Our long-term goal is to develop analytic models for the auditory processing of tones, noise, and speech in normal and impaired hearing. Precise quantitative models in the area of auditory science are important to effectively rehabilate hearing impairment and to provide better diagnostic tools. Our successful modeling of loudness perception in impaired listeners indicates that excitation patterns, which are derived from masking patterns, may serve as a general framework for modeling normal and impaired hearing. The proposed research tests the hypothesis that intensity discrimination, both in normal and impaired hearing, can be predicted on the basis of excitation patterns. In Phase I, we test and excitation-pattern model for intensity discrimination in normal hearing. Predictions derived on the basis of absolute thresholds and masking will be compared to intensity discrimination data from the same listeners. Intensity discrimination will be measured for pure tones over a wide range of frequencies and intensities as well as for two-tone complexes and multi-tone complexes as a function of bandwidth and level. All measurements will employ a 2I, 2AFC paradigm with an adaptive procedure. In addition to testing the model, these experiments will also provide a comprehensive, cohesive set of direly needed reference data for masking patterns and intensity discrimination in the same normal listeners. In Phase II, the model is extended to apply to listeners with impairments of predominantly cochlear origin. The model will be modified on the basis of threshold and masking measurements. Predictions thus derived will compared to these listeners' intensity discrimination performance for the set stimuli used in Phase I. For at least three listeners with unilateral impairments, these measurements will be performed in their impaired ears as well as in their normal ears in quiet and under masking with a noise spectrally shaped to simulate the hearing loss in the impaired ear. These measurements permit us to separately assess the effects of increased thresholds and reduced frequency selectivity on intensity discrimination in impaired hearing.