Age effects in discrimination of repeating sequence intervals

Differential sensitivity to speech rhythms in young and older adults

Sensitivity to the temporal properties of auditory patterns tends to be poorer in older listeners, and this has been hypothesized to be one factor contributing to their poorer speech understanding. This study examined sensitivity to speech rhythms in young and older normal-hearing subjects, using a task designed to measure the effect of speech rhythmic context on the detection of changes in the timing of word onsets in spoken sentences. A temporal-shift detection paradigm was used in which listeners were presented with an intact sentence followed by two versions of the sentence in which a portion of speech was replaced with a silent gap: one with correct gap timing (the same duration as the missing speech) and one with altered gap timing (shorter or longer than the duration of the missing speech), resulting in an early or late resumption of the sentence after the gap. The sentences were presented with either an intact rhythm or an altered rhythm preceding the silent gap. Listeners judged which sentence had the altered gap timing, and thresholds for the detection of deviations from the correct timing were calculated separately for shortened and lengthened gaps. Both young and older listeners demonstrated lower thresholds in the intact rhythm condition than in the altered rhythm conditions. However, shortened gaps led to lower thresholds than lengthened gaps for the young listeners, while older listeners were not sensitive to the direction of the change in timing. These results show that both young and older listeners rely on speech rhythms to generate temporal expectancies for upcoming speech events. However, the absence of lower thresholds for shortened gaps among the older listeners indicates a change in speech-timing expectancies with age. A further examination of individual differences within the older group revealed that those with better rhythm-discrimination abilities (from a separate study) tended to show the same heightened sensitivity to early events observed with the young listeners.

Top-down or bottom up: decreased stimulus salience increases responses to predictable stimuli of auditory thalamic neurons

KEY POINTS

Temporal imprecision leads to deficits in the comprehension of signals in cluttered acoustic environments, and the elderly are shown to use cognitive resources to disambiguate these signals. To mimic ageing in young rats, we delivered sound signals that are temporally degraded, which led to temporally imprecise neural codes. Instead of adaptation to repeated stimuli, with degraded signals, there was a relative increase in firing rates, similar to that seen in aged rats. We interpret this increase with repetition as a repair mechanism for strengthening the internal representations of degraded signals by the higher-order structures.

ABSTRACT

To better understand speech in challenging environments, older adults increasingly use top-down cognitive and contextual resources. The medial geniculate body (MGB) integrates ascending inputs with descending predictions to dynamically gate auditory representations based on salience and context. A previous MGB single-unit study found an increased preference for predictable sinusoidal amplitude modulated (SAM) stimuli in aged rats relative to young rats. The results suggested that the age-degraded/jittered up-stream acoustic code may engender an increased preference for predictable/repeating acoustic signals, possibly reflecting increased use of top-down resources. In the present study, we recorded from units in young-adult MGB, comparing responses to standard SAM with those evoked by less salient SAM (degraded) stimuli. We hypothesized that degrading the SAM stimulus would simulate the degraded ascending acoustic code seen in the elderly, increasing the preference for predictable stimuli. Single units were recorded from clusters of advanceable tetrodes implanted above the MGB of young-adult awake rats. Less salient SAM significantly increased the preference for predictable stimuli, especially at higher modulation frequencies. Rather than adaptation, higher modulation frequencies elicited increased numbers of spikes with each successive trial/repeat of the less salient SAM. These findings are consistent with previous findings obtained in aged rats suggesting that less salient acoustic signals engage the additional use of top-down resources, as reflected by an increased preference for repeating stimuli that enhance the representation of complex environmental/communication sounds.

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

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.

Effects of age and hearing loss on recognition of unaccented and accented multisyllabic words

The effects of age and hearing loss on recognition of unaccented and accented words of varying syllable length were investigated. It was hypothesized that with increments in length of syllables, there would be atypical alterations in syllable stress in accented compared to native English, and that these altered stress patterns would be sensitive to auditory temporal processing deficits with aging. Sets of one-, two-, three-, and four-syllable words with the same initial syllable were recorded by one native English and two Spanish-accented talkers. Lists of these words were presented in isolation and in sentence contexts to younger and older normal-hearing listeners and to older hearing-impaired listeners. Hearing loss effects were apparent for unaccented and accented monosyllabic words, whereas age effects were observed for recognition of accented multisyllabic words, consistent with the notion that altered syllable stress patterns with accent are sensitive for revealing effects of age. Older listeners also exhibited lower recognition scores for moderately accented words in sentence contexts than in isolation, suggesting that the added demands on working memory for words in sentence contexts impact recognition of accented speech. The general pattern of results suggests that hearing loss, age, and cognitive factors limit the ability to recognize Spanish-accented speech.

Recovery from forward masking in elderly cochlear implant users

OBJECTIVE

To compare temporal aspects of peripheral neural responses and central auditory perception between groups of younger adult and elderly cochlear implant users.

STUDY DESIGN

Cohort study.

SETTING

Academic hospital and cochlear implant center.

PATIENTS

Adult cochlear implant users aged 28 to 57 years in the younger group (n = 5) and 61 to 89 years (n = 9) in the elderly group. All subjects used Advanced Bionics devices.

INTERVENTION

Diagnostic.

MAIN OUTCOME MEASURES

Time constants of neural (i.e., electrically evoked compound action potentials [ECAPs]) and perceptual recovery from forward masking. Interstimulus intervals (ISIs) were varied in both experiments.

RESULTS

ECAP recovery rates were equivalent between groups, and no correlation was found between ECAP recovery and age. No correlations were found between ECAP recovery and speech perception. Psychophysical recovery was significantly slower in the elderly compared with the younger subjects (p < 0.0005), with a significant effect of age (R2 = 0.70, p < 0.0005). At the longest ISI (240 ms), elderly subjects experienced a mean maximum threshold shift of 35.2% (relative to 1 ms ISI) versus 14.8% for younger subjects. There was a significant positive relationship between psychophysical recovery and consonant-nucleus-consonant word scores (R2 = 0.62, p < 0.001), although no relationship was found with Hearing in Noise Test sentences.

CONCLUSION

These findings suggest that difficulties observed in speech perception by elderly CI users may be due to age-related changes in the central rather than peripheral auditory system. With further study, these results may provide information to allow clinicians to assess patients' temporal processing abilities and facilitate setting program parameters that will maximize their auditory perceptual experience with a cochlear implant.

Age-related changes in the use of regular patterns for auditory scene analysis

A recent approach to auditory processing suggests a close relationship of regularity processing in auditory sensory memory (ASM) and stream segregation, such that within-stream regularities can be used to stabilize stream segregation. The present study investigates age-related changes in how regular patterns are used for auditory scene analysis (ASA), when the stream containing the regularity is attended or unattended. In order to accomplish an intensity level deviant detection task, participants had to segregate the task-relevant pure tone sequence from an irrelevant distractor pure tone sequence, which randomly varied in level. In three conditions a simple spectro-temporal regularity ("Isochronous"), a more complex spectro-temporal regularity ("Rhythmic"), or no regularity ("Random") was embedded in either the attended target sequence (Experiment 1), or the unattended distractor sequence (Experiment 2). When the sequence containing the regularity was attended, older participants showed a similar increase of performance to younger adults in the conditions with regular patterns ("Isochronous" and "Rhythmic") compared to the "Random" condition. In contrast, when the sequence containing the regularity was unattended, older adults showed a specific performance decline compared to younger adults in the "Isochronous" condition. Results suggest a link between impaired automatic processing of regularities in ASM, and age-related deficits in the use of regular patterns for ASA.