What constitutes a meaningful signal for an organism in its ecological niche? This book considers this question with regard to how the structure and function of the nervous system allows perceptual categorization to occur. It addresses the overall theme under a variety of subtopics, including the recognition of biologically significant stimuli in the environment, the principles guiding development of patterns of anatomical and functional activity within the nervous system, and brain mechanisms in language. Theoretical as well as experimental approaches to understanding these subjects are included and the contributors represent disciplines ranging from psychology to cell biology.