Auditory cortical efferent actions upon inferior colliculus unitary activity in the guinea pig

Differential actions on inferior colliculus central nucleus (ICc) single cells spontaneous activity were observed with both ipsilateral and contralateral auditory cortical electrical stimulation (ACx stimulation). Following ACx stimulation, a firing depression of the spontaneous activity was obtained using contralateral or ipsilateral cortical stimulation, although a greater effect was elicited by the contralateral cortex. In contrast, ipsilateral ACx stimulation elicited more excitation with a shorter latency than contralateral stimulation. In units that failed to show spontaneous firing, the sound-evoked responses and ACx stimulation were studied; approximately 50% of them demonstrated firing depression to ACx stimulation on either side with either clicks or tone-bursts. Thirty percent of the units failed to show changes in response to any cortical stimulation. A temporary disruption of ICc-evoked neuronal discharge was elicited during contralateral cortex stimulation, as previously reported to occur during sleep. The demonstration that auditory cortices may differentially affect the same ICc unit activity, i.e. spontaneous and evoked, suggests that auditory processing may depend on the ongoing spontaneous activity plus the effects exerted from each auditory cortex activation.