Much of the research on differential hemispheric activation as a function of age or the presence of depression suggests that a relative decrease in left hemisphere activation is associated with depression, while a decrease of right hemisphere activation is associated with age. Recent research, however, has demonstrated the role of the right hemisphere in maintaining general behavioral arousal. Pilot data suggest that elderly people experience behavioral over-arousal when presented with stressful or novel environmental stimuli. Equally interesting is the finding in a single-case study that ambient light and noise have a differential effect on behavior presumed to be representative of the left and the right cerebral hemispheres. The left hemisphere appears to be more responsive to ambient light level, with the right hemisphere being more responsive to ambient noise level. The present study sought to provide further support of selective hemispheric activation to bright light, and to examine the relationships among hemi-activation, age and depression by using behavioral measures of lateral anterior (finger tapping rate) and posterior (dichotic listening) cortical functioning. Two identical experiments were employed to evaluate old and young, and depressed and non-depressed sUbjects. Direct evidence of right herni-aging effects on laterality was not significant in the first experiment, but data suggested the possibility of a ceiling effect for behavioral arousal in the older group which was not seen in younger subjects. The second experiment yielded no significant results between depressed and non-depressed groups. Hypotheses of under and over arousal in subjects are proposed to explain findings. Possible implications for treatment and recommendations for further research are proposed. / Ph. D.