Patrick Bouchet

Gamma‐range Activity Evoked by Coherent Visual Stimuli in Humans

We tested the hypothesis of a role of gamma‐range synchronized oscillatory activity in visual feature binding by recording evoked potentials from 12 subjects to three stimuli: two coherent ones (a Kanizsa triangle and a real triangle) and a non‐coherent one (a Kanizsa triangle in which the inducing disks had been rotated so that no triangle could be perceived). The evoked potentials were analysed by convoluting the signal for each subject and each stimulation type by Gabor wavelets centred from 28 up to 46 Hz, providing a continuous measure of frequency‐specific power over time. A first peak of activity was found around 38 Hz and 100 ms with a maximum at electrode Cz in each experimental condition. A second peak of activity occurred around 30 Hz and 230 ms, with a maximum at O1 in response to the real triangle and a maximum at Cz in the case of the illusory triangle. At 100 ms we did not find any variations of the gamma‐band component of the evoked potential with stimulation type, but the power of the 30 Hz component of the evoked potential between 210 and 290 ms differed from noise only in the case of a coherent triangle, no matter whether real or illusory. We thus found a 30 Hz component whose power correlates with stimulus coherency, which supports the hypothesis of a functional role of high‐frequency synchronization in feature binding. Moreover, this correlation seemed specific to the gamma‐frequency range, as neither the power of the low‐frequency components (0–25 Hz) nor the power of the electromyogram range components (60–80 Hz) showed the same effect.

Auditory selective attention in the human cochlea

According to current theories, auditory selective attention alters the sensory analysis of acoustic inputs only in the central auditory system. Despite numerous attempts, no evidence of attentional selection has been found in the auditory periphery. Measurements of evoked otoacoustic emissions (EOAEs) during a selective dichotic listening task showed that the EOAEs to tones in one ear had larger amplitude when attention was directed to this ear than when attention was directed to the opposite ear. The results indicate that genuine effects of auditory selective attention can be observed at the cochlear receptor.

Mismatch Negativity and N100 Monitoring: Potential Clinical Value and Methodological Advances

Continuous long-term recording of brainstem (BAEPs), middle-latency (MLAEPs) and long-latency auditory evoked potentials, including the mismatch negativity (MMN), brings additional information on the immediate functional state and the outcome of patients in coma or recovering after surgery, in relation with clinical observations and therapeutics. A recently designed monitoring system is introduced, aimed at the continuous recording of late auditory potentials (N100 and MMN) as well as BAEPs and MLAEPs. Specific methodological aspects are emphasized. Long-term monitoring data from one patient recorded in the recovery room after surgery are displayed, allowing an illustration of the techniques used and of the problems raised.

Effect of acetazolamide on ventilatory response in subjects with chronic mountain sickness

Although the effects of acetazolamide (ACZ) on ventilation during acute mountain sickness are well known, there are no studies assessing its effect on ventilatory response in chronic hypoxia. We studied this effect in patients with chronic mountain sickness (CMS). Subjects with CMS, living permanently at 4300 m, were assigned in a randomized, double-blind study to 250 mg/day (n=9) or to 500 mg/day (n=9) of ACZ. Resting end-tidal PET(O2) and end-tidal PET(CO2) were measured before and after 3 weeks of acetazolamide. Ventilatory responses were evaluated by the determination of sensitivity to hypoxia and to CO2. After treatment ventilatory response to hypoxia increased, resting PET(CO2) decreased, and ACZ caused a leftward shift in the position, but not a change in the slope of the ventilation (VE) versus PET(CO2) relationship. There were no differences between the two doses used. ACZ administration provides a beneficial effect on respiratory function of high altitude natives with CMS and thus it can be an effective therapy for the disease.

Auditory enhancement of visual perception at threshold depends on visual abilities.

Whether or not multisensory interactions can improve detection thresholds, and thus widen the range of perceptible events is a long-standing debate. Here we revisit this question, by testing the influence of auditory stimuli on visual detection threshold, in subjects exhibiting a wide range of visual-only performance. Above the perceptual threshold, crossmodal interactions have indeed been reported to depend on the subjects performance when the modalities are presented in isolation. We thus tested normal-seeing subjects and short-sighted subjects wearing their usual glasses. We used a paradigm limiting potential shortcomings of previous studies: we chose a criterion-free threshold measurement procedure and precluded exogenous cueing effects by systematically presenting a visual cue whenever a visual target (a faint Gabor patch) might occur. Using this carefully controlled procedure, we found that concurrent sounds only improved visual detection thresholds in the sub-group of subjects exhibiting the poorest performance in the visual-only conditions. In these subjects, for oblique orientations of the visual stimuli (but not for vertical or horizontal targets), the auditory improvement was still present when visual detection was already helped with flanking visual stimuli generating a collinear facilitation effect. These findings highlight that crossmodal interactions are most efficient to improve perceptual performance when an isolated modality is deficient.

Speech Auditory Brainstem Response through hearing aid stimulation.

Millions of people across the world are hearing impaired, and rely on hearing aids to improve their everyday life. Objective audiometry could optimize hearing aid fitting, and is of particular interest for non-communicative patients. Speech Auditory Brainstem Response (speech ABR), a fine electrophysiological marker of speech encoding, is presently seen as a promising candidate for implementing objective audiometry; yet, unlike lower-frequency auditory-evoked potentials (AEPs) such as cortical AEPs or auditory steady-state responses (ASSRs), aided-speech ABRs (i.e., speech ABRs through hearing aid stimulation) have almost never been recorded. This may be due to their high-frequency components requesting a high temporal precision of the stimulation. We assess here a new approach to record high-quality and artifact-free speech ABR while stimulating directly through hearing aids. In 4 normal-hearing adults, we recorded speech ABR evoked by a /ba/ syllable binaurally delivered through insert earphones for quality control or through hearing aids. To assess the presence of a potential stimulus artifact, recordings were also done in mute conditions with the exact same potential sources of stimulus artifacts as in the main runs. Hearing aid stimulation led to artifact-free speech ABR in each participant, with the same quality as when using insert earphones, as shown with signal-to-noise (SNR) measurements. Our new approach consisting in directly transmitting speech stimuli through hearing aids allowed for a perfect temporal precision mandatory in speech ABR recordings, and could thus constitute a decisive step in hearing impairment investigation and in hearing aid fitting improvement.

Non-verbal auditory cognition in patients with temporal epilepsy before and after anterior temporal lobectomy

For patients with pharmaco-resistant temporal epilepsy, unilateral anterior temporal lobectomy (ATL) – i.e. the surgical resection of the hippocampus, the amygdala, the temporal pole and the most anterior part of the temporal gyri – is an efficient treatment. There is growing evidence that anterior regions of the temporal lobe are involved in the integration and short-term memorization of object-related sound properties. However, non-verbal auditory processing in patients with temporal lobe epilepsy (TLE) has raised little attention. To assess non-verbal auditory cognition in patients with temporal epilepsy both before and after unilateral ATL, we developed a set of non-verbal auditory tests, including environmental sounds. We could evaluate auditory semantic identification, acoustic and object-related short-term memory, and sound extraction from a sound mixture. The performances of 26 TLE patients before and/or after ATL were compared to those of 18 healthy subjects. Patients before and after ATL were found to present with similar deficits in pitch retention, and in identification and short-term memorisation of environmental sounds, whereas not being impaired in basic acoustic processing compared to healthy subjects. It is most likely that the deficits observed before and after ATL are related to epileptic neuropathological processes. Therefore, in patients with drug-resistant TLE, ATL seems to significantly improve seizure control without producing additional auditory deficits.

Topographic recordings of auditory evoked potentials to speech: Subcortical and cortical responses

Topographies of speech auditory brainstem response (speech ABR), a fine electrophysiological marker of speech encoding, have never been described. Yet, they could provide useful information to assess speech ABR generators and better characterize populations of interest (e.g., musicians, dyslexics). We present here a novel methodology of topographic speech ABR recording, using a 32-channel low sampling rate (5 kHz) EEG system. Quality of speech ABRs obtained with this conventional multichannel EEG system were compared to that of signals simultaneously recorded with a high sampling rate (13.3 kHz) EEG system. Correlations between speech ABRs recorded with the two systems revealed highly similar signals, without any significant difference between their signal-to-noise ratios (SNRs). Moreover, an advanced denoising method for multichannel data (denoising source separation) significantly improved SNR and allowed topography of speech ABR to be recovered.

Boosting pitch encoding with audiovisual interactions in congenital amusia

The combination of information across senses can enhance perception, as revealed for example by decreased reaction times or improved stimulus detection. Interestingly, these facilitatory effects have been shown to be maximal when responses to unisensory modalities are weak. The present study investigated whether audiovisual facilitation can be observed in congenital amusia, a music-specific disorder primarily ascribed to impairments of pitch processing. Amusic individuals and their matched controls performed two tasks. In Task 1, they were required to detect auditory, visual, or audiovisual stimuli as rapidly as possible. In Task 2, they were required to detect as accurately and as rapidly as possible a pitch change within an otherwise monotonic 5-tone sequence that was presented either only auditorily (A condition), or simultaneously with a temporally congruent, but otherwise uninformative visual stimulus (AV condition). Results of Task 1 showed that amusics exhibit typical auditory and visual detection, and typical audiovisual integration capacities: both amusics and controls exhibited shorter response times for audiovisual stimuli than for either auditory stimuli or visual stimuli. Results of Task 2 revealed that both groups benefited from simultaneous uninformative visual stimuli to detect pitch changes: accuracy was higher and response times shorter in the AV condition than in the A condition. The audiovisual improvements of response times were observed for different pitch interval sizes depending on the group. These results suggest that both typical listeners and amusic individuals can benefit from multisensory integration to improve their pitch processing abilities and that this benefit varies as a function of task difficulty. These findings constitute the first step towards the perspective to exploit multisensory paradigms to reduce pitch-related deficits in congenital amusia, notably by suggesting that audiovisual paradigms are effective in an appropriate range of unimodal performance.

Verbal and musical short-term memory: Variety of auditory disorders after stroke.

HighlightsStroke patients have auditory verbal and musical short‐term memory deficits.Inferior fronto‐parietal operculum is involved in auditory short‐term memory.Cerebral networks for verbal and musical memory are to some extent distinct. Abstract Auditory cognitive deficits after stroke may concern language and/or music processing, resulting in aphasia and/or amusia. The aim of the present study was to assess the potential deficits of auditory short‐term memory for verbal and musical material after stroke and their underlying cerebral correlates with a Voxel‐based Lesion Symptom Mapping approach (VLSM). Patients with an ischemic stroke in the right (N = 10) or left (N = 10) middle cerebral artery territory and matched control participants (N = 14) were tested with a detailed neuropsychological assessment including global cognitive functions, music perception and language tasks. All participants then performed verbal and musical auditory short‐term memory (STM) tasks that were implemented in the same way for both materials. Participants had to indicate whether series of four words or four tones presented in pairs, were the same or different. To detect domain‐general STM deficits, they also had to perform a visual STM task. Behavioral results showed that patients had lower performance for the STM tasks in comparison with control participants, regardless of the material (words, tones, visual) and the lesion side. The individual patient data showed a double dissociation between some patients exhibiting verbal deficits without musical deficits or the reverse. Exploratory VLSM analyses suggested that dorsal pathways are involved in verbal (phonetic), musical (melodic), and visual STM, while the ventral auditory pathway is involved in musical STM.