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Olivier Bertrand

Publications and source records attributed to Olivier Bertrand.

At least 19 recordsLinked to original sources
PubMed2007

Neural correlates of consolidation in working memory.

Many of our daily activities rely on a brain system called working memory, which implements our ability to encode information for short-term maintenance, possible manipulation, and retrieval. A recent intracranial study of patients performing a paradigmatic working memory task revealed that the maintenance of information involves a distributed network of oscillations in the gamma band (>40 Hz). Using a similar task, we focused on the encoding stage and targeted a process referred to as short-term consolidation, which corresponds to the encoding of novel items in working memory. The paradigm was designed to manipulate the subjects' intention to encode: series of 10 letters were presented, among which only five had to be remembered, as indicated by visual cues preceding or following each letter. During this task we recorded the intracerebral EEG of nine epileptic patients implanted in mesiotemporal structures, perisylvian regions, and prefrontal areas and used time-frequency analysis to search for neural activities simultaneous with the encoding of the letters into working memory. We found such activities in the form of increases of gamma band activity in a set of regions associated with the phonological loop, including the Broca area and the auditory cortex, and in the prefrontal cortex, the pre- and postcentral gyri, the hippocampus, and the fusiform gyrus.

Adult↗
PubMed2007

Distinction between perceptual and attentional processing in working memory tasks: a study of phase-locked and induced oscillatory brain dynamics.

Working memory involves the short-term storage and manipulation of information necessary for cognitive performance, including comprehension, learning, reasoning and planning. Although electroencephalogram (EEG) rhythms are modulated during working memory, the temporal relationship of EEG oscillations with the eliciting event has not been well studied. In particular, the dynamics of the neural network supporting memory processes may be best captured in induced oscillations, characterized by a loose temporal link with the stimulus. In order to differentiate induced from evoked functional processes, the present study proposes a time-frequency analysis of the 3 to 30 Hz EEG oscillatory activity in a verbal n-back working memory paradigm. Control tasks were designed to identify oscillatory activity related to stimulus presentation (passive task) and focused attention to the stimulus (detection task). Evoked theta activity (4-8 Hz) phase-locked to the visual stimulus was evidenced in the parieto-occipital region for all tasks. In parallel, induced theta activity was recorded in the frontal region for detection and n-back memory tasks, but not for the passive task, suggesting its dependency on focused attention to the stimulus. Sustained induced oscillatory activity was identified in relation to working memory in the theta and beta (15-25 Hz) frequency bands, larger for the highest memory load. Its late occurrence limited to nonmatched items suggests that it could be related to item retention and active maintenance for further task requirements. Induced theta and beta activities displayed respectively a frontal and parietal topographical distribution, providing further functional information on the fronto-posterior network supporting working memory.

Adult↗
PubMed2006

Listening in silence activates auditory areas: a functional magnetic resonance imaging study.

Directing attention to some acoustic features of a sound has been shown repeatedly to modulate the stimulus-induced neural responses. On the contrary, little is known about the neurophysiological impact of auditory attention when the auditory scene remains empty. We performed an experiment in which subjects had to detect a sound emerging from silence (the sound was detectable after different durations of silence). Two frontal activations (right dorsolateral prefrontal and inferior frontal) were found, regardless of the side where sound was searched for, consistent with the well established role of these regions in attentional control. The main result was that the superior temporal cortex showed activations contralateral to the side where sound was expected to be present. The area extended from the vicinity of Heschl's gyrus to the surrounding areas (planum temporale/anterior lateral areas). The effect consisted of both an increase in the response to a sound delivered after attention was directed to detect its emergence and a baseline shift during the silent period. Thus, in absence of any acoustic stimulus, the search for an auditory input was found to activate the auditory cortex.

Acoustic Stimulation↗
PubMed2006

Experiencing and detecting happiness in humans: the role of the supplementary motor area.

Understanding emotions in others may involve neural structures implicated in both perception and action.1 Laughter is a socially important behavior and its neural correlates are poorly understood. Depth electrode electrophysiological techniques offer a rare opportunity to both record and stimulate neural structures involved in emotion processing. (2,3) This kind of direct electrophysiological investigation in humans is particularly determinant because laughter processing cannot be studied in animals.

Adult↗
PubMed2005

Localization of human supratemporal auditory areas from intracerebral auditory evoked potentials using distributed source models.

While source localization methods are increasingly developed to identify brain areas underlying scalp electro/magnetoencephalographic data (EEG/MEG), these methods have not yet been used to identify the sources of intracerebral signals which offer highly detailed information. Here, we adapted the minimum current estimates method to intracranial data in order to localize supratemporal sources of intracerebral auditory 1-kHz-tone-evoked potentials occurring within 100 ms after stimulus onset. After an evaluation of localization method and despite inter-subject variability, we found a common spatiotemporal pattern of activities, which involved the first Heschl's gyrus (H1) and sulcus (HS), the Planum Temporale (PT), H2/H3 when present, and the superior temporal gyrus (STG). Four time periods of activity were distinguished, corresponding to the time range of the scalp components P0, Na, Pa/Pb, and N100. The sources of the earliest components P0 (16-19 ms) and Na (20-25 ms) could be identified in the postero-medial portion of HS or H1. Then, several areas became simultaneously active after 25 ms. The Pa/Pb time range (30-50 ms) was characterized by a medio-lateral and postero-anterior propagation of activity over the supratemporal plane involving successively H1/HS, the Planum Temporale, H2/H3 when present, and the STG. Finally, we found to a large extent that the N100 (55-100 ms) involved almost the same areas as those active during the Pa/Pb complex, with a similar propagation of activities. Reconstructing scalp data from these sources on fictive EEG/MEG channels reproduced classical auditory evoked waveforms and topographies. In conclusion, the spatiotemporal pattern of activation of supratemporal auditory areas could be identified on the individual anatomy using current estimates from intracerebral data. Such detailed localization approach could also be used prior to epilepsy surgery to help identify epileptogenic foci and preserve functional cortical areas.

Acoustic Stimulation↗
PubMed2005

Listening to a walking human activates the temporal biological motion area.

A vivid perception of a moving human can be evoked when viewing a few point-lights on the joints of an invisible walker. This special visual ability for biological motion perception has been found to involve the posterior superior temporal sulcus (STSp). However, in everyday life, human motion can also be recognized using acoustic cues. In the present study, we investigated the neural substrate of human motion perception when listening to footsteps, by means of a sparse sampling functional MRI design. We first showed an auditory attentional network that shares frontal and parietal areas previously found in visual attention paradigms. Second, an activation was observed in the auditory cortex (Heschl's gyrus and planum temporale), likely to be related to low-level sound processing. Most strikingly, another activation was evidenced in a STSp region overlapping the temporal biological motion area previously reported using visual input. We thus propose that a part of the STSp region might be a supramodal area involved in human motion recognition, irrespective of the sensory modality input.

Acoustic Stimulation↗
PubMed2005

Functional significance of olfactory-induced oscillations in the human amygdala.

We recorded directly from the amygdalar nucleus of nine epileptic patients performing a delayed odor-matching recognition memory task. Time-frequency analysis of the responses to the odorants revealed that the stimulations elicited induced oscillatory responses, as well as already described olfactory evoked potentials. These oscillatory responses were composed of two frequency components--one in the beta band (15-25 Hz) and a faster one, in the low gamma band (25-35 Hz)--both of which lasted during the full duration of the inspiration. In pairs of identical odorants, the power of gamma oscillations was weaker for the second odorant (the target) than for the first one (the sample). We observed no such difference when the first and second odorants of a pair were different. Thus, gamma oscillations in the amygdala are weaker for repeated stimuli, a mechanism known as repetition suppression. This is consistent with an involvement of the human amygdala in the encoding and retrieval of olfactory information independently of its hedonic properties, at least in epileptic patients. Altogether, our results corroborate in humans evidence found in animals that oscillations serve as a common coding process of olfactory information.

Adaptation, Physiological↗
PubMed2005

The Kell protein of the common K2 phenotype is a catalytically active metalloprotease, whereas the rare Kell K1 antigen is inactive. Identification of novel substrates for the Kell protein.

The Kell blood group is a highly polymorphic system containing over 20 different antigens borne by the protein Kell, a 93-kDa type II glycoprotein that displays high sequence homology with members of the M13 family of zinc-dependent metalloproteases whose prototypical member is neprilysin. Kell K1 is an antigen expressed in 9% of the Caucasian population, characterized by a point mutation (T193M) of the Kell K2 antigen, and located within a putative N-glycosylation consensus sequence. Recently, a recombinant, non-physiological, soluble form of Kell was shown to cleave Big ET-3 to produce the mature vasoconstrictive peptide. To better characterize the enzymatic activity of the Kell protein and the possible differences introduced by antigenic point mutations affecting post-translational processing, the membrane-bound forms of the Kell K1 and Kell K2 antigens were expressed either in K562 cells, an erythroid cell line, or in HEK293 cells, a non-erythroid system, and their pharmacological profiles and enzymatic specificities toward synthetic and natural peptides were evaluated. Results presented herein reveal that the two antigens possess considerable differences in their enzymatic activities, although not in their trafficking pattern. Indeed, although both antigens are expressed at the cell surface, Kell K1 protein is shown to be inactive, whereas the Kell K2 antigen binds neprilysin inhibitory compounds such as phosphoramidon and thiorphan with high affinity, cleaves the precursors of the endothelin peptides, and inactivates members of the tachykinin family with enzymatic properties resembling those of other members of the M13 family of metalloproteases to which it belongs.

Antigens↗
PubMed2005

Dynamics of a temporo-fronto-parietal network during sustained spatial or spectral auditory processing.

Animal and human studies have suggested that posterior temporal, parietal, and frontal regions are specifically involved in auditory spatial (location and motion) processing, forming a putative dorsal "where" pathway. We used scalp EEG and current density mapping to investigate the dynamics of this network in human subjects presented with a varying acoustic stream in a two-factor paradigm: spatial versus pitch variations, focused versus diverted attention. The main findings were: (i) a temporo-parieto-frontal network was activated during the whole duration of the stream in all conditions and modulated by attention; (ii) the left superior temporal cortex was the only region showing different activations for pitch and spatial variations. Therefore, parietal and frontal regions would be involved in task-related processes (attention and motor preparation), whereas the differential processing of acoustic spatial and object-related features seems to take place at the temporal level.

Acoustic Stimulation↗
PubMed2004

Human Rhesus-associated glycoprotein mediates facilitated transport of NH(3) into red blood cells.

Rhesus (Rh) antigens are carried by a membrane complex that includes Rh proteins (D and CcEe), Rh-associated glycoproteins (RhAG), and accessory chains (LW and CD47) associated by noncovalent bonds. In heterologous expression systems, RhAG and its kidney orthologs function as ammonium transporters. In red blood cells (RBCs), it is generally accepted that NH(3) permeates by membrane lipid diffusion. We have revisited these issues by studying RBC and ghosts from human and mouse genetic variants with defects of proteins that comprise the Rh complex. In both normal and mutant cells, stopped-flow analyses of intracellular pH changes in the presence of inwardly directed methylammonium (CH(3)NH(+)(3)+CH(3)NH(2)) or ammonium (NH(+)(4)+NH(3)) gradients showed a rapid alkalinization phase. Cells from human and mouse variants exhibited a decrease in their kinetic rate constants that was strictly correlated to the degree of reduction of their RhAG/Rhag expression level. Rate constants were not affected by a reduction of Rh, CD47, or LW. CH(3)NH(2)/NH(3) transport was characterized by (i) a sensitivity to mercurials that is reversible by 2-mercaptoethanol and (ii) a reduction of alkalinization rate constants after bromelain digestion, which cleaves RhAG. The results show that RhAG facilitates CH(3)NH(2)/NH(3) movement across the RBC membrane and represents a potential example of a gas channel in mammalian cells. In RBCs, RhAG may transport NH(3) to detoxifying organs, like kidney and liver, and together with nonerythroid tissue orthologs may contribute to the regulation of the systemic acid-base balance.

Acid-Base Equilibrium↗
PubMed2004

Effects of the acyl coenzyme A:cholesterol acyltransferase inhibitor avasimibe on human atherosclerotic lesions.

BACKGROUND: Inhibition of the acyl coenzyme A:cholesterol acyltransferase (ACAT) enzyme may prevent excess accumulation of cholesteryl esters in macrophages. The ACAT inhibitor avasimibe was shown to reduce experimental atherosclerosis. This study was designed to investigate the effects of avasimibe on human coronary atherosclerosis. METHODS AND RESULTS: This randomized, double-blind, placebo-controlled trial assessed the effects of avasimibe at dosages of 50, 250, and 750 mg QD on the progression of coronary atherosclerosis as assessed by intravascular ultrasound (IVUS). All patients received background lipid-lowering therapy if necessary to reach a target baseline LDL level <125 mg/dL (3.2 mmol/L). IVUS and coronary angiography were performed at baseline and repeated after up to 24 months of treatment. Approximately equal percentages of patients across groups received concurrent statin therapy (87% to 89%). The mean total plaque volume at baseline was approximately 200 mm3, and the least squares mean change at end of treatment was 0.7 mm3 for placebo and 7.7, 4.1, and 4.8 mm3 for the avasimibe 50, 250, and 750 mg groups, respectively (adjusted P=0.17 [unadjusted P=0.057], 0.37, and 0.37, respectively). Percent atheroma volume increased by 0.4% with placebo and by 0.7%, 0.8%, and 1.0% in the respective avasimibe groups (P=NS). LDL cholesterol increased during the study by 1.7% with placebo but by 7.8%, 9.1%, and 10.9% in the respective avasimibe groups (P<0.05 in all groups). CONCLUSIONS: Avasimibe did not favorably alter coronary atherosclerosis as assessed by IVUS. This ACAT inhibitor also caused a mild increase in LDL cholesterol.

Acetamides↗
PubMed2004

Attention modulates gamma-band oscillations differently in the human lateral occipital cortex and fusiform gyrus.

We studied the existence, localization and attentional modulation of gamma-band oscillatory activity (30-130 Hz) in the human intracranial region. Two areas known to play a key role in visual object processing: the lateral occipital (LO) cortex and the fusiform gyrus. These areas consistently displayed large gamma oscillations during visual stimulus encoding, while other extrastriate areas remained systematically silent, across 14 patients and 291 recording sites scattered throughout extrastriate visual cortex. The lateral extent of the responsive regions was small, in the range of 5 mm. Induced gamma oscillations and evoked potentials were not systematically co-localized. LO and the fusiform gyrus displayed markedly different patterns of attentional modulation. In the fusiform gyrus, attention enhanced stimulus-driven gamma oscillations. In LO, attention increased the baseline level of gamma oscillations during the expectation period preceding the stimulus. Subsequent gamma oscillations produced by attended stimuli were smaller than those produced by unattended, irrelevant stimuli. Attentional modulations of gamma oscillations in LO and the fusiform gyrus were thus very different, both in their time-course (preparatory period and/or stimulus processing) and direction of modulation (increase or decrease). Our results thus suggest that the functional role of gamma oscillations depends on the area in which they occur.

Adult↗
PubMed2004

Early amygdala reaction to fear spreading in occipital, temporal, and frontal cortex: a depth electrode ERP study in human.

The amygdala involvement in fear processing has been reported in behavioral, electrophysiological, and functional imaging studies. However, the literature does not provide precise data on the temporal course of facial emotional processing. Intracranial event-related potentials to facial expressions were recorded in epileptic patients implanted with depth electrodes during a presurgical evaluation. Specific potentials to fear beginning 200 ms poststimulus were observed in amygdala, both individually in two patients and in a ten patient population study. These potentials occurred 100 ms earlier than potentials to disgust recorded in insula in a previous study. Potentials to fear were confined in amygdala during a first transient period and then, during a second period of sustained activity, spread to occipito-temporal, anterior temporal, and orbitofrontal cortex in two patients. This study clarifies the temporal course of the involvement of these structures known to be part of a neural network recruited to process emotional information.

Amygdala↗
PubMed2004

Attentional modulation of human auditory cortex.

Attention powerfully influences auditory perception, but little is understood about the mechanisms whereby attention sharpens responses to unattended sounds. We used high-resolution surface mapping techniques (using functional magnetic resonance imaging, fMRI) to examine activity in human auditory cortex during an intermodal selective attention task. Stimulus-dependent activations (SDAs), evoked by unattended sounds during demanding visual tasks, were maximal over mesial auditory cortex. They were tuned to sound frequency and location, and showed rapid adaptation to repeated sounds. Attention-related modulations (ARMs) were isolated as response enhancements that occurred when subjects performed pitch-discrimination tasks. In contrast to SDAs, ARMs were localized to lateral auditory cortex, showed broad frequency and location tuning, and increased in amplitude with sound repetition. The results suggest a functional dichotomy of auditory cortical fields: stimulus-determined mesial fields that faithfully transmit acoustic information, and attentionally labile lateral fields that analyze acoustic features of behaviorally relevant sounds.

Acoustic Stimulation↗
PubMed2004

Local landmark-based mapping of human auditory cortex.

Mammalian sensory cortex is functionally partitioned into cortical fields that are specialized for different processing operations. In theory, averaging functional and anatomical images across subjects can reveal both the average anatomy and the mean functional organization of sensory regions. However, this averaging process must overcome at least two obstacles: (1) the relative locations and sizes of cortical sensory areas vary in different subjects so that across-subject averaging introduces spatial smearing; (2) the relative locations and sizes of cortical areas vary between hemispheres, making it difficult to compare activations between hemispheres or to combine activations across hemispheres. These difficulties are particularly acute for small cortical regions such as auditory cortex. In whole-brain averaging procedures, considerable intersubject variance in the location and orientation of auditory cortex is introduced by variance of the size and shape of structures outside auditory cortex. Here, we compared these global methods with local landmark-based methods (LLMs) that use warping based on local anatomical landmarks. In comparison to maps made with global methods, LLMs produced anatomical maps of auditory cortex with clearer gyral and sulcal structure, and produce functional maps with improved resolution. These results suggest that LLMs have significant advantages over global mapping procedures in studying the details of auditory cortex organization.

Adult↗
PubMed2003

Human lateral geniculate nucleus and visual cortex respond to screen flicker.

The first electrophysiological study of the human lateral geniculate nucleus (LGN), optic radiation, striate, and extrastriate visual areas is presented in the context of presurgical evaluation of three epileptic patients (Patients 1, 2, and 3). Visual-evoked potentials to pattern reversal and face presentation were recorded with depth intracranial electrodes implanted stereotactically. For Patient 1, electrode anatomical registration, structural magnetic resonance imaging, and electrophysiological responses confirmed the location of two contacts in the geniculate body and one in the optic radiation. The first responses peaked approximately 40 milliseconds in the LGN in Patient 1 and 60 milliseconds in the V1/V2 complex in Patients 2 and 3. Moreover, steady state visual-evoked potentials evoked by the unperceived but commonly experienced video-screen flicker were recorded in the LGN, optic radiation, and V1/V2 visual areas. This study provides topographic and temporal propagation characteristics of steady state visual-evoked potentials along human visual pathways. We discuss the possible relationship between the oscillating signal recorded in subcortical and cortical areas and the electroencephalogram abnormalities observed in patients suffering from photosensitive epilepsy, particularly video-game epilepsy. The consequences of high temporal frequency visual stimuli delivered by ubiquitous video screens on epilepsy, headaches, and eyestrain must be considered.

Adult↗
PubMed2003

An attention modulated response to disgust in human ventral anterior insula.

The human brain is expert in analyzing rapidly and precisely facial features, especially emotional expressions representing a powerful communication vector. The involvement of insula in disgust recognition has been reported in behavioral and functional imaging studies. However, we do not know whether specific insular fields are involved in disgust processing nor what the processing time course is. Using depth electrodes implanted during presurgical evaluation of patients with drug-refractory temporal lobe epilepsy, we recorded intracerebral event-related potentials to human facial emotional expressions, that is, fear, disgust, happiness, surprise, and neutral expression. We studied evoked responses in 13 patients with insular contacts to specify the insular fields involved in disgust processing and assess the timing of their activation. We showed that specific potentials to disgust beginning 300 milliseconds after stimulus onset and lasting 200 milliseconds were evoked in the ventral anterior insula in four patients. The occurrence and latency of event-related potentials to disgust in the ventral anterior insula were affected by selective attention. The analysis of spatial and temporal characteristics of insular responses to disgust facial expression lead us to underline the crucial role of ventral anterior insula in the categorization of facial emotional expressions, particularly the disgust.

Attention↗