Paired-tone stimuli reveal reductions and alterations in temporal processing in inferior colliculus neurons of aged animals.

Abstract

Temporal processing deficits in the central auditory system of aged subjects are apparent from animal and human studies but could be due to peripheral hearing loss. Sequential paired tone stimuli reveal age-related changes in temporal processing properties of neurons in the central nucleus of the inferior colliculus (CIC). A greater proportion of CIC neurons exhibit suppression of excitability following pure tone stimulation in 20 month old ("aged") compared with 3-6 month "young" Long Evans rats. The duration (time constant of exponential curve fit to recovery of excitability) of suppression is also increased in aged compared with young rats, with more neurons exhibiting suppression with time constants over 100 ms. The time course of post-stimulatory suppression is not dependent on the duration or intensity of preceding stimuli and is not correlated with either initial magnitude of suppression or best frequency of IC neurons. Although the increase in unit thresholds is greater for high-frequency units in old animals, the largest post-stimulatory suppression changes occur in neurons with best frequencies of less than 10 kHz. Since the increase in duration of post-stimulatory suppression is not correlated with peripheral hearing loss, the difference is likely attributed to central auditory neuron changes in aging. In addition, the proportion of IC neurons exhibiting other temporal patterns of excitability (post-stimulatory facilitation and delayed-maximum excitability) is reduced in aged animals. Therefore, temporal processing of acoustic information is significantly altered in aged animals. The greater post-stimulatory suppression of excitability, reduced facilitation, and delayed facilitation is expected to reduce and alter the encoded information passing from the brainstem through the IC to higher structures. These changes correlate with reduced speech understanding in noise, elevated thresholds in noisy conditions, and reduced temporal processing capabilities in the elderly.