| Two interrelated themes have permeated neuroscience: neural information processing and experience-dependent plasticity. The central nervous system must maintain a consistent and reliable representation of the external environment while on the other hand, it must also be able to modify representations in order to adapt and learn. In the first part of this dissertation, I explored issues related to neural coding of acoustic information in the auditory thalamus, primary auditory cortex (A1), and posterior auditory field (PAF). I characterized response properties in each area to broad classes of sensory input ranging from tones, modulated trains of tones and noise, monkey vocalizations, and even human speech sounds. The results indicate that there are several physiologic differences in the way these areas represent these broad classes of sounds. We confirm that auditory responses to a sound are not static and that the activity of neurons is substantially influenced by stimulus history and spectrotemporal context on many timescales. Previous research suggests that changes in cortical processing contribute to the optimization of speech sound coding. Given the remarkable capacity of the mammalian brain to self-organize, I used a simple paradigm to study some of the principles that govern plasticity in A1. In the second part of this dissertation, electrical activation of the nucleus basalis was repeatedly paired with several different sounds to examine the potential strategies that cortical networks use to modify the representation of different inputs. The sounds paired with activation of nucleus basalis were tones, modulated trains of noise, a monkey vocalization, and human speech. In the plasticity studies presented in this thesis, I expand upon several earlier principles derived using this technique. In addition, I tested if and how known plastic properties of A1 neurons change after exposure to speech and vocalization sounds. Specifically, I examined how acoustic experience with natural sounds alters temporal masking patterns and the tonotopic map of frequency. These experiments were part of an effort to develop an animal model of vocalization-specific plasticity. Collectively, the results from these experiments demonstrate that both spectral and temporal responses of A1 neurons are mutable as a function of sensory experience. |
