Despite the large number of people who wear hearing aids (6.2 million; Kochkin, 2005), the specific effects of hearing aid amplification on the brain are largely unknown; however, it is known that the central auditory system (CAS) changes as a function of auditory deprivation and stimulation, reorganizing throughout the lifespan according to the auditory input that is available to the individual. Hearing aid amplification is one example of auditory stimulation, and experiments designed to examine the effect of amplification on the human CAS are beginning to emerge. By measuring CAS responses to amplified sound we can learn more about the effects of hearing loss and amplification on the CAS and, in turn, improve the science underlying auditory rehabilitation. Therefore, the long-term goal of this research program is to better understand the neural representation of amplified sound in adults with hearing loss. In theory, when sound is amplified by a hearing aid, neural response patterns should be larger in amplitude (strength) and shorter in latency (neural conduction travel-time) when compared with unaided neural responses. This theory is based on decades of neuroscience demonstrating the effects of incremental (as small as 2-3 dB) increases in intensity on electrophysiological recordings (e.g., Martin & Boothroyd, 2000; Rapin, Schimmel, et al., 1966); however, recent experiments conducted in our laboratory indicate that this is not the case. When 20 dB of gain was provided by a hearing aid, there were no significant differences between unaided and aided neural responses patterns (Billings, Tremblay & Souza, 2006; Tremblay, Billings et al., 2006). Presumably, some sort of interaction is occurring between the way sound is processed by the hearing aid and encoded in the auditory system, and it is necessary to understand this interaction before stimulation-related effects in the CAS, associated with hearing aid use, can be interpreted. Therefore, we introduce a program of research that examines the interaction between hearing-aid- processed acoustic input and neural encoding in people with and without hearing loss. The specific aims are: (1) determine the effects of stimulus intensity and amplification on neural responses in normal-hearing participants, (2) determine the same effects in hearing-impaired individuals, and (3) quantify signal- processing effects on stimuli acoustics using in-the-canal acoustic recordings. [unreadable] [unreadable] [unreadable] [unreadable]