Donald N. Elliott

Behavioral Investigation of Some Possible Effects of Sectioning the Crossed Olivocochlear Bundle

Due to the paucity of behavioral data bearing on the possible role of the efferent auditory system, an investigation of some possible functions of the crossed olivocochlear bundle (OCB) has been undertaken, using cats. Behavioral correlates of transection of the crossed OCB were sought in such measures as absolute thresholds in quiet, effective masking over differing noise levels and frequencies, and temporary threshold shifts. The following results were obtained after transection of crossed OCB: (1) Absolute thresholds were unchanged; (2) amount of masking at 1000 Hz and 2000 Hz increased (though the shift was not statistically significant with the small number of subjects tested); and (3) TTS did not differ greatly for the transected animals.

Determination of Absolute‐Intensity Thresholds and Frequency‐Difference Thresholds in Cats

While the cats basilar membrane is only two‐thirds the length of the human, its auditory‐frequency range is at least three times as great. Behaviorally defined absolute‐intensity and differential‐frequency thresholds have been determined for the cat and are compared with those of humans. The cats absolute thresholds lie well below those of humans over nearly all of the frequencies the two species respond to in common. The cats differential thresholds, however, are larger at all frequencies. But because of the cats larger frequency range, the total number of discriminable steps appear to be about equal. Counts of ganglion cells and hair cells for the cat are reported and compared with counts for humans. The greater density of the cats ganglion cells may explain its lower absolute thresholds, but there appears to be no direct relation between ganglion‐cell density and frequency discrimination. The lower density of hair cells for the cat, coupled with its shorter basilar membrane, may account for its poor frequency discrimination.

Effects of Intense Auditory Stimulation: Hearing Losses and Inner Ear Changes in the Squirrel Monkey

Twelve squirrel monkeys were exposed to pure‐tone stimuli at intensities and durations sufficient to produce, in six, repeated temporary threshold shifts (TTSs) from 15 to 25 dB and, in the remaining six, permanent threshold shifts (PTSs) from 10 to 20 dB. Thresholds were determined by means of an avoidance conditioning procedure similar to that of Clack and Herman (1963), in which observation intervals were signaled by the houselight and the animals required to press a bar if an auditory signal was presented, and to withold response if not. Following final behavioral testing, animals were sacrificed and the effect of the exposures upon hair cells was determined. This was accomplished with surface preparation techniques and phase microscopy. The organ of Corti was examined along its length and the number of missing and visibly damaged hair cells was determined for each of the inner and outer rows from a point 1 mm from the apex to a point 17 mm from the apex. There was no clear evidence of hair cell damag...

Thresholds of Audition for Three Species of Monkeys

Using various conditioning procedures, three species of monkeys have been conditioned in order to determine their audiograms. Squirrel monkeys have been conditioned in a double‐grill cage, using shock as the reinforcement, while squirrel monkeys, Rhesus monkeys, and Cynamolgus monkeys have been conditioned in a single‐bar reward and a single‐bar avoidance situation. In general, the audiograms obtained for the various species, and using the various conditioning procedures, agreed quite well. The audiograms of the individual animals, as well as the average performance for each species, are reported.

Effects of intense auditory stimulation: hearing losses and inner ear changes in the squirrel monkey. II

Five squirrel monkeys were exposed to pure tones at intensities and durations sufficient to produce permanent threshold shifts (PTS s). One animal was exposed to a 1 kHz tone at 130 dB SPL for 4 h. Two were exposed to a 1 kHz tone at 140 dB SPL, for respectively, 3 and 4 h. Two were exposed to a 2 kHz tone at 140 dB SPL, for respectively, 3 and 4 h. Pre‐ and post‐exposure behavioral testing was similar to that described in an earlier study (Hunter‐Duvar and Elliott, 1972). Following the exposure, testing was carried out at frequent intervals until recovery was no longer evident. Final thresholds were then determined. Animals were next sacrificed, their cochleas were fixed, and surface specimens prepared. Using a phase microscope, missing and severely damaged hair cells (those with fractures of the cuticular plate and obviously displaced nucleii) were determined for each of the inner and outer rows from a point 1 mm from the apex to a point 19 mm from the apex. Hair cells were inspected at all levels. Wide variations in hearing losses and hair‐cell damage were found, and were uncorrelated. Hearing losses ranged from less than 20 dB to greater than 50 dB; hair‐cell damage ranged from no apparent damage to complete loss of outer and moderate loss of inner cells. Little if any correlation was found between exposure‐tone characteristic (frequency, intensity, duration) and hearing loss or hair‐cell damage.

Adaptation and Loudness Decrement: A Reconsideration

The interpretation of data obtained with simultaneous dichotic loudness balance (SDLB) procedures as a reduction in the loudness response of the adapted ear to monaural stimulation is questioned. Since simultaneous homophonic presentation provides location cues as well as binaural loudness interaction cues, other interpretations than reduction in monaural loudness response are available. Further, the electrophysiological data cited to support the loudness‐reduction interpretation bear little similarity to the typical SDLB adaptation curve. The method of delayed balance, in which it is possible to obtain relatively uncontaminated judgments of loudness in the adapted and comparison ears, has been utilized. With such a procedure, no evidence of loudness decrement was evident at exposure intervals from 1 sec to more than 7 min, for an exposure intensity of 60 dB SL. It is concluded that SDLB judgments reflect a conglomerate of imperfectly related cues whose potency may vary for different Ss and probably does ...

Adaptation: Central or Peripheral?

Adaptation was measured using successive and simultaneous presentation of heterophonic stimuli in order to determine if adaptation is more a central than a peripheral phenomena. The first of the two studies reported compared the method of delayed balance, method of simultaneous dichotic balances (SDB) involving tracking for 15‐sec periods, and a modification of the SDB utilizing 1‐sec tones. Only the homophonic SDB involving tracking produced noticeable adaptation. In the second experiment several procedural variations of the heterophonic tracking SDB method were studied. None of the variations produced the intensity decrement customarily associated with adaptation. It was concluded that, when binaural interactions (loudness and/or lateralization) are reduced or eliminated by test procedures, the adapted ear shows little if any shift. Thus, the decrements observed with the typical SDB procedure probably reflect slowly developing changes in binaural interactions—and, consequently, are central rather than p...

Effect of Monaural Fatigue upon Pitch Matching and Discrimination

Using binaural pitch‐matching data, the effect of moderate poststimulatory fatigue upon the ears “tuning” was determined at several frequencies. In addition, the effect of the fatigue upon pitch discrimination was measured. Upward shifts in pitch were observed at TTSs of 20 dB at both 2800 and 5600 cps; however, the shift at 5600 cps is questionable since it was not statistically significant. From these data, Ruedis observation that the direction of postfatigue pitch shifts reverses (decreases rather than increases) for frequencies above 4000 cps is questioned. There was limited evidence that fatigue results in poorer pitch discrimination; however, such effects were not marked. Possibly, monaural data would have resulted in more‐clearcut effects.

Effect of Repeated Exposure to High‐Intensity Sound

Auditory fatigue was experimentally induced in eight subjects over a period of several weeks so that a systematic study could be made of the alteration in hearing that might occur as a result of such repeated exposures. Temporary threshold shifts and intensity‐difference thresholds were obtained from each of these subjects as indices of fatigue. In general, there were only chance variations in the TTS and the IDT over the period studied. It was concluded that within the limits of this study repeatedly exposing subjects does not basically alter the hearing mechanisms. However, as the sessions progressed, a trend toward a smaller TTS at the 1‐min postexposure point was noted. This change resulted in an accentuation of the “bounce” phenomenon, which did not disappear when the contralateral ear, not previously exposed, was fatigued. Thus, the data suggest that the change in the R‐1 recovery function was the result of a factor common to both ears.

Effects of Auditory Fatigue upon Intensity Discrimination

Although a majority of auditory fatigue studies have utilized the TTS as the primary index of change in functioning of the auditory system, it is clear that the threshold of audibility is not a completely adequate indication of the auditory systems functional status. Indices of discrimination, for example, are not related in any systematic fashion to the threshold of audibility, since various pathological conditions of the auditory system are reflected differentially in measures of the threshold of audibility and measures of differential thresholds. In the present study changes in the intensity discrimination threshold (IDT) as well as in the threshold of audibility have been determined. Significant decreases in the IDT were found to follow fatiguing exposure, such changes being related in a negatively decelerated manner to the TTS. The extent of the change in the IDT as a function of fatigue was found to be inversely related to the SPL of the signal to be discriminated. As a result of these two relation...