Tang J, Yang W, Suga N. Modulation of thalamic auditory neurons by the primary auditory cortex. J Neurophysiol 108: [935][936][937][938][939][940][941][942] 2012. First published May 2, 2012; doi:10.1152/jn.00251.2012.-The central auditory system consists of the lemniscal and nonlemniscal pathways or systems, which are anatomically and physiologically different from each other. In the thalamus, the ventral division of the medial geniculate body (MGBv) belongs to the lemniscal system, whereas its medial (MGBm) and dorsal (MGBd) divisions belong to the nonlemniscal system. Lemniscal neurons are sharply frequency-tuned and provide highly frequencyspecific information to the primary auditory cortex (AI), whereas nonlemniscal neurons are generally broadly frequency-tuned and project widely to cortical auditory areas including AI. These two systems are presumably different not only in auditory signal processing, but also in eliciting cortical plastic changes. Electric stimulation of narrowly frequency-tuned MGBv neurons evokes the shift of the frequency-tuning curves of AI neurons toward the tuning curves of the stimulated MGBv neurons (tone-specific plasticity). In contrast, electric stimulation of broadly frequency-tuned MGBm neurons augments the auditory responses of AI neurons and broadens their frequency-tuning curves (nonspecific plasticity). In our current studies, we found that electric stimulation of AI evoked tone-specific plastic changes of the MGBv neurons, whereas it degraded the frequency tuning of MGBm neurons by inhibiting their auditory responses. AI apparently modulates the lemniscal and nonlemniscal thalamic neurons in quite different ways. High MGBm activity presumably makes AI neurons less favorable for fine auditory signal processing, whereas high MGBv activity makes AI neurons more suitable for fine processing of specific auditory signals and reduces MGBm activity. corticofugal modulation; frequency tuning; medial geniculate body; nonspecific plasticity; tone-specific plasticity THE CENTRAL AUDITORY SYSTEM of mammals consists of the lemniscal and nonlemniscal systems. In the thalamus, the lemniscal system is composed of the ventral division of the medial geniculate body (MGBv) that projects to the primary auditory cortex (AI) and anterior auditory field (AAF). The nonlemniscal system is composed of the medial (MGBm) and dorsal (MGBd) divisions of the MGB and the posterior intralaminar nucleus. The MGBd projects to the cortical auditory areas surrounding AI, whereas the MGBm projects to all auditory areas including AI (Imig and Morel 1983 for review; Andersen et al. 1980; Winer et al. 1977). The cortical auditory areas project back to the subcortical auditory nuclei: AI and AAF project back to the MGBv; AII to MGBd; and all auditory areas, including AI, to MGBm (Andersen et al. 1980;Rouiller 1997 In general, the MGBv is tonotopically organized by sharply frequency-tuned neurons, whereas the MGBm and MGBd are poorly tonotopically organized by broadly frequency-tuned neurons. The MGBm is polysenso...