Temporal integration of sinusoidal increments in the absence of absolute energy cues

Classical temporal integration (TI) is often viewed as a frequency-dependent, energy-based detection process. Detection thresholds for brief sinusoidal increments in either a fixed-level or a random-level broadband pedestal are reported that refute this traditional perspective of TI, Instead, evidence is presented that indicates (a) detection of absolute energy is not necessary for the TI effect and (b) the frequency dependence of TI is consistent with variations across frequency in peripheral auditory tuning, rather than the integration process per se. When peripheral frequency selectivity is controlled, TI can be explained by a frequency-invariant integration process over at least the frequency range from 500 to 4,000 Hz. This process is characterized by threshold improvements of 8-9 dB per decade increase in duration for increment durations between 10 and 300 ms.

Exponential processes in human auditory excitation and adaptation

Peripheral auditory adaptation has been studied extensively in animal models, and multiple exponential components have been identified. This study explores the feasibility of estimating these component processes for human listeners with a peripheral model of adaptation. The processes were estimated from off-frequency masked detection data that probed temporal masking responses to a gated narrowband masker. The resulting response patterns reflected step-like onset and offset features with characteristically little evidence of confounding backward and forward masking. The model was implemented with linear combinations of exponential functions to represent the unadapted excitation response to gating the masker on and then off and the opposing effects of adaptation in each instance. The onset and offset of the temporal masking response were assumed to be approximately inverse operations and were modeled independently in this scheme. The unadapted excitation response at masker onset and the reversed excitation response at masker offset were each represented in the model by a single exponential function. The adaptation processes were modeled by three independent exponential functions, which were reversed at masker offset. Each adaptation component was subtractive and partially negated the unadapted excitation response to the dynamic masker. This scheme allowed for quantification of the response amplitude, action latency, and time constant for the unadapted excitation component and for each adaptation component. The results reveal that (1) the amplitudes of the unadapted excitation and reversed excitation components grow nonlinearly with masker level and mirror the 'compressive' input-output velocity response of the basilar membrane; (2) the time constants for the unadapted excitation and reversed excitation components are related inversely to masker intensity, which is compatible with neural synchrony increasing at masker onset (or offset) with increasing masker strength; (3) the composite strength of adaptation levels off at high masker levels; this 'saturation' response is consistent with a diminished contribution from peripheral neural adaptation processes at high sound levels; and (4) the response dynamics for two of the adaptation components correspond generally to those for the 'very rapid'/'rapid' processes and 'short-term' processes described in animal studies of peripheral neural adaptation. The action latency of a third adaptation component suggests the role of a second-order peripheral or central process. This modeling exercise (1) indicates that multiple adaptation processes, whatever their origins, contribute substantively to the form of the temporal masking response and (2) supports a sum-of-exponentials scheme for estimating properties of the component processes.

Celebrating a decade of evaluation and treatment: the University of Maryland Tinnitus & Hyperacusis Center

The University of Maryland Tinnitus & Hyperacusis Center in Baltimore was the first center in the United States dedicated to the evaluation and treatment of tinnitus and hyperacusis patients implementing an habituation-based protocol that has become known internationally as Tinnitus Retraining Therapy (TRT). A crucial feature of the model is the postulate that a number of systems in the brain are involved in the emergence of tinnitus. The cochlea and auditory periphery play only a secondary role. To facilitate the goal of habituation of the tinnitus signal, TRT implements both directive counseling to neutralize the negative emotional associations toward the tinnitus, and sound therapy to interfere with the signal. As an outgrowth of the work with tinnitus, the evaluation and treatment of hyperacusis has emerged as an increasingly important part of our program. This report describes the unique facility, staff, and services of the Center as we celebrate a decade of research and clinical management dedicated to the scientific understanding of tinnitus and hyperacusis.

Measurement of vestibular ocular reflex (VOR) time constants with a caloric step stimulus

A protocol is described for measuring responses to a broad-band (1-2 Hz) caloric step stimulus from which the vestibular ocular reflex (VOR) and adaptation time constants can be estimated. This novel stimulation is the caloric equivalent to a rotatory step of head acceleration. In this protocol, the ear is irrigated continuously for 5 min with water at a constant temperature. During the initial 2-min period of irrigation the subject is seated and leaning forward in a nonstimulable position (horizontal canals in a horizontal plane). This irrigation phase establishes a steady-state thermal gradient across the horizontal canal, effectively eliminating thermal dynamic properties of the caloric transmission as a confounding factor. At the end of this phase, the subject is rapidly reclined to a stimulable position (horizontal canals in vertical plane) that elicits the VOR nystagmus response to an on-step of force on the cupula. Consistent with adaptation processes, the VOR response first increases and then declines gradually over the 2-min period that the step of force is maintained. Four minutes after the onset of irrigation, the subject is rapidly returned to the nonstimulable position (off-step), which is then maintained for a final 1 min. The response after the off-step, which releases the force on the cupula, reveals reversed after-nystagmus due to adaptation. Five subjects provided caloric step responses for 26 caloric temperature conditions spanning the range from 28.4 to 43.0 degrees C. The resulting responses were fitted with an adaptation model similar to models applied to rotatory acceleration step responses. Estimates of the model parameters for robust caloric stimulation, including time constants for the VOR (18.3 sec) and for vestibular adaptation (153.2 sec), are considered in relation to corresponding values reported in the literature for rotatory and caloric vestibular stimulation. The results suggest that caloric step stimulation can be used successfully to probe VOR dynamics.

Enhancements of the edges of temporal masking functions by complex patterns of overshoot and undershoot

The purpose of this report is to present new data that provide a novel perspective on temporal masking, different from that found in the classical auditory literature on this topic. Specifically, measurement conditions are presented that minimize rather than maximize temporal spread of masking for a gated (200-ms) narrow-band (405-Hz-wide) noise masker logarithmically centered at 2500 Hz. Masked detection thresholds were measured for brief sinusoids in a two-interval, forced-choice (21FC) task. Detection was measured at each of 43 temporal positions within the signal observation interval for the sinusoidal signal presented either preceding, during, or following the gating of the masker, which was centered temporally within each 500-ms observation interval. Results are presented for three listeners; first, for detection of a 1900-Hz signal across a range of masker component levels (0-70 dB SPL) and, second, for masked detection as a function of signal frequency (fs = 500-5000 Hz) for a fixed masker component level (40 dB SPL). For signals presented off-frequency from the masker, and at low-to-moderate masker levels, the resulting temporal masking functions are characterized by sharp temporal edges. The sharpness of the edges is accentuated by complex patterns of temporal overshoot and undershoot, corresponding with diminished and enhanced detection, respectively, at both masker onset and offset. This information about the onset and offset timing of the gated masker is faithfully represented in the temporal masking functions over the full decade range of signal frequencies (except for fs=2500 Hz presented at the center frequency of the masker). The precise representation of the timing information is remarkable considering that the temporal envelope characteristics of the gated masker are evident in the remote masking response at least two octaves below the frequencies of the masker at a cochlear place where little or no masker activity would be expected. This general enhancement of the temporal edges of the masking response is reminiscent of spectral edge enhancement by lateral suppression/inhibition.

Measurement of vestibular ocular reflex (VOR) time constants with a caloric step stimulus*

A protocol is described for measuring responses to a broadband (1–2 Hz) caloric step stimulus from which the vestibular ocular reflex (VOR) and adaptation time constants can be estimated. This novel stimulation is the caloric equivalent to a rotatory step of head acceleration. In this protocol, the ear is irrigated continuously for 5 min with water at a constant temperature. During the initial 2-min period of irrigation the subject is seated and leaning forward in a nonstimulable position (horizontal canals in a horizontal plane). This irrigation phase establishes a steady-state thermal gradient across the horizontal canal, effectively eliminating thermal dynamic properties of the caloric transmission as a confounding factor. At the end of this phase, the subject is rapidly reclined to a stimulable position (horizontal canals in vertical plane) that elicits the VOR nystagmus response to an on-step of force on the cupula. Consistent with adaptation processes, the VOR response first increases and then declines gradually over the 2-min period that the step of force is maintained. Four minutes after the onset of irrigation, the subject is rapidly returned to the nonstimulable position (off-step), which is then maintained for a final 1 min. The response after the off-step, which releases the force on the cupula, reveals reversed after-nystagmus due to adaptation. Five subjects provided caloric step responses for 26 caloric temperature conditions spanning the range from 28.4 to 43.0 ∘ C. The resulting responses were fitted with an adaptation model similar to models applied to rotatory acceleration step responses. Estimates of the model parameters for robust caloric stimulation, including time constants for the VOR (18.3 sec) and for vestibular adaptation (153.2 sec), are considered in relation to corresponding values reported in the literature for rotatory and caloric vestibular stimulation. The results suggest that caloric step stimulation can be used successfully to probe VOR dynamics.

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.

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.

Evidence for an across-frequency, between-channel process in asymptotic monaural temporal gap detection

Monaurally measured temporal gap detection (TGD) thresholds characteristically increase as the frequency difference is increased over a range of about half an octave to an octave between two sinusoids that mark the onset and offset of the silent gap. For greater sinusoidal frequency separations, the TGD thresholds often become asymptotic. This pattern probably reflects two different processes. The first process likely reflects within-channel processing within a single auditory filter or channel. The second process is less certain, but may reflect between-channel processing of the silent gap stimulus across two or more independent frequency channels. To evaluate the hypothesis that asymptotic monaural gap detection can be explained by a simple between-channel process, TGD thresholds were measured as a function of frequency separation between a pregap sinusoid presented to the left ear (channel 1) and a postgap sinusoid, of higher frequency, presented to the right ear (channel 2). The rationale for dichotic presentation of the sinusoidal markers and gap signal followed from the fact that the gap detection task must be performed between two independent channels by combining the outputs from each channel (ear) and recovering the gap information centrally. The resulting TGD thresholds for pregap sinusoids from 250 to 4000 Hz were relatively invariant and increased only slightly with increasing marker frequency separation. The average TGD thresholds for four listeners were in the range of 30 to 40 ms, which corresponded closely with their asymptotic TGD thresholds for the same set of stimulus conditions measured monaurally. This correspondence of the two data sets supports an across-frequency, between-channel process for asymptotic monaural gap detection at marker frequency separations greater than about half an octave.

Evaluation of a maximum likelihood procedure for measuring pure-tone thresholds under computer control

An adaptive, maximum likelihood (ML) procedure was assessed as an automated tool for estimating audiometric pure-tone thresholds in the clinic under computer control. Pure-tone air-conduction thresholds were measured from 101 workmen who received annual hearing rechecks as part of their employee hearing conservation program. A pure-tone threshold was measured bilaterally for each of the standard audiometric frequencies in a 15-trial block to yield 60 percent correct detection with the ML procedure. The workmen were tested on a modified "yes-no" task. On a trial, the signal was presented in a visually cued 200-msec observation interval. Each workman then had 1000 msec to make a "yes" response. If the workman did not respond during the 1000-msec response period, then the computer assumed a "no" response. After either the "yes" or "no" response, the computer adjusted the signal level for the next trial. The thresholds measured by ML procedure compared favorably with thresholds measured from the same listeners by conventional (CONV) audiometry. The efficiency of the ML procedure was also compared in terms of the time necessary for an experienced audiologist to instruct the listener and perform CONV audiometry. CONV audiometry (3-4 minutes per listener) required about half of the time needed for the ML procedure (6-7 minutes per listener). The relatively longer time associated with measuring an audiogram with the ML procedure was due primarily to more trials being used to estimate threshold.

Labelling and discrimination of a synthetic fricative continuum in noise: a study of absolute duration and relative onset time cues

Categorical perception was evaluated for a nine-token voice onset time (VOT) continuum with endpoint tokens /feil/-/veil/. The synthetic speech continuum was presented in a random-level noise masker at different signal-to-noise ratios (SNR = 0, + 6, +12 dB) and overall presentation levels (50 and 70 dB HL). Overall labelling performance deteriorated as the SNR was reduced. Labelling results for the +12-dB-SNR condition reflected a category boundary at 87 ms for listeners with normal hearing sensitivity. The companion two-step discrimination function revealed better-than-chance performance between pairs of tokens labelled fail, chance performance between pairs of tokens labelled vail, and a slight performance peak at the labelling boundary between fail and vail. Listeners with high-frequency audiometric deficits produced labelling results for the +12-dB-SNR condition that were similar to normal functions measured for the 0-dB-SNR condition. These listeners were unable to discriminate two-step differences in voicing duration, but they produced a normal temporal labelling boundary. To try to understand the noncategorical discrimination data, a psychoacoustic analog for the speech continuum was evaluated. Relative onset time (ROT) difference limens (DLs) were measured as a function of the temporal onset delay of a low-frequency sawtooth waveform relative to the onset of a high-frequency noise burst. The ROT cue was used only when absolute stimulus duration could not be relied upon as a consistent cue, under conditions where a large range of random overall duration was presented to the listener. The ROT DLs were relatively invariant over a range of standard delays from 50 to 110 ms. The average DL was about 30 ms, which is consistent with the small performance peak in the synthetic speech discrimination function.

Masked detection thresholds and temporal integration for noise band signals

Masked detection thresholds were measured at a center frequency of 2500 Hz for a range of noise signal bandwidths (W = 62 to 6000 Hz) and durations (T = 10 to 480 ms) approximating that found acoustically in speech. The signals were presented in an uncorrelated 480-ms, 6000-Hz-wide masker. The masker was presented: (1) at a constant spectrum level (53 dB SPL/Hz) or (2) with the overall level varied randomly over a 50-dB range from interval to interval of a trial. Performance was disrupted in the random-level masker only for the condition where the signal and uncorrelated masker were gated on and off simultaneously and were matched spectrally. Time constants (tau) estimated from temporal integration functions fit to the masked detection threshold data were related inversely to W for W broader than the critical bandwidth. Sensitivity to the noise signals was evaluated in the context of an optimum-detection model (Green, 1960). The results did not follow the prediction of a constant bandwidth-duration (WT) product, but may be understood in terms of cues available to the listener from the relative combination of signal and masker parameters. At least three cues for detection were identified in these experiments: (1) a relative timing cue, (2) a spectral shape cue, and (3) a traditional energy cue compared across observation intervals. The relative timing cue and spectral shape cue together contributed as much as a 10- to 12-dB advantage relative to detection based on the traditional energy cue alone. A new multi-cue detection model for predicting the masked detection thresholds was derived. Predictions from the new model were highly correlated (r = 0.95) with the empirically measured masked detection thresholds.

Detection of silent temporal gaps between narrow-band noise makers having second-formantlike properties of voiceless stop/vowel combinations

Temporal gap detection thresholds were measured in narrow-band noise-burst markers having acoustic characteristics representative of isolated steady-state second-formant (F2) properties for/p,t,k/paired separately with/i,ae,u,o/. The results revealed that gap detection threshold increased systematically as the difference was increased between the simulated stop and vowel F2 frequencies. A strong positive correlation (r = 0.87) between gap detection threshold and linear marker center frequency difference was highly significant (p < 0.001). Differences in other stimulus features had little influence on gap detection performance. Implications for speech perception are discussed.

Measurement, analysis, and modelling of the caloric response. 2. Evaluation of mental alerting tasks for measurement of caloric-induced nystagmus

Management of the patient's level of arousal is one of the most important variables in obtaining consistent and strong caloric responses. The patient may suppress the caloric response and/or exacerbate beat-to-beat variability if some type of mental alerting (MA) task is not used to focus the patient's attention from his or her dizziness. An experiment was undertaken to evaluate simple MA tasks in terms of associated caloric response strength and variability. Warm caloric responses were measured while each of 10 normal subjects performed eight different MA tasks. The mental exercises included two math tasks, two quizzing tasks, two hand-motor tasks, and two alphabet tasks. One of the tasks in each complementary pair required the subject to interact with the examiner throughout the caloric response. Minimal or no interaction was required for the companion task. The relative ordering among the eight MA tasks was compared in terms of total sum of ranks, summed across 15 performance measures taken from caloric response indices. The highest-ranked altering task was an exercise requiring subjects to name or list local cities, states in the U.S.A., colors, etc. The lowest-ranked tasks were backward counting exercises and reflexive quizzing, which are the traditional tasks used in the clinic.

Measurement, analysis, and modelling of the caloric response. 1. A descriptive mathematical model of the caloric response over time

A mathematical model for describing the caloric response over time offers many important advantages over the commercially-available qualitatively-fitted curves that are now used by the clinician for evaluating caloric results. In this report advances in the development of a nonlinear least-squares mathematical model are discussed and the roles and derivations of fitting parameters and curve-derived indices are outlined. This model provides a rigorous and objective description of the caloric response in its entirety with four continuous parameters. These fitting parameters make it possible to 1) describe individual caloric responses precisely and uniquely, 2) compare pairs of individual caloric responses or groups of caloric responses statistically, 3) extract information not previously available, 4) quantify variability within the caloric response, and 5) model physical properties of the caloric stimulus and physiological variables affecting the caloric response. Results from this model are compared with the results from our earlier models and with traditional multiparameter caloric results.

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.

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 listener's attention from the temporal-gap cue.

Correlations between hearing thresholds and caloric responses among a heterogeneous sample of dizzy patients

Auditory and vestibular function were compared in a heterogeneous sample of dizzy patients (N = 52). Hearing thresholds for the conventional audiometric frequencies were measured for each patient and parceled into two frequency ranges, 0.25-1 kHz and 2-8 kHz. Hearing thresholds also were measured for each patient over an extended high-frequency range that included the frequencies 10, 12, and 14 kHz. Bithermal caloric responses for these patients were available and were grouped for unilateral weakness (UW) or no unilateral weakness (NOUW). Hearing thresholds ipsilateral to the side of unilateral weakness (UW) yielded significant (p less than .05) modest correlations (r = .39-.52) with UW for all three audiometric frequency ranges (N = 35). These findings do not suggest a strong tonotopic relation between the audiometric and UW data. Hearing thresholds for the frequency range 10-14 kHz, but not for the conventional audiometric frequencies, correlated with slow-phase eye velocity (SPV) when SPV was averaged across the four caloric conditions for each of 17 NOUW patients. The latter finding indicates a subtle trend for eye velocity to increase as a function of increasing hearing threshold.

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 audiometric 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 elderly 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.

Modulation threshold functions for chronically impaired Menière patients

Detection thresholds for sinusoidally amplitude-modulated broad-band noise were measured as a function of modulation frequency for both ears of 6 chronic Menière patients who suffered unilateral hearing impairments. Modulation thresholds were measured with an adaptive cued-standard forced-choice psychophysical method. Better-ear modulation thresholds were similar to normative data previously reported [Formby, C.: J. acoust. Soc.Am. 78:70-77, 1985], whereas 5 of the 6 patients exhibited deficits in modulation detection with their poorer ears. Modulation thresholds averaged across the patients' poorer ears were similar to the normative thresholds through 60-100 Hz; at higher modulation frequencies, sensitivity declined at approximately twice the normal attenuation rate (i.e., 6 vs. 3 dB per octave). The poorer-ear data can be described by the mathematical representation for a simple low-pass filter with a cutoff frequency of 60 Hz. This pattern of the Menière modulation thresholds is consistent with broadened peripheral tuning due to hydrops.

Changes in caloric nystagmus induced by vasodilating drugs

Systemic blood flow, blood flow to the peripheral skin, middle ear, and temporal bone, and that to the inner ear of normal volunteers was manipulated selectively through the use of vasodilator drugs. The effect of selective vasodilation was monitored via changes in the temporal course of the slow-phase velocity of nystagmus evoked by bithermal caloric irrigation. Drug influenced eye velocities were compared with control values measured before drug administration. Eye velocities measured after niacin (a peripheral vasodilator) administration generally were greater than control values, whereas eye velocities measured after papaverine (a central vasodilator) and histamine (a systemic vasodilator) administration typically were less than control values. These findings, with the exception of the cool caloric/niacin condition, are consistent with a model of vestibular dynamics and measurements of cochlear blood flow in animal models.

Frequency and rate discrimination by Menière patients

Discrimination tests were conducted for each ear of 6 patients with unilateral Menière's disease. Differential sensitivity to changes in pure-tone frequency (presumably containing temporal and place cues) was compared to differential sensitivity to changes in rate of sinusoidally amplitude-modulated noise (presumably containing only a temporal cue) at common low frequencies and modulation rates (less than 400 Hz). An adaptive forced-choice psychophysical procedure was used. The better-ear results for the Menière patients generally were similar to findings for normally hearing listeners. For the impaired ears, pure-tone frequency discrimination typically was impaired relatively more than was the rate discrimination up to 200 Hz. However, rate discrimination was difficult or impossible above 250-300 Hz. Absolute frequency difference limens (DLs) were, on average, smaller than or at least equivalent to absolute rate DLs. Moreover, frequency discrimination always was possible for the Menière patients.

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.

Frequency selectivity and thresholds of brief stimuli suitable for electric response audiometry

Auditory evoked potentials are nearly all on-effects and the 'effective stimuli' for them are necessarily brief. Their frequency specificity is therefore limited, especially for the brainstem responses, because of the well-known trade-off between duration and frequency specificity. Brainstem responses are of special interest because they are unchanged in the sleep-like sedation that is required for difficult-to-test children. The middle-latency responses do not meet this requirement. Two patterns of tone burst that are appropriate and promising for the slow cortical potentials and for brainstem potentials, respectively, have rise and fall times of 2 periods of the modulated tone and plateaus of 10 (or 7) periods and 1 period, respectively. Their behavioral thresholds are nearly insensitive to difference in repetition rate between 4 and 40 stimuli/s. Their peak equivalent SPL threshold values at 500, 1 000, 2 000 and 4 000 Hz have been determined for 16 otologically normal ears. Using these reference levels, audiograms have been obtained for subjects with impaired hearing. The audiograms for 'flat' hearing losses do not differ significantly from the corresponding conventional pure-tone audiograms. The slopes for steep high-frequency hearing losses are underestimated, however, particularly with the brief (2-1-2) pattern. Nevertheless, the 2-1-2 pattern appears to be close to the best possible compromise.

Long-term average speech spectra for normal and hearing-imparied adolescents

Acoustical aspects of the speech of hearing-imparied persons have usually been examined over brief periods of time for simple utterances. Of interest in this study was whether abnormal acoustical patterns would be observed when speech was examined over longer periods of time. Spectra were averaged for 20 hearing-impaired adolescents, for whom we had audiometric profiles and an assessment of speaker intelligibility, and also for ten normal-hearing adolescents. An analyzer having constant-frequency bandwidths of 60 Hz, with the center frequencies of the filters spaced at 40-Hz intervals across a 10-kHz range, was used in the measurements (averaging time 27 s). The normal spectra are characterized by a regular pattern of peaks occurring at multiples of the talkers' fundamental frequencies and by slopes declining at rates of -5 to -6 dB/octave. After correction for lip-radiation impedance, these slopes are similar to that reported for the normal glottal source ( - 12 dB/octave). The hearing-impaired adolescents produced spectra for which the harmonic structure ranged from the very well defined to the irregular and poorly defined; spectral slopes declined at rates equal to or greater than the normal rate, in some cases declining at twice the normal rate.

Frequency-discrimination ability of hearing-impaired listeners

Thresholds for frequency modulation were measured by an adaptive, two-alternative, forced-choice method with ten listeners: eight who showed varying degrees of sensorineural hearing impairment, and two with normal-hearing sensitivity. Results for test frequencies spaced at octave intervals between 125 and 4000 Hz showed that, relative to normal-hearing listeners, the ability of the hearing-impaired listeners to detect a sinusoidal frequency modulation: (1) is diminished above a certain level of hearing loss; and (2) is more disrupted for low-frequency tones than for high-frequency tones, given the same degree of hearing loss at the test frequency. The first finding is consistent with that of previous studies which show a general deterioration of frequency-discrimination ability associated with moderate, or worse, hearing loss. It is proposed that the second finding may be explained: 1) by differential impairment of the temporal and place mechanisms presumed to, encode pitch at the lower and higher frequencies, respectively; and/or, 2) for certain configurations of hearing loss, by the asymmetrical pattern of cochlear excitation that may lead to the underestimation, from measurements of threshold sensitivity, of hearing impairment for low-frequency tones and consequently to relatively large changes in frequency discrimination for small shifts in hearing threshold.

Some systematic observations on monaural diplacusis

Anomalies of monaural pitch perception, including pitch changes with level, roughness and beats, were mapped in the intensity-frequency plane by a listener using continuous-tone stimulation between approximately 1 900 and 2 750 Hz. A narrow region characterized by rapid threshold adaptation was also mapped between approximately 2 590 and 2 690 Hz. In the procedure used to localize the internally generated tones, which were interacting with the externally presented single tones to produce beats, the listener adjusted the frequency of a signal presented to this right ear so that the beat rate heard there was the same as a criterion beat rate produced by two external tones in his left ear. A plot in the intensity-frequency plane of adjustments for a constant-beat rate revealed a remarkable correspondence to the region of adaptation (between 2 590 and 2 690 Hz) in that the former curve was simply shifted in frequency away from the latter by an amount equal to the two-tone interval producing the beat rate. Thus, the locations of 'internal tones' correspond predictably to both the lower and upper-frequency boundaries of the region of adaptation. Pitch changes, as a function of signal level, at frequencies below the region of adaptation were also investigated and appear to reflect a change from interaction to cessation of interaction between the signal and the region of adaptation. The locus of these pitch-related abnormalities, over a broad range of frequencies, points to interaction between that range and a smaller-frequency region where abrupt transitions in sensitivity occur.

Psychophysical tuning curves for combination tones 2f1-f2 and f2-f1

A simultaneous masking paradigm was employed to demonstrate that combination tones are "stimulus-like" in nature. Three musically sophisticated listeners adjusted the level of pure tones at various frequencies to just mask a pulsing signal corresponding to a tone at 1200 Hz and 20 dB SPL. The signal was produced by four stimulus conditions: (1) a pulsing tone at 1200 Hz; (2) a combination tone at 2f1-f2 = 1200 Hz produced by a continuous f1 at 1650 Hz and a pulsing f2 at 2100 Hz; (3) a control for condition 2 with a pulsing tone at 1200 Hz and a continuous f1 at 1650 Hz; and (4) a combination tone at f2-f1 = 1200 Hz produced by a continuous f1 at 1900 Hz and a pulsing f2 at 3100 Hz. Across subjects and stimulus conditions the shapes of the tuning curves were similar; whether for an externally presented single tone or a combination tone, minimum masker level (which corresponds to the tuning-curve "tip," the frequency at which masking is most effective) was always at frequencies near 1200 Hz. These findings are consistent with previous psychophysical as well as recent physiological evidence which indicates that, regardless of how generated, each of the combination tones 2f1-f2 and f2-f1 is present in the motion of the basilar membrane in the cochlear region around the characteristic place of the combination-tone frequency.

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 Langenbeck's [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.