Neurophysiology of conversion disorders: a historical perspective.

The aim of this paper is to present a short historical perspective on the neurophysiological approach to hysteria and conversion disorders. The body of this paper will be constituted of three main parts. In the first part, we will present the significant progress due to some pioneers of neurology/psychiatry during the XIXth century. As we shall see, this period was particularly rich in personalities whose work gradually laid the foundations to a true medical approach to hysteria. In the first half of the XXth century, different factors have led to a long eclipse of the neurological approach to hysteria. In the second part, we will show how, by the 1960's-1970's, the conceptual and methodological advances in neurophysiology, as well as the turning point of cognitive sciences (and cognitive psychology in particular) allowed a gradual reinstatement of hysteria within the fields of neurology and clinical neurophysiology. Finally, and this is the third part of this paper, we will show how over the past three decades, an entirely new neurophysiological approach to hysteria and conversion disorders has emerged.

Visual processing of emotional expressions in mixed anxious-depressed subclinical state: an event-related potential study on a female sample.

AIMS: Controversy remains about the existence and the nature of a specific bias in emotional facial expression processing in mixed anxious-depressed state (MAD). MATERIAL AND METHODS: Event-related potentials were recorded in the following three types of groups defined by the Spielberger state and trait anxiety inventory (STAI) and the Beck depression inventory (BDI): a group of anxious participants (n=12), a group of participants with depressive and anxious tendencies (n=12), and a control group (n=12). Participants were confronted with a visual oddball task in which they had to detect, as quickly as possible, deviant faces amongst a train of standard neutral faces. Deviant stimuli changed either on identity, or on emotion (happy or sad expression). RESULTS: Anxiety facilitated emotional processing and the two anxious groups produced quicker responses than control participants; these effects were correlated with an earlier decisional wave (P3b) for anxious participants. Mixed anxious-depressed participants showed enhanced visual processing of deviant stimuli and produced higher amplitude in attentional complex (N2b/P3a), both for identity and emotional trials. P3a was also particularly increased for emotional faces in this group. CONCLUSION: Anxious state mainly influenced later decision processes (shorter latency of P3b), whereas mixed anxious-depressed state acted on earlier steps of emotional processing (enhanced N2b/P3a complex). Mixed anxious-depressed individuals seemed more reactive to any visual change, particularly emotional change, without displaying any valence bias.

The perception of fearful and happy facial expression is modulated by anxiety: an event-related potential study.

Anxiety is supposed to interfere with cognitive and emotional processing and high level of trait-anxiety has been associated with an attentional bias for fearful faces, even in sub-clinical anxiety. On the basis of the Spielberger State and Trait Anxiety Inventory (STAI), twenty students were grouped as low vs. high anxious. Pictures from the Ekman and Friesen series were used in an event-related potentials study to investigate the neurophysiological correlates of the emotional processing of fear and happiness in sub-clinical anxiety. Subjects were confronted with a visual oddball design, in which they had to detect, as quickly as possible, deviant happy or fearful faces amongst a train of standard stimuli (neutral faces). Anxiety does not modify early perceptual (N100, P100, N170, VPP) or attentional (N2b) component, but later components are affected. Indeed, high anxious subjects are faster to detect deviant faces as suggested by earlier reaction times and P3b component. However, they show a reduced ability to process the emotional content of faces, this deficit being indexed by a decreased N300 component. Indeed, N300 is supposed to be particularly sensitive to affective features of stimuli rather than to physical characteristics. We propose that the earlier P3b observed in high anxious subjects could be interpreted as a way to overcome the deficient emotional appraisal by a more salient conscious processing.

When audition alters vision: an event-related potential study of the cross-modal interactions between faces and voices.

Ten healthy volunteers took part in this event-related potential (ERP) study aimed at examining the electrophysiological correlates of the cross-modal audio-visual interactions in an identification task. Participants were confronted either to the simultaneous presentation of previously learned faces and voices (audio-visual condition; AV), either to the separate presentation of faces (visual, V) or voices (auditive, A). As expected, an interference effect of audition on vision was observed at a behavioral level, as the bimodal condition was performed more slowly than the visual condition. At the electrophysiological level, the subtraction (AV - (A + V)) gave prominence to three distinct cerebral activities: (1) a central positive/posterior negative wave around 110 ms, (2) a central negative/posterior positive wave around 170 ms, AND (3) a central positive wave around 270 ms. These data suggest that cross-modal cerebral interactions could be independent of behavioral facilitation or interference effects. Moreover, the implication of unimodal and multisensory convergence regions in these results, as suggested by a source localization analysis, is discussed.

Human gender differences in an emotional visual oddball task: an event-related potentials study.

Pictures from the Ekman and Friesen series were used in an event-related potentials study to define the timing of occurrence of gender differences in the processing of positive (happy) and negative (fear) facial expressions. Ten male and 10 female volunteers were confronted with a visual oddball design, in which they had to detect, as quickly as possible, deviant happy or fearful faces amongst a train of standard stimuli (neutral faces). Behavioral results suggest that men and women detected fearful faces more quickly than happy ones. The main result is that the N2b component, functionally considered as an attentional orienting mechanism, was delayed in men for happy stimuli as compared with fearful ones. Gender differences observed in the processing of emotional stimuli could then originate at the attentional level of the information processing system.

The electrophysiological correlates sustaining the retrieval of face-name associations: an ERP study.

An ERP study on 9 healthy participants was carried out to temporally constrain the neural network proposed by Campanella et al. (2001) in a PET study investigating the cerebral areas involved in the retrieval of face-name associations. Three learning sessions served to familiarize the participants with 24 face-name associations grouped in 12 male/female couples. During EEG recording, participants were confronted with four experimental conditions, requiring the retrieval of previously learned couples on the basis of the presentation of name-name (NN), face-face (FF), name-face (NF), or face-name (FN) pairs of stimuli. The main analysis of this experiment consisted in the subtraction of the nonmixed conditions (NN and FF) from the mixed conditions (NF and FN). It revealed two main ERP components: a negative wave peaking at left parieto-occipital sites around 285 ms and its positive counterpart recorded at left centro-frontal electrodes around 300 ms. Moreover, a dipole modeling using three dipoles whose localization corresponded to the three cerebral areas observed in the PET study (left inferior frontal gyrus, left medial frontal gyrus, left inferior parietal lobe) explained more than 90% of the variance of the results. The complementarity between anatomical and neurophysiological techniques allowed us to discuss the temporal course of these cerebral activities and to propose an interactive and original anatomo-temporal model of the retrieval of face-name associations.

[Neurophysiologic mechanisms of face recognition]. / A propos des mécanismes neurophysiologiques de la reconnaissance des visages.

During the last three decades, the mechanisms implicated in face perception and recognition, have been investigated with an interdisciplinary approach. On one hand, in psychology, models of cognitive architectures have been developed in which the different kinds of mental operations in the processing of face have been identified. On the other hand, a lot of studies in neuropsychology have analyzed the deficits in visual processing of face, specifically due to lesions or dysfunctions of the central nervous system. Finally, research in neurophysiology (cellular recording in animal and functional cerebral imagery in man) have tried to highlight the complex assembly of cerebral areas implicated in these different operations. Besides a short summary of the principal data collected in this interdisciplinary approach, we present in this contribution results obtained in our laboratory and in the context of an "Action de recherche concertée", in the fields of neuropsychology and cerebral functional imagery.

Expertise training with novel objects leads to left-lateralized facelike electrophysiological responses.

Scalp event-related potentials (ERPs) in humans indicate that face and object processing differ approximately 170 ms following stimulus presentation, at the point of the N170 occipitotemporal component. The N170 is delayed and enhanced to inverted faces but not to inverted objects. We tested whether this inversion effect reflects early mechanisms exclusive to faces or whether it generalizes to other stimuli as a function of visual expertise. ERPs to upright and inverted faces and novel objects (Greebles) were recorded in 10 participants before and after 2 weeks of expertise training with Greebles. The N170 component was observed for both faces and Greebles. The results are consistent with previous reports in that the N170 was delayed and enhanced for inverted faces at recording sites in both hemispheres. For Greebles, the same effect of inversion was observed only for experts, primarily in the left hemisphere. These results suggest that the mechanisms underlying the electrophysiological face-inversion effect extend to visually homogeneous nonface object categories, at least in the left hemisphere, but only when such mechanisms are recruited by expertise.

Discrimination of emotional facial expressions in a visual oddball task: an ERP study.

Several ERP studies have shown an orienting complex, the N2/P3a, associated to the detection of stimulus novelty. Its role consists in preparing the organism to process and react to biologically prepotent stimuli. Whether this N2/P3a: (1) could be obtained with complex visual stimuli, such as with emotional facial expressions; and (2) could take part in a complex discrimination process has yet to be determined. To investigate this issue, event-related potentials were recorded in response to repetitions of a particular facial expression (e.g. sadness) and in response to two different deviant (rare) stimuli, one depicting the same emotion as the frequent stimulus, while the other depicted a different facial expression (e.g. fear). As expected, deviant stimuli evoked an N2/P3a complex of larger amplitude than frequent stimuli. But more interestingly, when the deviant stimulus depicted the same emotion as the frequent stimulus the N2/P3a was delayed compared to the response elicited by the different-emotion deviant. The N2/P3a was thus implicated in the detection of physical facial changes, with a higher sensitivity to changes related to a new different emotional content, perhaps leading to faster adaptive reactions.

Categorical perception of happiness and fear facial expressions: an ERP study.

Behavioral studies have shown that two different morphed faces perceived as reflecting the same emotional expression are harder to discriminate than two faces considered as two different ones. This advantage of between-categorical differences compared with within-categorical ones is classically referred as the categorical perception effect. The temporal course of this effect on fear and happiness facial expressions has been explored through event-related potentials (ERPs). Three kinds of pairs were presented in a delayed same-different matching task: (1) two different morphed faces perceived as the same emotional expression (within-categorical differences), (2) two other ones reflecting two different emotions (between-categorical differences), and (3) two identical morphed faces (same faces for methodological purpose). Following the second face onset in the pair, the amplitude of the bilateral occipito-temporal negativities (N170) and of the vertex positive potential (P150 or VPP) was reduced for within and same pairs relative to between pairs. This suggests a repetition priming effect. We also observed a modulation of the P3b wave, as the amplitude of the responses for the between pairs was higher than for the within and same pairs. These results indicate that the categorical perception of human facial emotional expressions has a perceptual origin in the bilateral occipito-temporal regions, while typical prior studies found emotion-modulated ERP components considerably later.

Sensory and motor attentional modulation during the manual gap effect in humans: a high-density ERP study.

The present study investigated reaction time (RT) and event-related potential (ERP) differences between gap and step conditions using visual stimulation and manual responses. RTs during the gap condition were facilitated with respect to those of the step condition. The ERPs, which were obtained from electrodes placed at 58 scalp sites, showed differences when the gap and step conditions were compared for the following components: an early positive component centred at the vertex, an enhanced P1 component, a frontal negativity, a negative lateralized motor potential, and an increased P3. All these results suggest that the facilitation induced by the gap is mediated by a modulation of the neural circuits involved in sensory, motor, and cognitive functions.

Amplitude transition function of human express saccades.

The gap paradigm often promotes the occurrence of express saccades, which are supposed to be short latency, visually guided saccades, often forming a separate peak in saccadic latency distribution. We designed six experiments in which we compared the amplitudes of anticipatory, express and regular saccades, for various conditions of target eccentricities, target direction, and predictability. Then, saccadic amplitude was expressed as a continuous function of latency, for the various target eccentricities. From the obtained results, it is proposed that a saccade of a given amplitude is prepared during the gap period, on the basis of internal cues. The latency range of express saccades is a transition zone when the target begins to influence the already prepared saccade. The resulting amplitude will be a weighted average of the value determined during the gap and of the value defined by the target, the weighting being determined by the latency of the saccade. If the preprogrammed saccade is wrongly directed, the target will not be able to correct the saccadic amplitude and the express saccade will have the same amplitude as anticipatory saccades. Regular saccades are delayed sufficiently so that a wrongly directed preprogrammed saccade can be canceled or the amplitude of a rightly directed saccade can be adjusted according to the exact position of the visual target.

How does the brain discriminate familiar and unfamiliar faces?: a PET study of face categorical perception.

Where and how does the brain discriminate familiar and unfamiliar faces? This question has not been answered yet by neuroimaging studies partly because different tasks were performed on familiar and unfamiliar faces, or because familiar faces were associated with semantic and lexical information. Here eight subjects were trained during 3 days with a set of 30 faces. The familiarized faces were morphed with unfamiliar faces. Presented with continua of unfamiliar and familiar faces in a pilot experiment, a group of eight subjects presented a categorical perception of face familiarity: there was a sharp boundary in percentage of familiarity decisions between 40% and 60% faces. In the main experiment, subjects were scanned (PET) on the fourth day (after 3 days of training) in six conditions, all requiring a sex classification task. Completely novel faces (0%) were presented in Condition 1 and familiar faces (100%) in Condition 6, while faces of steps of 20% in the continuum of familiarity were presented in Conditions 2 to 5 (20% to 80%). A principal component analysis (PCA) indicated that most variations in neural responses were related to the dissociation between faces perceived as familiar (60% to 100%) and faces perceived as unfamiliar (0 to 40%). Subtraction analyses did not disclose any increase of activation for faces perceived as familiar while there were large relative increases for faces perceived as unfamiliar in several regions of the right occipito-temporal visual pathway. These changes were all categorical and were observed mainly in the right middle occipital gyrus, the right posterior fusiform gyrus, and the right inferotemporal cortex. These results show that (1) the discrimination between familiar and unfamiliar faces is related to relative increases in the right ventral pathway to unfamiliar/novel faces; (2) familiar and unfamiliar faces are discriminated in an all-or-none fashion rather than proportionally to their resemblance to stored representations; and (3) categorical perception of faces is associated with abrupt changes of brain activity in the regions that discriminate the two extremes of the multidimensional continuum.

Association of the distinct visual representations of faces and names: a PET activation study.

A PET study of seven normal individuals was carried out to investigate the neural populations involved in the retrieval of the visual representation of a face when presented with an associated name, and conversely. Face-name associations were studied by means of four experimental matching conditions, including the retrieval of previously learned (1) name-name (NN), (2) face-face (FF), (3) name-face (NF), and (4) face-name (FN) associations, as well as a resting scan with eyes closed. Before PET images acquisition, subjects were presented with 24 unknown face-name associations to encode in 12 male/female couples. During PET scanning, their task was to decide whether the presented pair was a previously learned association. The right fusiform gyrus was strongly activated in FF condition as compared to NN and Rest conditions. However, no specific activations were found for NN condition relative to FF condition. A network of three areas distributed in the left hemisphere, both active in (NF-FF) and (FN-NN) comparisons, was interpreted as the locus of the integration of visual faces and names representations. These three regions were localized in the inferior frontal gyrus (BA 45), the medial frontal gyrus (BA 6) and the supramarginal gyrus of the inferior parietal lobe (BA 40). An interactive model accounting for these results, with BA 40 seen as an amodal binding region, is proposed.

A pet study of human skill learning: changes in brain activity related to learning an orientation discrimination task.

Using 15O-water 3D positron emission tomography we investigated the effect of training in orientation discrimination upon cerebral activity in healthy human adults. When subjects are trained in this discrimination task, they learn the visuo-motor stimulus-response association required by the task and they increase their perceptual abilities in orientation discrimination. The present study was designed to investigate the rCBF modifications related to both these learning processes induced by training in orientation discrimination. PET data were acquired on two separate days (before and after training). Comparing the activation pattern related to orientation discrimination before and after the training period we observed activity decreases located in the left cerebellar cortex, in the right precentral gyrus and bilaterally in the fusiform gyri. The only region showing an activity increase was located in the body of the right caudate nucleus. These findings confirm the role of the neostriatum in skill learning and highlight the importance of mechanisms resulting in cortical and cerebellar neuronal activity decreases in this type of learning.

Current source density analysis of CNV during temporal gap paradigm.

The present report studied the contingent negative variation during Gap and Non-Gap conditions using visual stimulation and manual responses. The reaction times during the Gap condition were facilitated compared with those of the Non-Gap condition. The contingent negative variation component was obtained during the preparatory period from electrodes placed at 58 scalp sites for both Gap and Non-Gap conditions. The comparison between both conditions: Gap and non-gap did not show statistically significant differences during the preparatory period. The topography of the voltage and current source density maps showed three different foci: (i) an early negativity centred in electrodes overlying the supplementary motor area and cingulate motor areas, (ii) an activation over the primary motor cortex contralateral to the finger movement, and (iii) a bilateral activation on posterior sites. All these results suggest that the facilitation induced by the warning stimuli occurs in neural circuits that would be recruited for the subsequent processing of the imperative stimulus. The facilitation of the reaction times during the gap condition with respect to non-gap condition must be justified by neural events occurring during the gap period.

Right N170 modulation in a face discrimination task: an account for categorical perception of familiar faces.

Behavioral studies have shown that two different morphed faces belonging to the same identity are harder to discriminate than two faces stemming from two different identities. The temporal course of this categorical perception effect has been explored through event-related potentials. Three kinds of pairs were presented in a matching task: (1) two different morphed faces representing the same identity (within), (2) two other faces representing two different identities (between), and (3) two identical morphed faces (same). Following the second face onset in the pair, the amplitude of the right occipitotemporal negativity (N170) was reduced for within and same pairs as compared with between pairs, suggesting an identity priming effect. We also observed a modulation of the P3b wave, as the amplitude of the responses for within pairs was higher than for between and same pairs, suggesting a higher complexity of the task for within pairs. These results indicate that categorical perception of human faces has a perceptual origin in the right occipitotemporal hemisphere.

Hemispheric asymmetries for whole-based and part-based face processing in the human fusiform gyrus.

Behavioral studies indicate a right hemisphere advantage for processing a face as a whole and a left hemisphere superiority for processing based on face features. The present PET study identifies the anatomical localization of these effects in well-defined regions of the middle fusiform gyri of both hemispheres. The right middle fusiform gyrus, previously described as a face-specific region, was found to be more activated when matching whole faces than face parts whereas this pattern of activity was reversed in the left homologous region. These lateralized differences appeared to be specific to faces since control objects processed either as wholes or parts did not induce any change of activity within these regions. This double dissociation between two modes of face processing brings new evidence regarding the lateralized localization of face individualization mechanisms in the human brain.

The time-course of intermodal binding between seeing and hearing affective information.

Intermodal binding between affective information that is seen as well as heard triggers a mandatory process of audiovisual integration. In order to track the time course of this audiovisual binding, event related brain potentials were recorded while subjects saw facial expression and concurrently heard auditory fragment. The results suggest that the combination of the two inputs is early in time (110 ms post-stimulus) and translates as a specific enhancement in amplitude of the auditory NI component. These findings are compatible with previous functional neuroimaging results of audiovisual speech showing strong audiovisual interactions in auditory cortex in the form of magnetic response amplifications, as well as with electrophysiological studies demonstrating early audiovisual interactions (before 200 ms post-stimulus). Moreover, our results show that the informational content present in the two modalities plays a crucial role in triggering the intermodal binding process.

The N170 occipito-temporal component is delayed and enhanced to inverted faces but not to inverted objects: an electrophysiological account of face-specific processes in the human brain.

Behavioral studies have shown that picture-plane inversion impacts face and object recognition differently, thereby suggesting face-specific processing mechanisms in the human brain. Here we used event-related potentials to investigate the time course of this behavioral inversion effect in both faces and novel objects. ERPs were recorded for 14 subjects presented with upright and inverted visual categories, including human faces and novel objects (Greebles). A N170 was obtained for all categories of stimuli, including Greebles. However, only inverted faces delayed and enhanced N170 (bilaterally). These observations indicate that the N170 is not specific to faces, as has been previously claimed. In addition, the amplitude difference between faces and objects does not reflect face-specific mechanisms since it can be smaller than between non-face object categories. There do exist some early differences in the time-course of categorization for faces and non-faces across inversion. This may be attributed either to stimulus category per se (e.g. face-specific mechanisms) or to differences in the level of expertise between these categories.