Effects of intensity and location on sound location discrimination in macaque monkeys

Sound localization performance is degraded at low stimulus intensities in humans, and while the sound localization ability of humans and macaque monkeys appears similar, the effects of intensity have yet to be described in the macaque. We therefore defined the ability of four macaque monkeys to localize broadband noise stimuli at four different absolute intensities and six different starting locations in azimuth. Results indicate that performance was poorest at the lowest intensity tested (25 dB SPL), intermediate at 35 dB SPL, and equivalent at 55 and 75 dB SPL. Localization performance was best at 0 degree (directly in front of the animal) and was systematically degraded at more peripheral locations (+/-30 degrees and 90 degrees) and worst at a location directly behind the animal. Reaction times showed the same trends, with reaction times increasing with decreasing stimulus intensity, even under conditions where the monkey discriminated the location change with the same performance. These results indicate that sound level as well as position profoundly influences sound localization ability.     

Gap detection in Mongolian gerbils (Meriones unguiculatus)

Hearing thresholds for white-noise stimuli and temporal gap-detection thresholds in six Mongolian gerbils (Meriones unguiculatus) were determined in a GO/NOGO procedure using the method of constant stimuli. Gerbils were offspring of animals collected in the field and only bred in captivity for four generations or less. Hearing thresholds for 800 ms bursts of white noise ranged between -0.8 and 6.3 dB SPL. The median minimum-detectable gap centered in an 800 ms burst of white noise presented at 50 dB SPL was 2.1 ms. At levels of 40 dB SPL and above, gap-detection thresholds were independent of the sound-pressure level of the stimulus. At 30 dB SPL and below, the minimum-detectable gap increased with decreasing sound-pressure level. Near 5 dB sensation level, gap-detection thresholds ranged from 13.6 to 29.4 ms. The median threshold for the detection of gaps inserted 5 ms after the onset of an 800 ms burst of white noise of 50 dB SPL was 2.3 ms. The individuals' detection thresholds varied between 2.3 and 3.6 ms for stimuli in which the relative sound-pressure levels of the noise before and after the gap differed by up to 20 dB. The data found in the Mongolian gerbil match results from other mammal and bird species.     

Strain-dependence of age-related cochlear hearing loss in wild and domesticated Mongolian gerbils

The Mongolian gerbil (Meriones unguiculatus) is one of the animal models in auditory research that has been used in several studies on age-related hearing loss. The standard laboratory strain is domesticated as it was bred in captivity for more than 70 years. We compared properties of distortion product otoacoustic emissions (DPOAEs) in domesticated gerbils with wild-type gerbils from F6-F7 generations of a strain originating from animals trapped in Central Asia in 1995. Up to an age of 9months, DPOAE thresholds were comparable between both strains and were below 10dB SPL for f2 frequencies between 4 and 44kHz. In older domesticated animals, the thresholds were increased by up to 12dB. Significant increases were found at stimulus frequencies of 2kHz, 12-20kHz, and 56-60kHz. The best frequency ratio f2/f1 to evoke maximum DPOAE amplitude was larger in domesticated animals at the age of 9 months or older. While these data show that there is a deterioration of cochlear sensitivity due to domestication, the magnitude of the described changes is small. Thus, the general suitability of domesticated gerbils for auditory research seems not to be affected.     

Frequency discrimination for bands of noise.

Recoding of speech in order to improve discrimination for profoundly deaf people has been tried in various ways. One of these is frequency transposition of voiceless consonants. Some authors report positive and very promising results. Others, however, have not found any significant difference when using control groups with normal amplification. One reason might be a poor frequency discrimination. Is it possible to establish what minimum hearing capacity the listener must show for a successful use of frequency transposition? Using noise bands with different Q factors and varying the levels, the thresholds as minimum midfrequency shift have been measured. Great individual variations have been shown. There is a significant difference between (1) threshold values at different Q factors, (2) threshold values at different frequencies, and (3) threshold values for increasing and decreasing frequency, position of deltaf. There is a significant interaction between (1) frequency and position of deltaf, (2) Q factor and position of deltaf, and (3) between Q factor and frequency. There is also a significant triple interaction between Q factor, frequency, and position of deltaf.     

Expression of phospholipase C-gamma1 in experimental cholesteatoma using Mongolian gerbils

Phospholipase C (PLC)-gamma1 is known to play a central role in ligand-mediated signal transduction for cell proliferation. The purpose of this study was to elucidate the distribution of PLC-gamma1 in deep meatal skin, retroauricular skin and cholesteatoma matrix of experimental animals. We induced canal ligation cholesteatoma using Mongolian gerbils and investigated the expression of PLC-gamma1 in this model by using Western blot analysis and immunohistochemistry. By Western blot analysis, considerably higher levels of PLC-gamma1 protein were detectable in experimental cholesteatoma. On immunohistochemistry, experimental cholesteatoma showed more intense immunolabeling of PLC-gamma1 protein than deep meatal skin and retroauricular skin. In conclusion, overexpression of PLC-gamma1 may in part contribute to abnormal proliferation and differentiation of experimental cholesteatoma in Mongolian gerbils.     

Frequency discrimination in children: perception, learning and attention

It is generally believed that both sensory immaturity and inattention contribute to the poor listening of some children. However, the relative contribution of each factor, within and between individuals, and the nature of the inattention are poorly understood. In three experiments we examined the threshold and response variability of 6-11 y.o. children on pure tone frequency discrimination (FD) tasks. We first confirmed that younger children had both higher thresholds and greater within- and between-listener variability than older children and adults. Higher thresholds were mostly attributed to high response variability due to poor sustained attention. We next compared performance on the auditory FD task with that on visual spatial FD. No correlation was found between the thresholds or variability of individuals on the two tasks, suggesting involvement of modality-specific attention. Finally, we found lower thresholds for 8-9 y.o. children performing auditory FD training in a classroom than in the laboratory, possibly due to training session length or to a more familiar, motivating and focussed training environment. The adult-like performance of many younger children at times during their testing or training, together with the high response variability of immature performers, suggested that most elevated FD thresholds in children are due to inattention.     

Trained discrimination of temporal patterns: cochlear implants in gerbils

The purpose of this study was to establish an animal model for the discrimination of temporal order cues contained in electrical stimuli to the cochlea. Gerbils were deafened and implanted in the right cochlea with a single platinum stimulating electrode. Two groups of animals were trained in a two-way active avoidance shuttle box paradigm to discriminate downward from upward interval-modulated pulse trains (1-100 ms). One group consisted of naive animals. The other group had previously been trained in the same shuttle box (same behavioral meaning) to discriminate identical pulse trains presented acoustically. Significant discrimination performance was found in the group of naive animals. However, over the 6-day training period, animals with previous acoustic experience achieved no significant discrimination performance. This suggests that temporal order cues in pulse trains can be used in cochlear implants to transmit behaviorally relevant information but that this may be in conflict with relevant auditory preexperience.     

Behavioral and evoked-potential thresholds in young and old Mongolian gerbils (Meriones unguiculatus)

Two types of chickens are commercially available. Broiler birds are bred to develop quickly for meat production, while egg layers are bred to attain a smaller adult size. Because we have observed breed differences in the response of central auditory neurons to cochlear ablation in adult birds [Edmonds et al. (1999) Hear. Res. 127, 62-76], we examined cochleae from the two breeds for differences in integrity. We evaluated cochlear hair cell structure using scanning electron microscopy and cochlear hair cell function using distortion product otoacoustic emissions (DPOAEs) and the auditory brainstem response. We observed striking breed differences in cochlear integrity in adult but not hatchling birds. In adult broiler birds, all cochleae showed damage, encompassing at least the basal 29% of the cochlea. In 15 of 18 broiler ears, damage was observed throughout the basal 60% of the cochlea. In contrast, cochleae from egg layer adults were largely normal. Two thirds of egg layer ears showed no anatomical abnormalities, while in the remainder cochlear damage was seen within the basal 48% of the cochlea. DPOAEs recorded from egg layer birds showed loss of high frequency emissions in every ear for which the cochlea displayed anatomical damage. Average sound pressure levels in both commercial facilities were 90 dB, suggesting these two breeds may exhibit differential susceptibility to noise damage.     

Auditory frequency and intensity discrimination in pigmented rats

Auditory function was investigated in seven pigmented hooded rats (strain Long-Evans) with the aid of an operant conditioning procedure. Frequency difference limen was measured at frequencies from 0.5 to 64 kHz at 50 dB sensation level (SL). Weber ratios (frequency difference limen/frequency) in this range varied between 3.7 and 7.3%. The decline in the intensity of the stimulus from 50 to 10 dB SL was accompanied by a slight increase in the frequency difference limen. The frequency difference limen values were similar for frequency shifts upwards or downwards. Intensity discrimination was measured at 50 dB SL at frequencies of 2, 8 and 32 kHz. Intensity difference limen was frequency independent and amounted to 2.9 ± 0.5 dB in conditions of upward intensity shift. The values of intensity difference limen measured in conditions of downward intensity shift were significantly larger and amounted to 6.5 ± 1.6 dB. The characteristics of hearing function found in these experiments correspond with those described by other authors in albino rats and indicate that albinism in the rat has no significant influence on auditory frequency and intensity discrimination.     

The expression of apoptosis-related proteins in the aged cochlea of Mongolian gerbils

OBJECTIVE: Apoptotic changes have been reported in the aged gerbil cochlea and are speculated to be one of the principal causes of presbyacusis. The objective of the study was to determine the underlying mechanism of apoptotic change in the aged gerbil cochlea. STUDY DESIGN: Prospective controlled animal study. METHODS: We examined the tissue distribution of bcl-2, bax, caspase-3p20, and caspase-3p32 using immunohistochemical techniques in the young and aged gerbil cochlea, together with the measurement of the distortion product of otoacoustic emission (DPOAE). RESULTS: Aged gerbils showed a significant reduction of the DPOAE amplitude as compared with that of the young gerbils, suggesting a disturbance of the auditory function in the aged cochlea. There was a significant decrease in the number of bcl-2-positive cells in the aged gerbils. The expression of bax in the aged group was slightly increased but did not significantly differ from that in the young gerbils. A significantly increased number of caspase-3p20-positive cells was observed in the organ of Corti, spiral ganglion, and lateral wall of cochlea in the aged gerbils as compared with those of the young gerbils. There was no significant difference in the expression levels of caspase-3p32 between the young and aged groups. In the aged cochlea, the degree of deterioration of DPOAE responses was compatible with those of both the reduction of bcl-2 and the activation of caspase-3p20. CONCLUSION: These data suggest that the suppression of bcl-2 protein expression may lead to apoptosis-induced presbyacusis through activation of caspase-3 in the aged gerbil cochlea.     

Acoustic isolation reduces degeneration of the ventral cochlear nuclei in Mongolian gerbils

The role of auditory experience in the development of spongiform degeneration in the cochlear nuclei of Mongolian gerbils was studied by comparing results of animals exposed to either high or low levels of ambient noise. Gerbils reared in a typical vivarium experienced higher levels of ambient noise than animals reared in acoustic isolation chambers. Animals reared in the colony room showed a much greater number density and area density of spongiform lesions in the CN than did gerbils reared in acoustic isolation. The differences in the number and extent of spongiform lesions between the two groups of gerbils appeared to reflect their differences in exposure to ambient noise. These differences in lesion number and extent were most pronounced in the tonotopic regions of the PVCN which correspond to the greatest differences in the spectral characteristics of the ambient noise to which the animals were exposed. These results were compared with results previously obtained from gerbils with loss of hearing experimentally induced by a conductive block or by sensorineural damage. The lesion numbers and extent reflected the auditory experience of each group; in descending order, colony-reared, isolate, conductive-block, sensorineural loss. These results strongly support the hypothesis that this gerbilline encephalopathy is directly related to auditory functional activity.     

Auditory experience affects degeneration of the ventral cochlear nucleus in Mongolian gerbils

Gerbils exhibit a unique encephalopathy characterized by spongioform lesions in the neuropil of the cochlear nucleus and that others have recently described. The present results suggest that the course of this degenerative disorder is affected by acoustic experience. In gerbils in which acoustic stimulation was limited postnatally, the number and the extent of these lesions was dramatically reduced. Monaural deprivation reduced lesion number and extent only in the ipsilateral cochlear nucleus; binaural deprivation affected both cochlear nuclei. The lesions were most evident in certain portions of the cochlear nucleus, the caudal anterior ventral cochlear nucleus and the posterior ventral cochlear nucleus, sparing the rostral pole of the anterior ventral cochlear nucleus. In the affected regions the lesions were topologically restricted to the low and middle frequency regions. The apparent tonotopic distribution of lesions was associated with frequent exposure to low and middle frequency ambient noise.     

Immittance and otoacoustic emissions in rhesus monkeys and humans

The purpose of this study was to compare multifrequency tympanometry and otoacoustic emissions (OAEs) in rhesus monkeys (Macaca mulatta) and humans. Tympanometry and OAEs can be recorded efficiently in Macaca mulatta to assess peripheral auditory function with results comparable to those in humans. Differences include (1) greater admittances and conductances in humans from 226 to 630 Hz, the frequency range validly assessed; (2) larger amplitude transient evoked OAEs (TEOAEs) and noise levels in humans; (3) larger amplitude monkey 2f(1)-f(2) distortion product OAEs (DPOAES) (f(2)s>2 kHz); (4) more prominent DPOAEs other than 2f(1)-f(2) in monkeys; (5) more narrowly tuned human f(2)/f(1) X 2f(1)-f(2) amplitude functions at the lower frequencies tested; and (6) lower 2f(1)-f(2) DPOAE thresholds at f(2)=0.5 kHz and > or = 8 kHz in monkeys.     

Frequency discrimination deficits in people with specific language impairment: reliability, validity, and linguistic correlates.

The reliability and validity of a frequency discrimination (FD) task were tested in 16 people with specific language impairment (SLI) and 16 people with normal spoken language (controls). The FD thresholds of the 2 groups indicated that FD thresholds for 25-ms and 250-ms tones were remarkably stable across 18 months. The FD thresholds were lower for control listeners than for listeners with SLI for both duration conditions, and the FD thresholds for both groups of listeners were lower for 250-ms tones than for 25-ms tones. Moreover, the FD thresholds were influenced little by nonperceptual, task-related abilities (e.g., paired-associative learning, memory for temporal order, sustained attention, and control of attention) of the listener groups. The significant group difference between the mean FD thresholds of the SLI and control groups was explained by a subgroup of people with SLI who had particularly poor thresholds compared with those of controls and the majority of the SLI group. This subgroup did not differ from the remainder of the SLI sample in terms of age or nonverbal ability but was characterized by very poor reading that was associated with poor phonemic awareness.     

Recovery from prior stimulation. II: Effects upon intensity discrimination.

We obtained just-noticeable differences (jnds) for the intensity of pure tones following a forward masker. The masker was a 100-ms burst of narrow-band noise centered at 1000 Hz presented at 90 dB SPL; the pure-tone signal was at 1000 Hz and was 25 ms in duration. The masker-signal delay was 100 ms. Under these conditions, there is no threshold shift for the detection of the pure-tone signal following the forward masker. In contrast with the absence of a forward-masker effect upon detection thresholds, unusually large midlevel (40-60 dB SPL) jnds were observed. These large midlevel jnds were measured as a function of signal delay, revealing that they are not completely recovered to the normal (unmasked) values by 400 ms. We interpret these data as a consequence of the slower recovery of low-spontaneous rate, high-threshold neurons following prior stimulation (Relkin and Doucet, 1990). These experiments may therefore provide psychophysical evidence that the low-spontaneous rate, high-threshold neurons are a necessary physiological component in the coding of the large dynamic range for intensity. In addition, the present data provide evidence that the assumption that the effect of forward masking is limited to 100-200 ms is inappropriate, as this recovery time does not necessarily apply to suprathreshold tasks.     

Monaural and binaural intensity discrimination in normal and cochlear-impaired listeners

Monaural and binaural intensity difference limens for 75-dB SPL pure tones were determined for 6 normal subjects and for 6 subjects with cochlear hearing loss. The magnitude of binaural masking level difference (BMLD) was also determined. Both normal and hearing-impaired subjects showed a 0.45-dB binaural advantage for intensity discrimination. In contrast, the BMLD was appreciably reduced with hearing impairment. Results are discussed in terms of specific effects of hearing loss on binaural hearing.     

Effect of temporal separation on tone-sequence discrimination in monkeys

Perception of auditory spectral-temporal patterns was examined in two Japanese monkeys. The stimuli used were pairs of pure tones of different frequencies that were presented sequentially. The monkeys were required to discriminate whether the frequency of the second tone was higher or lower than the first tone. The performances of the monkeys deteriorated when a temporal gap (i.e., silence) was inserted between the component tones. A comparison experiment did not show such effects in human participants. The results suggested that monkeys use frequency transitions for tone-sequence discrimination, and that local characters are more dominant discrimination cues in monkeys than in humans.     

Frequency selectivity and speech discrimination in sensorineural hearing loss

Two supposed measures of auditory frequency selectivity--the critical band (CB) in loudness summation and the psychoacoustic tuning curve (PTC), Both measured at 1 kHz--were compared with the capacity for speech discrimination in patients with various cochlear disorders and a relatively flat audiometric pattern. The CB in loudness summation was correlated neither to the degree of hearing loss nor to the speech discrimination score. In contrast, the PTC changed with increasing hearing loss in the same manner as the electrophysiological tuning curve (FTC), i.e. rapidly deteriorating beyond normal limit values when the hearing loss exceeded 30--40 dB (HL). Nearly the same dependency of the degree of the hearing loss was demonstrated for the speech discrimination score (determined in noise and after filtering of the signal) and a significant correlation was present between this score and cochlear tuning, as expressed by the PTC. It is proposed that the PTC is a more valid measure of auditory frequency selectivity than the CB in loudness summation. If this is accepted the results seem to support the hypothesis of impaired frequency selectivity as a major cause for deteriorated speech discrimination in patients with cochlear disorders.