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

Abstract

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.