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The Air Force Office of Scientific Research (AFOSR) manages all the basic science activity for the U.S. Air Force. Research is supported in the Air Force Laboratory, and grants are provided to universities and research institutions. This presentation will outline the scope of AFOSR’s support for basic research in areas germane to human hearing. Topics of current interest include the complex of problems associated with spatial hearing, the psychophysical and perceptual foundations for the design of auditory displays, problems of auditory information overload, and neurologically informed computational models of auditory perceptual processing. To develop better methods of hearing protection, AFOSR also supports efforts to understand the biophysical basis of cochlear excitation via bone conduction. Opportunities to contribute to these and related topics will be described.

Methods of stimulating the skin by capacitive electrical discharges are described and properties of the stimulus waveforms are discussed. A neural excitation model is invoked to account for electrocutaneous sensitivity. The model gives a good account of threshold data from several experiments in which the methods of stimulation are compared. Sensitivity is shown to be charge-dependent for stimuli that are brief relative to the exponential time constant of the neural membrane model. Systematic departures from charge-dependence occur when the stimulus discharge time constant is made to exceed a depolarization time constant: these departures follow a strength]duration relationship predicted by the model.

This paper presents research on the sensory and electrical properties of human subjects in response to electrical stimuli from highvoltage capacitive discharges. Threshold data for single monophasic transient stimuli are analyzed. The psychophysical measurements are related to a neuroelectric model with parameters that depend on some of the basic properties of excitable membranes.

An experimental simulation technique is introduced to study memory constraints affecting comprehension of embedded sentences. Memory for embedded structure is measured in a pairing task that simulates requirements imposed by sentence comprehension. With increasing memory load (number of embeddings), the rate of performance decline is consistent across all (recall and comprehension) conditions, and reflects a loss of order (as opposed to item) information. Short-term memory overload, rather than linguistic complexity, appears sufficient to account for comprehension difficulty. Performance was imperfect even with singly embedded constructions. Recall showed no abrupt failure with multiple embeddings, indicating that memory constraints are not as discontinuous as English syntax might suggest.

A model for partial masking and other threshold-elevation effects is presented in the context of a sensation-matching paradigm. The model is applied to an electrocutaneous experiment in which the subjects adjusted stimulus intensity on the right-hand fingertip to match sensation levels of standard stimuli presented to the left fingertip. Concurrent mechanical stimulation on the right fingertip masked sensation magnitude in a way consistent with the model. Similarities between this tactile masking effect and analogous auditory phenomena are explored. When applied to loudness matching, the model describes the general shape of loudness contours and it shows that the steep slopes observed in auditory masking and "recruitment" can be a consequence of a threshold shift alone, without a supranormal growth in loudness. The model also shows that a small response bias can distort plots of sensation matching, leading to the suggestion that some varieties of loudness recruitment may not have a sensory basis.

Four experiments were conducted to develop and test a method of determining the frequency‐response characteristic of the observer when he listens for single‐frequency signals presented against a continuous background of wide‐band noise. After observers were trained to detect primary signals of a single frequency, probe signals of various other frequencies were presented infrequently, in lieu of the primary signal. Primary signals and all probe signals were presented with very similar amplitudes that would be expected to render them all equally detectable if presented alone in single‐frequency experiments. Estimates of the detectability of the signals of the various frequencies were obtained concurrently in a two‐alternative forced‐choice procedure. The results from 14 observers were quite similar and show differences in detection as a function of signal frequency when the primary signal was of 1000 or of 1100 Hz. In general, the primary signal was correctly detected 75%–90% of the time while signals with frequencies at approximately 150 to 200 Hz on either side of the primary‐signal frequency were detected at the chance level, 50% correct. In as few as three experimental sessions, the observer's frequency‐response characteristic was obtained using the probe‐signal method.

Uncertain frequency detection of brief sinusoids in noise was studied in a special Yes-No paradigm that included single-frequency trials randomly interleaved with trials on which either of two frequencies could be presented. Contrary to implications from previous studies, and to sensory filter models of attention, the Os’ deliberate attempts to impose narrow- or wide-band listening had only a small effect on the relative detectability of signals at 500 Hz and at 1100 Hz. If frequency selectivity is to be understood as sensitivity adjustment (rather than as a recognition process), the relevant parameters may be entirely stimulus determined.

124 subjects were tested in a procedure designed to measure sensitivity to transient currents applied cutaneously, and to assess individual characteristics accounting for sensitivity differences. College students (one male and one female group), female office workers, and male maintenance workers (electricians, carpenters, plumbers, and sheet metal workers) were tested. Perception and annoyance thresholds were determined for capacitive discharge stimuli to the fingertip and forearm. Nonsensory data were taken for each individual in an attempt to account for individual sensitivity differences (occupation, sex, age, height, weight, skin temperature, finger and forearm diameter, skin hardness, customary physical activity level, and prior degree of experience with electric shock). Of these, body size was the only significant correlate of electrical sensitivity. Apparent correlations with sex and occupation were found to be artifacts of the body size relationship. A regression equation relating sensitivity to body weight is presented.

A linear sensitivity model is presented for monophasic transients using electrical properties of excitable membranes. Strength/duration relationships derived from the model are in good agreement with our perceptual data using monophasic capacitive discharges. For biphasic stimuli the perceptual model must be extended to account for nonlinear membrane behavior.

Listeners with cochlear impairment were studied in an ABX pitch-matching paradigm. These listeners showed excellent ability to adjust one sinusoid (X) to match the frequency of another (B), except when a leading tone (A) was present. Pitch shifts induced by the leading tone exaggerate the pitch contrast between tones A and B, even in normal-hearing listeners, but the effect in sensorineural listeners is several times larger and indicates a severe vulnerability of pitch. The degree of pitch distortion does not appear to bear any simple relationship to the degree of hearing loss, or to stimulus amplitudes. The effect is obtained at ‘comfortable’ listening levels. Vulnerability of pitch, rather than a failure in discriminative capacity, may underlie some of the qualitative perceptual difficulties found in sensorineural hearing impairment.

A model of myelinated nerve axon was used to study the initiation and propagation of action potentials for a variety of extracellular electrical stimuli. Frankenhaeuser-Huxley nonlinearities were incorporated at each of several nodes in a longitudinal array, and the extracellular current pulse was modeled as a spatial distribution of voltage disturbance along the membrane. Results from the model were compared to data from human sensory experiments and from animal electrophysiological experiments. Effects of polarity, electrode position, pulse duration, and biphasic oscillation frequency were examined. Biphasic pulses have higher excitation thresholds than monophasic pulses, provided the duration of a single phase is short relative to the time constant of the membrane depolarization process. The shapes of strength/duration curves from sensory experiments conform well to model predictions for monophasic stimuli. Strength/frequency curves derived from the model are similar to those from sensory stimulation with sinusoidal currents. The shapes of strength/frequency curves can be explained by membrane integration effects at high frequencies and membrane leakage effects at low frequencies. The model predicts lower thresholds for cathodal than for anodal stimulation: the predicted degree of polarity selectivity is confirmed by direct stimulation of axons in animal experiments, but is at variance with the selectivity found in human transcutaneous stimulation.