Photopic and scotopic light detection in patients with seasonal affective disorder and control subjects

Background: Retinal sensitivity may play a role in the pathogenesis of seasonal affective disorder (SAD) and response to light therapy. Methods: Using a dark adaptation procedure, SAD patients and normal control subjects were tested in the winter and summer, with patients retested after light treatment. The eyes were preadapted to bright light followed by 30 min in darkness, during which subjects detected a dim signal titrated around the detection threshold. Photopic (cone-mediated) and scotopic (rod-mediated) components of the data were identified by nonlinear exponential curve fits to successive threshold estimates. Results: Patients (n = 24) showed significantly lower cone and rod thresholds in the summer than winter, while control subjects (n = 12) showed a similar trend. Relative to the control subjects, however, patients were supersensitive in winter (lower cone final threshold, faster rod recovery). Clinical responders to morning light showed a small summer-like increase in cone sensitivity, whereas nonresponders became subsensitive. In comparison to darker-eyed patients, blue-eyed patients showed a larger summertime increase in cone sensitivity and a similar trend after response to morning light. Conclusions: Heightened retinal sensitivity with increased light exposure, and supersensitivity of patients relative to control subjects in winter, may play roles in the pathogenesis of winter depression and the action of therapeutic light.