FUNCTIONAL ORGANIZATION OF INFERIOR COLLICULUS

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DESCRIPTION: (Adapted From The Applicant's Abstract.) The chief goal of these studies is to evaluate how the largely GABAergic inhibitory outputs of the dorsal nuclei of the lateral lemniscus (DNLLs) shape the monaural and binaural properties of EI neurons in the DNLL and inferior colliculus (ICc). The system to be used for these studies is the enlarged 60 kHz isofrequency contour of the mustache bat's DNLL and ICc. Pharmacological studies conducted during the previous grant period prompted a number of hypotheses about the roles that the ipsi- and contralateral DNLLs play in shaping the binaural properties and azimuthal receptive fields of ICc neurons. The goals of the proposed studies are to test those hypotheses with reversible inactivation of either the ipsi- or contralateral DNLLs while monitoring the response properties of EI cells in the opposite DNLL or ICc. Four sets of experiments are proposed to test these hypotheses: 1) The role of the commissure of Probst will be evaluated by recording the monaural and binaural properties of 60 kHz DNLL neurons and then assessing the same properties after the DNLL on the opposite side has been reversibly inactivated by the iontophoretic application of muscimol, an agonist for GABAa receptors. After recovery from inactivation, bicuculline will be iontophoretically applied to the neuron under study and the same properties will be determined again. 2) The same procedures will be used to evaluate the roles of the contra- and ipsilateral DNLLs on the monaural and binaural response properties of 60 kHz EI neurons in the ICc. The influence of the contra- and ipsilateral DNLLs will be assessed in separate experiments. 3) The effects of an initial sound on the binaural properties evoked by a second sound that follows shortly thereafter will be evaluated in ICc neurons. Initially the ICc cell's normal binaural properties with a single sound presented over a wide range of IIDs will be established. Subsequently, two successive binaural tone bursts will be presented. The IID of the first stimulus will be fixed and will favor the inhibitory ear while the second stimulus will be presented with a wide range of IIDs that would be produced when sounds emanate from different regions of space. The procedure will then be repeated but the first stimulus will have a fixed IID that favors the excitatory ear. The hypothesis predicts that the binaural properties evoked by the second signals: a) will be changed, and thus the response evoked by that sound will be different from those evoked by a single sound; b) will be different when the initial signal favors the ipsi ear from the properties obtained when the initial signal favors the contra ear; and c) will be due, in part, to the inhibition of either the ipsi- or contralateral DNLL that was evoked by the first signal. 4) To test the hypothesis that inhibition at the DNLL evoked by the first signal is responsible for the changes in binaural properties at the ICc evoked by a second signal, the same properties in ICc neurons will be assessed when either the ipsi- or contralateral DNLLs are inactivated by the iontophoretic application of muscimol.