Conducted 3 experiments, with 6 right-handed undergraduates in each, which examined hemispheric differences in reaction time (RT) to judge a set of items same or different (1 item differing from the rest), as a function of number of items in the set. When the items were letters, the left hemisphere yielded RTs increasing with the number of letters in the set, as in serial processing; the right hemisphere showed no increase of RT for larger numbers of letters, as in parallel or holistic processing. When the items were unnameable shapes, both hemispheres appeared to process in parallel. Thus, a serial vs. parallel processing difference between left and right hemispheres appears to be limited to linguistic material which can be analyzed either verbally or visuospatially. If verbal analysis forces a serial procedure, while visuospatial analysis permits parallel processing, then the results can be explained in terms of the lateralization of these modes of analysis. (24 ref.) (PsycINFO Database Record (c) 2006 APA, all rights reserved)