Against localisation

Localisation of brain function always had opponents. Gall's chief critic during his time was Pierre Jean Marie Flourens (1825). He was a pioneer in the use of the experimental method in neuroanatomy. By carrying out localised lesions in brains of rabbits and pigeons he claimed that the magnitude of deficit could not simply be linked to particular regions or even the total amount of brain tissue involved.

By 1906 Pierre Marie, a French neurologist had found that Broca's original claims were not justified, that each of his patients had had more extensive lesions than Broca had reported, and the range of their accompanying deficits had not been very precisely recorded. Localisation was called into question.

Shepherd Ivory Franz (American neuropsychologist, 1874-1933) developed the ideas of Flourens. He found:

• Lesion in the frontal lobes of mammals does not abolish learned behaviour unless the destruction of tissue is massive.

• Long-established habits tend to persist in any case

• Habits lost by extensive destruction can be re-learned

Karl Lashley (1890-1958), student of Franz, was the American neuropsychologist who was most influential in his arguments against behaviourism. Lashley (Brain Mechanisms and Intelligence, 1929 - BMI) argued that higher cognitive processes were not localised but distributed. BMI described studies with rats and mazes. Lashley removed portions of the cerebral cortex, varying from 10%-50% in an effort to study the role the cerebral cortex played in learning. Introduced two principles:

1. Principle of mass action: Based on the observation that the amount of cortex removed was critical to the learning abilities of the rats, Lashley concluded that the efficiency of learning, and generally that of higher cognitive abilities, is a function of the intact mass of the cortex. More tissue => more learning available

2. Equipotentiality: All areas of the cortex are equally important to learning - no area was proven to be more important than any other area. Also found evidence that brain regions can take on different functions as needed (Plasticity).

Lashley's search for the Engram:

Since behavior reflects actions of the nervous system, which is a physical-chemical system, changes in behavior from learning must be associated with physical-chemical alterations, particularly in the cerebral cortex - the importance of the cortex had been accepted since Pavlov.

Lashley (?) called these physical memory traces ``engrams.'' Lashley was never able to find the existence of an engram and concluded (??) that ``the necessary conclusion is that learning just is not possible.''

The concept of an engram, though now outdated, was important historically as it helped focus attention on the issue of brain and behaviour.

Lashley and other ``holist'' neurologists like him believed in the Gestalt assumption of the nervous system being organised in terms of ``neural fields'' operating across wide regions of the cortex (currently not a tenable hypothesis). Till this point skepticism about localisation was correlated in the minds of scientists with skepticism about reductionism.

To some extent the difference in perspectives came about through reliance on different types of data: the holists cited maze-running rat studies while the localisers looked at adult aphasia. It was realised later that maze-running involves too many complex motor and sensory capabilities and is thus not a suitable task to study localisation. The evidence for localisation is dependent on appropriate analysis of behavioural tasks. (sec. )

The extreme holist position was challenged by localisation found in the visual system. More evidence from neuroembryological studies by Paul Weiss (1952) and Roger Sperry (1951) showed that the developing neurons contain highly specific information: when budding limbs were surgically transplanted to a new position the nerves would still reconnect with the place of origin. There were limits to plasticity.