A COMPARISON OF INFERIOR COLLICULUS RESPONSES TO BAND PASSED NOISE IN YOUNG AND AGED RATS USING SINGLE UNIT RECORDINGS
Half of people over 75 in the United States suffer from age related hearing loss and have trouble understanding speech in a noisy background. Even older adults who have normal pure tone audiograms can have trouble understanding speech in a noisy background. Speech is a complex sound and therefore sounds more complex than pure tones are required to understand the differences in processing noisy speech in young and aged individuals. Band passed noise is easily controlled and is more complex than pure tones making it better stimulus for testing. The first place in the ascending auditory pathway that does complex processing is the inferior colliculus. Single unit recordings from the inferior colliculus of young and aged F344 rats were preformed using half octave band passed noise and pure tones. Firing rates, first spike latencies, the number of tuning peaks, normalized peak slope, bandwidth, and Q factors were all evaluated for each unit in response to band passed noise. For 54 of the units their responses to pure tones were also collected. Out of 286 units recorded from young animals, 218 were responsive and 178 of them had a band passed response. Out of 193 aged units, 145 were responsive and 134 had a band passed response. Young units had a significantly higher total firing rate (p = 0.008) and bandwidth (p = 0.004). The normalized peak slopes and Q factors were significantly lower in young units indicating sharper tuning in the aged units. Pure tones elicited a stronger response than band passed noise however, for many units the best frequency was similar for both stimuli. These results show that aged units are less responsive to stimuli containing multiple frequencies which may help explain why older adults have trouble understanding noisy speech.