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.

[Neural bases of perception and recognition of faces]

OBJECTIVE

To analyze the neural bases of perception and the recognition of faces. First of all we consider the concept of functional hemisphere specialization; then we look at the results obtained with regard to the neuroanatomy of processing faces and finally refer to the disorders of recognition of faces in humans. For this we review the clinical evidence obtained from the neuropsychological studies of prosopagnostic patients and the data derived from psychophysiological experiments done using intracranial recordings of nonhuman primates.

DEVELOPMENT

The agreement between the results analysed allows us conclude that in both cerebral hemispheres there are neural mechanisms specialized in the perception and recognition of faces, and in particular the ventral and posterior regions of the occipito-temporal cortex play a decisive part in these processes. Similarly experimental findings in nonhuman primates permits explanation of the neural nature of certain neuropsychological disorders seen in man, such as the case of the dissociation between disorders of the recognition of facial identity and disorders of recognition of emotional expression, and also dissociation between the difficulty in recognizing familiar faces and difficulty in recognizing non-familiar faces.

CONCLUSION

The use of modern neuroimaging techniques and electrophysiological studies using evoked potentials are necessary for greater understanding of these and other disorders related to processing facial information.

Making features relevant: learning faces and event-related potentials recording using an analytic procedure

We present a procedure designed for the learning of faces (represented by realistic drawings) and a task for the subsequent recording of Event-Related Potentials (ERPs), with the aim of investigating the psychobiological mechanisms involved in the recognition of familiar (or known) faces. The learning procedure consisted in a series of six sessions in which subjects familiarized themselves with the faces through a forced-choice features task: after the study of a set of 40 faces, each of these was presented at random in an incomplete way (without the eyes/eyebrows fragment), together with two sets of eyes and eyebrows for the subject to decide which of them corresponded to each face. This procedure is thought to facilitate for subjects the acquisition of the information related to the structural description of the faces (that is, without associated verbal/semantic information) using preferentially analytic processing strategies. The parameter d' of the Theory of Signal Detection allowed us to evaluate in the course of the learning sessions the degree of familiarization achieved with the studied faces. Subsequently, an ERPs' recording session was carried out, during which subjects executed a face-feature matching task. In this task, similar in structure to that carried out in the learning sessions, the subject had to decide whether the automatic completion of the faces that was made was correct or incorrect. ERPs' effects related to mismatching features were obtained, which indicate the existence of specific cerebral mechanisms involved in the recognition of faces.

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.