PubMed1987
It is not easy to see a simple outline in the progress of the idea of duality, because it did not develop evenly or reach the stage of general acceptance. From the seventeenth century there were shifts in some of the basic assumptions about how the brain and mind functioned, and there are some useful markers along the way to an era of more systematic studies. Descartes is the most convenient base. He had earlier firmly separated mind and matter in his philosophy, and is still chiefly known for that. But at the end of his life (1649) he tried to reconcile them by the device of a specific 'seat of the soul' in the brain through which information passed between brain and mind. Symmetry of the operation of the hemispheres was assumed. This theory had currency into the eighteenth century. At the end of that century Franz Gall of Austria and France was assigning discrete faculties to numerous parts of the brain on no strong evidence, and nothing the double form of the brain, without claiming independent action of the hemispheres. Hewett Watson in 1836 discussed duality more directly than had been the case before, and Arthur Wigan in 1844 asserted the duality of the mind roundly and treated the two hemispheres, not consistently, as two independent brains. He was not satisfied with independence, however, and tried various ways of allowing for joint action by the two sides of the brain, as well as for substitution, with one side having the power to act on behalf of both in cases of disease or injury. He also considered that one hemisphere, usually the left, was generally dominant; but he did not see the two hemispheres as differently constituted. Recognition of differentiation of function between the two sides came chiefly out of the largely French discussions, in the 1820s and after, about the location--frontal or not--of 'language', and out of the work and arguments of the middle of the century. Broca's left frontal language centre became widely known, though its experimental base was weak and he himself seems to have been more interested in the fact that it was frontal (the older debate) than in its one-sidedness. Brown-Séquard did not accept Broca's findings because of his general opposition to specific locations for particular functions; but he enthusiastically revived Wigan's notions of duality, without developing them further.(ABSTRACT TRUNCATED AT 400 WORDS)