Single-Unit Responses in the Inferior Colliculus of Decerebrate Cats I. Classification Based on Frequency Response Maps

Abstract

This study proposes a classification system for neurons in the central nucleus of the inferior colliculus (ICC) that is based on excitation and inhibition patterns of single-unit responses in decerebrate cats. The decerebrate preparation allowed extensive characterization of physiological response types without the confounding effects of anesthesia. The tone-driven discharge rates of individual units were measured across a range of frequencies and levels to map excitatory and inhibitory response areas for contralateral monaural stimulation. The resulting frequency response maps can be grouped into the following three populations: type V maps exhibit a wide V-shaped excitatory area and no inhibition; type I maps show a more restricted I-shaped region of excitation that is flanked by inhibition at lower and higher frequencies; and type O maps display an O-shaped island of excitation at low stimulus levels that is bounded by inhibition at higher levels. Units that produce a type V map typically have a low best frequency (BF: the most sensitive frequency), a low rate of spontaneous activity, and monotonic rate-level functions for both BF tones and broadband noise. Type I and type O units have BFs that span the cat's range of audible frequencies and high rates of spontaneous activity. Like type V units, type I units are excited by BF tones and noise at all levels, but their rate-level functions may become nonmonotonic at high levels. Type O units are inhibited by BF tones and noise at high levels. The existence of distinct response types is consistent with a conceptual model in which the unit types receive dominant inputs from different sources and shows that these functionally segregated pathways are specialized to play complementary roles in the processing of auditory information.

Midbrain Synchrony to Envelope Structure Supports Behavioral Sensitivity to Single-Formant Vowel-Like Sounds in Noise

“…The 10-dB bandwidth of the excitatory rate response to tones increased with increasing BF, while mean Q 10 (BF divided by 10-dB bandwidth; Fig. 3a, right) increased from 2.0 at BF of 1 kHz to 3.25 at BF of 4 kHz (similar to cat IC over the same BF range; Ramachandran et al 1999).…”

Monopolar Intracochlear Pulse Trains Selectively Activate the Inferior Colliculus

“…This is the opposite of what would be expected from neuronal adaptation. One possible mechanism that is consistent with this narrowing of sustained responses could be lateral inhibition, which has been shown to be present at multiple levels of central auditory processing (Shamma et al 1993;Ramachandran et al 1999;Young 2003). One implication of this narrowing of sustained responses is that comparisons of psychophysical and electrophysiological measures (for example, ECAPs) in CI users should ideally be made using similar pulse rates.…”

Statistical mapping of sound-evoked activity in the mouse auditory midbrain using Mn-enhanced MRI

“…In other mammals, the threshold levels in auditory neurons range over close to 40 dB SPL, and the dynamic ranges of the neuronal firing rate vs. sound level functions are variable (Kiang et al, 1965;Liberman et al, 1978;Aitkin, 1991). Some neurons in the auditory midbrain are known to have non-monotonic (not continuously increasing with sound level) firing characteristics (Ramachandran et al, 1999), but the spatial distribution and size of the non-monotonic population is currently unknown. Given this level of complexity, it is very difficult to predict the relationship between SPL and IC activity, and clearly more than three SPLs will be required to characterize the relationship between sound energy and MEMRI signal level and distribution.…”