Accumulative Effect of Stimulus-Specific Adaptation through the Ascending Pathway to the Auditory Cortex

The long-term goal of our research is to understand the interaction between the thalamus and the cerebral cortex. The thalamus is reciprocally connected with cerebral cortex. Both thalamocortical and corticothalamic projections send collaterals to the thalamic reticular nucleus (TRN) of the ventral thalamus. The TRN consists of only GABAergic inhibitory neurons that project back only to the medial geniculate body (MGB), not to the auditory cortex. Over the past decade, scientists have shown that neurons in different stages of the auditory ascending pathway exhibited stimulus-specific adaptation (SSA) [1-9]. It has been speculated that the SSA in auditory cortical neurons could be produced through interaction between the ascending pathway and reciprocal thalamocortical pathway[10]. Recently we found that the SSA of TRN neurons showed either facilitatory or inhibitory effect on MGB neurons [5,11]. However, SSA in the TRN was much greater than that in the MGB[5]. The SSA in the TRN was not affected much by the MGB neurons’ SSA[5]. The first question raised here is whether the SSA of cortical neurons has any correlation with that of their projection MGB neurons. The other questions are how much the SSA of cortical neurons is affected by that of MGB neurons and how much by TRN neurons. We would expect that the influence of the SSA of MGB neurons on that of cortical neurons is likely to be true, and is likely reflected on the early components of the responses, while the influence of the SSA of TRN neurons is reflected on the late components of the responses. In the present study, the questions will be addressed with both the extracellular and intracellular recordings from MGB and cortical neurons with oddball paradigm, while the TRN is manipulated with local drug application. Using intracellular recording, we would be able to measure the membrane potential change of cortical neurons at the subthreshold level to the acoustic stimulus and/or electrical stimulation of the MGB. The results of the proposed study would not only advance our understanding on the novelty detection function in the thalamocortical system, but also add an important piece of knowledge on potential functional implications of the highly integrative brain processes of attention.