Catherine Liégeois-Chauvel

Evoked potentials recorded from the auditory cortex in man: evaluation and topography of the middle latency components

The goal of this study is to determine and localize the generators of different components of middle latency auditory evoked potentials (MLAEPs) through intracerebral recording in auditory cortex in man (Heschls gyrus and planum temporale). The present results show that the generators of components at 30, 50, 60 and 75 msec latency are distributed medio-laterally along Heschls gyrus. The 30 msec component is generated in the dorso-postero-medial part of Heschls gyrus (primary area) and the 50 msec component is generated laterally in the primary area. The generators of the later components (60-75 msec) are localized in the lateral part of Heschls gyrus that forms the secondary areas. The localization of N100 generators is discussed.

The intracranial topography of the P3 event-related potential elicited during auditory oddball

In order to isolate the anatomical locus of neural activity primarily responsible for generating the scalp-recorded P3 (or P300), the topography of event-related potentials (ERPs) elicited during an auditory oddball task was compared between medial-to-lateral aspects of the frontal, parietal, and temporal lobes in 10 epileptic patients undergoing stereoelectroencephalography for seizure localization. Evidence of local ERP generation was obtained from each of these areas. Small amplitude P3-type potentials were sometimes observed to invert polarity across recording contacts in the frontal lobe. Large amplitude positive polarity P3-type components were observed in the lateral neocortex of the inferior parietal lobule (IPL), that rapidly attenuated in amplitude at more anterior, posterior, superior, inferior, and medial recording contacts. Large amplitude polarity inverting P3-type components were also observed to be highly localized to hippocampal contacts of temporal lobe electrodes. These data are discussed in the context of other recent studies of lesion effects, scalp topography, and intracranial recordings, and it is concluded that activity generated in the IPL is likely to make the major contribution to the scalp-recorded P3, with smaller contributions from these other sources. Finally, salient topographical differences between the intracranial distribution of the P3 and those of the N2 (or N200) and slow wave (SW) suggest that the generators of these components are not identical.

Neuromagnetic source localization of auditory evoked fields and intracerebral evoked potentials: a comparison of data in the same patients

OBJECTIVE To compare the localizations of different neural sources (a) obtained from intracerebral evoked responses and (b) calculated from surface auditory evoked field responses recorded in the same subjects. Our aim was to evaluate the resolving power of a source localization method currently used in our laboratory, which is based on a recent spatio-temporal algorithm used in magneto-encephalography (MEG). METHODS Auditory evoked responses were studied in 4 patients with medically intractable epilepsy. These responses were recorded from depth electrodes implanted in the auditory cortex for pre-surgical evaluation (stereo-electro-encephalography (SEEG)), as well as from surface captors (for MEG) placed on the scalp after removal of the depth electrodes. Auditory stimuli were clicks and short tone bursts with different frequencies. RESULTS All middle-latency components (from 13 to 70 ms post-stimulus onset) were recorded and localized (via SEEG) along Heschls gyrus (HG). MEG reliably localized Pam and P1m in the same area of HG that intracerebral recordings localized them in. No significant delay between SEEG and MEG latencies was observed. Both methods suggest that N1 is generated from different sources in the intermediate and lateral parts of the HG and in the planum temporale (PT). The source of P2 (PT and/or Area 22) remains unclear and was in one case, localized in different regions according to the method used. This latter component may therefore also be generated by different sources. CONCLUSIONS The results suggest that both techniques are useful and may be used together in a complementary fashion. Intracerebral recordings allow the researcher to validate and interpret surface recordings.

Single-trial analysis of oddball event-related potentials in simultaneous EEG-fMRI

There has recently been a growing interest in the use of simultaneous electroencephalography (EEG) and functional MRI (fMRI) for evoked activity in cognitive paradigms, thereby obtaining functional datasets with both high spatial and temporal resolution. The simultaneous recording permits obtaining event‐related potentials (ERPs) and MR images in the same environment, conditions of stimulation, and subject state; it also enables tracing the joint fluctuations of EEG and fMRI signals. The goal of this study was to investigate the possibility of tracking the trial‐to‐trial changes in event‐related EEG activity, and of using this information as a parameter in fMRI analysis. We used an auditory oddball paradigm and obtained single‐trial amplitude and latency features from the EEG acquired during fMRI scanning. The single‐trial P300 latency presented significant correlation with parameters external to the EEG (target‐to‐target interval and reaction time). Moreover, we obtained significant fMRI activations for the modulation by P300 amplitude and latency, both at the single‐subject and at the group level. Our results indicate that, in line with other studies, the EEG can bring a new dimension to the field of fMRI analysis by providing fine temporal information on the fluctuations in brain activity. Hum Brain Mapp, 2007.

Brain regions involved in the recognition of happiness and sadness in music

Here, we used functional magnetic resonance imaging to test for the lateralization of the brain regions specifically involved in the recognition of negatively and positively valenced musical emotions. The manipulation of two major musical features (mode and tempo), resulting in the variation of emotional perception along the happiness–sadness axis, was shown to principally involve subcortical and neocortical brain structures, which are known to intervene in emotion processing in other modalities. In particular, the minor mode (sad excerpts) involved the left orbito and mid-dorsolateral frontal cortex, which does not confirm the valence lateralization model. We also show that the recognition of emotions elicited by variations of the two perceptual determinants rely on both common (BA 9) and distinct neural mechanisms.

The contribution of frequency-specific activity to hierarchical information processing in the human auditory cortex

The fact that feed-forward and top-down propagation of sensory information use distinct frequency bands is an appealing assumption for which evidence remains scarce. Here we obtain human depth recordings from two auditory cortical regions in both hemispheres, while subjects listen to sentences, and show that information travels in each direction using separate frequency channels. Bottom-up and top-down propagation dominates in γ- and δ–β (<40 Hz) bands, respectively. The predominance of low frequencies for top-down information transfer is confirmed by cross-regional frequency coupling, which indicates that the power of γ-activity in A1 is modulated by the phase of δ–β activity sampled from association auditory cortex (AAC). This cross-regional coupling effect is absent in the opposite direction. Finally, we show that information transfer does not proceed continuously but by time windows where bottom-up or top-down processing alternatively dominates. These findings suggest that the brain uses both frequency- and time-division multiplexing to optimize directional information transfer.

Action Monitoring and Medial Frontal Cortex: Leading Role of Supplementary Motor Area

Oops, Thats Not Right… Evaluating our actions, and detecting our errors, is crucial for adaptive behavior. These fundamental executive functions are intensively studied in cognitive and social neuroscience, but their anatomical basis remains poorly characterized. Using intracerebral electroencephalography in patients being prepared for epilepsy surgery, Bonini et al. (p. 888) found that, contrary to what is widely assumed, the supplementary motor area, and not the anterior cingulate cortex, plays a leading role in these processes. The data provide a precise spatio-temporal description of the cortical network underlying action monitoring and error processing. Detection of a core brain region for performance monitoring and error detection in humans is shown. The capacity to evaluate the outcomes of our actions is fundamental for adapting and optimizing behavior and depends on an action-monitoring system that assesses ongoing actions and detects errors. The neuronal network underlying this executive function, classically attributed to the rostral cingulate zone, is poorly characterized in humans, owing to the limited number of direct neurophysiological data. Using intracerebral recordings, we show that the leading role is played by the supplementary motor area (SMA), which rapidly evaluates successful and erroneous actions. The rostral part of medial prefrontal cortex, driven by the SMA, was activated later and exclusively in the case of errors. This suggests a hierarchical organization of the different frontal regions involved in implementation of action monitoring and error processing.

Ultra-Rapid Sensory Responses in the Human Frontal Eye Field Region

Most of what we know about the human frontal eye field (FEF) is extrapolated from studies in animals. There is ample evidence that this region is crucial for eye movements. However, evidence is accumulating that this region also plays a role in sensory processing and that it belongs to a “fast brain” system. We set out to investigate these issues in humans, using intracerebral recordings in patients with drug-refractory epilepsy. Event-related potential recordings were obtained from 11 epileptic patients from within the FEF region while they passed a series of visual and auditory perceptual tests. No eye movement was required. Ultra-rapid responses were observed, with mean onset latencies at 24 ms after stimulus to auditory stimuli and 45 ms to visual stimuli. Such early responses were compatible with cortical routes as assessed with simultaneous recordings in primary auditory and visual cortices. Components were modulated very early by the sensory characteristics of the stimuli, in the 30–60 ms period for auditory stimuli and in the 45–60 ms period for visual stimuli. Although the frontal lobes in humans are generally viewed as being involved in high-level cognitive processes, these results indicate that the human FEF is a remarkably quickly activated multimodal region that belongs to a network of low-level neocortical sensory areas.

Brain regions underlying word finding difficulties in temporal lobe epilepsy

Word finding difficulties are often reported by epileptic patients with seizures originating from the language dominant cerebral hemisphere, for example, in temporal lobe epilepsy. Evidence regarding the brain regions underlying this deficit comes from studies of peri-operative electro-cortical stimulation, as well as post-surgical performance. This evidence has highlighted a role for the anterior part of the dominant temporal lobe in oral word production. These conclusions contrast with findings from activation studies involving healthy speakers or acute ischaemic stroke patients, where the region most directly related to word retrieval appears to be the posterior part of the left temporal lobe. To clarify the neural basis of word retrieval in temporal lobe epilepsy, we tested forty-three drug-resistant temporal lobe epilepsy patients (28 left, 15 right). Comprehensive neuropsychological and language assessments were performed. Single spoken word production was elicited with picture or definition stimuli. Detailed analysis allowed the distinction of impaired word retrieval from other possible causes of naming failure. Finally, the neural substrate of the deficit was assessed by correlating word retrieval performance and resting-state brain metabolism in 18 fluoro-2-deoxy-d-glucose-Positron Emission Tomography. Naming difficulties often resulted from genuine word retrieval failures (anomic states), both in picture and in definition tasks. Left temporal lobe epilepsy patients showed considerably worse performance than right temporal lobe epilepsy patients. Performance was poorer in the definition than in the picture task. Across patients and the left temporal lobe epilepsy subgroup, frequency of anomic state was negatively correlated with resting-state brain metabolism in left posterior and basal temporal regions (Brodmanns area 20-37-39). These results show the involvement of posterior temporal regions, within a larger antero-posterior-basal temporal network, in the specific process of word retrieval in temporal lobe epilepsy. A tentative explanation for these findings is that epilepsy induces functional deafferentation between anterior temporal structures devoted to semantic processing and neocortical posterior temporal structures devoted to lexical processing.

Asymmetric Function of Theta and Gamma Activity in Syllable Processing: An Intra-Cortical Study

Low-gamma (25–45 Hz) and theta (4–8 Hz) oscillations are proposed to underpin the integration of phonemic and syllabic information, respectively. How these two scales of analysis split functions across hemispheres is unclear. We analyzed cortical responses from an epileptic patient with a rare bilateral electrode implantation (stereotactic EEG) in primary (A1/BA41 and A2/BA42) and association auditory cortices (BA22). Using time-frequency analyses, we confirmed the dominance of a 5–6 Hz theta activity in right and of a low-gamma (25–45 Hz) activity in left primary auditory cortices (A1/A2), during both resting state and syllable processing. We further detected high-theta (7–8 Hz) resting activity in left primary, but also associative auditory regions. In left BA22, its phase correlated with high-gamma induced power. Such a hierarchical relationship across theta and gamma frequency bands (theta/gamma phase-amplitude coupling) could index the process by which the neural code shifts from stimulus feature- to phonological-encoding, and is associated with the transition from evoked to induced power responses. These data suggest that theta and gamma activity in right and left auditory cortices bear different functions. They support a scheme where slow parsing of the acoustic information dominates in right hemisphere at a syllabic (5–6 Hz) rate, and left auditory cortex exhibits a more complex cascade of oscillations, reflecting the possible extraction of transient acoustic cues at a fast (~25–45 Hz) rate, subsequently integrated at a slower, e.g., syllabic one. Slow oscillations could functionally participate to speech processing by structuring gamma activity in left BA22, where abstract percepts emerge.