Thalamocortical mechanism underlying cortical temporal information processing for sound perception

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In nature, more than one sound often impinges upon the eardrum in short time-intervals. Thus, it is necessary to have the ability to focus on one sound and ignore others to enable social interactions involving oral communication. Forward masking, a behavioral phenomenon typically measured by the two-tone stimulus paradigm, represents the capacity of how perception of a sound is inhibited by preceding ones. Forward suppression, the neural basis of forward masking, is well characterized in the auditory cortex but its mechanism remains unclear. Here, I focus on the pivotal locus for the transformation of temporal information to the cortex, specifically, the thalamocortical (TC) role in cortical forward processing. Use of conventional two-tone paradigm cannot characterize the TC function in intact animals due to several relays below the thalamus. We used the approach of in vivo focal electrical stimulation of the auditory thalamus and discovered that the TC system implements a homo-facilitation & hetero-suppression mechanism for the remodeling of cortical function. Here we hypothesize that the TC system implements a center-surround process for cortical forward processing in an input-specific manner. This work, designed in four aims, will complete our understanding of the TC role in cortical forward processing. We first establish the degree of cortical forward suppression that originates in the thalamus using classical tone-tone pair. The second aim measures the direct TC contribution to cortical forward suppression by substituting thalamic stimulus for either the preceding or succeeding tone in the two-tone paradigm. The third aim establishes the occurrence of cortical forward facilitation by paired thalamic stimuli in which the preceding and succeeding stimuli activate identical TC inputs. Finally, I examine the role of intracortical excitatory and inhibitory circuitry in cortical forward processing using advanced techniques - in vivo whole cell patch and iontophoretic injection of GABAergic agent to recorded neurons. Our data prompt a decisive role of TC transformation in cortical forward processing. We anticipate that the TC system anchors a mechanism of a self-strengthening and surround-weakening temporal processing for sound perception. This important concept advances my long-term goal of uncovering the neural mechanism involved in hearing. It will also benefit research and education in the fields of hearing, physiology, psychology and neuroscience.