Single-unit responses in the inferior colliculus of decerebrate cats II. Sensitivity to interaural level differences

Single-unit responses in the inferior colliculus of decerebrate cats. ii. S ensitivity to interaural level differences. J. Neurophysiol. 82: 164-175, 1 999. Single units in the central nucleus of the inferior colliculus (ICC) o f unanesthetized decerebrate cats can be grouped into three distinct types (V, I, and O) according to the patterns of excitation and inhibition reveal ed in contralateral frequency response maps. This study extends the descrip tion of these response types by assessing their ipsilateral and binaural re sponse map properties. Here the nature of ipsilateral inputs is evaluated d irectly using frequency response maps and compared with results obtained fr om methods that rely on sensitivity to interaural level differences (ILDs). In general, there is a one-to-one correspondence between observed ipsilate ral input characteristics and those inferred from LLD manipulations. Type V units receive ipsilateral excitation and show binaural facilitation (EE pr operties); type I and type O units receive ipsilateral inhibition and show binaural excitatory/inhibitory (EI) interactions. Analyses of binaural freq uency response maps show that these ILD effects extend over the entire rece ptive field of ICC units. Thus the range of frequencies that elicits excita tion from type V units is expanded with increasing levels of ipsilateral st imulation, whereas the excitatory bandwidth of type I and O units decreases under the same binaural conditions. For the majority of ICC units, applica tion of bicuculline, an antagonist for GABA(A)-mediated inhibition, does no t alter the basic effects of binaural stimulation; rather, it primarily inc reases spontaneous and maximum discharge rates. These results support our p revious interpretations of the putative dominant inputs to ICC response typ es and have important implications for midbrain processings of competing fr ee-field sounds that reach the listener with different directional signatur es.