PT  - JOURNAL ARTICLE
AU  - Aravindakshan Parthasarathy
AU  - Björn Herrmann
AU  - Edward L. Bartlett
TI  - Aging alters envelope representations of speech-like sounds in the inferior colliculus
AID  - 10.1101/266650
DP  - 2018 Jan 01
TA  - bioRxiv
PG  - 266650
4099  - http://biorxiv.org/content/early/2018/02/18/266650.short
4100  - http://biorxiv.org/content/early/2018/02/18/266650.full
AB  - Older listeners often experience difficulties understanding speech in the presence of background sound. These deficits are thought to reflect neural deficits in the central auditory pathway, as they can occur independent of changes in cochlear hearing thresholds. Here we used a systems-level (scalp recordings) and a microcircuit-level (extracellular recordings) approach in male Fischer-344 rats to investigate how aging affects sensitivity to the temporal envelopes of speech-like sounds in the inferior colliculus. Scalp-recorded potentials suggest an age-related increase in sensitivity to temporal regularity along the ascending auditory pathway. The underlying cellular changes in the midbrain were examined using extracellular recordings from inferior colliculus neurons. We used the local field potential (LFP) as a proxy for a neuron’s or neural population’s synaptic inputs, and unit activity as a measure of spiking output. We observed an age-related increase in sensitivity to the sound’s onset and temporal regularity (i.e., periodicity envelope) in the spiking output of inferior colliculus neurons, relative to their synaptic inputs. This relative enhancement for aged animals was most prominent for multi-unit (in contrast to single-unit) spiking activity. Spontaneous multi-unit, but not single-unit, activity was also enhanced in aged compared to young animals. Our results suggest that aging is associated with altered sensitivity to sound and a sound’s temporal regularities, and that these effects may be due to increased gain of neural network activity in the aged auditory midbrain.SIGNIFICANCE STATEMENT Older listeners commonly experience challenges understanding speech in the presence of background sound. The neural functional changes contributing to this problem remain unclear. Our study shows an increase in network-level neuronal activity in the auditory midbrain of aged animals that alters the sensitivity to temporal regularities in speech sounds. Sensitivity is abnormally heightened despite reduced synaptic input to neurons, which suggest aberrant gain control mechanisms that are associated with aging. Although an increased gain in the auditory pathway of aged listeners may support detection of temporal regularities in speech, it may come at the cost of reduced discrimination between multiple speakers, and thus may contribute to age-related difficulties in understanding speech.