From symmetry to complementary in hemisphere function: A long delayed conceptual transition
When first proposed, cerebral localization was presented as uniformly symmetrical (Gall). The irrefutable violation of this principle in the case of aphasia (Dax, Broca) did not lead to it being abandoned. Instead, the symmetry principle was patched along "exception proves the rule" lines by unilaterally superimposing language on an otherwise symmetrically organized cerebrum, rendering one hemisphere "dominant". Sporadic nineteenth century objections, both with regard to specific functions (Jackson) and to the right hemisphere as a whole (Wigan) left the dominant left hemisphere unscathed. Skilled observers, the early neuropsychologists nonetheless failed to recognize right hemisphere syndromes were attributed to bilateral cortical damage, even if there was no evidence of bilateral involvement.
Right hemisphere dominance was belatedly recognized after World War II (Duensing, Paterson and Zangwill). Even then, lateralization (right and left) was circumscribed. It was denied all animals, and denied human infants. Lateralization became an accolade to human exceptionality (of adult males in particular).
The tables were turned as of the sixties. Hemisphere differences, on an equal footing, or ever tilting toward the right (Galin, Ornstein) became acceptable. The inventory of lateralized functions continues to grow, to the point that it might be questioned whether any cognitive operation has a bisymmetric (rather than bicomplementary) cerebral base (Doty, Ringo). Infants, and species as modest as the mouse, have been conceded their lateralization, and minimal morphological asymmetries have been endowed with surplus meaning. Significant but minuscule laterality effects are interpreted as guides to what one hemisphere, and not the other, does.
I propose that it required a change in Zeitgeist (from autocratic/hierarchical to democratic/egalitarian) before the dogma of symmetry could be discarded. Does the current Zeitgeist, in turn, blind us to still more promising ways of conceiving cerebral function?
Session V -- 19th to 20th Centuries
Providence, Rhode Island, USA