[A comparison between group-average and individual evoked potential analysis]. / Etude comparative en potentiels évoqués des données fournies par le biais d'analyses de groupes et individuelles.

PURPOSE: Event-related potentials (ERPs) studies in human subjects have shown inter-individual response variations, probably linked to anatomical and functional brain disparities. The present study was conducted to compare the results obtained by a standard grand-average method and a single subject analysis of VEPs to faces. MATERIAL AND METHOD: Fifty-eight channel ERPs (analysis time: 1,024 ms) were recorded in 13 normal volunteers during gender or familiarity judgements on unknown and known faces, as well as on a control task using meaningless patterns. Data were then submitted to individual and group averages. RESULTS AND CONCLUSION: Three activities were identified by both procedures: a P1/N1 complex, a vertex positive potential (P2 or VPP) associated with a temporal negativity, and a N2 negativity. These peaks displayed a marked inter-individual topographical variability. Regarding the outcome of statistical analyses, a certain number of differences were found: on P1, in which individual analyses revealed a strong effect of experimental conditions, while the grand-average method did not; on VPP, in which grand-average analyses suggested an interaction between experimental conditions, face familiarity and cerebral lateralization, while individual analyses did not; and on N2, in which grand-average data showed a clear lateralization effect, while individual analyses did not. A P3 component (Pz, 250 ms) was also defined in grand-average data, but could not be clearly described in individual data. Statistical analyses on this P3 component were thus only performed on group data and revealed a right lateralization and an interaction between face familiarity and experimental conditions. These findings confirmed the existence of a marked topographical variability of ERPs to face and, therefore, question the validity of grand-average studies. Moreover, these results suggest a better efficiency of individual analyses for studying short and middle-latency peaks, while grand-averages appear to be better suited for studying late components.

Retrospective study of somatosensory evoked potential monitoring in deep hypothermic circulatory arrest.

BACKGROUND: We evaluated the efficiency of median-nerve somatosensory evoked potentials (SEPs) monitoring in determining the optimal level of hypothermia in 62 consecutive patients operated on under deep hypothermic circulatory arrest (CA). METHODS: CA was started at 1 degree C below the temperature at which both brainstem and cortical SEP components disappear. No additional method of cerebral protection was used. RESULTS: New neurological complications were observed in 15 patients: long-lasting in 11 and transient in 4. A retrospective analysis of SEP monitoring identified the origin of the complications in 12 cases: early CA with incomplete cooling due to emergency (3 cases); inefficient retrograde perfusion through the femoral artery during cooling due to the dissection flap effect (4 cases); embolism during rewarming (2 cases); delayed embolism (2 cases); hemorrhagic shock (1 case). In 2 cases, neurological sequelae involved the lower limbs (extracerebral origin). One case without intraoperative SEP changes was neurologically abnormal preoperatively and did not change postoperatively. There were no cases with sequelae due to excessive CA duration. CONCLUSIONS: The use of SEP monitoring to determine the optimal level of hypothermia efficiently prevents neurological sequelae of CA. It helps in monitoring the degree of cerebral protection during cooling (flap effect), and rewarming.

Task modulation of brain activity related to familiar and unfamiliar face processing: an ERP study.

In order to investigate stimulus-related and task-related electrophysiological activity relevant for face processing, event-related potentials (ERPs) from 58 electrodes at standard EEG sites were recorded while subjects performed a simple visual discrimination (control) task, in addition to various face processing tasks: recognition of previously learned faces and gender decision on familiar and unfamiliar faces. Three electrophysiological components or dipolar complex were recorded in all subjects: an occipital early component (P1, around 110 ms); a vertex positive potential (VPP; around 158 ms) which appeared to be specific to faces; and a negative central component, N2 (around 230 ms). Parametric analysis and source localization were applied to these components by means of a single-subject analysis methodology. No effect of familiarity was observed on any of these early component. While the VPP appears to be independent of the kind of processing performed, face task modulations of the early P1 and the N2 were observed, with a higher amplitude for the recognition than for the gender discrimination task. An attentional modulation of early visual areas is proposed for the first effect (P1 modulation), while the N2 seems to be related to general visual memory processing. This study strongly suggests that the VPP reflects an early visual stage of face processing in the fusiform gyrus that is strictly stimulus-related and independent of familiarity. It also shows that source localization algorithms may give reliable solutions on single subject averages for early visual components despite high inter-subject variability of the surface characteristics of ERPs.

[Sensitivity, specificity, and impact on the surgical strategy of the perioperative neuromonitoring of somatic evoked potentials in vascular surgery performed with circulatory arrest under deep hypothermia]. / Sensibilité, spécificité et impact sur la stratégie chirurgicale du neuromonitorage peropératoire par potentiels évoqués somesthésiques en chirurgie vasculaire pratiquée avec arrêt circulatoire sous hypothermie profonde.

AIM: Median nerve somatosensory evoked potentials (SEP) monitoring performed in deep hypothermic circulatory arrest is useful in determining when brain cooling may sufficiently permit circulatory arrest, and to immediately indicate when brain injury occurs. The aim of this retrospective study, including 58 patients, is to determine SEP sensitivity, specificity and impact on the surgical strategy. METHOD: Sensitivity was determined on patients presenting new neurological complications. Specificity was measured on the patients without any new neurological defect. The impact of intraoperative SEP on the surgical strategy was determined when the occurrence of SEP alterations had provoked a modification of the surgical procedure. RESULTS: Five patients died intraoperatively due to non-neurological complications. Fourteen patients presented new neurological complications. SEP sensitivity reaches 64.3% when all the complications are included, but increases to 100% in our series if late postoperative complications, paraplegia and tibial posterior nerve lesion are excluded because median nerve SEP are not able to detect such complications. SEP monitoring was uneventful in the 39 patients whose did not present neurological complication: thus SEP specificity is absolute in our series. SEP had an impact on the surgical procedure whenever neurological complications due to hemodynamic disturbances occurred. CONCLUSION: This retrospective study shows the interest of intraoperative SEP as an useful tool for promptly identifying and preventing the neurological complication of hypothermic circulatory arrest.