Psychometric function slopes and false positive rates were obtained from two groups of 10 subjects each: a normal group observed both with and without external auditory canal occlusion, and a group of subjects with otosclerotic hearing losses. The measures were obtained at 80, 125, and 1000 Hz. Analysis of subject responses revealed that steeper psychometric function slopes and fewer false positive responses were observed in the otosclerotic group as compared with the normal group. Interpretation of results supports a model of auditory functioning in which a substantial amount of physiological noise originates in the middle ear and creates confusion, uncertainty, and variability during threshold determination for normal listeners. This variability, reflected in a flattening of the psychometric function slope and an increase in occurrence of false positive responses, is not seen in subjects with otosclerosis, presumably because of attenuation of physiological noise arising in the middle ear.