Alternative mode of presentation of Kanizsa figures sheds new light on the chronometry of the mechanisms underlying the perception of illusory figures.

The mechanisms responsible for the perception of illusory modal figures are usually studied by presenting entire Kanizsa figures at stimulus onset. However, with this mode of presentation, the brain activity generated by the inducers (the 'pacmen') is difficult to differentiate from the activity underlying the perception of the illusory figure. Therefore, in addition to this usual presentation mode, we used an alternative presentation mode. Inducer disks remained permanently on the screen and the illusory figure was induced by just removing the notches from the disks. The results support the heuristic value of this alternative mode of presentation. The P1 deflection of the visual evoked potentials (VEPs) was found to be greater for the illusory modal figure than for its control and for an amodal figure. This modulation is one of the earliest direct evidences for a low-level processing of illusory forms in the human brain. Meanwhile, larger N1s were obtained for the control figures than for the illusory figures in the notch mode of presentation. While this new type of N1 modulation could shed some light on the stage of processing indexed by this deflection, several propositions are put forward to account for the P1 and N1 variations found.

N400-like potentials elicited by faces and knowledge inhibition.

Within the theoretical framework of reference, the brain errs in processing complex stimuli, such as faces. Thus, these stimuli not only activate accurate representations but also inaccurate representations corresponding to known persons who resemble the face stimulus, and hence knowledge about these known persons. Since more errors are made in processing unfamiliar than familiar stimuli, these inaccurate activations are assumed to be more frequent, and/or more intense, with unknown than with known faces. Moreover, top-down mechanisms favor representations of stimuli that are congruent with the context, and representations of known persons, even if inaccurate, receive an additional amount of activation in contexts wherein known faces are expected. Inaccurate representations have to be inhibited to achieve accurate recognition. Thus, more inhibition would be required for unknown than for known faces, and in contexts wherein known faces are expected. The aim of the present work is to study the hypothesis that the N400 component of the event-related potentials (ERPs) reflects the inhibition of knowledge, and to see whether this hypothesis accounts for the N400-like potential elicited by faces. To achieve that goal, ERPs to known and unknown faces were recorded while the richness in known faces of each experimental block, and thus the expectancy for known faces, was manipulated. Consistent with the hypothesis, the amplitudes of the N400-like components were greater in conditions where more inhibition was required, i.e. for unknown rather than for known faces, and in the context of the block rich in known faces. This context effect was larger for unknown than for faces, and in the context of the block rich in known faces. This context effect was larger for unknown than for known faces.

Dissociating memory processes involved in direct and indirect tests with ERPs to unfamiliar faces.

Event-related potentials (ERPs) were recorded during indirect and direct memory tests for unfamiliar faces. In both tests, ERPs displayed the usual positive shift known as the ERP repetition effect. In the indirect test, this effect includes parietal effect (the usual N400 effect) and a right fronto-central effect. Both effects are also present in the direct test. Two additional effects are present only in the direct test. These effects are an early fronto-polar effect and a late posterior effect (the usual P600 effect). These findings are taken as support for the distinction between 'associative' processes elicited in both the direct and indirect tests, and 'episodic' processes elicited only in the direct test. This task dissociation could well provide a scalp correlate of the distinction between the neocortical and cortico-limbic systems that have been shown to contribute respectively to associative and episodic processing. In addition, it is proposed that the dissociation between the two frontal effects could be accounted for by a distinction between the processing of intrinsic vs. extrinsic contextual attributes as a function of the task requirements.

ERPs and chronometry of face recognition: following-up Seeck et al. and George et al.

Seeck et al. found that event-related potentials (ERPs) evoked by repeated and non-repeated face photographs differ as early as 50-70ms post-onset. They thus suggested that faces are recognized at these latencies, in contrast with current opinions in ERP literature. However, the similar latencies obtained by George et al. for stimuli not perceived as faces suggest that Seeck et al.'s differences could index repetition rather than face recognition per se. To address this issue, we used matched faces of known and unknown persons. We found the earliest differences between the ERPs to these faces between 76 and 130 ms. These results, which are consistent with other data, suggest that the differentiation of faces takes approximately 100 ms of processing time in humans.

Memory impairment in schizophrenia: a study using event-related potentials in implicit and explicit tasks.

Although memory impairment is recognized as a major fact of schizophrenia, only a few studies have investigated memory impairments with specifically designed event-related potential (ERP) protocols. In this study, ERPs were recorded from 15 schizophrenia patients and 15 matched control subjects during implicit and explicit memory tasks for unfamiliar faces. The results showed that patients have a reduced modulation of an N400-like component in both the implicit and explicit tasks that suggests a deficient integration of incoming information with personal knowledge. Patients also displayed an enhanced frontally distributed activity in the explicit task that may represent an impairment in the integration of intrinsic contextual information, a disturbance in the ability to inhibit proactive interference or a combination of both processes. Finally, the modulation of the late positive component did not differ from that in control subjects in both implicit and explicit tasks, suggesting that the impairment in mnemonic binding processes suggested in schizophrenia is more qualitative, i.e. incomplete or inappropriate, due to the anomalies in antecedent processes. The correlations observed between impairments of ERP modulation and symptoms further support these interpretations.

Knowledge inhibition and N400: a study with words that look like common words.

In addition to their own representations, low frequency words, such as BRIBE, can covertly activate the representations of higher frequency words they look like (e.g., BRIDE). Hence, look-alike words can activate knowledge that is incompatible with the knowledge corresponding to accurate representations. Comparatively, eccentric words, that is, low frequency words that do not look as much like higher frequency words, are less likely to activate incompatible knowledge. This study focuses on the hypothesis that the N400 component of the event-related potential reflects the inhibition of incompatible knowledge. This hypothesis predicts that look-alike words elicit N400s of greater amplitudes than eccentric words in conditions where incompatible knowledge is inhibited. Results from a single item lexical decision experiment are reported which support the inhibition hypothesis.

[Capgras syndrome: evolution of the hypotheses]. / Syndrome de Capgras: évolution des hypothèses.

OBJECTIVE: to trace the evolution of hypotheses concerning Capgras' syndrome. METHODS: The data consist of slightly over 60 studies published between 1866 and 1994 which were selected in terms of their innovative nature and relevance to the clinical description of the syndrome and to psychodynamic, neurological and neuropsychological interpretations. RESULTS: Two partially overlapping major stages can be identified in the evolution of hypotheses regarding the mechanisms of the syndrome. The 1st, beginning in 1923, is characterized by the predominance of psychodynamic interpretations. The 2nd, resulting from the observation of organic dysfunctions in a high percentage of cases, is distinguished by the advent of neurological interpretations, and by a few mixed hypotheses. CONCLUSIONS: Overall, this review highlights the broad diversity of viewpoints concerning the syndrome. It will be used as a basis for the following study, which is designed to show that it is possible to test each of the viewpoints experimentally.

[Capgras syndrome: open perspectives for cognitive neuropsychology]. / Syndrome de Capgras: perspectives ouvertes par la neuropsychologie cognitive.

OBJECTIVE: To examine the way in which certain concepts regarding the physiopathology of Capgras' syndrome (1) have been tested neuropsychologically. METHODS: Data consist of approximately 30 studies selected for their relevance to the cerebral stages of face processing in patients with schizophrenia, patients with Capgras' syndrome and normal subjects. RESULTS: Study of this work shows: a) that with respect to patients, authors have focused on the stage of treatment corresponding to the facial recognition phase per se; b) but that it is also possible to study the phase corresponding to knowledge and beliefs relative to individuals and to evaluate the existence of the cleavage proposed by numerous psychodynamicians. CONCLUSIONS: Views from the field of neuropsychology, like those from the field of psychodynamics, can therefore be tested. By offering a means of developing testable predictions in experimental protocols, cognitive neuropsychology methods will, in short, make it possible to reject erroneous concepts and demonstrate accurate ones. Limited here to the example of Capgras' syndrome, we advocate that the same methods be applied to Capgras' syndrome as to each symptom of schizophrenia.

Spatial and temporal organization of episodic and semantic processes revealed by event-related potentials to unfamiliar faces.

This study investigated whether the semantic and episodic processes underlying the old/new effect can be dissociated by analyzing the spatial and temporal characteristics of event-related potentials (ERPs) evoked by unfamiliar face stimuli during direct and indirect memory tests. The ERP old/new effects obtained included three components. First, the classical posterior component, present in both tasks, which was thus interpreted as reflecting changes in the automatic activation of semantic representation. Second, an earlier frontal component, present in the direct test only, which was seen a consequence of the building of a contextual representation of presented items formed of a conjunction of elements stored in semantic memory. Third, a late right-temporal component, in the direct test, which was related to the integration of this conjunction into a unitized representation of the presented stimuli.

Neuropsychological impairments in the syndromes of schizophrenia: a comparison between different dimensional models.

This study investigates the associations between the different symptom dimensions of schizophrenia and neuropsychological performances. Globally, the results replicate previously described associations. The "negative" dimension correlates with impaired sustained attention and working memory, thus suggestive of dorsolateral frontal cortex dysfunction. "Disorganization" correlates with the ability to inhibit proactive interference. thus with ventromedial frontal dysfunction. The results also add support to the view that the "psychotic" dimension described in three-dimension models includes in fact two distinct dimensions, (1) "hallucinations," here associated to episodic memory measures, and (2) "delusions", here associated to visuospatial attention, thus suggestive of mediotemporal and posterior neocortical dysfunction, respectively.