Processing Of Acoustic Stimuli Research Articles

Exposure to 1800\xa0MHz LTE electromagnetic fields under proinflammatory conditions decreases the response strength and increases the acoustic threshold of auditory cortical neurons

Increased needs for mobile phone communications have raised successive generations (G) of wireless technologies, which could differentially affect biological systems. To test this, we exposed rats to single head-only exposure of a 4G long-term evolution (LTE)-1800 MHz electromagnetic field (EMF) for 2 h. We then assessed the impact on microglial space coverage and electrophysiological neuronal activity in the primary auditory cortex (ACx), under acute neuroinflammation induced by lipopolysaccharide. The mean specific absorption rate in the ACx was 0.5 W/kg. Multiunit recording revealed that LTE-EMF triggered reduction in the response strength to pure tones and to natural vocalizations, together with an increase in acoustic threshold in the low and medium frequencies. Iba1 immunohistochemistry showed no change in the area covered by microglia cell bodies and processes. In healthy rats, the same LTE-exposure induced no change in response strength and acoustic threshold. Our data indicate that acute neuroinflammation sensitizes neuronal responses to LTE-EMF, which leads to an altered processing of acoustic stimuli in the ACx.

Change in gene expression levels of GABA, glutamate and neurosteroid pathways due to acoustic trauma in the cochlea

The characteristic feature of noise-induced hearing loss (NIHL) is the loss or malfunction of the outer hair cells (OHC) and the inner hair cells (IHC) of the cochlea. 90–95% of the spiral ganglion neurons, forming the cell bodies of cochlear nerve, synapse with the IHCs. Glutamate is the most potent excitatory neurotransmitter for IHC-auditory nerve synapses. Excessive release of glutamate in response to acoustic trauma (AT), may cause excitotoxicity by causing damage to the spiral ganglion neurons (SGN) or loss of the spiral ganglion dendrites, post-synaptic to the IHCs. Another neurotransmitter, GABA, plays an important role in the processing of acoustic stimuli and central regulation after peripheral injury, so it is potentially related to the regulation of hearing function and sensitivity after noise. The aim of this study is to evaluate the effect of AT on the expressions of glutamate excitotoxicity, GABA inhibition and neurosteroid synthesis genes. We exposed 24 BALB/c mice to AT. Controls were sacrificed without exposure to noise, Post-AT(1) and Post-AT(15) were sacrificed on the 1st and 15th day, respectively, after noise exposure. The expressions of various genes playing roles in glutamate, GABA and neurosteroid pathways were compared between groups by real-time PCR. Expressions of Cyp11a1, Gls, Gabra1, Grin2b, Sult1a1, Gad1, and Slc1a2 genes in Post-AT(15) mice were significantly decreased in comparison to control and Post-AT(1) mice. No significant differences in the expression of Slc6a1 and Slc17a8 genes was detected. These findings support the possible role of balance between glutamate excitotoxicity and GABA inhibition is disturbed during the post AT days and also the synthesis of some neurosteroids such as pregnenolone sulfate may be important in this balance.

What studies of audio-visual integration do not teach us about audio-visual integration

Auditory perception depends on more than just the processing of acoustic stimuli. Visual stimuli can also have a profound influence on listening. Salient examples of such effects include spatial ventriloquism—in which the location percept of an auditory stimulus is “captured” by that of a simultaneous visual stimulus—as well as drastically improved understanding of speech in noise when the talker's face is visible to the listener. These phenomena are typically described as “audio-visual integration,” and are often well modeled, as in the case of the ventriloquist effect, by ideal Bayesian causal inference. However, there is an over-reliance in these studies on single pairs of stimuli (i.e., one auditory and one visual stimulus) and the nature in which cross-modal discrepancies are resolved. This talk will first discuss two problems resulting from that: first, there is ambiguity about whether the integration occurs from weighing two independent sensory estimates or a single bound percept, and second, the design is less useful for studying integration when the stimuli are congruent. The talk will then describe recent work from our lab focused on new designs using multiple stimuli in an attempt to alleviate these issues and inform better models of integration.

Language related differences of the sustained response evoked by natural speech sounds.

In tonal languages, such as Mandarin Chinese, the pitch contour of vowels discriminates lexical meaning, which is not the case in non-tonal languages such as German. Recent data provide evidence that pitch processing is influenced by language experience. However, there are still many open questions concerning the representation of such phonological and language-related differences at the level of the auditory cortex (AC). Using magnetoencephalography (MEG), we recorded transient and sustained auditory evoked fields (AEF) in native Chinese and German speakers to investigate language related phonological and semantic aspects in the processing of acoustic stimuli. AEF were elicited by spoken meaningful and meaningless syllables, by vowels, and by a French horn tone. Speech sounds were recorded from a native speaker and showed frequency-modulations according to the pitch-contours of Mandarin. The sustained field (SF) evoked by natural speech signals was significantly larger for Chinese than for German listeners. In contrast, the SF elicited by a horn tone was not significantly different between groups. Furthermore, the SF of Chinese subjects was larger when evoked by meaningful syllables compared to meaningless ones, but there was no significant difference regarding whether vowels were part of the Chinese phonological system or not. Moreover, the N100m gave subtle but clear evidence that for Chinese listeners other factors than purely physical properties play a role in processing meaningful signals. These findings show that the N100 and the SF generated in Heschl’s gyrus are influenced by language experience, which suggests that AC activity related to specific pitch contours of vowels is influenced in a top-down fashion by higher, language related areas. Such interactions are in line with anatomical findings and neuroimaging data, as well as with the dual-stream model of language of Hickok and Poeppel that highlights the close and reciprocal interaction between superior temporal gyrus and sulcus.

Auditory verbal hallucinations and the interhemispheric auditory pathway in chronic schizophrenia

Objectives. The interhemispheric auditory pathway has been shown to play a crucial role in the processing of acoustic stimuli, and alterations of structural and functional connectivity between bilateral auditory areas are likely relevant to the pathogenesis of auditory verbal hallucinations (AVHs). The aim of this study was to examine this pathway in patients with chronic schizophrenia regarding their lifetime history of AVHs. Methods. DTI scans were acquired from 33 healthy controls (HC), 24 schizophrenia patients with a history of AVHs (LT-AVH) and nine schizophrenia patients without any lifetime hallucinations (N-LT-AVH). The interhemispheric auditory fibre bundles were extracted using streamline tractography. Subsequently, diffusivity indices, namely Fractional Anisotropy (FA), Trace, Mode, Axial and Radial diffusivity, were calculated. Results. FA was decreased over the entire pathway in LT-AVH compared with N-LT-AVH. Moreover, LT-AVH displayed decreased FA and Mode as well as increased radial diffusivity in the midsagittal section of the fibre tract. Conclusions. These findings indicate complex microstructural changes in the interhemispheric auditory pathway of schizophrenia patients with a history of AVHs. Alterations appear to be absent in patients who have never hallucinated.

Changes on auditory physiology in response to the inactivation of amygdala nuclei in high anxiety rats expressing learned fear

The inferior colliculus (IC) is primarily involved in the processing of acoustic stimuli, including those emitted by prey and predators. The role of the central nucleus of the IC (CIC) in fear and anxiety has been suggested based on electrophysiological, behavioral and immunohistochemical studies. The reactivity of high-anxiety rats (HA) to diverse challenges is different from low-anxiety ones (LA). In humans and laboratory animals, pathological anxiety is often accompanied by heightened vigilance and alertness, hyperactivity of the amygdala (AM), and increased amplitude of the auditory evoked potentials (AEP) from the IC. This study aims to evaluate the influence of the inactivation of the central (CEA) and basolateral (BLA) nuclei of the amygdala, after local infusions of the full GABAA agonist muscimol (1nmol/0.2μl), on the AEP elicited in the CIC of rats tested under a learned fear state. Our results showed that both BLA and CEA inactivation change the expression of conditioned fear, in a paradigm using the context as the conditioned stimulus (CS). These changes are correlated to the innate anxiety levels of the animals. It is supposed that this shortcoming is in addition to the imbalance between the regulatory role of the top-down and bottom-up processes in the control of anxiety.

Enhanced representation of spectral contrasts in the primary auditory cortex

The role of early auditory processing may be to extract some elementary features from an acoustic mixture in order to organize the auditory scene. To accomplish this task, the central auditory system may rely on the fact that sensory objects are often composed of spectral edges, i.e., regions where the stimulus energy changes abruptly over frequency. The processing of acoustic stimuli may benefit from a mechanism enhancing the internal representation of spectral edges. While the visual system is thought to rely heavily on this mechanism (enhancing spatial edges), it is still unclear whether a related process plays a significant role in audition. We investigated the cortical representation of spectral edges, using acoustic stimuli composed of multi-tone pips whose time-averaged spectral envelope contained suppressed or enhanced regions. Importantly, the stimuli were designed such that neural responses properties could be assessed as a function of stimulus frequency during stimulus presentation. Our results suggest that the representation of acoustic spectral edges is enhanced in the auditory cortex, and that this enhancement is sensitive to the characteristics of the spectral contrast profile, such as depth, sharpness and width. Spectral edges are maximally enhanced for sharp contrast and large depth. Cortical activity was also suppressed at frequencies within the suppressed region. To note, the suppression of firing was larger at frequencies nearby the lower edge of the suppressed region than at the upper edge. Overall, the present study gives critical insights into the processing of spectral contrasts in the auditory system.

GABAergic regulation of auditory sensory gating in low- and high-anxiety rats submitted to a fear conditioning procedure

The inferior colliculus (IC) is primarily involved in the processing of acoustic stimuli, being in a position to send auditory information to motor centers that participate in behaviors such as prey catching and predators' avoidance. The role of the central nucleus of the IC (CIC) on fear and anxiety has been suggested on the basis that rats are able to engage in tasks to decrease the aversiveness of CIC stimulation, increased Fos immunolabeling during diverse aversive states and increased CIC auditory evoked potentials (AEP) induced by conditioned fear stimuli. Additionally, it was shown that brainstem AEP, represented by wave V, for which the main generator is the IC, is increased during experimentally-induced anxiety. Rats segregated according to their low or high emotional reactivity have been used as an important tool in the study of fear and anxiety. The IC contains a high density of GABA receptors. Since the efficacy of an anxiolytic compound is a function of the animal's anxiety level, it is possible that GABA-benzodiazepine (Bzp) agents affect LA and HA animals differently. In this study we investigated the GABA-Bzp influence on the modulation of AEP in rats with low- (LA) or high-anxiety (HA) levels, as assessed by the elevated plus-maze test (EPM). GABA-Bzp modulation on the unconditioned AEP response was analyzed by using intra-CIC injections (0.2 μl) of the GABA-Bzp agonists muscimol (121 ng) and diazepam (30 μg), or the GABA inhibitors bicuculline (10 ng) and semicarbazide (7 μg). In a second experiment, we evaluate the effects of contextual aversive conditioning on AEP using foot-shocks as unconditioned stimuli. On the unconditioned fear paradigm GABA inhibition increased AEP in LA rats and decreases this measure in HA counterparts. Muscimol was effective in reducing AEP in both LA and HA rats. Contextual fear stimuli increased the magnitude of AEP. In spite of no effect obtained with diazepam in LA rats the drug inhibited AEP in HA animals. The specificity of the regulatory mechanisms mediated by GABA-Bzp for the ascending neurocircuits responsible for the acquisition of aversive information in LA and HA animals shed light on the processing of sensory information underlying the generation of defensive reactions.

Habituation of the Auditory Startle Response in Cervical Dystonia and Parkinson’s Disease

The auditory startle response (ASR) is a brainstem reflex elicited by an unexpected acoustic stimulus. In focal dystonia (FD), the excitability of brainstem neurons is abnormally enhanced. To identify a possible impact of this pathology on the processing of acoustic stimuli, we studied the habituation of the ASR in patients (n = 11) with FD and compared the findings to those of patients with Parkinson’s disease (PD; n = 11) and controls (n = 11). Latencies in FD patients did not differ from those of controls but were delayed in PD patients compared to controls (p < 0.0001). Habituation was normal at the orbicularis oculi muscles but reduced at the sternocleidomastoid muscles in FD (p = 0.005). Habituation in PD was comparable to controls. Normal latencies and sequence activation indicate intact neural pathways mediating the ASR in FD. Impaired habituation of the ASR points towards a reduced inhibition of acoustic stimuli in FD.

Processing of acoustic stimuli during NREM sleep: Results from a simultaneous fMRI and EEG study

Processing of acoustic stimuli during NREM sleep: Results from a simultaneous fMRI and EEG study