Corticofugal modulation on both ON and OFF responses in the nonlemniscal auditory thalamus of the guinea pig.

Corticofugal modulation on both ON and OFF responses in various nuclei in the medial geniculate body (MGB) was examined by locally activating the auditory cortex and looking for effects on the neuronal responses to acoustic stimuli. In contrast with a major corticofugal facilitatory effect on the ON neurons in the lemniscal nucleus of the MGB of the guinea pigs, of 132 ON neurons tested in three conditions with cortical activation through each of three implanted electrodes, the majority of the tested conditions (319/396) that were sampled from the nonlemniscal nuclei of the MGB received inhibitory modulation from the activated cortex. This inhibitory effect was >50% for 99 cases while the auditory cortex was activated. Most of the OFF and ON-OFF MGB neurons (44/54) showed a facilitatory effect of 111.4 +/- 99.9%, and three showed a small inhibitory effect of 25.7 +/- 5.8% on their OFF responses. Thirty neurons in the border region between the lemniscal and nonlemniscal MGB showed mainly facilitatory corticofugal effects on both ON and OFF responses. Meanwhile, cortical stimulation induced almost exclusive inhibitory effects on the ON response and facilitatory effects on the OFF response in the MGcm. It is suggested that the OFF response is produced as a disinhibition from the inhibitory input of the auditory stimulus. The present results provide a possible explanation for selective gating of the auditory information through the lemniscal MGB while switching off other unwanted sensory signals and the interference from the limbic system, leaving the other auditory cortex prepared to process only the auditory signal.