CORTICOFUGAL AMPLIFICATION OF SUBCORTICAL RESPONSES TO SINGLE TONE STIMULI IN THE MOUSTACHED BAT

Since 1962, physiological data of corticofugal effects on subcortical auditory neurons have been controversial: inhibitory, excitatory, or b oth. An inhibitory effect has been much more frequently observed than an excitatory effect. Recent studies performed with an improved experi mental design indicate that corticofugal system mediates a highly focu sed positive feedback to physiologically ''matched'' subcortical neuro ns, and widespread lateral inhibition to ''unmatched'' subcortical neu rons, in order to adjust and improve information processing. These res ults lead to a question: what happens to subcortical auditory response s when the corticofugal system, including matched and unmatched cortic al neurons, is functionally eliminated? We temporarily inactivated bot h matched and unmatched neurons in the primary auditory cortex of the mustached bat with muscimol (an agonist of inhibitory synaptic transmi tter) and measured the effect of cortical inactivation on subcortical auditory responses. Cortical inactivation reduced auditory responses i n the medial geniculate body and the inferior colliculus. This reducti on was larger (60 vs. 34%) and faster(11 vs. 31 min) for thalamic neur ons than for collicular neurons. Our data indicate that the corticofug al system amplifies collicular auditory responses by 1.5 times and tha lamic responses by 2.5 times on average. The data are consistant with a scheme in which positive feedback from the auditory cortex is modula ted by inhibition that may mostly take place in the cortex.