Brain and Cognition 63 (2007) 145–158 www.elsevier.com/locate/b&c 0278-2626/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.bandc.2006.10.007 BeneWts of interhemispheric collaboration can be eliminated by mixing stimulus formats that involve diVerent cortical access routes Urvi J. Patel ¤ , Joseph B. Hellige Department of Psychology, SGM 501, University of Southern California, Los Angeles, CA 90089-1061, USA Accepted 31 October 2006 Available online 15 December 2006 Abstract Previous studies indicate that the beneWts of dividing an information processing load across both cerebral hemispheres outweigh the costs of interhemispheric transfer as tasks become more diYcult or cognitively complex. This is demonstrated as better performance when two stimuli to be compared are presented one to each visual Weld and hemisphere than when both stimuli are presented to the same single hemisphere (an across-hemisphere advantage). Two experiments indicate that this Wnding does not generalize to complex tasks that require matching numeric quantities represented by two very diVerent visual formats whose processing involves somewhat diVerent corti- cal areas: digits and dice-like dot patterns. In fact, mixing these stimulus formats consistently produces a within-hemisphere advantage. We propose that, when two simultaneously presented stimuli are presented in suYciently diVerent visual formats, identiWcation of the two stimuli may take place in parallel, via diVerent cortical access routes and with little or no interference, even when they are presented to the same cerebral hemisphere.  2006 Elsevier Inc. All rights reserved. Keywords: Corpus callosum; Interhemispheric interaction; Interhemispheric transfer; Interhemispheric cooperation; Interhemispheric collaboration; Matching task; Numeric quantity stimuli; Divided visual Weld 1. Introduction A prominent characteristic of human brains is division of the cerebral cortex into left and right hemispheres, each constituting a somewhat separate information processing system with its own abilities, propensities and biases. Col- laboration of the two hemispheres is facilitated by a rich network of interhemispheric connections, with the largest Wber tract being the corpus callosum. Studies using a wide range of techniques and subject populations have begun to illustrate ways in which the left and right hemispheres inter- act to produce unity of thought and action. Although there are processing costs associated with transfer of information from one hemisphere to the other, there are also beneWts of dividing the information processing load across both hemi- spheres. A predominant view is that the beneWts of distrib- uting the load across both hemispheres outweigh the costs of interhemispheric transfer as tasks become more diYcult or complex. The present experiments were designed to test the limits of this view by examining how the beneWcial eVect of dividing the processing load across both hemispheres is inXuenced by mixing stimulus formats that are believed to involve diVerent processing routes, even within a single hemisphere. The present experiments take advantage of the fact that the visual projection pathways of humans are such that information from the right visual half-Weld (RVF) projects directly to the left cerebral hemisphere and information from the left visual half-Weld (LVF) projects directly to the right cerebral hemisphere. Thus, simultaneous lateraliza- tion of two stimuli to the same or opposite visual half-Welds can determine whether interhemispheric collaboration is needed to determine whether the two stimuli match accord- ing to some criterion. The critical comparison involves * Corresponding author. E-mail address: upatel@usc.edu (U.J. Patel).