The Speed of Individual Face Categorization

Early Visually Evoked Electrophysiological Responses Over the Human Brain (P1, N170) Show Stable Patterns of Face-Sensitivity from 4 years to Adulthood

Whether the development of face recognition abilities truly reflects changes in how faces, specifically, are perceived, or rather can be attributed to more general perceptual or cognitive development, is debated. Event-related potential (ERP) recordings on the scalp offer promise for this issue because they allow brain responses to complex visual stimuli to be relatively well isolated from other sensory, cognitive and motor processes. ERP studies in 5- to 16-year-old children report large age-related changes in amplitude, latency (decreases) and topographical distribution of the early visual components, the P1 and the occipito-temporal N170. To test the face specificity of these effects, we recorded high-density ERPs to pictures of faces, cars, and their phase-scrambled versions from 72 children between the ages of 4 and 17, and a group of adults. We found that none of the previously reported age-dependent changes in amplitude, latency or topography of the P1 or N170 were specific to faces. Most importantly, when we controlled for age-related variations of the P1, the N170 appeared remarkably similar in amplitude and topography across development, with much smaller age-related decreases in latencies than previously reported. At all ages the N170 showed equivalent face-sensitivity: it had the same topography and right hemisphere dominance, it was absent for meaningless (scrambled) stimuli, and larger and earlier for faces than cars. The data also illustrate the large amount of inter-individual and inter-trial variance in young children's data, which causes the N170 to merge with a later component, the N250, in grand-averaged data. Based on our observations, we suggest that the previously reported “bi-fid” N170 of young children is in fact the N250. Overall, our data indicate that the electrophysiological markers of face-sensitive perceptual processes are present from 4 years of age and do not appear to change throughout development.

Human ability to detect kinship in strangers' faces: effects of the degree of relatedness

The resemblance between human faces has been shown to be a possible cue in recognizing the relatedness between parents and children, and more recently, between siblings. However, the general inclusive fitness theory proposes that kin-selective behaviours are also relevant to more distant relatives, which requires the detection of larger kinship bonds. We conducted an experiment to explore the use of facial clues by 'strangers', i.e. evaluators from a different family, to associate humans of varying degrees of relatedness. We hypothesized that the visual capacity to detect relatedness should be weaker with lower degrees of relatedness. We showed that human adults are capable of (although not very efficient at) assessing the relatedness of unrelated individuals from photographs and that visible facial cues vary according to the degree of relatedness. This sensitivity exists even for kin pair members that are more than a generation apart and have never lived together. Collectively, our findings are in agreement with emerging knowledge on the role played by facial resemblance as a kinship cue. But we have progressed further to show how the capacity to distinguish between related and non-related pairs applies to situations relevant to indirect fitness.

Common cortical responses evoked by appearance, disappearance and change of the human face

Background

To segregate luminance-related, face-related and non-specific components involved in spatio-temporal dynamics of cortical activations to a face stimulus, we recorded cortical responses to face appearance (Onset), disappearance (Offset), and change (Change) using magnetoencephalography.

Results

Activity in and around the primary visual cortex (V1/V2) showed luminance-dependent behavior. Any of the three events evoked activity in the middle occipital gyrus (MOG) at 150 ms and temporo-parietal junction (TPJ) at 250 ms after the onset of each event. Onset and Change activated the fusiform gyrus (FG), while Offset did not. This FG activation showed a triphasic waveform, consistent with results of intracranial recordings in humans.

Conclusion

Analysis employed in this study successfully segregated four different elements involved in the spatio-temporal dynamics of cortical activations in response to a face stimulus. The results show the responses of MOG and TPJ to be associated with non-specific processes, such as the detection of abrupt changes or exogenous attention. Activity in FG corresponds to a face-specific response recorded by intracranial studies, and that in V1/V2 is related to a change in luminance.

Effect of configural distortion on a face-related ERP evoked by random dots blinking

Using random dots blinking (RDB), which reflects the activity of the higher visual area related to face perception, the following stimuli were presented. (1) Upright: a schematic face; (2) Inverted: the Upright stimulus inverted; and (3) Scrambled: the same contour and features as in Upright but with the spatial relation distorted. Clear negative components (N-ERP250) were identified at approximately 250 ms after stimulus onset. At the T5 and T6 electrodes, the peak latency was significantly longer for Inverted and Scrambled than for Upright. At the P4 electrode, the maximum amplitude was significantly larger for Scrambled than for Upright and Inverted. These results indicate that the delayed latency for Inverted and Scrambled reflects the involvement of the additional analytic processing caused by the configural distortion, and that the increase in amplitude for Scrambled indicates the existence of further processing caused by the distortion of the spatial relationship between the contour and features.

Time course of superior temporal sulcus activity in response to eye gaze: a combined fMRI and MEG study

The human superior temporal sulcus (STS) has been suggested to be involved in gaze processing, but temporal data regarding this issue are lacking. We investigated this topic by combining fMRI and MEG in four normal subjects. Photographs of faces with either averted or straight eye gazes were presented and subjects passively viewed the stimuli. First, we analyzed the brain areas involved using fMRI. A group analysis revealed activation of the STS for averted compared to straight gazes, which was confirmed in all subjects. We then measured brain activity using MEG, and conducted a 3D spatial filter analysis. The STS showed higher activity in response to averted versus straight gazes during the 150-200 ms period, peaking at around 170 ms, after stimulus onset. In contrast, the fusiform gyrus, which was detected by the main effect of stimulus presentations in fMRI analysis, exhibited comparable activity across straight and averted gazes at about 170 ms. These results indicate involvement of the human STS in rapid processing of the eye gaze of another individual.

Using spatial frequency scales for processing face features and face configuration: an ERP analysis

In the present study we examined the influence of spatial filtering on the N170-effect, a relatively early face-selective ERP difference associated with face detection. We compared modulation of the N170-effect using spatially filtered stimuli that either facilitated feature analysis or impeded configural analysis. The salience of inner face components was enhanced by presenting them in isolation. Configural processing was manipulated by face inversion. The N170-effects elicited by upright faces and isolated inner components were similar across low- and high-spatial frequency scales. In contrast, the inversion effect (enhanced N170 amplitude for inverted compared with upright faces) was only observed with broadband and low-spatial frequency stimuli. These findings demonstrate that the N170-effect can be influenced by both low- and high-spatial frequency channels. Moreover, they indicate that different configural manipulations (isolated features vs. face inversion) affect face detection in distinct ways, consistent with separate processing mechanisms for different types of configural encoding.

Neural correlates of colour categories

This study used an electrophysiological marker of visual detection to investigate adults' processing of colour difference. Event-related potentials were collected from the identical colour (green: G0) presented as the frequent or infrequent stimulus within different colour contexts. Critically, we compared differences within the same colour category (G0 vs. green: G1) to differences between colour categories (G0 vs. blue and G0 vs. red). All differences showed a change-related positivity with similar scalp distribution. It was, however, not simply the magnitude of colour difference that reduced the latencies of the change-related positivity. A change in colour category without a magnitude difference also reduced latency of the event-related potential. Thus, for the first time we report an independent neural correlate of a colour category.

Electrophysiological neural mechanisms for detection, configural analysis and recognition of faces

Despite ample explorations the nature of neural mechanisms underlying human expertise in face perception is still undetermined. Here we examined the response of two electrophysiological signals, the N170 ERP and induced gamma-band activity (>20 Hz), to face orientation and familiarity across two blocks, one in which the face identity was task-relevant and one in which it was not. N170 amplitude to inverted faces was higher than to upright faces and was not influenced by face familiarity or its task relevancy. In contrast, induced gamma activity was higher for upright than for inverted faces and for familiar than unfamiliar faces. The effect of face inversion was found in lower gamma frequency band (25-50 Hz), whereas familiarity affected amplitudes in higher gamma frequency band (50-70 Hz). For gamma, the relevance of face identity to the task modulated both inversion and familiarity effects. These findings pinpoint three functionally dissociated neural mechanisms involved in face processing, namely, detection, configural analysis, and recognition.

Sex-contingent face aftereffects depend on perceptual category rather than structural encoding

Many studies have used visual adaptation to investigate how recent experience with faces influences perception. While faces similar to those seen during adaptation phases are typically perceived as more 'normal' after adaptation, it is possible to induce aftereffects in one direction for one category (e.g. female) and simultaneously induce aftereffects in the opposite direction for another category (e.g. male). Such aftereffects could reflect 'category-contingent' adaptation of neurons selective for perceptual category (e.g. male or female) or 'structure-contingent' adaptation of lower-level neurons coding the physical characteristics of different face patterns. We compared these explanations by testing for simultaneous opposite after effects following adaptation to (a) two groups of faces from distinct sex categories (male and female) or (b) two groups of faces from the same sex category (female and hyper-female) where the structural differences between the female and hyper-female groups were mathematically identical to those between male and female groups. We were able to induce opposite aftereffects following adaptation between sex categories but not after adaptation within a sex category. These findings indicate the involvement of neurons coding perceptual category in sex-contingent face aftereffects and cannot be explained by neurons coding only the physical aspects of face patterns.

Early electrophysiological responses to multiple face orientations correlate with individual discrimination performance in humans

Picture-plane inversion dramatically impairs face recognition. Behavioral and event-related potential (ERP) studies suggest that this effect takes place during perceptual encoding of the face stimulus. However, the relationship between early electrophysiological responses to upright and inverted faces and the behavioral face inversion effect remains unclear. To address this question, we recorded ERPs while presenting 10 subjects with face photographs at 12 different orientations around the clock (30 degrees steps) during an individual face matching task. Using the variability in the electrophysiological responses introduced by the multiple orientations of the target face, we found a correlation between the ERP signal at 130-170 ms on occipito-temporal channels, and the behavioral performance measured on the probe stimulus. Correlations between ERP signal and behavioral performance started about 10 ms earlier in the right hemisphere. Significant effects of orientation were observed already at the level of the visual P1 (peaking at 100 ms), but the ERP signal was not correlated with behavior until the face-sensitive N170 time window. Overall, these observations indicate that plane-inversion affects the perceptual encoding of faces as early as 130 ms in occipito-temporal regions, leading directly to an increase in error rates and RTs during individual face recognition.

Hemispheric asymmetries in image-specific and abstractive priming of famous faces: evidence from reaction times and event-related brain potentials

We investigated hemispheric differences in image-specific and abstractive immediate repetition priming of famous faces. Participants performed speeded familiarity decisions for centrally presented famous and unfamiliar target faces. Target faces were preceded by lateralized primes (150 ms), presented either in the left or right visual field (LVF or RVF). Primes were either an identical photograph of the famous target face (image-specific priming), a different image of the famous target face (abstractive priming) or a different familiar face (unprimed condition). Reaction times (RTs) revealed significant effects of priming for both image-specific and abstractive priming overall. In addition, image-specific priming was more than twice the magnitude for targets following LVF primes as compared to RVF primes. By contrast, no hemispheric differences emerged for abstractive face priming across different images. Whereas ERPs revealed no evidence that priming affected the N170 component, both image-specific and abstractive priming significantly modulated the amplitudes of a right temporal N250r and a parietal N400 component. Behavioural and electrophysiological evidence for hemispheric differences in image-specific and abstractive face priming are discussed with respect to current theories of how the human left and right ventral temporal cortices represent abstractive and form-specific visual information.

Rapid Adaptation of the M170 Response: Importance of Face Parts

Face perception is often characterized as depending on configural, rather than part-based, processing. Here we examined the relative contributions of configuration and parts to early "face-selective" processing at the M170, a magnetoencephalographic response approximately 170 ms after stimulus onset, using adaptation. Previously (Harris and Nakayama 2007), we showed that rapid successive presentation of 2 stimuli (stimulus-onset asynchrony < 800 ms) attenuates the M170 response. Such adaptation is face-selective, with greater attenuation when faces are preceded by other faces than by houses. This technique therefore provides an independent method to assess the nature of this early neurophysiological marker. In these experiments, we measured the adapting power of face configurations versus parts using upright and inverted faces (Experiment 1), face-like configurations of black ovals versus scrambled nonface configurations of face parts (Experiment 2), and isolated face parts (Experiment 3). Although face configurations alone do not produce face-selective adaptation, scrambled and even isolated face parts adapt the M170 response to a similar extent as full faces. Thus, at least for the relatively early face-selective M170 response, face parts produce face-selective adaptation but face configurations do not. These results suggest that face parts are important at relatively early stages of face perception.

Neural Correlates of Human and Monkey Face Processing in 9-Month-Old Infants

Behavioral and electrophysiological evidence suggests a gradual, experience-dependent specialization of cortical face processing systems that take place largely in the first year of life. To further investigate these findings, event-related potentials (ERPs) were collected from typically developing 9-month-old infants presented with pictures of familiar and unfamiliar monkey or human faces in two different orientations. Analyses revealed differential processing across changes in monkey and human faces. The N290 was greater for familiar compared to unfamiliar faces, regardless of species or orientation. In contrast, the P400 to unfamiliar faces was greater than to familiar faces, but only for the monkey condition. The P400 to human faces differentiated the orientation of both familiar and unfamiliar faces. These results suggest more specific processing of human compared to monkey faces in 9-month-olds.