This study examined the contribution of glycinergic inhibition to the time-sensitive spectral integration performed by neurons in the inferior colliculus of the mustached bat (Pteronotus parnellii ). These neurons are sometimes called combination-sensitive because they display facilitatory (or inhibitory) responses to the combination of distinct spectral elements in sonar or social vocalizations. Present in a wide range of vertebrates, their temporally and spectrally selective integration is

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... ught to endow them with the ability to discriminate among social vocalizations or to analyze particular cues concerning sonar targets. The mechanisms that underlie these responses or the sites in the auditory system where they are created are not known. We examined combination-sensitive neurons that are facilitated by the presentation of two different harmonic elements of the bat's sonar call and echo. Responses of 24 single units were recorded before and during local application of strych-nine, an antagonist of glycinergic inhibition. For each of the 24 units, strychnine application eliminated or greatly reduced temporally sensitive facilitation. There was no difference in this effect for neurons tuned to frequencies associated with the frequencymodulated or the constant-frequency sonar components. These results are unusual because glycine is considered to be an inhibitory neurotransmitter, but here it appears to be essential for the expression of combination-sensitive facilitation. The findings provide strong evidence that facilitatory combination-sensitive response properties present throughout the mustached bat's auditory midbrain, thalamus, and cortex originate through neural interactions in the inferior colliculus. Communication by sound uses spectrally and temporally complex signals, and their analyses in the CNS require integration across the spectral and temporal elements in the signals. One form of integration is performed by neurons responding best when distinct spectral elements in signals are presented in specific temporal relationships. These neurons are sometimes called combinationsensitive. Although general features of combination-sensitive responses are common in a wide range of vertebrate species, the species-specific features of the integration create response preferences to a bird's own song (Margoliash and Fortune, 1992; Doupe, 1997) , to pulse-echo delay in mustached bats (O'Neill and Suga, 1982; Olsen and Suga, 1991) , and among different social vocalizations in frogs (Fuzessery and Feng, 1983 ) and mammals (Rauschecker et al., 1995; Ohlemiller et al., 1996) . The neural interactions that underlie these response properties are poorly understood, but have been generally thought to originate in the auditory forebrain (Olsen