Although the lesion in sensorineural hearing loss (SNHL) is almost always in the cochlea, the abnormal activity patterns of auditory nerve fibers in a damaged ear cause adaptive changes in the central nervous system as well. The work proposed here will study two aspects of the central neural representation of auditory stimuli. The goal is to answer questions about perceptual deficits observed in persons with SNHL.
The first aim i s to study deficits in intensity perception. In persons with SNHL, loudness grows more steeply over some part of the range of sound levels, called recruitment. Although recruitment is consistent with changes occurring in the cochlea, our studies of auditory nerve fibers do not show a corresponding steepening of their response growth. Thus it is not clear how the cochlear effects are communicated to the brain.
This aim i s motivated by the hypothesis, supported by some existing evidence, that the central auditory system has an abnormally steep intensity sensitivity in cases of SNHL, accounting for some aspects of loudness recruitment. Neurons in the cochlear nucleus of cats with noise-induced SNHL will be characterized with a variety of measures of intensity sensitivity. A second problem accompanying SNHL is loss of frequency selectivity. Frequency selectivity of central auditory neurons is governed by both the peripheral sensitivity of the cochlea and the degree of spread of anatomical connections along the frequency axis in central circuits. We will analyze the receptive fields of neurons in the cochlear nucleus and inferior colliculus to characterize deficits of frequency analysis that go beyond peripheral tuning changes. We will also use a new method for characterizing connectivity in auditory circuitry to look for deficits in wiring specificity of the cochlear nucleus. Finally, cats are used as the animal model for this research, but the degree to which the preparation that we use suffer from loudness recruitment is unknown. Behavioral determination of response latency as a function of stimulus intensity will be used to characterize the growth of perceptual stimulus intensity in normal cats and cats with noise-induced SNHL. These data will be used to connect human perceptual data with data on cat auditory neurophysiology.
