Responses of young and aged rat inferior colliculus neurons to sinusoidally amplitude modulated stimuli

The inferior colliculus (IC) is a processing center for monaural and binaur al auditory signals. Many units in the central nucleus of the inferior coll iculus (CIC) respond to amplitude and frequency modulated tones, features f ound in communication signals. The present study examined potential effects of age on responses to sinusoidally amplitude modulated (SAM) tones in CIC and external cortex of the inferior colliculus (ECIC) units in young and a ged F344 rats. Extracellular recordings from 154 localized single units of aged (24 month) rats were compared to recordings from 135 TC units from you ng adult (3 month) animals. SAM tones were presented at 30 dB above thresho ld. Comparisons were made between CIC and ECIC regarding the percentage of units responding to SAM stimuli, the relationship between SAM responsivenes s and temporal response patterns, maximum discharge rates and maximum modul ation gains, shapes of rate transfer functions and synchronization modulati on transfer functions (MTFs) in response to SAM tones. Sixty percent of uni ts in young and aged rat IC were selectively responsive to SAM stimuli. Eig hty-one percent of units classified as onset temporal response patterns wer e not tonically responsive to SAM stimuli. Median maximum discharge rate in response to SAM tones was 17.6/s in young F344 rats; median maximum modula tion gain was 3.85 dB. These measurements did not change significantly with age. Thirty-seven percent of young rat units displayed bandpass MTFs and 5 3%, had lowpass MTFs. There was a significant age-related shift in the dist ribution of MTF shapes in both the CIC and ECIC. Aged animals showed a lowe r percentage of bandpass functions and a higher percentage of lowpass funct ions. Age-related changes observed in SAM coding may reflect an altered bal ance between excitatory/inhibitory neurotransmitter efficacy in the aged ra t IC, and/or possibly a change in the functional dynamic range of IC neuron s. (C) 2001 published by Elsevier Science B.V.