Acoustic input to the lateral pontine nuclei.

Axon and terminal degeneration were studied in the cat dorsolateral pontine nucleus (DLPN) after lesion of the inferior colliculus. In separate experiments the acoustic responses of 111 units of the lateral pontine nuclei were studied in cats anesthetized with chloralose-urethane. Lesions of all three nuclei of the inferior colliculus (central, pericentral and external) lead to a very similar pattern of terminal degeneration in a discrete region of ipsilateral DLPN. This is suggestive of a highly convergent projection in which topography may be blurred. Most units responded to binaural stimulation, and the most common binaural response consisted of excitatory inputs from each ear which were facilitated at some binaural intensity levels and occluded at others. Discharge rates changed as a result of alterations in the number of spikes evoked at the onset of the stimulus, and sustained discharges were rarely encountered. Units were broadly and irregularly tuned; binaural inhibition was very uncommon. Unit response characteristics suggested that, while the projection from the inferior colliculus was highly convergent, only a subclass of inferior colliculus neurons may be involved. However, the acoustic properties of lateral pontine units were strikingly similar to those of the cerebellar vermis, a region to which DLPN is known to project.