The ability to discern intonation or amplitude modulation of syllables in the individual words is of vital importance to speech perception. This application is for support to investigate how amplitude modulation in temporally discrete sounds are represented in the inferior colliculus (IC) and the auditory cortex (AC). The study will be carried out in echo locating bats for which survival depends on having a fine ability to discriminate amplitude and frequency modulations in echoes of their own biosonar vocalizations. Specifically, for myotic lucifugus, different insects induce different amplitude-modulation patterns in a sequence of temporally discrete echoes; these patterns can be used as signatures for discriminating the different insects. Electrophysiological experiments will be carried out to address the following questions: 1. How well do single neurons in the inferior colliculus and auditory cortex represent amplitude-modulations that span across successive sound pulses (""""""""AM"""""""")? 2. Is the representation of """"""""AM frequency"""""""" a function of a unit's temporal discharge pattern, or its characteristic frequency? If so, how? 3. Is the representation of """"""""AM frequency"""""""" dependent upon the rate at which sound pulses are presented (repetition rate)? If so, how? 4. How do neurons in the tow levels of the central auditory system differ in their representations of the """"""""AM frequency""""""""? Is there a transformation in the representation along the collilculo-thalamo- cortical pathway? 5. Is there an topographically ordered representation of """"""""AM frequency"""""""" in either the inferior colliculus or the auditory cortex? Results of these physiological studies are expected to generate insights into how behaviorally meaningful amplitude-modulated stimuli are encoded in the auditory system of echolocating bats and to shed light on the mechanisms by which the nervous system of echolocating bats and to shed light on the mechanisms by which the nervous system represents amplitude variations over discrete components of complex sounds such as speech.
