BME Seminar Series: Nathanial Greene, PhD Candidate

Decoding sound localization: The role of the LSO in the interaural level difference processing pathway

Abstract

Neurons in the central nucleus of the inferior colliculus (ICC) of decerebrate cats show three major response patterns when tones of different frequencies and sound pressure levels (i.e. a response map) are presented to the contralateral ear. The frequency response maps of type I units are uniquely defined by a narrow V-shaped excitatory area at best frequency (BF) with flanking inhibition at higher and lower frequencies. Units that produce type I maps typically have high BFs (>3 kHz), high rates of spontaneous activity (˜10 spikes/s), and monotonic rate-level curves for BF-tones and noise, with BF-tone thresholds approaching the lowest values of the auditory nerve at all frequencies.

Additionally, these units receive ipsilateral inhibition, and display binaural excitatory/inhibitory (EI) interactions, thus are sensitive to the difference in level between the two ears (ILD). Given this constellation of properties, it has been hypothesized that the contralateral lateral superior olive (LSO) provides the dominant excitatory input to type I units.

All previous quantitative studies of the LSO have been performed in anesthetized preparations; here, we report on the monaural response properties of single units in the LSO of decerebrate cats. The results reveal that LSO units form a relatively homogeneous population. The response maps of LSO units show relatively narrow V-shaped excitatory tuning, with flanking inhibition visible in units with spontaneous activity.

LSO units generally have high BFs, low spontaneous rates (< 1 spike/s), and monotonic rate-level curves for BF-tones and noise. These results suggest that LSO units are generally a suitable source of the principal excitatory drive to ICC type I units. However, units with BFs below approximately 3 kHz exhibit BF-tone thresholds at least 30 dB above the best values of the auditory nerve, rendering these units incapable of providing the principal source of this excitatory drive.

The remaining response properties suggest that additional excitatory and inhibitory inputs are required to sharpen the tuning and decrease rate-level curve monotonicity in order to account for all response properties seen in ICC type I units. These results are largely consistent with prior observations, and support the conjecture that a crossed LSO projection is the primary source of excitation for high frequency type I units. Supported by NIDCD grant R01 DC 05161-07.