Ela I. Olivares

Different scalp topography of brain potentials related to expression and identity matching of faces

Event-related potentials (ERPs) and behavioral data were recorded while subjects performed two tasks on the same set of faces (presented in pairs). One task was identity matching and the other expression matching. Two groups of subjects participated, one familiar and the other unfamiliar with the faces. Subjects were less accurate in matching expressions than identity. Familiarity facilitated identity but not expression matching. ERPs to mismatches in both tasks elicited a negativity around 400 ms, which was similar in latency and amplitude in the two tasks, but differed in scalp topography. Whereas the mismatch negativity had the same landscape over the left hemisphere for both tasks, the component related to expression had larger amplitudes over the right-temporal regions. Familiarity had no effect on these negativities, although it affected a late positivity (LP). These results support the idea of distinct neural systems subserving face processing, and agree with a role of the right hemisphere for the processing of emotional expressions.

Differential effects of object-based attention on evoked potentials to fearful and disgusted faces

Event-related potentials (ERPs) were used to investigate the role of attention on the processing of facial expressions of fear and disgust. Stimuli consisted of overlapping pictures of a face and a house. Participants had to monitor repetitions of faces or houses, in separate blocks of trials, so that object-based attention was manipulated while spatial attention was kept constant. Faces varied in expression and could be either fearful or neutral (in the fear condition) or disgusted or neutral (in the disgust condition). When attending to faces, participants were required to signal repetitions of the same person, with the facial expressions being completely irrelevant to the task. Different effects of selective attention and different patterns of brain activity were observed for faces with fear and disgust expressions. Results indicated that the perception of fear from faces is gated by selective attention at early latencies, whereas a sustained positivity for fearful faces compared to neutral faces emerged around 160ms at central-parietal sites, independent of selective attention. In the case of disgust, ERP differences began only around 160ms after stimulus onset, and only after 480ms was the perception of disgust modulated by attention allocation. Results are interpreted in terms of different neural mechanisms for the perception of fear and disgust and related to the functional significance of these two emotions for the survival of the organism.

Searching for face-specific long latency ERPs: a topographic study of effects associated with mismatching features

In a previous study [E. Olivares, M.A. Bobes, E. Aubert, M. Valdés-Sosa, Associative ERPs effects with memories of artificial faces, Cogn. Brain Res. 2 (1994) 39-48] we reported the presence of a negativity associated with mismatching features when subjects carried out a face-feature matching task whilst their evoked potentials were recorded. Since the stimuli used were learned faces (realistic drawings), for which the subjects possessed no semantic information or associated verbal labels, the mismatch negativity obtained was considered a face-specific N400. In this work we present a new experiment to study the topographic distribution of these mismatch effects. As in the above-mentioned study, in each trial the subjects observed previously an incomplete (without the eyes/eyebrows fragment) familiar face, which served as a structural context for the face recognition. The face was then completed by grafting either matching (learned) features or mismatching features (from another face). In line with neuropsychological studies on prosopagnosia and electrophysiological findings in humans and non-human primates, we found as one of the most relevant items of data that the most-posterior (principally, left occipital) cortices appear to be a region in which are located the possible neural generators of the negativity associated with the detection of incongruencies in the structure of familiar faces. We also reported a late positivity, distributed in more anterior regions, which follows the mismatch negativity. This complex N-P is interpreted as reflecting a dual process of retrieval and integration of information in memory.

Long-Latency ERPs and Recognition of Facial Identity

N400 brain event-related potential (ERP) is a mismatch negativity originally found in response to semantic incongruences of a linguistic nature and is used paradigmatically to investigate memory organization in various domains of information, including that of faces. In the present study, we analyzed different mismatch negativities evoked in N400-like paradigms related to recognition of newly learned faces with or without associated verbal information. ERPs were compared in the following conditions: (1) mismatching features (eyes-eyebrows) using a facial context corresponding to the faces learned without associated verbal information (pure intradomain facial processing); (2) mismatching features using a facial context corresponding to the faces learned with associated occupations and proper names (nonpure intradomain facial processing); (3) mismatching occupations using a facial context (cross-domain processing); and (4) mismatching names using an occupation context (intra-domain verbal processing). Results revealed that mismatching stimuli in the four conditions elicited a mismatch negativity analogous to N400 but with different timing and topo-graphical patterns. The onset of the mismatch negativity occurred earliest in Conditions 1 and 2, followed by Condition 4, and latest in Condition 3. The negativity had the shortest duration in Task 1 and the longest duration in Task 3. Bilateral parietal activity was confirmed in all conditions, in addition to a predominant right posterior temporal localization in Condition 1, a predominant right frontal localization in Condition 2, an occipital localization in Condition 3, and a more widely distributed (although with posterior predominance) localization in Condition 4. These results support the existence of multiple N400, and particularly of a nonlinguistic N400 related to purely visual information, which can be evoked by facial structure processing in the absence of verbal-semantic information.

Event-Related Potentials Elicited By Face Identity Processing In Elderly Adults With Cognitive Impairment

Background/Study Context: Patients with mild Alzheimers disease and mild cognitive impairment show selective loss of knowledge regarding facial identification. Methods: The authors focus on decline effects on event-related potentials (ERPs) P100, N170, N250, and N400, associated with the processing of facial identity. Different famous and unknown faces were presented in explicit and implicit familiarity tasks. Results: Patients with cognitive impairment showed modulations on P100 and N170 and greater activity in prefrontal areas in the earlier component. In healthy elderly individuals, but not in patients, famous faces modulated the long-latency ERPs N250 and N400, related to the access and retrieval of stored facial-related information, respectively. Conclusion: ERPs have potential as markers of neurodegenerative disease such as dementia. The neural systems supporting facial identification may differ in normal and cognitively impaired older adults.

Brain potentials and integration of external and internal features into face representations

In this ERP study we analyzed how different orders of presentation of external and internal features influence the integration of facial components into face gestalts. Participants carried out a face-feature matching task in which, in each trial, external (E) and internal (I) facial features were presented separately and in sequence, followed by a complete unfamiliar face (matching or mismatching). For the E-I group of participants the order of presentation was external features, internal features and then the complete face. The I-E group viewed the internal features first. Mismatch effects in complete faces were more conspicuous and lasted from about 300 to 600 ms with the order E-I, while those with the order I-E were scarcely observable, and significant only from 450 to 470 ms. The external features tended to elicit a larger P150, while the N170 was preferentially associated with the internal ones. A P360, probably indicating stimulus relevance, was present in both groups for external features, while it was associated with internal features only in the I-E group. These results suggest that in the E-I order the binding of facial features into a single face representation occurs according to a stepwise process which facilitates the integration. In turn, in the I-E order the processing related to the two sets of features appears more dissociated and is of a more componential nature. Moreover, we propose that the external features may be especially relevant for object categorization, while internal features would be more closely related to subsequent configural mechanisms.

Brain Signals of Face Processing as Revealed by Event-Related Potentials

We analyze the functional significance of different event-related potentials (ERPs) as electrophysiological indices of face perception and face recognition, according to cognitive and neurofunctional models of face processing. Initially, the processing of faces seems to be supported by early extrastriate occipital cortices and revealed by modulations of the occipital P1. This early response is thought to reflect the detection of certain primary structural aspects indicating the presence grosso modo of a face within the visual field. The posterior-temporal N170 is more sensitive to the detection of faces as complex-structured stimuli and, therefore, to the presence of its distinctive organizational characteristics prior to within-category identification. In turn, the relatively late and probably more rostrally generated N250r and N400-like responses might respectively indicate processes of access and retrieval of face-related information, which is stored in long-term memory (LTM). New methods of analysis of electrophysiological and neuroanatomical data, namely, dynamic causal modeling, single-trial and time-frequency analyses, are highly recommended to advance in the knowledge of those brain mechanisms concerning face processing.

Cognitive decline effects at an early stage: evidence from N170 and VPP.

The perceptual processing of faces was studied using event-related potentials (ERPs) in 12 elderly patients with cognitive impairment, 15 elderly adults and 16 young adults in order to explore the sensitivity of N170/VPP to the cognitive decline associated to Alzheimers disease. Famous and unknown faces were presented in a familiarity categorization task. Eight patients and 11 elderly adults repeated this task to obtain longitudinal data. Topographical effects were analyzed using PCA. The posterior N170 showed reduced amplitude in patients with cognitive impairment and elderly adults, compared to young adults, which could indicate perceptual impairment in configural face-encoding processing. The anterior VPP showed enhanced amplitude in patients with cognitive impairment, compared to young and elderly adults, which might relate to the prefrontal dysfunction associated to mild dementia. These preliminary findings suggest that N170/VPP could be modulated by the decline related to pathological cognitive aging.

Event-Related Potentials Elicited by the Explicit and Implicit Processing of Familiarity in Faces

Brain activity underlying explicit and implicit processing of face familiarity was assessed by Event-Related Potentials (ERPs) elicited by famous and unknown faces with happy or neutral expressions. A set of faces was presented in a familiarity judgment (explicit) task and another in an expression judgment (implicit familiarity) task. After recording, these tasks were repeated exchanging the stimuli, and post-recording behavioral data from the familiarity task were used for re-averaging EEG segments from the expression task. Both explicit and implicit processing of famous faces resulted in an enhanced N250. Explicit processing of famous faces was specifically associated with earlier N400 and P600, with increased activity within brain areas involved in identity processing around 250 and 450 ms. These findings suggest different brain dynamics for explicit and implicit face processing, and that implicit processing of the identity in the context of an expression task is mainly associated with the transient activation of face representations in memory.

Source Reconstruction of Brain Potentials Using Bayesian Model Averaging to Analyze Face Intra-Domain vs. Face-Occupation Cross-Domain Processing

We investigated the neural correlates of the access to and retrieval of face structure information in contrast to those concerning the access to and retrieval of person-related verbal information, triggered by faces. We experimentally induced stimulus familiarity via a systematic learning procedure including faces with and without associated verbal information. Then, we recorded event-related potentials (ERPs) in both intra-domain (face-feature) and cross-domain (face-occupation) matching tasks while N400-like responses were elicited by incorrect eyes-eyebrows completions and occupations, respectively. A novel Bayesian source reconstruction approach plus conjunction analysis of group effects revealed that in both cases the generated N170s were of similar amplitude but had different neural origin. Thus, whereas the N170 of faces was associated predominantly to right fusiform and occipital regions (the so-called “Fusiform Face Area”, “FFA” and “Occipital Face Area”, “OFA”, respectively), the N170 of occupations was associated to a bilateral very posterior activity, suggestive of basic perceptual processes. Importantly, the right-sided perceptual P200 and the face-related N250 were evoked exclusively in the intra-domain task, with sources in OFA and extensively in the fusiform region, respectively. Regarding later latencies, the intra-domain N400 seemed to be generated in right posterior brain regions encompassing mainly OFA and, to some extent, the FFA, likely reflecting neural operations triggered by structural incongruities. In turn, the cross-domain N400 was related to more anterior left-sided fusiform and temporal inferior sources, paralleling those described previously for the classic verbal N400. These results support the existence of differentiated neural streams for face structure and person-related verbal processing triggered by faces, which can be activated differentially according to specific task demands.