Effects of age and frequency disparity on gap discrimination

Cortical evoked response to gaps in noise: within-channel and across-channel conditions

OBJECTIVES

The objective of this study was to describe the cortical evoked response to silent gaps in a group of young adults with normal hearing using stimulus conditions identical to those used in psychophysical studies of gap detection. Specifically, we sought to examine the P1-N1-P2 auditory evoked response to the onsets of stimuli (markers) defining a silent gap for within-channel (spectrally identical markers) and across-channel (spectrally different markers) conditions using four perceptually-equated gap durations. It was hypothesized that (1) P1, N1, and P2 would be present and consistent for 1st marker (before the gap) onsets; (2) for within-channel markers, P1, N1, and P2 would be present for 2nd marker (after the gap) onsets only when the gap was of a duration equal to or larger than the behaviorally measured gap detection threshold; and (3) for the across-channel conditions, P1, N1, and P2 would be present for 2nd marker onsets regardless of gap duration. This is expected due to the additional cue of frequency change following the gap.

DESIGN

Twelve young adults (mean age 26 years) with normal hearing participated. Within-channel and across-channel gap detection thresholds were determined using an adaptive psychophysical procedure. Next, cortical auditory evoked potentials (P1-N1-P2) were recorded with a 32-channel Neuroscan electroencephalogram system using within-channel and across-channel markers identical to those used for the psychophysical task and four perceptually weighted gap durations: (1) individual listener's gap detection threshold; (2) above gap detection threshold; (3) below gap detection threshold; and (4) a 1-ms gap identical to the gap in the standard interval of the psychophysical task. P1-N1-P2 peak latencies and amplitudes were analyzed using repeated-measures analyses of variance. A temporal-spatial principal component analysis was also conducted.

RESULTS

The latency of P2 and the amplitude of P1, N1, and P2 were significantly affected by the acoustic characteristics of the 2nd marker as well as the duration of the gap. Larger amplitudes and shorter latencies were generally found for the conditions in which the acoustic cues were most salient (e.g., across-channel markers, 1st markers, large gap durations). Interestingly, the temporal-spatial principal component analysis revealed activity elicited by gap durations equal to gap detection threshold in the latency regions of 167 and 183 ms for temporal-parietal and right-frontal spatial locations.

CONCLUSIONS

The cortical response to a silent gap is unique to specific marker characteristics and gap durations among young adults with normal hearing. Specifically, when the onset of the 2nd marker is perceptually salient, the amplitude of the P1-N1-P2 response is relatively larger and the P2 latency is relatively shorter than for nonsalient 2nd marker onsets, providing noninvasive, nonbehavioral indicators of the neural coding of this important temporal cue in the thalamic-cortical region of the central auditory system. Gap duration appears to be most clearly indicated by P1 and T-complex amplitude.

Age-related differences in discrimination of an interval separating onsets of successive tone bursts as a function of interval duration

The study measured listener sensitivity to increments in the inter-onset interval (IOI) separating pairs of successive 20-ms 4000-Hz tone pulses. A silent interval between the tone pulses was adjusted across conditions to create reference tonal IOI values of 25-600 ms. For each condition, a duration DL for increments of the tonal IOI was measured in listeners comprised of young normal-hearing adults and two groups of older adults with and without high-frequency hearing loss. Discrimination performance of all listeners was poorest for the shorter reference IOIs, and improved to stable levels for longer reference intervals exceeding about 200 ms. Temporal sensitivity of the young listeners was significantly better than that of the elderly listeners in each condition, with the largest age-related differences observed for the shortest reference interval. Age-related differences were also observed for duration DLs measured using single 4000-Hz tone bursts set to three reference durations in the range 50-200 ms. The tone DLs of all listeners were smaller than the corresponding tone-pair IOI DLs, particularly for the shorter reference stimulus durations. There were no significant performance differences observed between the older listeners with and without hearing loss for either discrimination task.

Effect of spectral frequency range and separation on the perception of asynchronous speech

The use of across-frequency timing cues and the effect of disrupting these cues were examined across the frequency spectrum by introducing between-band asynchronies to pairs of narrowband temporal speech patterns. Sentence intelligibility by normal-hearing listeners fell when as little as 12.5 ms of asynchrony was introduced and was reduced to floor values by 100 ms. Disruptions to across-frequency timing had similar effects in the low-, mid-, and high-frequency regions, but band pairs having wider frequency separation were less disrupted by small amounts of asynchrony. In experiment 2, it was found that the disruptive influence of asynchrony on adjacent band pairs did not result from disruptions to the complex patterns present in overlapping excitation. The results of experiment 3 suggest that the processing of speech patterns may take place using mechanisms having different sensitivities to exact timing, similar to the dual mechanisms proposed for within- and across-channel gap detection. Preservation of relative timing can be critical to intelligibility. While the use of across-frequency timing cues appears similar across the spectrum, it may differ based on frequency separation. This difference appears to involve a greater reliance on exact timing during the processing of speech energy at proximate frequencies.

An Adaptive Clinical Test of Temporal Resolution

Purpose

It has been recommended that diagnostic and screening test batteries for auditory processing disorder (APD) include a measure of temporal gap detection using broadband noise stimuli. Although psychophysical laboratory procedures exist for the measurement of temporal resolution, none are clinically feasible. This study was designed to obtain preliminary data on a new clinical measure of gap detection, the Adaptive Test of Temporal Resolution (ATTR).

Method

The ATTR, a currently available clinical test (Random Gap Detection Test), and a standard psychophysical laboratory procedure were used to measure gap detection thresholds (GDTs) from a group of 30 young adults with normal hearing.

Results

Mean ATTR GDTs were 2.2 ms, consistent with GDTs measured using the psychophysical laboratory procedure (3.2 ms) and significantly smaller than those measured using the Random Gap Detection Test (7.0 ms).

Conclusions

Because it incorporates standard adaptive psychophysical methodology in a computer application that can be used on any desktop computer but does not depend on specialized hardware for application, the ATTR promises to be a clinically feasible addition to the APD test battery.

Effects of age and sequence presentation rate on temporal order recognition

The experiments examined the ability of younger and older listeners to identify the temporal order of sounds presented in tonal sequences. The stimuli were three-tone sequences that spanned two-octave frequency range, and listeners identified random permutations of tone order using labels of relative pitch. Some of the sequences featured uniform timing characteristics, and the sequence duty cycle was varied across conditions to examine the relative influence of tonal durations and intertone interval on recognition performance across a range of sequence presentation rates. Other stimulus sequences featured nonuniform timing with unequal tone durations and intertone intervals. The listeners were groups of younger and older persons with or without hearing loss. Results indicated that temporal order recognition was influenced primarily by sequence presentation rate, independent of tonal duration, tonal interval spacing, or sequence timing characteristics. The performance of older listeners was poorer than younger listeners, but the age-related recognition differences were independent of sequence presentation rate. There were no consistent effects of hearing loss on temporal ordering performance.

Temporal masking curves for hearing-impaired listeners

The decay of forward masking was investigated for three subjects with moderate sensorineural hearing loss. For such subjects, compression on the basilar membrane (BM) is thought to be largely absent, enabling one to determine the decay of masking without the influence of compression. Temporal masking curves (TMCs), plots of the masker level at threshold against delay between masker offset and signal onset, were measured for delays of 0, 15, 30, 45, 60, and 75 ms, for signal frequencies, fs, of 500, 1000, 2000, 4000, and 6000 Hz. Masker frequencies were 0.5, 0.8, 1.0, 1.15, and 1.3 times fs. Most of the TMCs were well fitted with single-segment straight lines, which, except for high masker levels, were roughly parallel for each fs, supporting the belief that BM compression was largely absent in these subjects. However, the slopes of the TMCs were greater for fs = 500 and 1000 Hz than for higher frequencies, which may indicate that the decay of forward masking is not the same for all signal frequencies. The results suggest that it may not be valid to infer BM compression at low signal frequencies by using a reference TMC for a high fs.

Temporal processing deficits in the pre-senescent auditory system

This study tested the hypothesis that temporal processing deficits are evident in the pre-senescent (middle-aged) auditory system for listening tasks that involve brief stimuli, across-frequency-channel processing, and/or significant processing loads. A gap duration discrimination (GDD) task was employed that used either fixed-duration gap markers (experiment 1) or random-duration markers (experiment 2). Independent variables included standard gap duration (0, 35, and 250 ms), marker frequency (within- and across-frequency), and task complexity. A total of 18 young and 23 middle-aged listeners with normal hearing participated in the GDD experiments. Middle age was defined operationally as 40-55 years of age. The results indicated that middle-aged listeners performed more poorly than the young listeners in general, and that this deficit was sometimes, but not always, exacerbated by increases in task complexity. A third experiment employed a categorical perception task that measured the gap duration associated with a perceptual boundary. The results from 12 young and 12 middle-aged listeners with normal hearing indicated that the categorical boundary was associated with shorter gaps in the young listeners. The results of these experiments indicate that temporal processing deficits can be observed relatively early in the aging process, and are evident in middle age.

Age-related changes in within- and between-channel gap detection using sinusoidal stimuli

Pure tone gap stimuli with identical (within-channel) or dissimilar (between-channel) marker frequencies of 1 and 2 kHz were presented to young and old listeners in a two-interval forced choice gap detection task. To estimate the influence of extraneous duration cues on gap detection, thresholds in the between-channel conditions were obtained for two different sets of reference stimuli: reference stimuli that were matched to the overall duration of the gap stimulus, i.e., two markers plus the gap, and reference stimuli that were fixed at the combined duration of the two markers excluding the gap. Results from within-channel conditions were consistent with previous studies, i.e., there were small but highly reliable age differences, smaller gap thresholds at longer marker durations, and an interaction between the two variables. In between-channel conditions, however, age differences were not as clear cut. Rather, the effect of age varied as a function of duration cue and was more pronounced when stimuli were matched for overall duration than when the duration of the reference tone was fixed.

Age-related differences in identification and discrimination of temporal cues in speech segments

This study investigated age-related differences in sensitivity to temporal cues in modified natural speech sounds. Listeners included young noise-masked subjects, elderly normal-hearing subjects, and elderly hearing-impaired subjects. Four speech continua were presented to listeners, with stimuli from each continuum varying in a single temporal dimension. The acoustic cues varied in separate continua were voice-onset time, vowel duration, silence duration, and transition duration. In separate conditions, the listeners identified the word stimuli, discriminated two stimuli in a same-different paradigm, and discriminated two stimuli in a 3-interval, 2-alternative forced-choice procedure. Results showed age-related differences in the identification function crossover points for the continua that varied in silence duration and transition duration. All listeners demonstrated shorter difference limens (DLs) for the three-interval paradigm than the two-interval paradigm, with older hearing-impaired listeners showing larger DLs than the other listener groups for the silence duration cue. The findings support the general hypothesis that aging can influence the processing of specific temporal cues that are related to consonant manner distinctions.

Combining temporal-envelope cues across channels: effects of age and hearing loss

The goal of this study was to examine the ability to combine temporal-envelope information across frequency channels. Three areas were addressed: (a) the effects of hearing loss, (b) the effects of age and (c) whether such effects increase with the number of frequency channels. Twenty adults aged 23-80 years with hearing loss ranging from mild to severe and a control group of 6 adults with normal hearing participated. Stimuli were vowel-consonant-vowel syllables. Consonant identification was measured for 5 conditions: (a) 1-channel temporal-envelope information (minimal spectral cues), (b) 2-channel, (c) 4-channel, (d) 8-channel, and (e) an unprocessed (maximal spectral cues) speech condition. Performance of listeners with normal hearing and listeners with hearing loss was similar in the 1-channel condition. Performance increased with the number of frequency channels in both groups; however, increasing the number of channels led to smaller improvements in consonant identification in listeners with hearing loss. Older listeners performed more poorly than younger listeners but did not have more difficulty combining temporal cues across channels than in a simple, 1-channel temporal task. Age was a significant predictor of nonsense syllable identification, whereas amount of hearing loss was not. The results support an age-related deficit in use of temporal-envelope information with age, regardless of the number of channels.

Effect of age on detection of gaps in speech and nonspeech markers varying in duration and spectral symmetry

Gap detection thresholds for speech and analogous nonspeech stimuli were determined in younger and older adults with clinically normal hearing in the speech range. Gap detection thresholds were larger for older than for younger listeners in all conditions, with the size of the age difference increasing with stimulus complexity. For both ages, gap detection thresholds were far smaller when the markers before and after the gap were the same (spectrally symmetrical) compared to when they were different (spectrally asymmetrical) for both speech and nonspeech stimuli. Moreover, gap detection thresholds were smaller for nonspeech than for speech stimuli when the markers were spectrally symmetrical but the opposite was observed when the markers were spectrally asymmetrical. This pattern of results may reflect the benefit of activating well-learned gap-dependent phonemic contrasts. The stimulus-dependent age effects were interpreted as reflecting the differential effects of age-dependent losses in temporal processing ability on within- and between-channel gap detection.

GIN (Gaps-In-Noise) Test Performance in Subjects with Confirmed Central Auditory Nervous System Involvement

OBJECTIVE

The purpose of the present study was to investigate the value of a new gap detection procedure called Gaps-In-Noise (GIN) for assessment of temporal resolution in a clinical population.

DESIGN

The test consists of 0 to 3 silent intervals ranging from 2 to 20 msec embedded in 6-sec segments of white noise. The location, number, and duration of the gaps per noise segment vary throughout the test for a total of 60 gaps presented in each of four lists. The GIN procedure was administered to 50 normal-hearing listeners (group I) and 18 subjects with confirmed neurological involvement of the central auditory nervous system (group II).

RESULTS

Results showed mean approximated gap detection thresholds of 4.8 msec for the left ear and 4.9 msec for the right ear for group I. In comparison, results for group II demonstrated a statistically significant increase in gap detection thresholds, with approximated thresholds of 7.8 msec and 8.5 msec being noted for the left and right ears, respectively. Significant mean differences were also observed in the overall performance scores (i.e., the identification of the presence of the gaps within the noise segments) of the two groups of subjects. Finally, psychometric functions, although similar for short and long duration gaps, were highly different for gaps in the 4- to 10-msec range for the two groups.

CONCLUSIONS

A variety of psychoacoustic procedures are available to assess temporal resolution; however, the clinical use of these procedures is minimal at best. Results of the present study show that the GIN test holds promise as a clinically useful tool in the assessment of temporal resolution in the clinical arena.

Time-compressed visual speech and age: a first report

OBJECTIVE

The purpose of this research was to investigate the effects of age on the ability to identify temporally altered visual speech signals.

DESIGN

Two groups of adult lipreaders, older (N = 20) and younger (N = 15), were tested on perception of visual-only speech signals. Identification performance was measured for time-compressed, time-expanded, and unaltered versions of words with visual only speech.

RESULTS

An overall reduction in lipreading ability was observed as a function of age. However, in contrast to results with time-altered auditory speech, older adults did not show a disproportionate change to speeded or slowed visual speech.

CONCLUSIONS

The absence of age effects in the identification of temporally altered visual speech signals stands in contrast to the considerable evidence that older adults are disproportionately affected by temporal alterations of auditory speech signals. These results argue against a generalized slowing of information processing in older adults and instead point to modality specific changes in temporal processing abilities.

Effects of age and hearing loss on gap detection and the precedence effect: narrow-band stimuli

Deficits in temporal resolution and/or the precedence effect may underlie part of the speech understanding difficulties experienced by older listeners in degraded acoustic environments. In a previous investigation, R. Roberts and J. Lister (2004) identified a positive correlation between measures of temporal resolution and the precedence effect, specifically across-channel gap detection (as measured dichotically) and fusion. Across-channel gap detection may also be measured using frequency-disparate markers. Thus, the present investigation was designed to determine if the relation is specific to dichotic gap detection or may generalize to all types of across-channel gap detection. Gap-detection thresholds (GDTs) for fixed-frequency and frequency-disparate markers and lag-burst thresholds (LBTs) were measured for 3 groups of listeners: young with normal hearing sensitivity (YNH), older with normal hearing sensitivity (ONH), and older with sensorineural hearing loss (OIH). Also included were conditions of diotic and dichotic GDT. Largest GDTs were measured for the frequency-disparate markers, whereas largest LBTs were measured for the fixed-frequency markers. ONH and OIH listeners exhibited larger frequency-disparate and dichotic GDTs than YNH listeners. Listener age and hearing loss appeared to influence temporal resolution for frequency-disparate and dichotic stimuli, which is potentially important for the resolution of timing cues in speech. Age and hearing loss did not significantly influence fusion as measured by LBTs. Within each participant group, most GDTs and LBTs were positively, but not significantly, correlated. For all participants combined, across-channel GDTs and LBTs were positively and significantly correlated. This suggests that the 2 tasks may rely on a common across-channel temporal mechanism.

Effects of age and hearing loss on gap detection and the precedence effect: broadband stimuli

Older listeners with normal-hearing sensitivity and impaired-hearing sensitivity often demonstrate poorer-than-normal performance on tasks of speech understanding in noise and reverberation. Deficits in temporal resolution and in the precedence effect may underlie this difficulty. Temporal resolution is often studied by means of a gap-detection paradigm. This task is similar to binaural fusion paradigms used to measure the precedence effect. The purpose of this investigation was to determine if within-channel (measured with monotic and diotic gap detection) or across-channel (measured with dichotic gap detection) temporal resolution is related to fusion (measured with lag-burst thresholds; LBTs) under dichotic, anechoic, and reverberant conditions. Gap-detection thresholds (GDTs) and LBTs were measured by means of noise-burst stimuli for 3 groups of listeners: young adults with normal-hearing sensitivity (YNH), older adults with normal-hearing sensitivity (ONH), and older adults with impaired-hearing sensitivity (OIH). The GDTs indicated that across-channel temporal resolution is poorer than within-channel temporal resolution and that the effects of age and hearing loss are dependent on condition. Results for the fusion task indicated higher LBTs in reverberation than for the dichotic and anechoic conditions, regardless of group, and no effect of age or hearing loss for the nonreverberant conditions. However, higher LBTs were observed in the reverberant condition for the ONH listeners. Further, there was a correlation between across-channel temporal resolution and fusion in reverberation. Gap detection and fusion may not necessarily reflect the same underlying processes; however, across-channel gap detection may influence fusion under certain conditions (i.e., in reverberation).

Effect of age on silent gap discrimination in synthetic speech stimuli

The difficulty that older listeners experience understanding conversational speech may be related to their limited ability to use information present in the silent intervals (i.e., temporal gaps) between dynamic speech sounds. When temporal gaps are present between nonspeech stimuli that are spectrally invariant (e.g., noise bands or sinusoids), older listeners are less able to resolve temporal gaps than are younger listeners. It has also been demonstrated that temporal gap perception deteriorates as the frequency difference between the sounds bordering the silent gap increases, and this effect becomes more pronounced with age (J. Lister, J. Besing, and J. Koehnke, 2002; J. Lister, J. Koehnke, and J. Besing, 2000). In this study, the effect of age on the ability to discriminate temporal gaps in dynamic stimuli (i.e., changing in frequency and duration over time) was measured in a gap duration discrimination (GDD) task. The participants were two groups of listeners with normal hearing sensitivity through at least 4000 Hz: (a) ages 21-38 years and (b) ages 50-72 years. Stimuli simulated the frequency characteristics of 1 consonant (/s/), a steady-state vowel (/a/), a weak diphthong (/eI/), and a voiced bilabial plosive (/burst/) in 6 combinations: (a) /s-a/, (b) /s-eI/, (c) /a-a/, (d) /eI-eI/, (e) /burst-a/, and (f) /burst-eI/. For each of the 6 phoneme combinations, 2 conditions of stimulus duration were used: (a) fixed, meaning that the durations of leading and trailing noises were fixed at values typical for the speech sounds being simulated, and (b) random, meaning that duration varied randomly within a range acceptable for accurate perception of the speech contrasts. Gap duration difference limens (GDDLs) were significantly larger for the older listeners than for younger listeners. For both groups, GDDLs were poorer for the spectrally dynamic marker pairs (e.g., /burst-eI/) than for the marker pairs that were relatively stable in frequency over time (/a-a/). GDD performance also was poorer for the random conditionthan for the fixed condition. Listener age and hearing sensitivity were significantly correlated, and age was not significantly correlated with GDD when controlling for hearing sensitivity. The authors conclude that temporal resolution (as measured by gap discrimination) is affected by age, stimulus complexity, and, perhaps, by hearing sensitivity in a speech context.