewing photographs of Caucasian, Asian, and African faces in upright and inverted orientations. During a race categorization task, reaction times were faster for African than Asian faces, and both of them faster than Caucasian ones, independent of their orientation. The face-sensitive N170 component was low in amplitude for Caucasian, intermediate for Asian, and maximal for African faces. The face inversion effect was observed for all ethnic groups on N170 amplitudes, but was more evident for Caucasian faces. According to the perceptual expertise hypothesis, our results indicate that SR faces involve more configural/holistic processing OR faces.

The speed of recognition of personally familiar faces

Despite the generally accepted notion that humans are very good and fast at recognising familiar individuals from their faces, the actual speed with which this fundamental brain function can be achieved remains largely unknown. Here, two groups of participants were required to respond by finger-lift when presented with either a photograph of a personally familiar face (classmate), or an unfamiliar one. This speeded manual go/no-go categorisation task revealed that personally familiar faces could be categorised as early as 380 ms after presentation, about 80 ms faster than unfamiliar faces. When response times were averaged across all 8 stimulus presentations, we found that minimum RTs for both familiar and unfamiliar face decisions were substantially lower (310 ms and 370 ms). Analyses confirmed that stimulus repetition enhanced the speed with which faces were categorised, irrespective of familiarity, and that repetition did not affect the observed benefit in RTS for familiar over unfamiliar faces. These data, representing the elapsed time from stimulus onset to motor output, put constraints on the time course of familiar face recognition in the human brain, which can be tracked more precisely by high temporal resolution electrophysiological measures.

Early electrophysiological correlates of adaptation to personally familiar and unfamiliar faces across viewpoint changes

Behavioral studies have shown that matching individual faces across depth rotation is easier and faster for familiar than unfamiliar faces. Here we used event-related potentials (ERPs) to clarify the locus of this behavioral facilitation, that is whether it reflects changes at the level of perceptual face encoding, or rather at later stages of processing. We used an identity adaptation paradigm in ERPs, during which a first (adapting) face (~3000 ms) rotated 30° in depth was followed by a second full front face (200 ms) which was either the same or a different identity as the first face. For unfamiliar faces, the early face-sensitive N170 component was reduced for immediately repeated as compared to different unfamiliar faces in the right hemisphere only. However, for personally familiar faces, the effect was absent at right hemisphere electrode sites and appeared instead over the left hemisphere at the same latency. Later effects of face identity adaptation were also present on the scalp, but from about 300 to 400 ms over fronto-central regions, and slightly later on occipito-temporal regions, there was a strong adaptation effect only for familiar faces. These observations suggest that the perceptual encoding of familiar and unfamiliar faces may be of different nature, as indicated by early (N170) hemispheric differences for identity adaptation effects depending on long-term familiarity. However, the behavioral advantage provided by familiarity to match faces across viewpoints might rather be related to processes that are closer in time to the behavioral response, such as semantic associations between the faces to match.

Face familiarity decisions take 200 msec in the human brain: Electrophysiological evidence from a go/no-go speeded task

Recognizing a familiar face rapidly is a fundamental human brain function. Here we used scalp EEG to determine the minimal time needed to classify a face as personally familiar or unfamiliar. Go (familiar) and no-go (unfamiliar) responses elicited clear differential waveforms from 210 msec onward, this difference being first observed at right occipito-temporal electrode sites. Similar but delayed (by about 40 msec) responses were observed when go response were required to the unfamiliar rather than familiar faces, in a second group of participants. In both groups, a small increase of amplitude was also observed on the right hemisphere N170 face-sensitive component for familiar faces. However, unlike the post-200 msec differential go/no-go effect, this effect was unrelated to behavior and disappeared with repetition of unfamiliar faces. These observations indicate that accumulation of evidence within the first 200 msec poststimulus onset is sufficient for the human brain to decide whether a person is familiar based on his or her face, a time frame that puts strong constraints on the time course of face processing.

Early holistic face-like processing of Arcimboldo paintings in the right occipito-temporal cortex: evidence from the N170 ERP component.

The properties of the face-sensitive N170 component of the event-related brain potential (ERP) were explored through an orientation discrimination task using natural faces, objects, and Arcimboldo paintings presented upright or inverted. Because Arcimboldo paintings are composed of non-face objects but have a global face configuration, they provide great control to disentangle high-level face-like or object-like visual processes at the level of the N170, and may help to examine the implication of each hemisphere in the global/holistic processing of face formats. For upright position, N170 amplitudes in the right occipito-temporal region did not differ between natural faces and Arcimboldo paintings but were larger for both of these categories than for objects, supporting the view that as early as the N170 time-window, the right hemisphere is involved in holistic perceptual processing of face-like configurations irrespective of their features. Conversely, in the left hemisphere, N170 amplitudes differed between Arcimboldo portraits and natural faces, suggesting that this hemisphere processes local facial features. For upside-down orientation in both hemispheres, N170 amplitudes did not differ between Arcimboldo paintings and objects, but were reduced for both categories compared to natural faces, indicating that the disruption of holistic processing with inversion leads to an object-like processing of Arcimboldo paintings due to the lack of local facial features. Overall, these results provide evidence that global/holistic perceptual processing of faces and face-like formats involves the right hemisphere as early as the N170 time-window, and that the local processing of face features is rather implemented in the left hemisphere.