Keith E Rice - The Brain

the lesion. Lashley then put forward the notion that the cortex as a whole is equipotential for some processes such as learning or problem-solving.

The Principle of Multiple Control clearly applies to some parts of the brain which are involved in several different behaviours. Eg: the hypothalamus is implicated in both eating/ drinking and aggressive behaviour. Certain behaviours may, therefore, be produced with the involvement of multiple areas of the brain.

The brain is within the skull and the spinal cord is within the vertebrae. The brain and spinal cord are protected by 3 membranes called meninges; the space between the 2 inner meninges is filled with cerebrospinal fluid.

The brain contains around 80 billion neurons - approximately 80% of all neurons - and about 10 times as many glia cells which provide nutrition and waste removal.

The brain is divided into 3 main areas.

The forebrain or cerebrum has two symmetrical halves - left and right cerebral hemispheres which are joined by fibres including the corpus callosum. Any information reaching one hemisphere first is rapidly and automatically transferred to the other hemisphere via the corpus callosum. Each half has 4 lobes - frontal, parietal, occipital and temporal. The grey outer 6 mm of the cerebrum is the cerebral cortex. This is the center of higher mental processes, where sensations are registered, voluntary actions initiated, decisions made, and plans formulated.

Within the cerebrum are various subcortical structures, including the thalamus, the limbic system and the basil ganglia. This last is involved in aspects of memory and emotional expression as well as planning sequences of behaviour.

The limbic system is concerned with actions that satisfy basic needs and with emotion. A key part of it is the hypothalamus which regulates endocrine activity via the pituitary gland and such life-maintaining processes as metabolism and temperature control. It produces the responses of anger, sexual arousal or fear via its control of the ANS. The amygdala is another part of the limbic system which plays a key role in motivation and emotional expression - eg: fear and anger - while the hippocampus is involved in motivation, learning and emotional memory.)

The second main area of the brain is the midbrain which contains part of the reticular formation and part of the brainstem. It is involved with (amongst other things) selective attention.

The third part is the hindbrain which contains the cerebellum, the pons and the medulla oblongata. The pons, together with the midbrain, activates the muscles of the eye – whether voluntarily or involuntarily (as in REM sleep). The cerebellum co-ordinates the muscles so that movement and delicate hand co-ordination is smooth and precise.

Localisation of Function in the Cerebral Cortex

Building on the work of John Hughlings Jackson’s (1865) observations of the effects of strokes on control of the human body and Gustav Fritsch & Eduard Hitzig’s (1870) experiments in electrically stimulating areas of dogs’ brains to produce movements, Wilder Penfield & Edwin Boldrey (1937) stimulated human brains (exposed during surgery) to produce movement in certain body parts. Over the next two decades, Penfield continued and extended such experiments, leading to what was effectively a typographical map of the motor and somatosensory cortexes (Penfield & Theodore Rasmussen, 1950).

Penfield’s work seemed to establish three clear principles with regard to the localisation of function in the brain:-

The left hemisphere of the brain controls the muscles of the right side of the body and the right hemisphere the left side - ie: contralateral
The body is represented upside down in the brain so that stimulation near the top of the head produces movement of the lower body while stimulation lower in the motor area led to movement of the upper body
Some areas of the body have large areas of the brain devoted to them while others had only small representations - this difference relating not size of the body area but to the degree of control, coordination and sensitivity required.

However, the Law of Mass Action and the Law of Equipotentiality, as proposed by Karl Lashley (1929), work against the concept of localisation, as demonstrated by Penfield. The Law of Mass Action was derived by Lashley experimenting with making lesions in the cortexes of rats which had been trained to run a maze: it wasn’t the location of the lesion which affected the rats’ memory but the size of

This has been confirmed in some instances by PET scans.

More recently the idea of Connectionism (distributed function) has taken hold. This is a holistic perspective in which the brain functions as a whole. All areas are inter-connected and have multiple tasks to carry out. Information is distributed in networks made up of millions of neurons.

This concept allows for cortical specialisation but the interconnections mean no one area has overall control.

However, even this attempt at an all-encompassing view of brain activity falls down in face of the fact that some brain functions are strictly localised. Eg: no evidence has yet been produced to

suggest that temperature control is the responsibility of any other part of the brain other than the hypothalamus.

Brain Lateralisation & Gender

The brain’s two hemispheres do, in fact, engage in some notable specialisations. Which means, at times, one half will be ‘cerebrally dominant’. For example, the left hemisphere specialises in language while the right is concerned with visuospatial tasks.

Generally speaking, male brains are said to be more lateralised - having a preference fo the right side. This would help explain why males are better at Mathematics and map-reading.

Females are usually thought to use both hemispheres more equally than males. Females often have a larger corpus callosum, meaning their two hemispheres are better connected so they can use the two halves together more. This could explain superior female fluency in speech, thought, body language...and, of course, multitasking!

Another sex difference frequently found in brain structure is that the anterior commissure (which communicates sensory information) is larger in women and male homosexuals.

Evidence for gender differences in brain lateralisation includes:-

Some studies show that damage to one side of the brain can lead to difficulties with verbal tasks (damage to the left hemisphere) or difficulties in visuospatial tasks (damaged right hemisphere) for men, but not women

Women with Turner’s Syndrome (X0 genotype) use both sides of the brain even more than normal and display very feminine’ behaviour
Men who do not have normal exposure to androgens in the womb tend to use both sides of the brain more
From studies using rats, it is thought that high levels of testosterone slow neuron development in the left hemisphere
Females are generally acknowledged to do better at language tasks, tasks in verbal fluency, speed of speaking, and tasks involving mathematical calculation. Males usually perform better at spatial tasks such as maze performance and mental rotation tasks
Males with damage to the left hemisphere generally have more problems with speech than females with equivalent damage

One problem in mapping brain activity to (self-reported) thoughts and (observable) behaviour is that, although blood flow can be measured to indicate neural activity, the activity may occur not specifically due to different brain structure but because different strategies are employed.

Another difficulty in attempting to attribute the causes of thought and behaviour is that nurture is often involved as well as nature - see Nature-Nurture - with the differences between male and female brains often being a combination of biology and environment.

The nervous system is the network of all the neurons (nerve cells) in the body. It subdivides into the central nervous system (CNS) and the peripheral nervous system (PNS). The brain and the spinal cord together comprise the CNS, the function of which is to analyse information arriving from the PNS and initiate appropriate responses to be sent via the PNS to the muscles and organs of the body.