Plasticité développementale dans le cortex auditif : La résultante de l’état de maturation cortical et des caractéristiques sonores de l’environnement

International audienceChez l’animal, la plasticité développementale du système auditif n’a été que sporadiquement étudiée jusqu’au début des années 2000. Depuis, plusieurs séries de travaux ont démontré l’existence d’une période critique néonatale pour le fonctionnement du cortex auditif et ont révélé les conséquences drastiques que peuvent avoir des environnements bruités au cours de cette période. Néanmoins, après la fin de cette période critique néonatale, la susceptibilité du système auditif aux environnements bruités reste encore élevée chez l’animal juvénile surtout si ces environnements présentent des caractéristiques acoustiques particulières qui ne sont pas présentes dans les milieux naturels

Robust Neuronal Discrimination in Primary Auditory Cortex Despite Degradations of Spectro-temporal Acoustic Details: Comparison Between Guinea Pigs with Normal Hearing and Mild Age-Related Hearing Loss

International audienceThis study investigated to which extent the primary auditory cortex of young normal-hearing and mild hearing-impaired aged animals is able to maintain invariant representation of critical temporal-modulation features when sounds are submitted to degradations of fine spectro-temporal acoustic details. This was achieved by recording ensemble of cortical responses to conspecific vocalizations in guinea pigs with either normal hearing or mild age-related sensorineural hearing loss. The vocalizations were degraded using a tone vocoder. The neuronal responses and their discrimination capacities (estimated by mutual information) were analyzed at single recording and population levels. For normal-hearing animals, the neuronal responses decreased as a function of the number of the vocoder frequency bands, so did their discriminative capacities at the single recording level. However, small neuronal populations were found to be robust to the degradations induced by the vocoder. Similar robustness was obtained when broadband noise was added to exacerbate further the spectro-temporal distortions produced by the vocoder. A comparable pattern of robustness to degradations in fine spectro-temporal details was found for hearing-impaired animals. However, the latter showed an overall decrease in neuronal discrimination capacities between vocalizations in noisy conditions. Consistent with previous studies, these results demonstrate that the primary auditory cortex maintains robust neural representation of temporal envelope features for communication sounds under a large range of spectro-temporal degradations

Age-related Changes in Auditory Cortex Without Detectable Peripheral Alterations: A Multi-level Study in Sprague–Dawley Rats

International audienceAging is often considered to affect both the peripheral (i.e. the cochlea) and central (brainstem and thalamus-cortex) auditory systems. We investigated the effects of aging on the cochlea, brainstem and cortex of female Sprague-Dawley rats. The auditory nerve threshold remained stable between the ages of nine and 21 months, as did distortion product otoa-coustic emissions and the number of ribbon synapses between inner hair cells and nerve fibers. The first clear signs of aging appeared in the brainstem, in which response amplitude decreased, with thresholds remaining stable until the age of 15 months, and increasing slightly thereafter. The responses of primary auditory cortex neurons revealed specific effects of aging: at 21 months, receptive fields were spectrally narrower and the temporal reliability of responses to communication sounds was lower. However, aging had a null or even positive effect on neuronal responses in the presence of background noise, responses to amplitude-modulated sounds, and responses in gap-detection protocols. Overall, inter-animal variability remained high relative to the variability across groups of different ages, for all parameters tested. Beha-vioral performance for the modulation depth of amplitude modulation noise was worse in 21-month old animals than in other animals. Age-related alterations of cortical and behavioral responses were thus observed in animals displaying no signs of aging at the peripheral level. These results suggest that intrinsic, central aging effects can affect the perception of acoustic stimuli independently of the effects of aging on peripheral receptors

Usefulness and pitfalls of MAA SPECT/CT in identifying digestive extrahepatic uptake when planning liver radioembolization

International audiencePURPOSE: Identifying gastroduodenal uptake of (99m)Tc-macroaggregated albumin (MAA), which is associated with an increased risk of ulcer disease, is a crucial part of the therapeutic management of patients undergoing radioembolization for liver tumours. Given this context, the use of MAA single photon emission computed tomography (SPECT)/CT may be essential, but the procedure has still not been thoroughly evaluated. The aim of this retrospective study was to determine the effectiveness of MAA SPECT/CT in identifying digestive extrahepatic uptake, while determining potential diagnostic pitfalls. METHODS: Overall, 139 MAA SPECT/CT scans were performed on 103 patients with different hepatic tumour types. Patients were followed up for at least 6 months according to standard requirements. RESULTS: Digestive, or digestive-like, uptake other than free pertechnetate was identified in 5.7% of cases using planar imaging and in 36.6% of cases using SPECT/CT. Uptake sites identified by SPECT/CT included the gastroduodenal region (3.6%), gall bladder (12.2%), portal vein thrombosis (6.5%), hepatic artery (6.5%), coil embolization site (2.1%) as well as falciform artery (5.0%). For 2.1% of explorations, a coregistration error between SPECT and CT imaging could have led to a false diagnosis by erroneously attributing an uptake site to the stomach or gall bladder, when the uptake actually occurred in the liver. CONCLUSION: SPECT/CT is more efficacious than planar imaging in identifying digestive extrahepatic uptake sites, with extrahepatic uptake observed in one third of scans using the former procedure. However, more than half of the uptake sites in our study were vascular in nature, without therapeutic implications. The risk of coregistration errors must also be kept in mind

The alpha 2-adrenergic antagonist idazoxan enhances the frequency selectivity and increases the threshold of auditory cortex neurons

International audienceIdazoxan (IDA), an alpha 2 antagonist of adrenoceptors, has been shown to increase cortical release of norepinephrine (NE) by an action mediated primarily by the alpha 2 autoreceptors located on the NE terminals. In the present experiment, IDA application was used to increase the cortial concentration of NE. Single unit activity (n = 107) was recorded in the rat auditory cortex, and the neurons' frequency receptive fields (FRF) were determined before and after systemic (intraperitoneal or intravenous) or local application of IDA. In the whole population (n = 107) there was a decrease in spontaneous activity and/or evoked activity for 84% of the recordings (90/107 cells). Decreased tone-evoked responses were obtained after systemic injections (n = 39), as well as after local applications (n = 68) of IDA. These effects were not observed after either systemic injections (n = 13) or local applications (n = 9) of saline. The signal-to-noise ratio (the mean evoked responses divided by the spontaneous activity) was slightly decreased after systemic injections and slightly increased after local applications. However, after both systemic and local injections the frequency selectivity of the neuronal responses was increased. For a group of neurons (n = 27), testing the FRF at three intensities indicated that this increased selectivity can be expressed at high or middle range intensity but not at low intensity. For 37 cells, the intensity function was tested at the best frequency before and after IDA application, and the threshold for excitatory responses was determined in 28 cases. An increased threshold was observed in 16 of 28 cases after IDA application. Thus, using a pharmacological procedure to increase the extracellular concentration of NE, the dominant inhibitory effect on the auditory cortex neurons led to an enhancement of the frequency selectivity, but also an increase in the threshold of these neurons

Beyond traditional approaches to understanding the functional role of neuromodulators in sensory cortices

Over the last two decades, a vast literature has described the influence of neuromodulatory systems on the responses of sensory cortex neurons (review in Gu 2002; Edeline 2003; Weinberger 2003; Metherate 2004, 2011). At the single cell level, facilitation of evoked responses, increases in signal-to-noise ratio, and improved functional properties of sensory cortex neurons have been reported in the visual, auditory and somatosensory modality. At the map level, massive cortical reorganizations have been described when repeated activation of a neuromodulatory system are associated with a particular sensory stimulus. In reviewing our knowledge concerning the way the noradrenergic and cholinergic system control sensory cortices, I will point out that the differences between the protocols used to reveal these effects most likely reflect different assumptions concerning the role of the neuromodulators. More importantly, a gap still exists between the descriptions of neuromodulatory effects and the concepts that are currently applied to decipher the neural code operating in sensory cortices. Key examples that bring this gap into focus are the concept of cell assemblies and the role played by the spike timing precision (i.e., by the temporal organization of spike trains at the millisecond time-scale) which are now recognized as essential in sensory physiology but are rarely considered in experiments describing the role of neuromodulators in sensory cortices. Thus, I will suggest that several lines of research, particularly in the field of computational neurosciences, should help us to go beyond traditional approaches and, ultimately, to understand how neuromodulators impact on the cortical mechanisms underlying our perceptual abilities

Etude comparee des modifications des activites cellulaires dans la voie auditive et dans l'hippocampe au cours de l'acquisition et de la retention d'un apprentissage

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

LE NOYAU RETICULAIRE THALAMIQUE (ROLE DANS LA GENESE DES OSCILLATIONS EVOQUEES THALAMO-CORTICALES CONTROLE DU CODAGE DES FREQUENCES ET DES INTENSITES DANS LE THALAMUS)

NOUS AVONS D'ABORD ETUDIE LES OSCILLATIONS EVOQUEES PAR UN STIMULUS ACOUSTIQUE DANS LA BOUCLE AUDITIVE THALAMO-CORTICALE. SUR DES RATS ANESTHESIES A L'URETHANE, CES OSCILLATIONS SONT SYNCHRONES PAR RAPPORT AU STIMULUS ET DANS LA GAMME DE FREQUENCES 5-14 HZ. L'INTERVALLE INTER-SOUS CONTROLE L'APPARITION DES OSCILLATIONS ET LA TEMPERATURE CONTROLE LEUR FREQUENCE ET LEUR DUREE. EN INACTIVANT CHAQUE STRUCTURE DE LA BOUCLE THALAMO-CORTICALE A L'AIDE D'UN AGONISTE GABA A, LE MUSCIMOL, NOUS AVONS MONTRE QUE CES OSCILLATIONS NECESSITENT A LA FOIS LA PARTICIPATION DU NOYAU RETICULAIRE THALAMIQUE (RE) ET DU THALAMUS AUDITIF, MAIS PAS CELLE DU CORTEX. NOUS AVONS EVALUE LA PRESENCE DE CES OSCILLATIONS SUR DES ANIMAUX VIGILES. EN VEILLE ET EN SOMMEIL PARADOXAL, AUCUNE OSCILLATION N'A ETE OBSERVEE. EN SOMMEIL A ONDES LENTES, NOUS N'AVONS PRATIQUEMENT OBSERVE QUE DES OSCILLATIONS ASYNCHRONES. LORSQUE LES MEMES ANIMAUX SONT SOUMIS A DES INJECTIONS DE VALIUM, DE NEMBUTAL OU D'URETHANE, ON OBSERVE A LA FOIS DES OSCILLATIONS SYNCHRONES ET ASYNCHRONES. DANS TOUS LES CAS, LES OSCILLATIONS SONT PRINCIPALEMENT DANS LA GAMME DES BASSES FREQUENCES (5-15 HZ). NOUS AVONS ENSUITE ETUDIE LE ROLE DU RE DANS LE CODAGE SENSORIEL. DES MODIFICATIONS SPATIALEMENT RESTREINTES DE L'ACTIVITE DU RE AUDITIF ONT ETE EFFECTUEES PAR DES INJECTIONS IONTOPHORETIQUES DE GABA OU DE GLUTAMATE. L'ANALYSE A PORTE SUR L'ENSEMBLE DES CELLULES THALAMIQUES ET SUR LES CELLULES LES PLUS SUSCEPTIBLES D'ETRE AFFECTEES : (I) LES CELLULES QUI PRESENTENT DES VARIATIONS D'ACTIVITE EVOQUEE OU SPONTANEE DANS LE SENS ATTENDU, (II) LES CELLULES SELECTIONNEES SELON LES EFFETS AU SITE D'INJECTION, ET SELON LE CHEVAUCHEMENT DE LEURS COURBES D'ACCORD AVEC CELLES DU RE. NOUS AVONS MONTRE QU'UNE ACTIVATION DU RE ENTRAINE UNE AUGMENTATION DE LA SELECTIVITE ET DU SEUIL DANS LE THALAMUS, UNE INHIBITION ENTRAINE UNE DIMINUTION DE LA SELECTIVITE ET DU SEUIL ACOUSTIQUE.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Age-related changes in the guinea pig auditory cortex : relationship with peripheral changes and comparison with tone-induced hearing loss.

International audienceElderly people often show degraded hearing performance and have difficulties in understanding speech, particularly in noisy environments. Although loss in peripheral hearing sensitivity is an important factor in explaining these low performances, central alterations also have an impact but their exact contributions remained unclear. In this study, we focus on the functional effects of aging on auditory cortex responses. Neuronal discharges and local field potentials were recorded in the auditory cortex of aged guinea pigs (> 3 years), and several parameters characterizing the processing of auditory information were quantified: the acoustic thresholds, response strength, latency and duration of the response, and breadth of tuning. Several of these parameters were also quantified from auditory brainstem responses collected from the same animals, and recordings obtained from a population of animals with trauma-induced hearing loss were also included in this study. The results showed that aging and acoustic trauma reduced the response strength at both brainstem and cortical levels, and increased the response latencies more at the cortical level than at the brainstem level. In addition to the brainstem hearing loss, aging induced a 'cortical hearing loss' as judged by additive changes in the threshold and frequency response seen in the cortex. It also increased the duration of neural responses and reduced the receptive field bandwidth, effects that were not found in traumatized animals. These effects substantiate the notion that presbycusis involves both peripheral hearing loss and biological aging in the central auditory system