Biomedical subjects

L I Hellstrom

Publications and source records attributed to L I Hellstrom.

6 recordsLinked to original sources

PubMed1996

Measures of tuning and suppression in single-fiber and whole-nerve responses in young and quiet-aged gerbils.

Tuning and suppression were examined in the auditory nerves of young-control and quiet-aged gerbils. Tuning curves obtained from single-fiber responses were compared to those obtained with the compound action potential (CAP) using masking procedures. Tuning was measured in terms of the high-frequency slope of the tuning curve and Q values obtained 10 dB and 40 dB (Q10 dB and Q40 dB, respectively) above the threshold at its best frequency. The study had two objectives: first, how CAP measures of tuning and suppression correspond to single-fiber measures in presbyacusic cochleas; and second, how the above measures of tuning change with age without the confounding effects of noise exposure. It was found that measures derived from the CAP responses in aged gerbils remained similar to the trends of the single-fiber data, despite the many morphological changes that are known to occur in the presbyacusic ear. Thus, CAP procedures remain an appropriate alternative to single-fiber recording in aged animals where the stability of the preparation can be tenuous. With regard to tuning and suppression, our results show that single-fiber and CAP tuning curves in quiet-aged animals are similar to those of young controls except for higher thresholds near the best frequency. The tail response remained largely stable with age, and no instances of tail hypersensitivity were found. The slopes of the high-frequency legs of the curves were similar in young and aged animals, and were associated with the presence of two-tone suppression on the high-frequency side. In a few instances high-side suppression was absent in the aged gerbils and was associated with tuning curves with shallow high-frequency slopes and characteristic frequency (CF) threshold shifts of 30 dB or greater. Mean high-frequency slopes and Q values increased with best frequency in young controls, but were fairly constant with best frequency in quiet-aged animals. Thus, in comparison to controls, the frequency selectivity of quiet-aged gerbils tends to decrease somewhat at frequencies above 4 kHz. Many of these changes with age may be ascribed to a chronic decline of the endocochlear potential (EP) acting on an intact hair-cell system in quiet-aged gerbils. With this assumption, CF thresholds are most sensitive to the EP decline, whereas boundaries of tuning and two-tone suppression taken at more intense levels are least affected.

Aging↗

PubMed1991

Rate/level functions of auditory-nerve fibers in young and quiet-aged gerbils.

Steady-state rate/level functions of single auditory-nerve fibers to characteristic frequency (CF) tone bursts were measured in quiet-aged (35-37 months) and young control (4-7 months) gerbils. Rate/level functions of aged gerbils are different from those of young controls in that the thresholds are shifted to higher sound levels, but otherwise the shapes of the aged and young rate/level functions are similar. Specifically, there is little difference in the slope of the dynamic range portion of the rate/level functions when comparing aged gerbils to young controls. This is in contrast to whole-nerve input/output (I/O) functions of aged gerbils, which exhibit slopes that are less steep than those of the young controls (Hellstrom and Schmiedt, 1990b). Thus, it is likely that the deterioration of the CAP I/O function in aged animals is not due to a deterioration of rate/level functions in single units, but rather to other factors such as spiral ganglion cell degeneration or a loss of synchrony.

Acoustic Stimulation↗

PubMed1991

Age-related changes in cochleas of mongolian gerbils.

The effects of aging on the gerbil cochlea were studied in 16 animals raised in a quiet environment. Animals were tested at ages ranging from 33 to 36 months, the approximate average lifespan of gerbils in our colony. Hearing sensitivity was assessed by measures of whole-nerve compound action potential (CAP) thresholds and surface preparations of the organ of Corti were subsequently examined by light microscopy for losses of sensory hair cells. These quiet-aged animals showed a wide range of hair-cell losses and threshold shifts. Outer hair cells often showed significant losses while inner hair cells were rarely absent. All animals had some threshold shift, especially at frequencies above 4 kHz. These shifts ranged from 1 to 68 dB. At high frequencies, threshold shifts often occurred without hair-cell losses at corresponding cochlear locations. At low frequencies, threshold shifts seldom reflected the losses of hair cells commonly found in the cochlear apex. Thus, the correlation of specific hair-cell losses and CAP threshold shifts at corresponding frequencies was poor. On the other hand, the total number of missing hair cells, irrespective of location, was a good, general indicator of the hearing capacity in a given ear. It appears that the factor or factors that makes cochleas susceptible to hair-cell loss with increasing age also affects other cochlear mechanisms that are necessary for normal functioning of the ear.

Action Potentials↗

PubMed1990

Compound action potential input/output functions in young and quiet-aged gerbils.

Auditory-nerve compound action potentials (CAP) and cochlear microphonic (CM) potentials were measured with round window electrodes in two sets of quiet-reared gerbils: young (N = 9 ears, 4-7 months) and aged (N = 11 ears, 35-37 months). CAP thresholds, measured at probe frequencies from 0.5 to 25.6 kHz, are plotted as audibility curves. Input/output (I/O) functions were derived from CAP and CM amplitude measurements at six frequencies. When compared to young controls, CAP audibility curves from aged animals all show some degree of threshold shift, ranging from minimal to severe, as well as increased variability. Our data suggest that some of the variability in the aged-animal audibility curves can be attributed to variations in individual genetic factors. Maximum CAP amplitudes for the aged animals average significantly less than those of the young controls at all frequencies tested. Young control I/O functions are generally steeper than those of the aged gerbils. Differences in the CM amplitudes of the young and aged gerbils are not as clear cut as the differences in the CAP. Possible mechanisms explaining the decrease in amplitudes and slopes of the CAP I/O functions in aged animals include changes in numbers or thresholds of primary ganglion cells, or a decrease in synchrony in discharges of auditory-nerve fibers.

Action Potentials↗

PubMed1989

Physiological responses to the pulsation threshold paradigm. I: Pulsation threshold patterns do not reproduce physiological rate profiles of high-pass and low-pass noise maskers.

This article compares psychophysical measures of human processing of acoustic stimuli with one neurophysiological representation (normalized discharge rate profiles) of those stimuli. Psychophysical pulsation threshold patterns (PTPs) were derived for high-pass and low-pass noise maskers. Spectral features of both maskers are clearly evident in the PTPs. However, while the representation of high-pass noise in the PTPs becomes sharper with increasing masker level, the representation of low-pass noise degenerates as masker level is increased. One assumption that has been used previously to interpret pulsation threshold data is that PTPs reflect the profile of activity in primary neural elements in response to the masking stimulus. To investigate this hypothesis, normalized-rate profiles of responses to both maskers were derived from populations of auditory-nerve fibers in cats. Normalized-rate profiles do not exhibit the same behavior as PTPs for high-pass noise maskers in that the neural representation of the band edge degenerates as sound level increases. Furthermore, the distinction between the passband and the stop band is lost in the neural rate profiles, whereas the distinction improves in the high-pass noise PTPs.

Acoustic Stimulation↗

PubMed1989

Physiological responses to the pulsation threshold paradigm. II: Representations of high-pass noise in average rate measures of auditory-nerve fiber discharge.

In a companion article [L. I. Hellstrom, J. Acoust. Soc. Am. 85, 230-242 (1989)], it was shown that psychophysical pulsation threshold masking patterns (PTPs) for high-pass noise maskers are not a simple transformation of the profile of activity evoked in the auditory nerve by the masker. In this article, PTPs are compared with neural representations in which interactions of masker and probe are considered. It is hypothesized that, at pulsation threshold, some criterion value of rate change occurs when the stimulus switches from masker to probe. The iso-rate probe level, defined for single auditory-nerve fibers, is the probe level at which this rate change is zero. Iso-rate probe levels are lowest when probe frequency equals best frequency (BF) of the fiber. Profiles of iso-rate probe level versus BF (equal to probe frequency) are qualitatively similar to PTPs but differ quantitatively, e.g., in the rate of growth of probe level with masker level (1.2 dB/dB for PTPs, 0.54 dB/dB for iso-rate profiles). Quantitative differences can be further reduced by requiring a positive rate criterion. These results suggest that PTPs are not solely a reflection of the internal representation of the masker, but reflect responses to the probe tone as well.

Acoustic Stimulation↗