The Areas of Localisation on the Cerebral Cortex

The Areas of Localisation on the Cerebral CortexOf all portions of the brain the cerebral cortex possesses the most direct interest to the psychologist. In it the afferent and efferent arms of the long reflex pathways, which represent the highest type of reflex arcs, meet, and in it also many sensations are mediated. Further it is the region where consciousness and memory are located, and where the complex nervous processes which find expression in the highest forms of psychical activity take place.

From the manner in which the nervous system has gradually progressed to its great complexity in the higher animals, it is obvious that different portions of it are specialised for different functions.

This general localisation was all that was known to the older physiologists, who recognised that the spinal cord presided over the execution of movements, that the cerebellum was concerned in their co-ordination, and that in the upper parts of the brain voluntary movements were initiated.

It is since 1870 that direct proof of the functional differences between different areas of the cerebral cortex has been advanced. The first result of this work led to the belief that the cortex could be mapped out into small areas anatomically independent of one another and differing in physiological function.

Later experiments and observations have shown that although there are marked differences of function between individual parts of the cortex, yet every part of the cortex is so intimately connected with every other portion that such independence is only relative. In other words interference with one part will produce effects in several other regions.

This applies even to such widely different functions as sensation and motion, for in the cortex the neurons of these are so intimately associated that an insensible transition from one to the other is effected.

Three methods have given valuable results in the study of cortical localisation:

(1) Physiological;
(2) Histological;
(3) Clinical.

(1) Physiological. - The first convincing experiments performed in animals were those of stimulation of different areas of the cortex. The cortex was exposed by removal of a portion of the skull, and stimulated electrically. By this method a region was demarcated, stimulation of portions of which evoked definite muscular movements, and the presence of a motor area was thus established. Previous to this the chief mode of experimenting was by removing or destroying portions of the cortex, and then after a time carefully examining the voluntary musqular paralyses or interferences with sensation which followed. By a combination of the two methods greater accuracy has been ensured. In a third series of experiments, portions remote from the cortex were destroyed, and the resulting changes (if any) in the cortex noted.

(2) Histological. - The older method of histological investigation is an extension of that already described (p. 31) in connection with the tracing of the nerve tracts, viz. observing the different periods at which the white fibres running to and from known areas of the cortex acquire their medullary sheaths.

While it had been known for some time that the types of cells and their arrangement in layers were not exactly alike in all the regions of the cortex, yet it is only within the last few years that careful systematic observations have been inade to determine whether these differences had any definite relation to cortical localisation.

It has now been shown that such a relationship does exist; certain areas can be accurately differentiated by microscopic examination of their characteristics, but the proof of their functions necessarily rests on the other methods. Much has yet to be done in this direction.

(3) Clinical. - In man, and to a lesser extent in animals, important information has been gained by studying the effects of known injuries and diseases of the brain. Destruction of the cerebral cortex may be due to external injuries, to abnormal growths, or frequently to the rupture of bloodvessels within the skull, and in such cases the amount of injury can be determined roughly by naked-eye inspection; in a crude manner motor and sensory areas may be thus delimited. In cases of obscure nervous diseases or of mental affections, no gross surface differences may be observable; the great motor and sensory tracts may be but little affected, although all the higher psychical activities are interfered with.

By the use of these various methods, singly or in combination, it is possible to map out on the surface of the cerebral cortex certain areas with well-defined functions - thus, motor areas, sensory areas, and association areas are described.

The motor areas have been determined with much accuracy; some of the sensory areas are also well known; the association areas, necessarily from their great complexity, are as yet but imperfectly understood. In a general way the cortex in front of the Rolandic fissure may be regarded as the great efferent area, while the rest of the cortex is specialised for the reception of sensory impressions. It must not be forgotten, however, that the motor area has many afferent (but not sensory) fibres, and it is by impulses through these that the motor cells are thrown into activity; hence the motor area is more accurately designated the psycho-motor area (Figs. 77 and 78).