Effect of stimulus frequency and stimulation site on the N1m response of the human auditory cortex.

The aim of the present study was to investigate the functional organization of the auditory cortex for pure tones of 1, 2, 4, 6, 8 and 12 kHz. Ten subjects were tested with a whole-head magnetometer (151 channels). The location, latency and amplitude of the generators of the N1m (the main component of the response, peaking approximately at 100 ms) were explored simultaneously in the right and left hemispheres under monaural stimulation. Our results revealed that tonotopy is a rather complex functional organization of the auditory cortex. From 1 to 12 kHz, tonotopic maps were found for contralateral as well as for ipsilateral stimulation: N1m generators were found to be tonotopically organized mainly in an anterior-posterior direction in both hemispheres, whatever the stimulated ear, but also in an inferior-superior direction in the right hemisphere. Furthermore, latencies were longer in the left than in the right hemisphere. Two different representations of spectral distribution were found in the right auditory cortex: one for ipsilateral and one for contralateral stimulation.

Evaluation of the nonorganic hearing loss suspect.

OBJECTIVE: To examine the utility of distortion-product otoacoustic emissions (DPOAEs) in the assessment of the nonorganic suspect because DPOAE analysis constitutes an objective test of hair-cell function that yields audiometriclike data. STUDY DESIGN: Retrospective study of clinical findings. SETTING: Audiology outpatient clinic of our university's medical center. PATIENTS: The study cohort comprised 30 patients who presented with a profile of suspicion for nonorganic hearing loss. Most cases were found, with the aid of DPOAE testing, to be nonorganic or to have nonorganic overlays to organic hearing loss. INTERVENTIONS: Interventions were diagnostic only. MAIN OUTCOME MEASURES: Observed audiometric findings and changes thereof. RESULTS: Statistically significant decreases in thresholds occurred in subgroups of those cases deemed to be truly nonorganic in origin. CONCLUSIONS: Especially considering test efficiency, the results support the inclusion of DPOAE analysis in the diagnostic management of the suspect-nonorganic patient.

Relative effective frequency response of bone versus air conduction stimulation examined via masking.

An adaptation of the sensorineural acuity level procedure was employed to obtain thresholds under bone- (BC) vs. air-conducted (AC) white noise masking. For the BC masking condition, a Radioear B71 was placed on one mastoid. An Etymotic ER3 with a foam tip placed in the ear on the same side was used to deliver the pure-tone probe stimulus. This transducer was chosen to approximate the Telephonics TDH-39 earphone response characteristic while reducing occlusion effect. For the AC masking condition, the masker and probe were mixed electrically and delivered to the earphone. Masked threshold data, transformed into frequency response curves, demonstrated greater variance of BC vs. AC response across frequency but less high-frequency roll-off than expected from coupler measurements obtained using an artificial mastoid and 6-cm3 cavity, respectively.

Correlations of neuroanatomical measures to auditory brain stem response latencies.

Magnetic resonance imaging (MRI) was used to obtain measures presumed to scale the dimensions of the lower auditory pathway in humans for the purpose of further defining the relationship between length of the auditory pathway and auditory brain stem response (ABR) latencies. Specifically, measurements of soft tissue structures, that is, the eighth nerve and brain stem, were made for comparison with skull dimensions and ABR latencies. It was hypothesized that the brain stem dimensions covary significantly with skull dimensions and that the ABR parameters covary with both skull and brain stem dimensions. In general, only weak correlations were obtained with coefficients failing to reach statistical significance for most comparisons. These findings suggest that variance in ABR latencies cannot be attributed completely to variance in brain stem dimensions and raise suspicion that skull dimensions do not directly reflect brain stem dimensions.

The frequency-following response and the onset response: evaluation of frequency specificity using a forward-masking paradigm.

The onset and frequency-following response (FFR) components of the auditory brain stem response were investigated using a tone on tone forward-masking paradigm, as an alternate strategy to simultaneous masking, in an effort to further clarify the issue of place specificity of these components when elicited by a low-frequency stimulus (500 Hz tone burst). The results of this preliminary investigation suggest that both the onset and FFR components yield frequency-specific information, that is, they reflect activity along a limited apicalward region of the cochlea, but both are biased somewhat basalward to the place of the probe frequency. Also, of the two, the FFR is the relatively more place-specific response.

Changes in BAEP under hypoglycemia: temperature-related?

Latencies of the brain-stem auditory evoked potentials were observed to increase in subjects whose plasma glucose levels were reduced. These changes appeared to be attributable to reduced body temperature, rather than direct effects of hypoglycemia on the auditory nerve or the brain-stem. The results suggest the need for caution in interpreting evoked potential measurements under hypoglycemia.

Psychophysical tuning functions for brief stimuli: preliminary report.

Observations from clinical and research applications of brain stem evoked response audiometry and electrocochleography have suggested that tone bursts of only a few milliseconds' duration can elicit frequency specific responses. Because such brief stimuli are not perceived as being total, the extent to which the auditory system can "tune" to such stimuli was questioned. In order to investigate this question, psychophysical tuning functions were determined for brief tone bursts (i.e., 1. to 2 msec. duration). The results suggest that in working near normal thresholds (and therefore at relatively low sound pressure levels), brief tone bursts do yield rather frequency specific information. However, the range of sound pressure levels of the stimulus for which the tuning is resonably sharp is limited. the tuning functions show signs of deterioration, particularly in the "tip region," with only modest increases in the sound pressure level above threshold (i.e., typically with 20 dB.). Differences also may be seen between tuning functions for these very brief tone bursts, which in turn are characterized by a so-called click pitch, and those of stimuli long enough to have tonal pitch (i.e., 20 msec.). Finally, tuning functions also were examined for click stimulation, which also revealed some degree of tuning at relatively low levels of stimulation.

Computer identification of waves in the auditory brain stem evoked potentials.

The high reliability with which the waves I through V can be identified in auditory brain stem potential (BAEP) recordings suggests the possibility of using the computer to identify the peaks. This paper examines the application of a smoothing algorithm in which detection of zero crossings in the derivative array of the averaged potentials is used to identify waves I--V in clinical BAEP recordings. A comparison of computer wave identification with human evaluation is presented. The results suggest that computer evaluation cannot entirely replace 'manual' evaluation of the waves, but can expedite analysis and alert the clinician to potentially abnormal responses.