Intercollicular modulation of auditory processing in the inferior colliculus

Abstract

The inferior colliculi (ICs) are the principal nuclei of the auditory midbrain. Each IC processes converging inputs from numerous brainstem nuclei as well as from thalamus and cortex. The ICs are interconnected in mirror image by one of the largest afferent inputs to each IC, the commissure of the inferior colliculus. There is exiguous knowledge about how each IC influences the processing of auditory information in its contralateral counterpart. This thesis investigates how one IC modulates the neural representation of sounds in the contralateral IC. To this end, I established and validated an experimental model in anaesthetised guinea pig whereby neuronal activity in one IC was selectively and reversibly deactivated. Cryoloop cooling produced temperature changes sufficient to deactivate spiking activity in the dorsal half of one IC, whilst leaving other centres in the auditory pathway unaffected. Single units were recorded in one IC before, during and after deactivation of the other IC. The characteristic frequency (CF) of IC neurons was unaffected during cooling, but the threshold of the population was raised. The area of non-V-shaped frequency response areas (FRAs) changed more than V-shaped FRAs. Differential changes were also observed in the firing rate of units with different temporal response patterns. Onset responders increased their firing rate whilst the firing of Chopper units was reduced. The temporal firing patterns of all neurons were unchanged by cooling. Changes in first spike latency (FSL) were negatively correlated with changes in firing rate. These data indicate that each IC differentially modulates the frequency selectivity, sensitivity, firing rate and FSL, but not the temporal firing pattern or CF of neurons in the contralateral IC. These findings demonstrate that the analysis of auditory stimuli in each IC is dependent on intercollicular processing. The ICs should therefore be viewed as working cooperatively rather than independently.