Cerebral Cortex

The cerebral cortex is a paired structure in the forebrain that is found only in mammals and that is largest (relative to body size) in humans (Herrick 1926; Jerison 1973). Its most distinctive anatomical features are (i) the very extensive internal connections between one part and another part, and (ii) its arrangement as a six-layered sheet of cells, many of which cells are typical pyramidal cells. Although the crumpled, folded surface of this sheet is responsible for the very characteristic appearance of the brains of large mammals, the cortex of small mammals tends to be smooth and unfolded. Where folds occur, each fold, or gyrus, is about 1/2 cm in width (Sarnat and Netsky 1981).

Imagine the crumpled sheet expanded to form a pair of balloons with walls 2.5 mm thick, each balloon with a diameter of 18 cm and a surface area close to 1000 cm2. The pair weighs about 500 grams, contains about 2 × 10 ¹⁰ cells connecting with each other through some 1014 synapses, and through a total length of about 2 × 10⁶ km of nerve fiber -- more than five times the distance to the moon (Braitenberg and Schuz, 1991). The two balloons connect to each other through the corpus callosum, a massive tract of nerve fibers. Almost all the synapses and nerve fibers connect cortical neurons to each other, but it is of course the connections with the rest of the animal that allow the cortex to control the animal's highest behavior. The main connections are as follows.

The olfactory input from the nose probably represents the input to the primordial structure from which the cortex evolved. It goes directly to layer 1, the outermost layer, of a special region at the edge of the cortical sheet. Touch, hearing, and vision relay through thalamic nuclei that are situated near what would be the necks of the balloons, and these nuclei receive a large number of feedback connections from the cortical sheet as well as inputs from the sense organs. An important output pathway comes from the motor area, which is the region believed to be responsible for voluntary movement. The CEREBELLUM should be mentioned here because it is also concerned with the regulation and control of muscular movement; it has profuse connections to and from the cortex, and enlarged with it during evolution. It has recently become clear that it is concerned with some forms of CONDITIONING (Yeo, Hardiman, and Glickstein 1985; Thompson 1990).

The second set of pathways in and out of the cortex pass through two adjacent regions of modified cortical sheet called the archicortex and paleocortex that flank the six-layered neocortex (Sarnat and Netsky 1981). Paleocortex lies below the necks of the balloons and contains the rhinencephalon (nose brain), where smell information enters; it connects with other regions thought to be concerned with mood and EMOTION. Archicortex developed into the HIPPOCAMPUS, and is thought to be concerned with the laying down of memories. Like the paleocortex, it has connections with regions involved in mood, emotion, and endocrine control.

The words used to describe the higher mental capacities of animals with a large neocortex include CONSCIOUSNESS, free will, INTELLIGENCE, adaptability, and insight, but animals with much simpler brains learn well, so LEARNING should not be among these capacities (Macphail 1982). The comparative anatomists Herrick and Jerrison emphasize that neocortically dominated animals show evidence of having acquired extensive general knowledge about the world, but laboratory learning experiments are usually designed to prevent previously acquired knowledge from influencing results, and are therefore not good tests for stored general knowledge. The evidence for such knowledge is that cortically dominant animals take advantage of the enormously complicated associative structure of their environments, and this could come about in two ways. A species could have genetically determined mechanisms, acquired through evolutionary selection, for taking advantage of the regular features of the environment, or they could have learned through direct experience. It seems most likely that animals with a dominant neocortex have a combination of the two means -- they have the best of both worlds by combining genetic and individual acquisition of knowledge about their environments (Barlow 1994).

Neuropsychologists who have studied the defects that result from damage and disease to the cerebral cortex emphasize localization of function (Phillips, Zeki, and Barlow 1984): cognitive functions are disrupted in a host of different ways that can, to some extent, be correlated with the locus of the damage and the known connections of the damaged part. But there can be considerable recovery of a function that has been lost in this way, particularly following damage in infancy or childhood: in the majority of adults the left hemisphere is absolutely necessary for speech and language, but these capacities can develop in the right hemisphere following total loss of the left hemisphere in childhood.

Neurophysiologists have recorded the activity of individual nerve cells in the cortical sheet. This work leads to the view that the cortex represents the sensory input and the animal is receiving, processes it for OBJECT RECOGNITION, and selects an appropriate motor output. Neurons in the primary visual cortex (also called striate cortex or V1) are selectively responsive to edge orientation, direction of MOTION, TEXTURE, COLOR, and disparity. These are the local properties of the image that, according to the laws of GESTALT PERCEPTION, lead to segregation of figure from ground. The neurons of V1 send their axons to adjacent extra-striate visual cortex, where the precise topographic mapping of the visual field found in V1 becomes less precise and information is collected together according to parameters (such as direction and velocity of motion) of the segregating features (Barlow 1981). These steps can account for the first stages of object recognition, but what happens at later stages is less clear.

Although there are considerable anatomical differences between the different parts of the cortical sheet, there are also great similarities, and its evolution as a whole prompts one to seek a similar function for it throughout. Here the trouble starts, for it is evident that comparative anatomists say it does one thing, neuropsychologists another, and neurophysiologists yet something else (Barlow 1994). These divergences result partly from the different spatial and time scales of the observations and experiments in different fields, for neurophysiology follows the activity of individual neurons from second to second, neuropsychologists are concerned with chronic, almost permanent defects resulting from damage to cortical areas containing several million cells, and behavioral observation is concerned with functions of the whole animal over intermediate periods of seconds and minutes. But the cells everywhere have an unusual and similar form, which suggests they have a common function; an attractive hypothesis is that this is prediction.and

Sense organs are slow, but an animal's competitive survival often depends upon speedy response; therefore, a representation that is up to the moment, or even ahead of the moment, would be of very great advantage. Prediction depends upon identifying a commonly occurring sequential pattern of events at an early stage in the sequence and assuming that the pattern will be completed. This requires knowledge of the spatio-temporal sequences that commonly occur, and the critical or sensitive periods, which neurophysiologists have studied in the visual cortex (Hubel and Wiesel 1970; Movshon and Van Sluyters 1981) but which are also known to occur in the development of other cognitive systems, may be periods when spatio-temporal sequences that occur commonly encourage the development of neurons with a selectivity of response to these patterns. If such phase sequences, as HEBB (1949) called them, were recognized by individual cells, one would have a computational unit that, with appropriate connections, would be of selective advantage in an enormous range of circumstances. The survival value of neurons with the power of prediction could have led to the explosive enlargement of the neocortex that culminated in the human brain.

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-- Horace Barlow

References

Barlow, H. B. (1981). Critical limiting factors in the design of the eye and visual cortex. The Ferrier Lecture, 1980. Proceedings of the Royal Society, London Series B 212:1-34.

Barlow, H. B. (1994). What is the computational goal of the neocortex? In C. Koch and J. Davis, Eds., Large Scale Neuronal Theories of the Brain. Cambridge, MA: MIT Press.

Braitenberg, V., and A. Schuz. (1991). Anatomy of the Cortex: Statistics and Geometry. Berlin: Springer-Verlag.

Hebb, D. O. (1949). The Organisation of Behaviour. New York: Wiley.

Herrick, C. J. (1926). Brains of Rats and Men. Chicago: University of Chicago Press.

Hubel, D. H., and T. N. Wiesel. (1970). The period of susceptibility to the physiological effects of unilateral eye closure in kittens. Journal of Physiology 206:419-436.

Jerison, H. J. (1973). Evolution of the Brain and Intelligence. New York: Academic Press.

Macphail, E. (1982). Brain and Intelligence in Vertebrates. New York: Oxford University Press.

Movshon, J. A., and R. C. Van Sluyters. (1981). Visual neural development. Annual Review of Psychology 32:477-522.

Phillips, C. G., S. Zeki, and H. B. Barlow. (1984). Localisation of function in the cerebral cortex. Brain 107:327-361.

Sarnat, H. B., and M. G. Netsky. (1981). Evolution of the Nervous System. New York: Oxford University Press.

Thompson, R. F. (1990). Neural mechanisms of classical conditioning in mammals. Philosophical Transactions of the Royal Society of London Series B 329:171-178.

Yeo, C. H., M. J. Hardiman, and M. Glickstein. (1985). Classical conditioning of the nictitating membrane response of the rabbit (3 papers). Experimental Brain Research 60:87-98; 99 - 113; 114 - 125.

Cerebral Cortex

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References

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