Pollak's principal interests are in elucidating how circuits in the mammalian brain transform information in sensory systems and how populations of neurons then represent features of the external world. Specifically, I study the mammalian auditory system and evaluate how the auditory system processes species-specific communication signals and the cues that enable animals to associate a sound with its location in space. The animals that I use for experimental subjects are echolocating bats. The reasons for using bats are that they are mammals, and thus their auditory systems are mammalian in design, but due to the high premium they place on hearing,, their auditory systems are greatly enlarged and express features with exceptional clarity. Like all animals, bats have to know what a sound is and where in space it came from. Projects that address "what the sound is" investigate how the brain decodes and represents species-specific social communication signals. We do this by recording from individual nerve cells to evaluate how they respond to these complex signals. We then dissect how those response features are created either by iontophoresing drugs that block inhibitory receptors or by reversibly inactivating lower nuclei that innervate the neurons from which we record. In this way we determine what rules the auditory system employs to create response selectivity and diversity and how each nucleus in the circuit contributes to the implementation of those rules. We also focus on how the brain processes and represents sound location. Sound localization is of particular interest because the nervous system has to compute the location of a sound source from on acoustic cues received at the two ears. Thus, my research efforts are directed at evaluating how the cues from the two ears are processed by the circuits in the auditory system and how those cues are represented in the higher regions of the auditory system by populations of neurons.