Processing of auditory information in forebrain regions after hearing loss in adulthood: Behavioral and electrophysiological studies in a rat model

Slight and hidden hearing loss differentially affect short- and long-term memory in young rats

Mild forms of hearing loss (HL) have been linked to cognitive impairments in children, yet the neurobiological mechanisms underlying this connection remain unclear. Existing research using animal models mostly focuses on more severe levels of HL or investigates only limited aspects of cognition. To gain a broader understanding of how slight/hidden HL affects cognitive behaviors, we induced HL in 4-week-old Wistar rats through noise exposure. Auditory brainstem response measurements confirmed slight and hidden HL, but this auditory impairment did not alter the density of inner hair cells or their synapses with the spiral ganglion (primary auditory) neurons. Both short- and long- term memory formation were tested using the object location, novel object recognition, and social recognition task. Behaviorally, rats with slight/hidden HL performed better than normal hearing (NH) rats during short-term cognition tests. However, long-term memory was impaired in rats with slight/hidden HL when compared to NH controls. Slight/hidden HL also did not consistently affect (social) exploration. In conclusion, this study demonstrates that slight and hidden HL differentially affect short- and long-term cognitive processes in an animal model of early (noise-induced) HL, without affecting (social) exploration. These results suggest a nuanced relationship between slight and hidden HL and both short- and long-term memory formation, underscoring the importance of broader cognitive phenotyping and further investigation into the neurobiological structures linking hearing impairment with cognitive function.

Auditory working memory in noise in cochlear implant users: Insights from behavioural and neuronal measures

OBJECTIVE

We investigated auditory working-memory using behavioural measures and electroencephalography (EEG) in adult Cochlear Implant (CI) users with varying degrees of CI performance.

METHODS

24 adult CI listeners (age: M = 61.38, SD = 12.45) performed the Sternberg auditory-digit-in-working-memory task during which EEG, accuracy, and promptness were captured. Participants were presented with 2, 4, or 6 digits at Signal-to-Noise Ratios (SNR) of 0, +5 and +10dB. They had to identify a probe stimulus as present in the preceding sequence. ANOVA models were used to compare conditions.

RESULTS

ANOVA revealed that increasing memory load (ML) led to decreased task performance and CI performance interacted with ML and SNR. Centro-parietal alpha power increased during memory encoding but did not differ between conditions. Frontal alpha power was positively correlated with accuracy in conditions most affected by SNR (r = 0.57, r = 0.52) and theta power in conditions most affected by ML (r = 0.55, r = 0.57).

CONCLUSIONS

While parietal alpha power is modulated by the task, it is frontal alpha that relates quantitatively to sensory aspects of processing (noise) and frontal theta to memory load in this group of CI listeners.

SIGNIFICANCE

These results suggest that alpha and theta show distinct relationships to behaviour, providing additional insight into neurocognitive (auditory working-memory) processes in CI users.

Exploring the cognitive effects of hearing loss in adult rats: Implications for visuospatial attention, social behavior, and prefrontal neural activity

Age-related hearing loss in humans has been associated with cognitive decline, though the underlying mechanisms remain unknown. We investigated the long-term effects of hearing loss on attention, impulse control, social interaction, and neural activity within medial prefrontal cortex (mPFC) subregions. Hearing loss was induced in adult rats via intracochlear neomycin injection (n = 13), with non-operated rats as controls (n = 10). Rats were tested for motor activity (open field), coordination (Rotarod), and social interaction (including ultrasonic vocalization, USV) before surgery and at weeks 1, 2, 4, 8, 16, and 24 post-surgery. From week 8 on, rats were trained in the five-choice serial reaction time task (5-CSRTT) to assess visuospatial attention and impulse control. Finally, oscillatory neuronal activity in mPFC subregions was recorded with multielectrode arrays during anesthesia, followed by immunohistological staining for NeuN+ and Parvalbumin+ cells. Deafened rats were more active than controls, whereas social interaction and USV were temporarily reduced. They also had difficulties to learn the concept of the 5-CSRTT paradigm and made more incorrect responses. Electrophysiology showed decreased power in theta, alpha, and beta frequency, and enhanced high gamma band in the mPFC in deafened rats, which was most pronounced in the cingulate subregion (Cg1). The number of NeuN+ and Parvalbumin+ cells, however, did not differ between groups. The behavioral deficits together with the altered neuronal activity found in the Cg1 subregion of the mPFC in adult deafened rats may be used as an endophenotype to elucidate the mechanisms behind the cognitive decline seen in older patients with hearing loss.

Conductive hearing loss does not affect spatial learning and memory in middle-aged guinea pigs

Hearing loss (HL) in mid-life has been suggested as a risk factor for cognitive decline. It is unclear whether this relationship is due to deprivation of auditory input alone, degenerative processes, or a combination. Animal models are useful to investigate underlying neural mechanisms as human studies can be confounded by various factors. However, most animal studies use young animals and often exclude females. We used middle-aged guinea pigs of both sexes to investigate whether 8 weeks of auditory deprivation due to conductive HL caused spatial learning and memory impairments. Forty guinea pigs (20 M, 20 F, ~ 12 months) were tested in the Morris Water Maze (MWM) to assess baseline spatial learning and memory. In 20 of these animals (10 M, 10 F) the ear canal was plugged and 8 weeks later, animals were again assessed in MWM. No deficits in spatial learning or memory were observed in either sex. HL caused a small decline in body weight suggesting some stress associated with conductive HL, although adrenal weight, corrected for body weight, did not change. Our data suggest that auditory input deprivation alone does not affect spatial cognition in middle-age, in line with recent human data suggesting that additional risk factors need to be present.

Age-dependent deficits of auditory brainstem responses in juvenile Neurexin1α knockout rats

Abnormal sensory processing is core to neuropsychiatric and neurodevelopmental disorders, such as schizophrenia and autism spectrum disorders. Developing efficient therapies requires understanding the basic sensory pathways and identifying circuit abnormalities during early development. Auditory brainstem responses (ABRs) are well-established biomarkers for auditory processing on the brainstem level. Beyond their advantage of being easily applicable in clinics (given their non-invasive nature), ABRs have high reproducibility in rodents and translate well to humans (e.g. wave identity), despite species differences (e.g. wave features). We hypothesized that ABRs would reveal sensory abnormalities in neurodevelopmental models with construct validity, such as Neurexin1α knockout (Nrxn1α KO) rats during their development. In a previous study, adult Nrxn1α KO rats showed altered cortical auditory-evoked potentials and impaired prediction error to auditory stimuli (Janz in Transl Psychiat, 12:455, 2022 ). This study used ABR measurements to assess brainstem physiology during auditory processing in Nrxn1α KO rats and their wild-type littermates. Therefore, we followed the development trajectories of ABRs from the age of 3 weeks to 12 weeks longitudinally. We found that juvenile Nrxn1α KO rats (3 weeks of age) show altered ABRs, which normalized during further development. This alteration was confined to increased latency in waves II, III, and IV of the ABRs, suggesting impaired auditory processing on the level of the superior olivary complex and inferior colliculus. In conclusion, our results suggest that early but transient deficits in the processing of auditory information on the level of the brainstem are present in Nrxn1α KO rats, which may contribute to later cortical auditory processing deficits observed in adulthood. Our study emphasizes the value of ABRs as a functional readout of auditory brainstem circuit function with potential value as a translational biomarker.

Alpha and theta oscillations on a visual strategic processing task in age-related hearing loss

Introduction

Emerging evidence suggests changes in several cognitive control processes in individuals with age-related hearing loss (ARHL). However, value-directed strategic processing, which involves selectively processing salient information based on high value, has been relatively unexplored in ARHL. Our previous work has shown behavioral changes in strategic processing in individuals with ARHL. The current study examined event-related alpha and theta oscillations linked to a visual, value-directed strategic processing task in 19 individuals with mild untreated ARHL and 17 normal hearing controls of comparable age and education.

Methods

Five unique word lists were presented where words were assigned high- or low-value based on the letter case, and electroencephalography (EEG) data was recorded during task performance.

Results

The main effect of the group was observed in early time periods. Specifically, greater theta synchronization was seen in the ARHL group relative to the control group. Interaction between group and value was observed at later time points, with greater theta synchronization for high- versus low-value information in those with ARHL.

Discussion

Our findings provide evidence for oscillatory changes tied to a visual task of value-directed strategic processing in individuals with mild untreated ARHL. This points towards modality-independent neurophysiological changes in cognitive control in individuals with mild degrees of ARHL and adds to the rapidly growing literature on the cognitive consequences of ARHL.

Hearing loss in juvenile rats leads to excessive play fighting and hyperactivity, mild cognitive deficits and altered neuronal activity in the prefrontal cortex

Background

In children, hearing loss has been associated with hyperactivity, disturbed social interaction, and risk of cognitive disturbances. Mechanistic explanations of these relations sometimes involve language. To investigate the effect of hearing loss on behavioral deficits in the absence of language, we tested the impact of hearing loss in juvenile rats on motor, social, and cognitive behavior and on physiology of prefrontal cortex.

Methods

Hearing loss was induced in juvenile (postnatal day 14) male Sprague-Dawley rats by intracochlear injection of neomycin under general anesthesia. Sham-operated and non-operated hearing rats served as controls. One week after surgery auditory brainstem response (ABR) measurements verified hearing loss or intact hearing in sham-operated and non-operated controls. All rats were then tested for locomotor activity (open field), coordination (Rotarod), and for social interaction during development in weeks 1, 2, 4, 8, 16, and 24 after surgery. From week 8 on, rats were trained and tested for spatial learning and memory (4-arm baited 8-arm radial maze test). In a final setting, neuronal activity was recorded in the medial prefrontal cortex (mPFC).

Results

In the open field deafened rats moved faster and covered more distance than sham-operated and non-operated controls from week 8 on (both p < 0.05). Deafened rats showed significantly more play fighting during development (p < 0.05), whereas other aspects of social interaction, such as following, were not affected. Learning of the radial maze test was not impaired in deafened rats (p > 0.05), but rats used less next-arm entries than other groups indicating impaired concept learning (p < 0.05). In the mPFC neuronal firing rate was reduced and enhanced irregular firing was observed. Moreover, oscillatory activity was altered, both within the mPFC and in coherence of mPFC with the somatosensory cortex (p < 0.05).

Conclusions

Hearing loss in juvenile rats leads to hyperactive behavior and pronounced play-fighting during development, suggesting a causal relationship between hearing loss and cognitive development. Altered neuronal activities in the mPFC after hearing loss support such effects on neuronal networks outside the central auditory system. This animal model provides evidence of developmental consequences of juvenile hearing loss on prefrontal cortex in absence of language as potential confounding factor.

Adult mice with noise-induced hearing loss exhibited temporal ordering memory deficits accompanied by microglia-associated neuroplastic changes in the medial prefrontal cortex

Acquired peripheral hearing loss in midlife is considered the primary modifiable risk factor for dementia, while the underlying pathological mechanism remains poorly understood. Excessive noise exposure is the most common cause of acquired peripheral hearing loss in modern society. This study was designed to investigate the impact of noise-induced hearing loss (NIHL) on cognition, with a focus on the medial prefrontal cortex (mPFC), a brain region that is involved in both auditory and cognitive processes and is highly affected in patients with cognitive impairment. Adult C57BL/6 J mice were randomly assigned to a control group and seven noise groups: 0HPN, 12HPN, 1DPN, 3DPN, 7DPN, 14DPN, and 28DPN, which were exposed to broadband noise at a 123 dB sound pressure level (SPL) for 2 h and sacrificed immediately (0 h), 12 h, or 1, 3, 7, 14, or 28 days post-noise exposure (HPN, DPN), respectively. Hearing assessment, behavioral tests, and neuromorphological studies in the mPFC were performed in control and 28DPN mice. All experimental animals were included in the time-course analysis of serum corticosterone (CORT) levels and mPFC microglial morphology. The results illustrated that noise exposure induced early-onset transient serum CORT elevation and permanent moderate-to-severe hearing loss in mice. 28DPN mice, in which permanent NIHL has been verified, exhibited impaired performance in temporal order object recognition tasks concomitant with reduced structural complexity of mPFC pyramidal neurons. The time-course immunohistochemical analysis in the mPFC revealed significantly higher morphological microglial activation at 14 and 28 DPN, preceded by a remarkably higher amount of microglial engulfed postsynaptic marker PSD95 at 7 DPN. Additionally, lipid accumulation in microglia was observed in 7DPN, 14DPN and 28DPN mice, suggesting a driving role of lipid handling deficits following excessive phagocytosis of synaptic elements in delayed and sustained microglial abnormalities. These findings provide fundamentally novel information concerning mPFC-related cognitive impairment in mice with NIHL and empirical evidence suggesting the involvement of microglial malfunction in the mPFC neurodegenerative consequences of NIHL.