Nineteenth-century ideas on hemisphere differences and “duality of mind”

Some historical notes orienting towards brain mechanisms that could underlie hemispheric asymmetries

The first minor aim of this synthetical historical survey consisted in showing that the discovery of the internal organization of language within the left hemisphere has been mainly determined by theoretical models and cultural factors, whereas the discovery of the left lateralisation of language and of the right lateralization of emotions and of other cognitive and perceptual functions has been mainly determined by empirical observations. A second more relevant aim of the survey consisted in discussing historical and more recent data suggesting that the different lateralisation of language and emotions has influenced not only the asymmetrical representation of other cognitive, affective and perceptual functions, but also (thank to the shaping influence of language on human cognition) of asymmetries regarding more general aspects of thought (such as the distinctions between 'propositional vs automatic' and 'conscious vs unconscious' ways of functioning). In the last part of the review, these data will be included in a more general discussion, concerning the brain functions that could be subsumed by the right hemisphere for three main reasons: (a) to avoid conflicts with the language mediated activities of the left hemisphere; (b) because of unconscious and automatic aspects of its non-verbal organisation or (c) due to the competition for cortical space determined by the development of language within the left hemisphere.

Can the mind be split? A historical introduction

The idea that the mind might be composed of distinct conscious entities goes back at least to the mid-19th century, and was at first based on the bilateral symmetry of the brain, with each side seemingly a mirror-image replica of the other. This led to early speculation as to whether section of the forebrain commissures might lead to separate, independent consciousnesses. This was not put to the test until the 1960s, first in commissurotomized cats and monkeys, and then in humans who had undergone commissurotomy for the relief of intractable epilepsy. Initial results did indeed suggest independent consciousness in each separated hemisphere, but later findings have also revealed a degree of mental unity, especially in some perceptual functions and in motor control. Some of these findings might be interpreted in terms of subcortical connections or external cross-cuing, and also address questions about the nature of consciousness in a more concrete way.

How Asymmetries Evolved: Hearts, Brains, and Molecules

Humans belong to the vast clade of species known as the bilateria, with a bilaterally symmetrical body plan. Over the course of evolution, exceptions to symmetry have arisen. Among chordates, the internal organs have been arranged asymmetrically in order to create more efficient functioning and packaging. The brain has also assumed asymmetries, although these generally trade off against the pressure toward symmetry, itself a reflection of the symmetry of limbs and sense organs. In humans, at least, brain asymmetries occur in independent networks, including those involved in language and manual manipulation biased to the left hemisphere, and emotion and face perception biased to the right. Similar asymmetries occur in other species, notably the great apes. A number of asymmetries are correlated with conditions such as dyslexia, autism, and schizophrenia, and have largely independent genetic associations. The origin of asymmetry itself, though, appears to be unitary, and in the case of the internal organs, at least, may depend ultimately on asymmetry at the molecular level.

Franz Joseph Gall on hemispheric symmetries

Franz Joseph Gall believed that the two cerebral hemispheres are anatomically and functionally similar, so much so that one could substitute for the other following unilateral injuries. He presented this belief during the 1790s in his early public lectures in Vienna, when traveling through Europe between 1805 and 1807, and in the two sets of books he published after settling in France. Gall seemed to derive his ideas about laterality independently of French anatomist Marie François Xavier Bichat (1771-1802), who formulated his "law of symmetry" at about the same time. He would, however, later cite Bichat, whose ideas about mental derangement were different from his own and who also attempted to explain handedness, a subject on which Gall remained silent. The concept of cerebral symmetry would be displaced by mounting clinical evidence for the hemispheres being functionally different, but neither Gall nor Bichat would live to witness the advent of the concept of cerebral dominance.

Humanity and the left hemisphere: The story of half a brain

Until fairly late in the nineteenth century, it was held that the brain was bilaterally symmetrical. With the discovery of left-brain dominance for language, the so-called "laws of symmetry" were revoked, and asymmetry was then seen as critical to the human condition, with the left hemisphere, in particular, assuming superordinate properties. I trace this idea from the early discoveries of the late nineteenth century through the split-brain studies of the 1960s, and beyond. Although the idea has persisted, the evidence has revealed widespread cerebral asymmetries in nonhuman animals, and even language and its asymmetries are increasingly understood to have evolved gradually, rather than in a single speciation event. The left hemisphere nevertheless seemed to take over a role previously taken by other structures, such as the pineal gland and the hippocampus minor, in a determined effort to place humans on a pedestal above all other species.

Chapter 51: recovery of function: redundancy and vicariation theories

Redundancy and vicariation theories were employed by 19th-century practitioners and animal researchers to account for what seemed to be sparing and recovery of function after brain damage. Those individuals believing in redundancy maintained that there are duplicate or back-up areas that can mediate a function after brain damage, such as the homologous region on the opposite side of the brain. In contrast, vicariation theorists argued that brain areas with different functions could sometimes assume or "take over" the functions of injured areas. This chapter looks at the history and early evidence for these two different views, and how theorizing changed as more was learned about cortical localization of function. It reveals that there were subtle variations on these basic themes and that certain factors, such as age at the time of brain injury, were often brought into the equation. With limited knowledge and inadequate methodologies, the debates about recovery of function that flared up during the late-19th century would not be quickly or easily rectified.

Reversed hemispheric asymmetry during simple visual perception in schizophrenia

Processing of sensory information in the human brain progresses from primary areas, dedicated to a single sensory feature, to multimodal areas, which integrate many features across sensory modalities. For some of these processes hemispheric dominance has developed. Here we report the results of a passive viewing task using positron emission tomography. Subjects were scanned twice while staring at a stationary visual noise pattern. Normal subjects showed a significant reduction of regional cerebral blood flow (rCBF) in a distributed right hemisphere network of brain regions during the second visual task. Schizophrenic subjects, however, showed significant increases of right hemisphere rCBF during the second visual task and showed significant decreases only in the left hemisphere. These results are consistent with the notion of reversed hemispheric asymmetry during the processing of sensory information in schizophrenia.

The role of right hemisphere dysfunction in psychiatric disorders

In 1861 Broca, a Parisian surgeon, reported a postmortem he had carried out on a man who had lost his ability to speak 20 years previously and who in the intervening period had only regained the use of one phrase: “Tan-tan”. Broca found that this man had an old infarct in his left hemisphere, affecting the posterior portions of the second and third frontal gyri.

"Whole brain" politics and brain laterality research

The paper discusses a recent international conference in Munich (FRG) on brain-laterality research. An open letter on the goals of the conference is considered. In this letter, the "automatic" moral and intellectual decay of "left brain" modern society was predicted, and a clarion call for the reinsertion of "whole brain" values into our over-mechanized culture was sounded. Discussion of the conference itself focuses on the political and social risks involved in selling irrationalist ideologies - even well-meaning ones - in the name of science. The paper calls for a more responsible attitude among modern brain-laterality researchers who would participate in scientific conferences that blur the distinction between description and social prescription.