Abstract 60, Date 1:00 pm, Wednesday, February 14, 2007 (24 hours)
Session W8: Poster
Rate-Intensity Functions of Ventral Cochlear Nucleus Neurons in Normal and Hearing-Impaired Cats and Their Possible Relationships to Loudness Recruitment
*Shanqing Cai, Wei-Li Ma, Benjamin Letham, Eric D. Young
Loudness recruitment, an abnormally rapid increase in loudness with sound intensity, is commonly seen in sensorineural hearing loss. Its physiological mechanism remains obscure. Previous research in our lab revealed that auditory nerve fiber (ANF) population rate responses in cats with noise-induced hearing loss (NIHL) did not show recruitment, suggesting that recruitment reflects changes in central neurons.

The current study examined rate-intensity relationships of ventral cochlear nucleus (VCN) neurons in normal-hearing and deafened cats. Cats were deafened with exposure to 2-kHz noise bands (50 Hz wide, 111-112 dB SPL for 4 hours). Peripheral hearing thresholds were measured with the compound action potential. Simple and complex stimuli, including tones, broadband noise and speech tokens, were presented to construct rate functions ipsilateral to the site of recording. A pseudo-population approach was used to address the problem of limited data yield. Rate functions were analyzed by a linear spline regression algorithm.

Noise exposure caused 30-50 dB threshold shift in a range of frequencies surrounding 2-kHz. VCN units showed elevated thresholds and broadened tuning. Generally, the slopes of individual rate functions were not significantly different in normal and impaired ears. But NIHL caused decreased occurrence of non-monotonic rate functions. On the population level, NIHL reduced the range of unit thresholds and led to a larger degree of overlap between the dynamic ranges of different neurons. Similar effects were also observed in archived ANF data, though to a lesser degree. As a result, the population average rate functions showed steeper slopes and compressed dynamic ranges in the deaf ear. Simulated loudness matching between the normal and deaf populations produced curves with slopes greater than 1 dB/dB, consistent with recruitment. Full recruitment (rate catching-up at 80-90 dB SPL) was observed in a number of situations. (Supported by NIH grant DC00109)