C. Formby

Tonotopic organization in human auditory cortex revealed by positron emission tomography

Positron emission tomography (PET) was used to map alterations in local neuronal activity induced in human primary auditory cortex by pure-tone stimulation. Patterns of blood flow were observed in specific regions on the superior temporal plane showing systematic changes in activity depending on the frequency of a stimulating pure tone. The orientation of these regions agrees well with data for non-human primates.

Modulation and gap detection for broadband and filtered noise signals

Modulation detection thresholds (as a function of sinusoidal amplitude modulation frequency) and temporal gap detection thresholds were measured for three low-pass-filtered noise signals (fc = 1000, 2000, and 4000 Hz), a high-pass-filtered noise signal (fc = 4000 Hz), and a broadband signal. The two latter noise signals were effectively low-pass filtered (fc = 6500 Hz) by the earphone. Each of the filtered signals was presented with a complementary filtered noise masker. Modulation and gap detection thresholds were lowest for the broadband and high-pass signals. Thresholds were significantly higher for the low-pass signals than for the broadband and high-pass signals. For these tasks and conditions, the high-frequency content of the noise signal was more important than was the signal bandwidth. Sensitivity (s) and time constant (tau) indices were derived from functions fitted to the modulation detection data. These indices were compared with gap detection thresholds for corresponding signals. The gap detection thresholds were correlated inversely (rho = -1.0, p less than 0.05) with s (i.e., smaller gap detection thresholds were correlated with greater sensitivity to modulation), but were not correlated significantly with tau, which was relatively invariant across signal conditions.

Temporal gap detection measured with multiple sinusoidal markers: Effects of marker number, frequency, and temporal position

Detection thresholds were measured for silent temporal gaps within combinations of two, three, or four sinusoidal markers (i.e., combinations of one or two pre-gap markers with one or two post-gap markers). The markers were selected from the frequency range 2000-3100 Hz. Sinusoidal frequencies F1 and F4 were used as pre-gap markers, while F2 and F3 served as post-gap markers. Temporal gap detection (TGD) thresholds were measured from sets of three normal-hearing adults who tracked 70.7% correct detection thresholds adaptively across blocks of 50 two-interval, two-alternative, forced-choice trials. For symmetric marker conditions, where pre- and post-gap markers were equivalent in frequency (e.g., F1 = F2 or F1 = F2 and F3 = F4), TGD thresholds were < 10 ms. However, for asymmetric marker frequency alignments across the silent gap, including stimulus configurations where only three markers were presented on a trial (e.g., F1 = F2, F2 not equal to F3, no F4), performance was highly variable and was dramatically disrupted by the presentation of a second post-gap marker. The multiple-marker results reveal that TGD depends greatly on the number of markers presented, both in terms of the marker temporal position before and after the silent gap signal and the marker frequency alignment (symmetry) across the gap. These results, which cannot be predicted from models of the auditory periphery, may reflect perceptual mechanisms that are important in grouping and organizing auditory images.

The role of frequency selectivity in measures of auditory and vibrotactile temporal resolution

The purpose of this study was to compare the role of frequency selectivity in measures of auditory and vibrotactile temporal resolution. In the first experiment, temporal modulation transfer functions for a sinusoidally amplitude modulated (SAM) 250-Hz carrier revealed auditory modulation thresholds significantly lower than corresponding vibrotactile modulation thresholds at SAM frequencies greater than or equal to 100 Hz. In the second experiment, auditory and vibrotactile gap detection thresholds were measured by presenting silent gaps bounded by markers of the same or different frequency. The marker frequency F1 = 250 Hz preceded the silent gap and marker frequencies after the silent gap included F2 = 250, 255, 263, 310, and 325 Hz. Auditory gap detection thresholds were lower than corresponding vibrotactile thresholds for F2 markers less than or equal to 263 Hz, but were greater than the corresponding vibrotactile gap detection thresholds for F2 markers greater than or equal to 310 Hz. When the auditory gap detection thresholds were transformed into filter attenuation values, the results were modeled well by a constant-percentage (10%) bandwidth filter centered on F1. The vibrotactile gap detection thresholds, however, were independent of marker frequency separation. In a third experiment, auditory and vibrotactile rate difference limens (RDLs) were measured for a 250-Hz carrier at SAM rates less than or equal to 100 Hz. Auditory RDLs were lower than corresponding vibrotactile RDLs for standard rates greater than 10 Hz. Combination tones may have confounded auditory performance for standard rates of 80 and 100 Hz. The results from these experiments revealed that frequency selectivity influences auditory measures of temporal resolution, but there was no evidence of frequency selectivity affecting vibrotactile temporal resolution.

Detection of time- and bandlimited increments and decrements in a random-level noise.

The purpose of this study was to compare detection of increments and decrements occurring over limited regions of time and frequency within a 500-ms broadband (0-6000 Hz) noise. Three listeners tracked detection thresholds adaptively in a two-interval, two-alternative forced-choice task. Thresholds were measured for both increments and decrements in level [delta L = 10 log10(1 + delta N0/N0) dB, where N0 is the spectral power density of the noise] as a function of signal duration (T = 30-500 ms) for a range of signal bandwidths (W = 62-6000 Hz) that were logarithmically centered around 2500 Hz. Listeners were forced to rely on temporal- and spectral-profile cues for detection due to randomization of overall presentation level from interval to interval, which rendered overall energy an inconsistent cue. Increments were detectable for all combinations of W and T, whereas decrements were not consistently detectable for W < 500 Hz. Narrow-band decrements were not detectable due to spread of excitation from the spectral edges of the noise into the decrements. Increment and decrement thresholds were similar for W > or = 1000 Hz. Temporal- and spectral-integration effects were observed for both increments and decrements. The exceptions were for random-level conditions in which the signal matched the bandwidth or duration of the standard. A multicue decision process is described qualitatively to explain how the combination of temporal- and spectral-profile cues can produce temporal- and spectral-integration effects in the absence of overall-energy cues.

Modification of Loudness Discomfort Level: Evidence for Adaptive Chronic Auditory Gain and Its Clinical Relevance

A vexing problem in audiology has been the modification of sound tolerance. Sound intolerance and restricted dynamic ranges are common conditions that audiologists encounter daily in the hearingimpaired population, especially in the fitting of hearing aids. To date, no clinical protocol has proven to be successful for modifying sound tolerance among the hearing-impaired population. This report describes the use of low-level, broadband sound in a habituation-based treatment protocol termed Tinnitus Retraining Therapy (TRT). Over the past decade, TRT has become increasingly popular as an intervention for severe tinnitus and hyperacusis. One of the primary treatment effects from TRT is that over the course of the intervention the patient’s loudness discomfort levels (LDLs) routinely shift to higher levels. Ostensibly, the resulting higher LDLs reflect treatment effects associated with the resetting of a plastic and adaptive auditory gain process that somehow regulates the suprathreshold sensitivity of the auditory system in response to chronic changes in the sound input from the auditory periphery to the central auditory pathways. Tinnitus patients with significant hearing losses and sound tolerance problems respond successfully to TRT and, as a consequence, their LDLs are usually elevated (improved) and, concomitantly, their dynamic ranges are expanded. Many of these patients, who before the TRT intervention could not tolerate amplified sound, then have been able to make a comfortable transition into appropriate amplification after treatment. In principle, TRT would seem to offer a viable intervention strategy for modifying sound tolerance in the general hearing-impaired population. If so, then clinical applications of TRT principles may extend well beyond the treatment of tinnitus and hyperacusis. T hi s do cu m en t w as d ow nl oa de d fo r pe rs on al u se o nl y. U na ut ho riz ed d is tr ib ut io n is s tr ic tly p ro hi bi te d. 180 SEMINARS IN HEARING/VOLUME 23, NUMBER 2 2002 In the abstract and body of the original article, the terminology “threshold” was inadvertently used in association with loudness discomfort level (LDL). The abstract has been reprinted here to correct this mistake, which is an erratum also applicable to the corresponding terminology in the body of the report. Page 31, 1st column, 1st paragraph, last three sentences: Consider, for example, the discussion by Allen and his colleagues of the results for their Loudness Growth in ⁄2 Octave Bands (LGOB) protocol.83 These investigators described findings for a pair of audiometrically normal subjects who appeared to show a form of loudness recruitment based upon their abnormally reduced response patterns for LOUD and TOO LOUD sound levels.83(p 752) The anomaly they were describing was likely hyperacusis rather than loudness recruitment.

Effects of Randomizing Signal Level and Duration on Temporal Gap Detection

Although discriminable changes in stimulus energy or overall duration may accompany the silent temporal gap, there is little evidence that these extraneous cues confound the measurement of temporal gap detection threshold. In this report we show that (1) under conditions where gap detection thresholds are large in relation to the duration of the standard, extraneous cues may confound gap detection leading to underestimates of the true threshold, and (2) randomization of overall stimulus level and duration can successfully remove confounding energy and duration cues without distracting the listeners attention from the temporal-gap cue.

The effects of continuous phonation on 133xenon-inhalation air curves (of the kind used in deriving regional cerebral blood flow).

Regional cerebral blood flow (rCBF) may be measured with inhalation techniques that use end-expired values of radioactivity to estimate the isotope concentration in arterial blood. These end-expired data are used as an input function in a mathematical equation to derive rCBF. End-expired air is assumed normally to be in equilibrium with the arterial blood at the alveolar surface of the lung during regular (passive) breathing; this assumption may not be valid during continuous phonation. We therefore have analyzed breathing (inhalation/exhalation) patterns and end-expired radioactivity (133Xe) during (1) speaking, (2) singing, and (3) humming of the national anthem, and also during (4) passive breathing. Statistically significant differences in breathing patterns were measured between a group of nonmusicians and two groups of musicians (singers) during the phonation tasks: The nonmusicians breathed more often (and more rapidly) and exhibited less variability in their breathing patterns than did the musicians. Notwithstanding these differences, the shapes of smoothed functions derived from the end-expired values were not influenced appreciably during phonation (except possibly during talking). The latter finding suggests that estimates of rCBF derived with these data should not be confounded seriously because of the continuous phonation.

Hearing loss among stroke patients.

Pure-tone hearing thresholds were measured by air conduction for 140 right hemiplegic and 103 left hemiplegic patients who had suffered a single stroke. Statistical and informal testing of the mean audtometric thresholds revealed that: (1) the ability of the aphasic patient to attend to and to perform the audiometric test was independent of the severity of the aphasic impairment; (2) aphasic patients did not suffer disproportionately greater hearing losses than did other right hemiplegic stroke patients; (3) no relation was apparent between the severity of the hearing loss and the test ear or side of hemiplegia after stroke; (4) the severity of high-frequency hearing loss among stroke patients was consistent with that found in the eiderly male population at Framingham, MA (Moscicki, Elkins, Baum, and McNamara, Ear Hear 1985;6:184–90); and (5) the prevalence of hearing loss among the stroke patients was greater than that reported at Framingham, but was consistent with rates found among elderly nursing home residents.

Pure-Tone Masking of Tinnitus

Experiments were initiated to determine the frequency most closely associated with a continuous atonal tinnitus reported by a listener with a sloping sensorineural hearing loss in his left ear. The procedure was modeled after that used to obtain a psychophysical tuning curve. Pure tones presented binaurally and monaurally between 521 and 3629 Hz were adjusted in level by the listener to just mask his tinnitus, which functioned as a signal. Masking curves for binaural and monaural masker presentation were found to be of similar shape but they varied in terms of absolute level, variability of estimates, and with regard to fine detail of the curves. The magnitude of the tinnitus as also estimated by a loudness match with a pure tone to the opposite ear. The masking levels required to mask the tinnitus were found to be consistent with those reported necessary to mask the equally loud pure tone. These findings are consistent with Langenbecks [1953, 1965] hypothesis that tinnitus arising from the inner ear should be masked in a manner similar to an externally presented tone at the same effective level. Information derived from the masking of tinnitus by pure tones may be useful in fitting tinnitus-masking devices and also for diagnosing the site of lesion associated with tinnitus.