Descending connections of auditory cortex to the midbrain and brain stem

Abstract

Descending pathways in the brain have been known, since the end of the nineteenth century (Held 1891) but their significance was unappreciated due to the focus on ascending pathways and the unsuitability of the tract tracing methods then available to reveal these projections. Renewed interest was triggered by the discovery of the olivocochlear bundle (Rasmussen 1946, 1953), and interest surged as the magnitude of the descending pathways emerged (Bourassa et al. 1997; Winer 2006). The auditory cortex (AC) projects to a wide range of subcortical targets in the auditory pathway (Winer 2006; Winer and Lee 2007). By far, the projections to the auditory thalamus and midbrain are the largest and the projections to subcollicular nuclei such as nucleus sagulum, the paraleminscal regions, superior olivary complex (SOC), cochlear nuclear complex (CNC), and pontine nuclei (PN) were not appreciated until recently (Feliciano and Potashner 1995; Weedman and Ryugo 1996; Doucet et al. 2002; Doucet et al. 2003; Meltzer and Ryugo 2006; Perales et al. 2006). The AC also projects to subcortical forebrain structures such as the amygdala (Romanski and LeDoux 1993), the basal ganglia, and premotor structures including the striatum (Beneyto and Prieto 2001), superior colliculus (Paula-Barbosa and Sousa-Pinto 1973), and central gray (Winer et al. 1998), suggesting that the AC has an important role not only in sensory processing of audition, but also in motor behavior, autonomic function, and state dependent changes (Winer 2006).