Response characteristics of cochlear nucleus neurons to 500-Hz tones and noise: findings relating to frequency-following potentials.

Abstract

The response properties of low and high best-frequency (BF) cochlear nucleus neurons that phase-locked to 500-Hz tones were studied in order to understand the frequency specificity of the scalp-recorded 500-Hz frequency-following potential (FFP) in kangaroo rat. Thresholds to 500 Hz were determined for neurons having BF between 200 Hz-10.0 kHz. The effects of broad-band (40-Hz high pass) and 2.0-kHz high-pass noise (96 dB/octave slope) on the 500-Hz phase-locked discharges of neurons with BF near 500 Hz, and those with BF above 2.0 kHz, were evaluated. The neuronal findings are related to characteristics of the 500-Hz volume-conducted FFP that were recorded from the same animals using exactly the same stimuli. At FFP threshold levels (15- to 20-dB sound pressure level (SPL)), only neurons with BF near 500 Hz were driven by 500 Hz. At 20-25 dB (re FFP threshold), neurons having BF between 200 Hz and 1.5 kHz were activated and, at 40-45 dB (re FFP threshold), neurons with BF up to .apprx. 5.0 kHz were responsive to 500-Hz tones. At tone levels 40-45 dB above FFP threshold, the 500-Hz phase-locked responses from neurons with BF near 500 Hz and those with BF > 2.0 kHz were differentially affected by noise. The levels of broad-band noise necessary for desynchronization were greater for the low BF than for the high BF neurons. Broad-band and 2.0-kHz high-pass noise had comparable effects on discharge rate and synchrony of neurons with BF above 2.0 kHz. Neurons having BF near 500 Hz maintained a fixed pattern of discharge synchrony to 500 Hz in the presence of 2.0-kHz high-pass noise. The discharge rates to tones of these low BF neurons were slightly decreased when the 2.0-kHz high-pass noise was intense. Apparently the 500-Hz FFP at threshold (15-20 dB SPL) and moderate levels (55-65 dB SPL) is generated primarily by the synchronous activity of neurons with best frequencies below 1.5-2.0 kHz. The results permit better understanding of the levels of noise needed for appropriate masking of the basal portions of the cochlea by depicting the effects of broad-band and 2.0-kHz high-pass noise on 500 Hz phase-locked discharges of low and high BF neurons.