Measurement of the depth-dependent resonance of water-loaded human lungs.

An experiment was conducted to determine the response of the human lung to water-borne sound in the range of 20 to 500 Hz. A small pool inside a hyperbaric chamber was used to simulate four ambient pressure conditions spanning the range of recreational diving depths. Ten subjects were tested on two occasions each using three separate measures to evaluate the response of the subjects' lungs. With some notable exceptions, results were consistent between subjects and between measures. These indicate that human lungs can be reasonably modeled as a lumped single-degree-of-freedom system over the lower portion of the band of interest. Here, the surrounding fluid provides the dominant mass and the dominant stiffness is provided by the entrapped air with a small additional contribution from tissue elasticity. Measured resonances increase with the square root of ambient pressure from an average of 40 Hz with a quality factor of 1.8 at near-surface pressure to 73 Hz with a quality factor of 2.6 at an equivalent depth of 36.4 m. There is evidence of other resonances within or near the band of interest that may be attributable to nonvolumetric chest/lung modes, Helmholtz resonance, and/or resonance of gastrointestinal bubbles.

Hearing function in a hyperbaric environment.

Navy divers' hearing function was assessed as part of three saturation deep dives to 1,000 feet of sea water (fsw) to determine explanations for threshold shifts observed under hyperbaric conditions. Across the three deep dives, different aspects of the ear were evaluated, including air- and bone-conduction pure-tone thresholds, real ear probe microphone measurements, auditory evoked potentials, and central auditory processing assessments. Attempts to measure middle ear function and cochlear function (through otoacoustic emissions) were unsuccessful. Baseline measurements were obtained at 0 fsw in air before and after the saturation deep dives. Results showed that some aspects of hearing function remained unchanged with increases in depth. In general, audiometric thresholds at depth were similar to those measured on the surface at 500, 1,000, 2,000, 3,000, and 4,000 Hz. However, hearing sensitivity actually improved at depth at 6,000 and 8,000 Hz. The use of a specially designed sound booth for a pressurized heliox environment yielded significantly lower ambient noise levels and improved the accuracy of threshold measurement. Auditory evoked potential measurements and central auditory processing function were relatively unaffected by changes in depth. Significant changes at depth were seen in ear canal resonance which shifted up in frequency; this finding was attributed to the effect of helium on the hearing mechanism. Because objective measurement of middle ear and inner ear function were not methodologically possible, questions still remain regarding the interpretation of middle and inner ear function at depth. Nonetheless, our overall findings suggest that most aspects of hearing functioning are similar under high atmospheric pressures and in heliox as they are on the surface, with the exception of shifts in ear canal resonance and improvements in audiometric thresholds at high frequencies.

Audio-visual consonant recognition with the 3M/House cochlear implant.

Eight experienced 3M/House cochlear implant users' consonant recognition was evaluated with videotaped vowel-consonant vowel lists presented in auditory implant only (A), visual (V), and auditory-visual (AV) conditions. All subjects' scores were better than chance. Results revealed that the AV scores were significantly better than the V scores, which were better than the A scores. Sequential Information Analysis of the consonant errors revealed that different features were transmitted better in each condition. Sonorant and voicing features were transmitted well for the A condition, but features related to high-frequency and place cues were not. Place features were transmitted best in the V condition, but acoustic features were not. Both place and acoustic features were transmitted in the AV condition, but they were influenced most by visual cues.

Acoustic reflex temporal summation measured at threshold.

This study measured the threshold of the acoustic reflex as a function of reflex-activating stimulus duration. Acoustic reflex thresholds (ARTs) were measured for tonal stimuli at frequencies of 1000 and 3000 Hz and broadband noise at six durations between 20 and 500 msec for both normal and hearing-impaired subjects. Traditional techniques and response averaging were used. Results suggest that these methods have a significant influence on ART and reflex temporal summation at threshold. Lowest thresholds and almost no temporal summation were found with a response averaging technique and a bidirectional shift criterion, whereas clinical procedures yielded highest thresholds and greater summation. These results suggest a need to examine the methodology used for measurement of the acoustic reflex and the interpretation of clinical pathology based on measurements of reflex temporal summation.

The time course of the acoustic reflex.

The time course of the impedance changes caused by the contralateral acoustic reflex was investigated in five normal hearing, young adult subjects The acoustic reflex function was measured with equipment having a time constant of 15 milliseconds which permitted precise temporal measures. Temporal intervals from signal onset and termination to various portions of the acoustic reflex function were derived using an analysis procedure based on the slope function underlying the acoustic reflex response. These temporal intervals are related to physiological events underlying the acoustic reflex.

Qualitative analysis of scopolamine-induced amnesia.

The neurochemistry of memory remains to be determined. Acetylcholine may be one of the neuotransmitters which mediates memory function, since the anticholinergic drug scopolamine produces amnesia in man. This study of scopolamine-induced memory deficits further defines those cognitive processes which are disrupted. The drug does not diminish attention, as assessed with an auditory vigilance task, or initial signal detection. More complex auditory decoding is affected, however. Scopolamine impairs aspects of initial memory acquisition (e. g., encoding and consolidation) and spontaneous memory retrieval. Retention is unaffected. Precise delineation of the neurochemistry of human memory will require comparative studies of amnesia-producing compounds, systematically examining the neuropsychological processes impaired by each.