BRAIN AND LANGUAGE 9, 78-92 (1980) Cognitive Task Effects on Hemispheric Blood Flow in Humans: Evidence for Individual Differences in Hemispheric Activation RUEIENC.GUR University of Pennsylvanifr AND MARTIN REIVICH Cerebrovascular Research Center, University of Pennsylvania Evidence from two indirect measures of hemispheric activity, EEG a and conjugate lateral eye movements, has pointed to the existence of individual differences in hemispheric activation. Results from a more direct indicator of hemispheric activity, regional cerebral blood flow as measured by the rs3Xe inhalation method, show that such individual differences can be detected in the distribution of blood in the two hemispheres and that the amount of increase in blood flow in the right relative to the left hemisphere is correlated with perfor- mance on a spatial task. These results corroborate and extend the findings from the EEG and eye movement studies and suggest that the dimension of individual differences in hemispheric activation may exert significant influence on cognitive performance and on problem-solving strategies. A number of indirect measures of hemispheric activation such as EEG (Doyle, Omstein, & Galin, 1974; Galin & Ornstein, 1972; Galin & Ellis, 1975; McKee, Humphrey, & McAdam, 1973; Morgan, Macdonald, & Hilgard, 1974), evoked potentials (Buchsbaum & Fedio, 1970; Cohn, 1971; McAdam & Whitaker, 1971; Morrell& Salamy, 1971; Wood, Goff, & Day, 1971; Shucard, Shucard, & Thomas, 1977), and eye movement directionality (Gur, Gut-, & Harris, 1975; Kocel, Galin, Ornstein, & Mer- This research was supported by USPHS Grant NS-10939-06. Requests for reprints should be sent to Ruben C. Gur, Department of Psychology, University of Pennsylvania, 3815 Walnut Street, Philadelphia, PA 19104. We thank Herbert Goldberg, John McGhee, Walter Obrist, Marcel Saucy, Jesse Wein- berger, and Walter W. Wind&h, for their help in conducting the research. 78 OO93-934X/80/010078-15$02.00/O Copyright 0 1980 by Academic Press, Inc. All rights of reproduction in any form reserved.