Loud noise-exposure changes the firing frequency of subtypes of layer 5 pyramidal neurons and Martinotti cells in the mouse auditory cortex

Introduction

Loud noise-exposure can generate noise-induced tinnitus in both humans and animals. Imaging and in vivo studies show that noise exposure affects the auditory cortex; however, cellular mechanisms of tinnitus generation are unclear.

Methods

Here we compare membrane properties of layer 5 (L5) pyramidal cells (PCs) and Martinotti cells expressing the cholinergic receptor nicotinic alpha 2 subunit gene (Chrna2) of the primary auditory cortex (A1) from control and noise-exposed (4-18 kHz, 90 dB, 1.5 h, followed by 1.5 h silence) 5-8 week old mice. PCs were furthermore classified in type A or type B based on electrophysiological membrane properties, and a logistic regression model predicting that afterhyperpolarization (AHP) and afterdepolarization (ADP) are sufficient to predict cell type, and these features are preserved after noise trauma.

Results

One week after a loud noise-exposure no passive membrane properties of type A or B PCs were altered but principal component analysis showed greater separation between type A PCs from control and noise-exposed mice. When comparing individual firing properties, noise exposure differentially affected type A and B PC firing frequency in response to depolarizing current steps. Specifically, type A PCs decreased initial firing frequency in response to +200 pA steps (p = 0.020) as well as decreased steady state firing frequency (p = 0.050) while type B PCs, on the contrary, significantly increased steady state firing frequency (p = 0.048) in response to a + 150 pA step 1 week after noise exposure. In addition, L5 Martinotti cells showed a more hyperpolarized resting membrane potential (p = 0.04), higher rheobase (p = 0.008) and an increased initial (p = 8.5 × 10^-5) and steady state firing frequency (p = 6.3 × 10^-5) in slices from noise-exposed mice compared to control.

Discussion

These results show that loud noise can cause distinct effects on type A and B L5 PCs and inhibitory Martinotti cells of the primary auditory cortex 1 week following noise exposure. As the L5 comprises PCs that send feedback to other areas, loud noise exposure appears to alter levels of activity of the descending and contralateral auditory system.

Intrinsic Noise Improves Speech Recognition in a Computational Model of the Auditory Pathway

Noise is generally considered to harm information processing performance. However, in the context of stochastic resonance, noise has been shown to improve signal detection of weak sub- threshold signals, and it has been proposed that the brain might actively exploit this phenomenon. Especially within the auditory system, recent studies suggest that intrinsic noise plays a key role in signal processing and might even correspond to increased spontaneous neuronal firing rates observed in early processing stages of the auditory brain stem and cortex after hearing loss. Here we present a computational model of the auditory pathway based on a deep neural network, trained on speech recognition. We simulate different levels of hearing loss and investigate the effect of intrinsic noise. Remarkably, speech recognition after hearing loss actually improves with additional intrinsic noise. This surprising result indicates that intrinsic noise might not only play a crucial role in human auditory processing, but might even be beneficial for contemporary machine learning approaches.

Tinnitus: still 'A Ghost in the Machine' or a Darwinian survival phenomenon?

INTRODUCTION

The causation of tinnitus continues to intrigue. Despite the plethora of publications there is no definitive path available to concentrate our efforts, in alleviating the symptom. Several mechanical theories are available in standard tinnitus literature with varying empiricism. Purpose/aim of the study: To investigate a possible way forward.

MATERIALS AND METHODS

Employing a forensic methodology ("crime scene analysis" technique) and utilizing available evidence from the related sciences, inductive and abstract reasoning, a pragmatic model incorporating the known features of tinnitus is available.

RESULTS

A plausible evolutionary explanation of the origins and functions for the causation of tinnitus is offered with a possible link to its evasive nature, in our search for a cause.

CONCLUSION

The functional value of tinnitus may be provided by our evolutionary history. It is possible that tinnitus was a protective adaptive phenomenon in earlier forms but in our current environment merely contributes to nuisance value.

Chronic Cervicogenic Tinnitus Rapidly Resolved by Intermittent Use of Cervical Collar

INTRODUCTION

Cervicogenic tinnitus is not a generally accepted pathogenetic subtype, which might be subsumed under the concept of somatosensory tinnitus. After the personal experience of therapy-resistant tinnitus in context with a cervical pain syndrome (CPS) and successful add-on treatment with cervical collar (CC), the idea was pursued in several individual treatments in patients.

PATIENTS AND METHODS

Reporting one particular case with chronic tinnitus, considered untreatable, that rapidly improved with exclusive treatment by CC use. Thereafter, tinnitus was experimentally replicated by head inclination, the respective neck-head angles, and cerebral blood flow was measured.

RESULTS

Chronic subjective tinnitus of a 20 years duration completely disappeared within 4 weeks with an intermittent short time application of CC. Thereafter, tinnitus was deliberately again induced by head inclination, set on with anterior tilt of 14°, reaching maximum strength by 23°. Tinnitus stopped with return to neutral head position. Blood flow in the vertebral arteries on both sides was unchanged during head inclination with prevalent tinnitus; however, blood flow was physiologically reduced with head rotation though not accompanied by tinnitus.

DISCUSSION

In a single case of chronic tinnitus, we found that treatment with CC rapidly led to full remission. Blood flow reduction in vertebral arteries was unrelated to tinnitus. However, tinnitus could be resumed by constrained head postures. Experimental tinnitus replication (by inclination) points to an underscored role of upper posterior cervical muscle groups, matching with animal experiments, also in concert with other triggers including psychological factors.

Increases in Spontaneous Activity in the Dorsal Cochlear Nucleus Following Exposure to High Intensity Sound: A Possible Neural Correlate of Tinnitus

The purpose of this study was to test the effects of intense tone exposure on the spontaneous activity of multiunit clusters in the mammalian dorsal cochlear nucleus (DCN). Adult hamsters (60-101 days of age) were exposed to a 10 kHz tone at levels between 125 and 130 dB SPL for a period of 4 hours. The effects of tone exposure were studied following a recovery period of 30-58 days and were quantified by measuring the spontaneous rates, response thresholds and frequency tuning properties of neural clusters at the surface of the DCN. Measures were performed at each of 10-15 sites along the tonotopic axis of the DCN. The effects of the tone exposure were examined by comparison with identical measures obtained from normal unexposed animals. Results indicate that tone exposure induced major chronic increases in the spontaneous activity of the DCN. Such increases were broadly distributed across the tonotopic range of the DCN and were generally found in tonotopic map areas characterized by tone-induced elevations of neural thresholds. Mean spontaneous rate reached its maximum value at or close to the tonotopic locus which normally represents the frequency of the exposure tone. The increased activity induced by tone exposure resembled the heightened activity in normal animals during presentation of a moderate level continuous tone. These changes in spontaneous activity indicate that central auditory neurons are in a state of elevated activity for extended periods following intense sound exposure and suggest that the affected neurons may signal the presence of acoustic stimulation even though such stimulation is not present. Possible mechanisms of these changes and their relation to the clinical problem of tinnitus are discussed.