Operation and interaction of the descending control and ascending sensory pathways in audition

Neural theories of human auditory perception often oversimplify the interaction of afferent and efferent projections in the corticofugal pathway. However, investigations of the effects of long-term experience on the auditory brainstem response demonstrate that cortical systems can affect early processing of acoustic information through top-down efferent projections (Krishnan et al., 2005; Wong et al., 2007). The immediate or real time effects of these top-down pathways on activity in the auditory brainstem, however, are less clear (Galbraith et al., 1998; Hoormann et al., 1994). One approach to investigating possible corticofugal interactions is to examine whether the suppressive effects of motor behavior exhibited in primary auditory cortex (Houde et al., 2002; Schneider et al., 2014) can be demonstrated at the level of the auditory brainstem and to determine whether these effects are driven by divided attention between the motor and auditory systems. Chapters 2 and 3 tested for motor behavior versus attention effects on neural activity in the auditory brainstem resulting from the presentation of a simple tone or a brief synthetic syllable, with the hypothesis that motor behavior or separable attention demands could reduce the auditory brainstem response to the acoustic stimulus. Chapter 4 held motor behavior constant and varied the demands on attention. In all cases, attention to moments in time enhanced spectral responses in the auditory brainstem response to acoustic stimuli, rejecting the hypothesis that there is descending cortical inhibition of auditory responses in the brainstem due to motor behavior, and the hypothesis that there are descending effects from the cortex of dividing attention. The results suggest that a system-wide attention network that directs attention to specific events or moments in time exerts control over the descending auditory pathway. Importantly, the results demonstrate the need to include the bidirectional projections between cortical networks and subcortical sensory structures in neural accounts of human auditory and speech perception.