Corticofugal Modulation Of Neural Responses In The Auditory Thalamus Of Awake Marmosets

The auditory system is the second most important sensory pathway for human beings to perceive and understand the world.How the brain processes auditory information is one of the core questions in auditory research,which is essential for understanding the neural mechanisms underlying biological relevant sound processing,such as human speech,music as well as natural sounds.Due to the limitation of methodology,most of the past auditory studies were carried out in anesthetized animals.However,accumulating studies have shown that the function of the auditory system,especially the auditory cortex,is greatly affected by anesthesia,such as the neural processing of temporal information embedded in sounds.Therefore,it is of great significance to study the neural circuity mechanisms underlying auditory processing in awake animals.In the present study,single unit recordings were performed in medial geniculate body(MGB)of awake marmosets while the primary auditory cortex(Al)were reversibly inactivated by using the cooling technique.We found that:(1)cooling of Al has great influence over the sustained(55-205 ms)instead of on-set(5-55 ms)response in response to acoustic stimuli.Our results suggest that the sustained response that was prevalent in the auditory cortex was generated by intracortical inputs instead of thalamocortical inputs;(2)cooling of Al affected the tuning bandwidth of MGB neurons and the modulation effects are location specific;(3)cooling of Al has gained modulation on MGB neurons,however,the spontaneous firing rate of some MGB neurons increased although the sound elicited responses decreased after cortical cooling,indicating that the auditory cortex has complex nonlinear regulation on the thalamus;(4)cooling of Al affected the temporal processing of MGB neurons,especially the non-synchronized response.The sustained firing of non-synchronized MGB neurons in response to fast click diminished after cortical cooling,which suggests that cortical feedback control changed the temporal coding schemes of MGB neurons to time-varying stimuli.In the current project,we have developed the first reversible chronic cooling technique in the auditory cortex of awake marmosets.We have found that cooling of the auditory cortex has strong and complex corticofugal effects on MGB neurons.The study will improve our understanding of corticofugal modulation of neural response in MGB neurons under awake condition and temporal processing in corticothalamic pathway.