§ 6. The diagnostics of thinking

5.6.1. The dependence of the type of thinking on the functional asymmetry of cerebral hemispheres

The active studies of the dependence of the type of thinking on the functional asymmetry of cerebral hemispheres began not so long ago. First of all it is connected with the name of the American psycho-neurologist R. Sperry, who made an operation on disconnecting the links between the cerebral hemispheres with the aim of treating a complicated form of epilepsy at the end of 60-s. During the observations of his patients R. Sperry discovered that the two hemispheres of a uniform brain act as two completely different brains. A man, whose right hemisphere was disconnected, but the left hemisphere worked, conserved his ability to speak, adequately reacted to the words, numbers and other signs, but he was often helpless when he was supposed to do something with material things or their pictures. When the left hemisphere was disconnected, the patient could easily manage with such tasks, he understood some pieces of art, melodies and speech intonations, he could orient in space, but he lost his ability to understand complicated verbal constructions and could not speak coherently at all.

The explanation of reasons, which led to the hemisphere specialization in the process of evolution, suggested by Doreen Kimura and her colleagues (1973) is rather interesting. Basing on the fact that the speech function of left hemisphere is connected with the leading right hand movements, she supposes that the specialization of left hemisphere for speech is not so much the consequence of asymmetric development of symbolical functions as the development of some particular motive skills, which help in the process of communication. The language appeared, because the left hemisphere was more suitable for some kinds of motive activity.

The link between the left hemisphere and some particular type of movement is well known in clinic. The hand corresponding to the hemisphere with speech center (the right hand of right-handed people) shows more abilities to make delicate movements than the hand connected with non-dominating hemisphere. The patients with left hemisphere dysfunction, but having no right-side paralysis, find it difficult to reproduce a composite sequence of motions and composite finger positions. The left hemisphere dysfunction of deaf-mutes is accompanied by the decay of sign language, which is similar to the speech decay of normally speaking people.

D. Kimura suggests (1973) that in the process of evolution it was the development of the hand as a sign language organ and the development of its manipulative abilities that led to the development of the left hemisphere. Later this function was acquired by voice muscles.

Probably the functional asymmetry, which appeared due to the evolution, allowed using the space for information procession more economically, avoiding double functions. So the dominance of one hemisphere in speech function prevents a possible competition between the hemispheres for the tongue, upper and lower lips and mouth. The process of evolution is characterized not only by the quantitative accumulation of asymmetry elements, but also by a qualitative saltus – the development of a deep specialization of the left and right hemisphere function saving certain potential abilities of compensating one hemisphere by another one (N.L.Bianki, 1985; M.Gazaniga, 1974).