Part 2: Looking beyond ‘simple audition’

Vishakha W. Rawool, PhD, is a member of the faculty in the Department of Speech Pathology and Audiology at West Virginia University, Morgantown, WVa. Part 1 of this article appears in the May HR.

Deficits in temporal processing can affect the ability to understand speech, especially in noisy surroundings. This article, a follow-up to Part 1 in the May edition of HR, provides a review of the literature related to the potential effects of sensorineural hearing loss (referred to here as simply ‘hearing loss’) on temporal processing and a summary of how deficits in temporal processing may be related to speech recognition.

Hearing care professionals need to consider these deficits during the provision of services to individuals with hearing loss. Amplification and other auditory rehabilitation services with a focus on temporal processing deficits will be discussed in a future article.

Temporal Resolution
Gap detection. The ability to detect gaps between two stimuli in the presence of hearing loss is partially determined by the degree to which the stimuli used for the gap detection task vary randomly in amplitude and the sound level of the stimuli.

Gap detection tends to be normal when stimuli are sinusoids.1,2 However, when stimuli have random amplitude fluctuations (eg, bands of noise), individuals with hearing loss have more difficulty in detecting these gaps than normal-hearing individuals.3 People with hearing loss often suffer from recruitment or abnormal growth of loudness. Recruitment can make them more sensitive to the random fluctuations or dips occurring in narrow-bands of noise, and these dips can be confused with gaps that the individuals are expected to detect.4

If gap detection is compared among individuals with hearing loss and those with normal hearing using the same sound presentation levels (eg, 50 dB SPL), those with hearing loss seem to have poorer temporal resolution; they need larger gaps to detect the gaps. However, at higher presentation levels or at sensation levels similar to those used in normal-hearing individuals, the gap detection is normal.1,2,3 Since most individuals with significant hearing loss will hear environmental sounds at lower presentation levels without amplification, they can be expected to have difficulty in detecting gaps in common noisy backgrounds.

Temporal modulation detection. Individuals with high frequency hearing loss appear to have reduced sensitivity to high rates of modulation when compared to normal-hearing individuals, probably because of inaudible high frequencies.5 Listeners with flat hearing loss appear to show modulation detection performance that is similar to that of individuals with normal hearing when stimuli are presented at equal sound pressure levels. When stimuli are presented at equal sensation levels to normal-hearing and hearing-impaired listeners, individuals with hearing loss appear to be better at detecting modulations.6

Thus, temporal modulation detection appears to depend on the bandwidth audible to the listener. If the entire bandwidth is audible to the listener, temporal modulation detection is expected to be normal. However, if the high frequency bands are inaudible, temporal modulation detection can be expected to be poorer.

Duration discrimination. Duration discrimination is important for accurate speech recognition. For example, fricatives (eg, ‘f’ as in fine) can be differentiated from affricates (eg, ‘ch’ as in chap) based on the difference in the duration of the fricative noises associated with these sounds.7

The effect of hearing loss on duration discrimination is somewhat controversial. Some investigators have reported no effect of hearing loss,8,9 whereas others have reported that duration discrimination is poorer in the presence of hearing loss.10 Duration discrimination may be poorer when the duration of the standard stimulus is lower (eg, 20 ms as compared to 200 ms), and this factor may interact with hearing loss.

Gap-duration discrimination. Dor­man et al11 summarized the importance of gap-duration discrimination in identifying phonemes. Gap duration discrimination is important for discriminating fricatives and affricates, for identifying the presence or absence of a stop consonant in a consonant cluster (eg, the ‘p’ sound in the word ‘spray’), for detecting voicing of a stop consonant in word-medial position, and for discriminating between single and double stop consonants. Gap-duration discrimination appears to be poorer in the presence of hearing loss.10

Temporal Asynchrony
A rapid sequence of sounds is not always perceived as a coherent whole. The sounds may actually be divided into different groups according to their general attributes, such as loudness, perceived location, pitch, closeness to each other, temporal synchrony of different frequency bands, etc. The different frequency components from a particular sound source tend to start and finish together. Thus, if the different frequency bands start together or have onset synchrony, then they tend to be perceived as part of the same sound source; otherwise, they form separate auditory streams. This ability can allow us to concentrate on the signal of interest and ignore the signals that are not of interest.

The effect of hearing loss on temporal asynchrony detection may be dependent on the particular asynchrony task, the configuration of hearing loss (flat or sloping), and the degree of hearing loss. Some investigators have suggested that mild to moderate hearing loss does not appear to affect people’s ability to make use of relatively gross temporal asynchrony cues. This suggests that they retain the ability to group auditory objects into separate streams based on temporal asynchronies.12

Temporal Ordering
Individuals with mild to moderate hearing loss tend to show good performance on a duration pattern recognition task13 as long as the signal is audible and the different durations within the pattern can be discriminated. It should be noted that in the clinical version of the duration pattern test, 1000 Hz tone-burst stimuli are used; the shorter tone is 250 msec in duration and the longer tone is 500 msec in duration. Most individuals with cochlear hearing loss will not have difficulty in discriminating between such durations.

Temporal Masking
Some investigators have reported that people with hearing loss exhibit more forward masking than would be predicted based on the test results of normal-hearing individuals. Additionally, as masker levels increase, the growth of forward masking tends to be steeper in individuals with hearing loss. The difference is more obvious for brief masker durations.14

Many listeners with moderate to severe sensorineural hearing loss show backward and forward masking, sometimes extending as far as 200 ms. In addition, there are large individual differences in the degree and extent of temporal masking.15

Temporal Integration/Summation
Duration-induced improvement of auditory thresholds. The improvement in auditory thresholds with increased duration of signals tends to be less in individuals with hearing loss than that apparent in those with normal-hearing.16 Reduced temporal integration is more apparent at frequencies in which there is greater hearing loss.14 Thus the ability of the auditory system to integrate acoustic energy in brief sounds appears to be reduced in the presence of hearing loss.17,18

Rate-induced improvement of auditory thresholds. In individuals with normal-hearing, thresholds improve when you increase the signal-rate. This improvement is reduced in individuals with hearing loss.19

Temporally Degraded Speech
Time-compressed speech. The perception of time-compressed speech is poorer in the presence of high-frequency hearing loss.20 This is true even after the sentences are presented at high levels, suggesting that individuals with hearing loss have difficulty understanding fast speech.

Reverberant speech. Reverberation can cause distortion of the speech signal due to the temporal smearing of the original signal by reflections of the original signal. Individuals with hearing loss perform more poorly than individuals with normal hearing when presented with reverberant speech.20 Furthermore, poor reverberant speech scores are associated with increased gap-duration discrimination thresholds.

Speech with temporal asynchrony. Healy and Bacon21 suggested that reverberation times for low frequency sounds can be longer than high frequency sounds, because low frequency sounds are absorbed less efficiently. Thus, low frequency components can persist longer in a reverberant field than high frequency components. This can create cross-channel asynchrony for the indirect signals. The poor tolerance of temporal asynchrony in individuals with hearing loss can contribute to difficulty in perceiving speech in reverberant environments.

Speech in competing background. Individuals with hearing loss have difficulty in recognizing speech in the presence of background of signals that have temporal variations.22 Speech recognition in competing noise tends to be difficult, even in the presence of mild hearing loss in the mid-frequencies (500 Hz, 1000 Hz, and 2000 Hz).23

Individuals with hearing loss have relatively higher speech reception thresholds in noise.24 Conversely, individuals with normal hearing are able to take advantage of the fluctuations that can occur in masking noise that make it easier to listen to speech signals during the ‘dips’ in the noise level. This ability is limited in individuals with hearing loss, partly because the audibility of the speech signal is limited during the relatively silent periods in the masker (the noise).

Reduced comodulation masking release may also be a factor.25 Co-modulation masking release is apparent in normal-hearing individuals, and refers to the reduction of threshold for a signal in a comodulated masker following the introduction of a second comodulated masker.

Binaural temporal processing. Binaural temporal processing requires the processing of stimuli over time by both ears. For this type of processing to occur, stimuli presented to two ears must be compared at some central location in the auditory system.

Precedence effect. Individuals with hearing loss show poorer performance on the precedence effect tasks than those with normal hearing.26 It has been suggested that the ability to fuse direct sounds and early reflections is poorer in individuals with hearing impairment, and this may contribute to the difficulties experienced by these listeners in reverberation.27

Sound localization. Hearing loss affects sound localization in the median vertical plane, except when hearing sensitivity is very good in the higher frequencies. Relatively better hearing in the low-to-middle frequencies can allow good frontal-horizontal localization.28

Interaural time difference (ITD) discrimination. In this task, the test stimulus differs from the standard stimulus in interaural delay (the difference in time-arrival of the sound at two ears). Listeners with high-frequency hearing loss generally have more difficulty with ITD discrimination for high-frequency stimuli, but they can also have poor ITD discrimination for lower frequencies.29

Masking level difference. The size of the masking level difference is generally reduced in the presence of hearing loss.30,31

Spatial separation of speech and noise sources. The binaural advantage occurring due to the spatial separation of speech and noise sources (eg, speech in front and noise in back) decreases with increasing high-frequency hearing loss.24 At higher frequencies, there is a greater interaural intensity difference. This is why, in individuals with normal hearing, noise on the contralateral side will be attenuated compared to the speech on the ipsilateral side, and this can improve speech perception.

Beyond Simple Audition
The above should make it clear that speech understanding is not always completely reliant on audition; temporal processing makes a difference in how people hear and can be extremely important for hearing speech in noise. Understanding the effects of hearing loss on temporal resolution allows us to appreciate the fact that the perception of fast-changing signals—such as speech—is difficult for hearing-impaired listeners, and that this difficulty will be enhanced in the presence of background noise.

Forward masking effects appear to play a role in the perception of speech in quiet.32 In average conversational speech, the intensity of sounds can vary by about 30 dB. The more severe effect of forward masking in individuals with hearing loss may cause frequent masking of a weaker sound when it is followed by a relatively intense sound. Difficulties in gap-duration discrimination can also lead to difficulties in understanding speech in quiet.

Gap detection thresholds appear to be related to speech understanding in noise.33 Real-life background noises often fluctuate in intensity, allowing for the extraction of useful information from the signal of interest during nulls in the background noise. However, poor gap detection in individuals with hearing loss may not allow them to take advantage of these nulls, or softer levels, in the background noise.

Other factors involving temporal processing that predict poor performance in the presence of background noise include poor sound localization, poor separation of speech and noise, poor interaural time-difference discrimination, and poor masking level differences.

The poor ability of individuals with hearing loss to fuse the direct sound and early reflections of that sound and poor tolerance of speech with temporal asynchrony predicts poor speech recognition in the presence of reverberation. It has also been shown that individuals who have difficulty in discriminating between gap-durations have difficulty resolving the variable temporal fluctuations in the reverberant speech waveform.34

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Correspondence to Vishakha W. Rawool, Dept of Speech Pathology and Audiology, West Virginia University, PO Box 6122, Morgantown, WVa 26506-6122; e-mail: [email protected].