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Seasonal affective disorder (SAD), winter type, is characterized by recurrent winter episodes of depressed mood, overeating, carbohydrate craving, hypersomnia, increased sleepiness, sadness, difficulty in performing daily activities, and decreased concentration (Rosenthal, 1993; Rosenthal et al., 1984). Cognitive difficulties, such as decreased concentration and short-term memory loss, are often reported by patients as being among the most problematic aspects of their condition (Rosenthal, 1993). Providing clinicians and patients with a more detailed description of these cognitive problems would be useful in further understanding the difficulties that these patients experience. In addition, objective measures of cognitive deficits could provide researchers with a useful tool for evaluating treatment effects and further elucidating the psychobiological underpinnings of SAD.Impairments on visuospatial tasks and abnormalities in right hemisphere function have been reported in the only studies of cognitive processes in SAD performed to date. Using positron emission tomography, a discrete region of the right hemisphere was found to have significantly increased metabolic activity in SAD patients compared with a control group (Cohen et al., 1992). In light of a recent finding indicating an inverse relationship between metabolic brain activity and visuospatial/motor performance (Haier et al., 1992), this localized area of increased activity might correspond to decreased visuospatial performance in SAD patients. Mate-Kole et al. (1993) found that SAD patients showed impairment on neuropsychological tests of visuospatial memory, construction, fluency, and cognitive failures compared with normal control subjects. In another study, SAD patients showed no deficit in attention, but were impaired on a visual memory task of spatial recognition and showed deficits in delayed matching to sample tests in comparison to healthy controls (O'Brian et al., 1993). In addition, this study found a highly significant correlation between spatial recognition latency and residual depressive symptoms. In a study of hemispheric language lateralization in SAD (Volf et al., 1993), winter depression was associated with a shift of laterality from the left to the right hemisphere. Due to the known involvement of the right hemisphere in spatial processing, it is possible this seasonal shift in lateralization could have some effect on this function. Together, these studies provide evidence for decreased performance in the processing of visuospatial information and disturbances with right hemisphere function in SAD.The relative dearth of investigations of cognitive function in SAD stands in contrast to an extensive literature of cognitive investigations of nonseasonal depression. Impairments in visuospatial tasks, such as digit symbol, design learning, and object learning, are among the most frequently and consistently reported abnormalities in depression (Cassens et al., 1990). In addition, depressed patients have shown impairments in attentional tasks such as trail making (Gray et al., 1987), the Stroop task (Raskin et al., 1982), digit span (Breslow et al., 1980), and cancellation (Neville and Folstein, 1979). The investigations cited above may not, however, be applicable to patients with SAD, who are often more mildly affected than their nonseasonal counterparts, in that the former generally have a pattern of reverse vegetative symptoms, such as hypersomnia and overeating, and have the unique property of being highly reactive to the duration and intensity of environmental light.The purpose of this study was to further the investigation of cognitive impairments in seasonal depression by assessing the performance of SAD patients and controls on a pattern recognition task and the Stroop color-word test of attentional interference (Stroop, 1935), in winter, and to determine whether any impairments found would persist following summer remission.