Index of content:
Volume 106, Issue 1, July 1999
- PHYSIOLOGICAL ACOUSTICS 
106(1999); http://dx.doi.org/10.1121/1.427055View Description Hide Description
Acoustic impedance/reflectance measurements were made at various ear-canal pressures in 20 subjects with a clinical acoustic immittance instrument and an experimental impedance/reflectance system. Measurements were made over a frequency range of 226–2000 Hz with the clinical system and 125–11 310 Hz with the experimental system. For frequencies ⩽2.0 kHz, tympanograms obtained with the two systems are similar, with patterns that progress through the same orderly sequence with increasing frequency. Eardrum impedance measurements were also similar. There are small gender differences in middle-ear impedance. Reflectance patterns (reflectance versus frequency) at ambient ear-canal air pressure are characterized by high reflectance at low frequencies, two distict minima at 1.2 and 3.5 kHz, increasing reflectance to 8.0 kHz, and decreasing reflectance above that frequency. Ear-canal pressure increases reflectance at low frequencies, decreases reflectance in the region of the minimum, and increases reflectance slightly at high frequencies. Reflectance tympanograms (reflectance versus ear-canal pressure) progress through a sequence of three patterns. At low frequencies, reflectance tympanograms are “V” shaped, indicating that pressure increases reflectance. At frequencies near the minimum reflectance, the pattern inverts, indicating that pressure decreases reflectance. At high frequencies, the patterns are flat, indicating that ear-canal pressure has little effect. Results presented for one patient suggest that reflectance tympanometry may be useful for detecting middle-ear pathology.
106(1999); http://dx.doi.org/10.1121/1.427056View Description Hide Description
A commonly accepted physiological model for lateralization of low-frequency sounds by interaural time delay (ITD) stipulates that binaural comparison neurons receive input from frequency-matched channels from each ear. Here, the effects of hypothetical interaural frequency mismatches on this model are reported. For this study, the cat’s auditory system peripheral to the binaural comparison neurons was represented by a neurophysiologically derived model, and binaural comparison neurons were represented by cross-correlators. The results of the study indicate that, for binaural comparison neurons receiving input from one cochlear channel from each ear, interaural CF mismatches may serve to either augment or diminish the effective difference in ipsilateral and contralateral axonal time delays from the periphery to the binaural comparison neuron. The magnitude of this increase or decrease in the effective time delay difference can be up to 400 μs for CF mismatches of 0.2 octaves or less for binauralneurons with CFs between 250 Hz and 2.5 kHz. For binaural comparison neurons with nominal CFs near 500 Hz, the 25-μs effective time delay difference caused by a 0.012-octave CF mismatch is equal to the ITD previously shown to be behaviorally sufficient for the cat to lateralize a low-frequency sound source.
Human auditory cortex electrophysiological correlates of the precedence effect: Binaural echo lateralization suppression106(1999); http://dx.doi.org/10.1121/1.427057View Description Hide Description
Echoes lagging shortly after a sound and originating from a different location blend with the sound source perceptually. The location of the fused “” is dominated by the source, suggesting suppression of echo localization. This effect is diminished monaurally, implying involvement of binaural processing. The neural substrates underlying the echo localization suppression are still unclear. The electrophysiological indications of primary auditory cortex involvement in binaural suppression of echo lateralization are presented. Position judgment and auditory-evoked potentials (AEPs) were recorded to single- and pairs of binaural and monaural clicks. The pairs simulated a source and its echo. The binaural position judgment was dominated by the source at small echo lags. With lag increase, it shifted toward the echo. The AEPs were studied for binaural processes specific to a “real” echo, as opposed to an identical single sound (a “virtual” echo). A reduction in binaural peak amplitude and area and an increase in binaural peak latency were detected specifically in the real echo-evoked middle-latency component Pa. The binauralecho Pa suppression depended upon echo lag, although less strongly than the psychoacoustic position judgment. Pa is associated with primary auditory cortex activity, thus suggesting primary cortical involvement in echo-lateralization suppression.
Near-field responses from the round window, inferior colliculus, and auditory cortex of the unanesthetized chinchilla: Manipulations of noiseburst level and rate106(1999); http://dx.doi.org/10.1121/1.427058View Description Hide Description
Few studies have compared the response properties of near-field potentials from multiple levels of the auditory nervous system of unanesthetized animals. The purpose of this study was to investigate the effects of brief-duration noisebursts on neural responses recorded from electrodes chronically implanted at the round window, inferior colliculus and auditory cortex of chinchillas. Responses were obtained from seven unanesthetized chinchillas to a noiseburst-level and noiseburst-rate series. For the noiseburst-rate series, a 70 dB pSPL noiseburst was varied in rate from 10 to 100 Hz using conventional averaging procedures, and from 100 to 500 Hz using pseudorandom pulse trains called maximum length sequences (MLSs). Response thresholds were similar for the compound action potential (CAP), inferior colliculus potential (ICP) and auditory cortex potential (ACP). With decreasing noiseburst level, there were decreases in the amplitudes and increases in the latencies of the CAP, ICP and ACP. The shapes of the mean normalized amplitude input/output (I/O) functions were similar for the ICP and ACP, while the normalized I/O functions for the first positive peak and first negative peak of the CAP differed from each other and from the ICP and ACP. The slopes of the latency/intensity functions were shallowest for the CAP, intermediate for the ICP, and steepest for the ACP. With increasing rate, the latency shift was least for the CAP, intermediate for the ICP and greatest for the ACP. The amplitude of of the CAP varied little with rate. All other potentials showed a pronounced decrease in amplitude at high stimulation rates. Excluding CAP proportional amplitude decrease with rate was greatest for the ACP, intermediate for of the CAP and least for the ICP. Responses were present in most animals at all recording sites, even for the highest rate (500 Hz) used in this study. For all potentials, the MLS procedure allowed the collection of a response at rates well above those where sequential responses would have overlapped using conventional averaging procedures.