Modulatory effects of regional cortical activation on the onset responses of the cat medial geniculate neurons.

Corticofugal modulation on activity of the medial geniculate body (MGB) was examined by locally activating the primary auditory cortex (AI) and looking for effects on the onset responses of MGB neurons to acoustic stimuli. Of 103 MGB neurons recorded from 13 hemispheres of 11 animals, 91 neurons (88%) showed either a facilitatory or inhibitory effect or both; of these neurons, 72 showed facilitatory effects and 25 inhibitory effects. The average facilitatory effect was large, with a mean increase of 62.4%. Small inhibitory effects (mean: -16.2%) were obtained from a few neurons (6 of 103) when a pure tone stimulus was used, whereas the effect became larger and more frequent when a noise burst stimulus was used (mean: -27.3%, n = 22 of 27 neurons). Activation of an AI site having the same best frequency (BF) as the MGB neuron being recorded from produced mainly a facilitatory effect on MGB neuronal responses to pure tones. Activation of AI at a site neighboring the BF site produced inhibitory effects on the MGB response when noise burst stimuli were used. We found that the effective stimulation sites in AI that could modulate MGB activity formed patchlike maps with a diameter of 1.13 +/- 0.09 (SE) mm (range 0.6-1.9 mm, n = 15) being larger than the patches of thalamocortical terminal fields. Examining the effects of sound intensities, of 18 neurons tested 9 neurons showed a larger effect for low-sound-intensity stimuli and small or no effects for high-sound-intensity stimuli. These were named low-sound-intensity effective neurons. Five neurons showed high sound intensity effectiveness and four were non-intensity specific. Most low-sound-intensity effective neurons were monotonic rate-intensity function neurons. The AI cortical modulatory effect was frequency specific, because 15 of 27 neurons showed a larger facilitatory effect when a BF stimulus was used rather than a stimulus of any other frequency. The corticothalamic connection between the recording site in MGB and the most effective stimulation site in AI was confirmed by injecting wheat germ agglutinin-horseradish peroxidase tracer at the stimulation site and producing a small lesion in the recording site. The results suggest that 1) the large facilitation effects obtained by AI activation at the region that directly projected to the MGB could be the result mainly of the direct projection terminals to the MGB relay neurons; 2) the large size patches of the effective stimulation site in AI could be due to widely ramifying corticothalamic projections; and 3) the corticofugal projection selectively gates auditory information mainly by a facilitatory effect, although there is also an inhibitory effect that depends on the sound stimulus used.