Experiments which have investigated differences of function of the right and left cerebral hemispheres by studying ear asymmetry are reviewed, and literature relating to the two click threshold, the task used in this series of experiments, is summarised. Two reaction time studies establish a perceptual factor in the ear asymmetry effect and subsequent experiments explore the nature of this perceptual asymmetry_ A signal detection method is developed which, unlike the psychophysical threshold method, reveals greater sensitivity in detection of two clicks presented to the left ear than to the right ear when clicks are presented with a contralateral burst of white noise, or with preceding shock, but little difference between ears without white noise or shock. Caffeine citrate also increases the right/left ear difference. The effects of temporal intervals between white noise onset or shock and click presentation, and of laterality of white noise or shock are investigated. Interaction effects are again obtained, and periodicity effects are noted which suggest the role of attention mechanisms in the ear asymmetry effect. It is suggested that two major factors are involved in the task: detection of two clicks and pitch discrimination. The former is considered to be an index of cortical arousal. The possibility of hemispheric differences in arousal is discussed and a method for investigating individual differences in arousal is suggested. Thelatter is considered to reveal differentiation of function of the cerebral hemispheres in an auditory discrbnination task mainly involving pitch discrimination. The phenomenon is interpreted as being primarily perceptual, but attention mechanisms are also probably involved. Although the results obtained are reliable, the interpretation is tentative and much further investigation is suggested