Functional Age-Related Changes Within the Human Auditory System Studied by Audiometric Examination

Abstract

Age related hearing loss (presbycusis) is one of the most common sensory deficits in the aging population. The main subjective ailment in the elderly is the deterioration of speech understanding, especially in a noisy environment, which cannot solely be explained by increased hearing thresholds. The examination methods used in presbycusis are primarily focused on the peripheral pathologies (e.g., hearing sensitivity measured by hearing thresholds), with only a limited capacity to detect the central lesion. In our study, auditory tests focused on central auditory abilities were used in addition to classical examination tests, with the aim to compare auditory abilities between an elderly group (elderly, mean age 70.4 years) and young controls (young, mean age 24.4 years) with clinically normal auditory thresholds, and to clarify the interactions between peripheral and central auditory impairments. Despite the fact that the elderly were selected to show natural age-related deterioration of hearing (auditory thresholds did not exceed 20 dB HL for main speech frequencies) and with clinically normal speech reception thresholds (SRTs), the detailed examination of their auditory functions revealed deteriorated processing of temporal parameters [gap detection threshold (GDT), interaural time difference (ITD) detection] which was partially responsible for the altered perception of distorted speech (speech in babble noise, gated speech). An analysis of interactions between peripheral and central auditory abilities, showed a stronger influence of peripheral function than temporal processing ability on speech perception in silence in the elderly with normal cognitive function. However, in a more natural environment mimicked by the addition of background noise, the role of temporal processing increased rapidly.

Keywords: central hearing loss; cognition; laterogram; presbycusis; temporal processing.

Figure 1

Example of a laterogram plot in 3D space [interaural time difference (ITD) × interaural level difference (ILD) × response value] evaluated by means of non-linear surface fitting.

Figure 2

Average audiograms of elderly and young, with the broken line expressing the normalized values of elderly according to correction from Jilek et al. (2014) suggesting physiologic auditory thresholds in our elderly group.

Figure 3

All graphs show the comparison of different threshold-related values used in audiometry, showing significant differences between young and elderly. (A) Comparison of pure tone average (PTAV; 0.5, 1, 2 kHz). (B) Comparison of hearing loss according to Fowler correction. (C) Comparison of weighted averaged thresholds computed over the whole frequency range (***p < 0.001).

Figure 4

All graphs show the comparison of different threshold-related values. (A) Comparison of click threshold. Comparison of parameters of exponential fits of thresholds for 1 kHz tones depending on their duration expressed as exponential parameter A (B) and exponential parameter k (C; ***p < 0.001).

Figure 5

Speech perception tests show significant differences between young and elderly. (A) Speech reception thresholds (SRTs). (B) Speech-to-noise ratio at which participants reach 50% recognition score (negative SNR values mean higher noise level). (C) Proportion of signal in the gated speech needed for 50% recognition score (***p < 0.001).

Figure 6

An example of a laterogram of one subject. (A) Averaged laterograms of young controls and elderly group (B) showing a shallower slope of the trading function in the elderly.

Figure 7

Several parameters of the laterogram suggest significant differences in extracting binaural auditory information at subcortical levels. (A) Slope of the laterogram compares the ability to trade between ITD and ILD and is significantly shallower in the elderly. This is probably due to the reduced ability to process the temporal differences between the auditory inputs indicated by the different ITD lateralization slope (B) without any pathology in the processing of the intensity parameter, the ILD lateralization slope (C; ***p < 0.001).

Figure 8

(A) Processing of the temporal parameter of sound (gap in noise detection threshold) at cortical level (***p < 0.001). (B) Relationship between the gap detection threshold (GDT) and PTAV values for individual participants of the elderly group, providing an option for the further characterization of presbycusis subpopulations.

Figure 9

Correlations of GDT with SIN (A) and with gated speech (B) suggest the importance of the temporal processing factor for speech understanding in a complex listening environment.

Figure 10

Comparison of three groups of elderly subjects (GDT impaired, best hearing, worst hearing) based on their peripheral (PTAV) and central (GDT) functions inspired by Figure 8B; shows significant differences between worst hearing and both other groups in SIN (p < 0.01; C) and SRT (p < 0.05) parameters (B) and close to statistical significance [Cliff’s d was medium-to-large (d = 0.62)] in gated speech (D). In none of the other parameters [ITD lateralization slope (A) Montreal Cognitive Assessment (MoCA) (E)] was a significant difference present. Comparison of best hearing and GDT impaired did not show any significant differences (*p < 0.05, **p < 0.01).