Evidence for an auditory subsector within the zona incerta of rats

This study explores the patterns of connections between the zona incerta (ZI) of the thalamus and the major auditory centres of the rat brain. Two series of experiments were performed using neuroanatomical tract tracing methods (biotinylated dextran or cholera toxin subunit b). First, tracers were injected into the ZI and resultant patterns of labelling in the auditory centres were examined. Labelling was seen in distinct subdivisions of the temporal cortex (Te3), medial geniculate complex (medial division; MGm), inferior colliculus (external cortex; ICe) and cochlear nucleus (dorsal and ventral divisions). For the most part, these subdivisions are involved in more associative or integrative aspects of audition. In a second series of experiments, three of these centres (Te3, ICe, cochlear nucleus) were injected with tracers and the labelling patterns formed in the ZI were explored. These results show that each of these connections makes a very distinct territory or subsector within the most lateral region of the ZI. This subsector of connectivity with the auditory centres does not respect the well defined cytoarchitectonic sectors of ZI, being made up of small slabs in both the dorsal and ventral sectors. Overall, the results suggest that ZI may integrate auditory information together with the other exteroceptive and interoceptive information that it receives and then influences subsequently global states of arousal and/or attention.