Response properties of inferior colliculus neurons in young and very old CBA/J mice

Extracellular recordings were obtained from inferior colliculus (IC) neurons on young (2-month) and very old (greater than 2-year) CBA/J mice in response to contralateral tone and noise stimuli. The old mice had a small loss of spiral ganglion cells throughout the cochlea and moderate hearing loss, manifested as an elevation of neuronal thresholds throughout the IC. There was an age-related increase (3% to 22%) in 'sluggish' neurons (auditory, but poorly driven by sound); however, most neurons responded robustly to sound. Nine response types were derived from post-stimulus time histograms (PSTHs); all types were found in both age groups with no difference in their relative incidence. Sustained responses remained vigorous in old neurons, as indicated by spike counts and temporal discharge patterns. The percentage of neurons that were spontaneously active increased with age in the ventral IC (the area most sensitive to high frequencies, including most of the central nucleus) and decreased in the dorsal IC (the area most sensitive to lower frequencies, including much of the dorsal cortex). Parameters of response areas (range, upper frequency cutoff, best frequency, and rate-best frequency) showed modest age differences, while rate-level functions showed little age-related change. While a significant correlate of old age was attrition of IC neurons from the population capable of responding robustly to sounds, the majority of individual neurons demonstrated a remarkable degree of normalcy in their responses.