Thalamocortical forward modulation in the auditory cortex
Date
2017-12-18
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Abstract
Auditory masking is a psychoacoustic phenomenon with strong physiological correlates in the central auditory system such as in the primary auditory cortex. The caveat of conventional two-tone paradigms in testing the neurophysiology of forward masking (i.e., forward suppression) is that tones are processed at every auditory center, meaning the recorded activities do not represent the pure characteristics of the recorded site. Thus, there is a great need for a specialized protocol to tease out and uncover the pure suppression characteristics at each site, particularly the thalamocortical system - the primary sensory input to the auditory cortex. This study implemented an in vivo thalamocortical model in C57 mice along with common electrophysiological techniques to examine the pure role of the thalamocortical system in cortical forward modulation; focal electrical stimulation (ES) of the ventral division of the medial geniculate body (MGBv, ESMGBv) substituted for one or both tones of the conventional two-tone stimulus paradigm. With this ameliorative approach, I demonstrated that ESMGBv facilitated succeeding ESMGBv while suppressing succeeding tones. This suggests that the default state of the thalamocortical system is facilitation while maintaining the capability of forward suppression through heterosynaptic thalamocortical inputs. Forward suppression of tones by ESMGBv was comparatively weaker than forward suppression of ESMGBv by tone stimulus. The period of complete suppression in the cortex for all stimuli was approximately 100 ms. Furthermore, the thalamocortical forward suppression spectral areas were found to be mirrored images of the receptive fields of recorded cortical neurons. Together, these data suggest that thalamocortical forward facilitation and suppression in the recorded cortical neurons can be accomplished without the involvement of complex cortical circuitry. This idea is further supported by the finding that cortical GABAA inhibition was not required for the thalamocortical forward suppression. Additionally, it was shown that two-tone stimulus resulted in complete MGBv suppression for approximately 75 ms. This means that the thalamus provides the cortex with a 75 ms “silent window” during which cortical mechanisms, such as GABAA inhibition, are not necessary for forward suppression. Following careful investigation of literature, a novel "single-cell” model appears to account for thalamocortical forward modulation.
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Keywords
single-unit model of thalamocortical forward modulation, auditory temporal processing, in vivo electrophysiology, auditory forward masking, thalamocortical system
Citation
Xiong, C. (2017) Thalamocortical forward modulation in the auditory cortex (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.