Bilaterality: Hemispheric Specialisation and Integration

The two cerebral hemispheres are identical in appearance. However, ever since Broca discovered in 1861 that left hemisphere damage results in ‘expressive’ aphasia, neuroscientists have realised that the brain is functionally asymmetrical (Ornstein, 1997). A few years later, Wernicke identified that left hemisphere damage leads to ‘receptive’ aphasia. As a result, neuroscientists held the belief that the left hemisphere was superior to the right because it was the language centre, and thus the seat of reason and intellect. Eventually, however, Hughlings-Jackson, in the 1870s, realised that the right hemisphere plays a central role in comprehension of the world together with the left. During this same period, many neuroscientists were influenced by popular notions regarding the essential nature of man, such as those depicted in Robert Louis Stevenson's 1886 story of Dr Jekyll and Mr Hyde. For these neuroscientists, it was as if humans had two separate minds, even two ‘consciousnesses’, a ‘cultivated’ left one and a ‘primitive’ right one.

In this century, in the 1930s, Lange revitalised interest in the central importance of the right hemisphere. He recognised that the right hemisphere provides a general contextual background to a person's world view. During the 1960s the initial work of Sperry and Gazzaniga (Gazzaniga et al., 1962) at first seemed to corroborate the ‘two minds, two consciousnesses’ view held by some late nineteenth-century scientists. Subsequent work, however, resulted in the now generally accepted ‘asymmetric but integrated’ perspective. The right and left cerebral cortex are each lateralised for specialised functions, but in the healthy brain the two hemispheres share their information and operate collaboratively (Joseph, 1996; Deacon, 1997; Gazzaniga, 1995; Ornstein, 1997).

Research regarding lateralisation of specialised functions pertains mostly to cortical functions. However it must be remembered that specialised functions of