NEUROPSYCHOLOGY

Brain Structure, Function andCerebral Dominance

Dr. Malcolm Hughes

Within the areas of study that constitute neuro-psychology, the understanding of the brain and its constituent parts have led to substantial contributions to the relationship between brain and behaviour.

Included among these areas are the physiological mechanisms associated with the central nervous system.

An overall understanding of the anatomy of the brain structure reflects the diversity of the individual’s mental processes and behaviour. Some structures can highlight these effects quite significantly.

As well as understanding the functions of the brain, it is important to recognise that the two sides of the brain are not identical to each other, as research into “lateralisation” of brain function and the “split-brain” effects will demonstrate

Midline structure of the human brain

Detailed part cross section of the brain stem

Compartmentalisation of the Human Brain

The brain, (as distinct from the spinal cord), consists of three major divisions:

The Hindbrain – the most posterior part of the brain

The Midbrain – proportionally the smallest part

The Forebrain – the most prominent part of the brain

The Hindbrain:

Consists of three structures – the medulla, the pons and the cerebellum.

The medulla, pons, midbrain and the central parts of the forebrain constitute the brain stem.

The Medulla:

Situated just above the spinal cord (almost and extension of that structure) and controls a number of vital reflexes - notably heart rate, breathing, vomiting, salivation and sneezing – through the cranial nerves.

Damage to the medulla can easily be fatal; particularly susceptible to certain drugs e.g. opiates.

The Pons:

Contains the nucleii for several cranial nerves. Is a section of the nervous system where axons can cross from one side of the body to the opposite side.

Both the pons and medulla contain the reticular formation and the raphe system – these systems send axons throughout the forebrain and control the overall state of nervous system arousal.

The Cerebellum:

Is a large hindbrain structure – contributes to the control of movement, including balance and orientation.

In addition, lateral parts of the cerebellum contribute to the speed and skill of acquiring language and cognition (Leiner & Dow, 1989). Thus, individuals with cerebellar damage can have problems with their memory and finding the right word.

The Midbrain:

Although this structure starts in the middle of the brain, it is eventually dwarfed and surrounded by the forebrain.

The roof of the midbrain is called the tectum. On either side of the roof are two swellings, the superior colliculus and the inferior colliculus – both are routes for sensory information.

Under the tectum is the tegmentum – is involved with the pathways between the forebrain and spinal cord/hindbrain. It also includes the nucleii for the 3rd and 4th cranial nerves.

Another structure is the substantia nigra, an area whose cells and axons deteriorate in Parkinson’s disease.

The Forebrain:

This is the most prominent portion of the human brain, comprising the cerebral cortex, limbic system and other structures including the thalamus, hypothalamus and hippocampus.

The limbic system and sub-cortical structures

a) The hypothalamus

Has widespread connections with the rest of the fore brain and midbrain and contains a number of distinct nucleii. Damage to one of the hypothalmic nucleii can lead to abnormalities of one or more motivated behaviours,

e.g. feeding, drinking, temperature regulation, level of activity.

The hypothalamus also regulates the secretion of hormones from the pituitary gland. Is also associated with psychobiological reactions to stress and onset of psychosomatic illness.

b) Basal ganglia

These are a group of subcortical stuctures left and right of the thalamus these are the caudate nucleus, the putamen and the globus pallidus.

The basal ganglia tend to be damaged in Parkinson’s disease, Huntingdon’s disease and other conditions affecting movement.

Although the basal ganglia do not control movement directly or send axons to the medulla or spinal cord, they do send messages to the thalamus and midbrain which relay information to the cerebral cortex.

The basal ganglia

c) The Hippocampus

This is a large structure between the thalamus and the cerebral cortex (mostly towards the posterior of the forebrain). Two major axon tracts, the FORNIX and the FIMBRIA, link the hippocampus with the hypothalamus.

Evidence implicates this structure with memory. If damaged, learning about new events (episodic memory) and facts (semantic memory) become severely impaired.

In humans there are left-right differences in the function of the hippocampus:

i) Left temporal lobe and hippocampal damaged impairs verbal memory tasks e.g. word-paired associations.

ii) Right temporal lobe and hippocampal damage affects spatial response learning.

d) The Thalamus

This structure is the main source of input to the cerebral cortex and almost the only source of sensory information.

Can be described a a ‘way station’ for information going to the cerebral cortex. Each nucleus of the thalamus sends its axons to, and receives axons from, a particular part of the cerebral cortex.

e) The Ventricles

The cerebral ventricles are fluid-filled cavities within the brain and extend into the central canal of the spinal cord.

The fluid is cerebrospinal fluid (CSF), similar to blood plasma. Its function is to cushion the brain against mechanical shock when the head moves. It also provides a reservoir of hormones and nutrition for the brain and spinal cord.

Sometimes the flow of CSF is obstructed and accumulates within the ventricles or subarachnoid space, thus increasing the pressure on the brain – condition known as hydrocephalus, usually associated with mental retardation.

Photo showing part of the hippocampus and thalamus

The Cerebral Cortex and Corpus Callosum

The surface of the forebrain consists of two cerebral hemispheres (one left, one right) which cover all the other forebrain structures.

Each hemisphere is organised to receive sensory information from the contralateral (opposite) side of the body through axons to the spinal cord and cranial nerve nucleii.

Neurons in each hemisphere communicate with neurones in the corresponding part of the other hemisphere by two bundles of axons, the Corpus callosum and the smaller anterior commissure.

The microscopic structure of the cells of the cerebral cortex varies substantially from one cortical area to another – due to differences in function. The structures can be subdivided into four major areas:

a) Frontal lobe – involved with planning of movement, aspects of memory, inhibition of inappropriate behaviours. b) Occipital lobe – involved with vision (visual cortex).

c) Parietal lobe – body sensations.

d) Temporal lobe – deals with hearing and advanced visual processing.

Coronal section though the human brain,

showing the location of the amygdala

The major subdivisions of the human cerebral cortex, with indications of a

few of their primary functions.

Lateralisation of Brain Function

The hemispheres of the cerebral cortex are not mirror images of each other – have a division of labour known as “lateralisation”.

Via the Corpus callosum, each hemisphere deals with information from both sides of the body.

It is only through damage to the C. callosum that we come to recognise the effects of lateralisation.

Damage to the C. callosum is sometimes used to minimise the effects of epilepsy (in v. severe cases); has the effect of reducing the effects of epileptic seizures crossing from one hemisphere to the other.

Maturation of the Corpus callossum

Matures over 5 to 10 years – is one of the last brain structures to attain full maturity; due to this part of the brain developing more axons in the C. callossum than are actually needed at maturity.

The brain then selects certain axons, then discards the remainder.

Only those axons that connect to similar cells actually survive.

Two views of the corpus callosum, a large set of axons conveying information between the two hemispheres.

Brain development without the Corpus callosum

Such a condition can be due to:

a) genetic factors

b) toxins during pregnancy

Is a very rare condition; however, a person born without a C. callossum is very different from a person who looses this structure as a result of split-brain surgery.

The person born without a C. callossum can perform tasks that the split-brain person cannot.

e.g. can verbally describe what they feel with either hand or describe what they see in either visual field.

It is therefore possible that such people develop alternative connections across the hemispheres by means of the Anterior commisure or Hippocampal commisure.

- Such individuals can co-ordinate movements but only v. slowly – e.g their hands may be receiving conflicting information from the two hemispheres of the brain.

Demonstration of the effects of damage to the corpus callosum:

When the word “hatband” is flashed on a screen, a person with a split-brain can report only what their left hemisphere saw, “band”.

However, with their left hand, they can point to a hat which is what the right hemisphere saw.

The Split-brain Phenomon

Based primarily on the work of Roger Sperry (1960’s onwards).

The work revealed subtle behavioural effects when stimuli were limited to one side of the body or another.

The two hemispheres of a split-brain person (SBP) can process information and answer questions independently of each other.

e.g. Preilowski (1975) described a case where the individual buttoned his shirt with his right hand, but unbottoned it with his left.

Such neurological conflict appears more common after neurosurgery (Bogen, Schulz & Vogel, 1988) – important to note that the C. callossum, once cut, cannot grow back (this applies to all nervous tissue as nerve cells do not have the capacity to undergo cellular division).

However, the brain can utilise other sub-cortical structures to compensate.

Function of the Right Hemisphere

Believed to support the left (major) hemisphere, but is subordinate to it; sometimes dependant on the nature of the functions involved.

e.g. in people with intact, healthy brains, the right hemisphere is less active than the left during speech; however, the right hemisphere contributes to the emotional content of speech (Shapiro & Danly, 1985). Right hemispherical damage results in poor facial expression and difficulty in the understanding of others facial expressions (of emotion).In addition:

i) the right hemisphere is also more adept at recognising and dealing with complex visual patterns – notable in left-handed people;

ii) neurologists have long recognised that right hemispherical damaged people have difficulty finding their way from one place to another

iii) The split-brain person arranges puzzle pieces more accurately with the left hand than with the right;

iv) The left hand does better at drawing items e.g. box, bicycle, etc.

Based upon research evidence (Levy & Sperry, 1968), the right hemisphere appears to be specialised for the majority of complex visual and spatial tasks.

Hemispherical Differences and Cognitive Style

There is a doubtful assumption that any given individual relies consistently on one hemisphere or another, regardless of the task or situation.

Rather an overstated view as anyone with an intact brain makes use of both hemispheres for every task – each one may be more active for certain specific tasks.

Lateralisation and Handedness

Question as to the relationship between handedness and hemispherical dominance for speech:

Geschwind & Levitsky (1968) reported that one section of the temporal cortex, the planum temporale, (area for speech and language) is larger:-

a) on the left side for 65% of people;

b) is equal on each side for 24% of people and

c) is larger on the right side for 11% of people.

Location of the hippocampus in relation to the temporal lobes

A study of children who died before 3 month of age revealed that the planum temporale was larger even before language development occurs (on average, 2× the size on the left).

Cerebral Dominance and Handedness

10% of people are left-handed (over 90 % of prehistoric drawings indicate right handedness).

Most left-handers are partly ambidextrous.

The brain of l.h.p. is different from that of a r.h.p. but not simply the reverse.

For 98% of r.h.p., the left hemisphere is strongly dominant for speech – the planum temporale is decidedly larger on the left rather than the right side.

The right hemisphere is dominant for speech in approx. 35 – 40% of left-handers; the left hemisphere is dominant among the remainder.

The C. callosum is 11% thicker in l.h.p. – believed to be due to facilitating cross-hemisphere communication and bilateral representation of brain function.

Reasons for Left-handedness

Possible causes: Genetics;

Biological factors e.g. hormones which modify other parts of the body;

Organic brain damage – minimal cerebral dysfunction.

Geschwind & Garaburda, 1985) – evidence indicated that the hormone testorsterone contributed to left-handedness.

High levels of testosterone during the formative years may delay maturation of the left hemisphere and retard growth of the thymus gland and related structures implicated with the immune system.

Also, high levels of testosterone appear to result in a more highly developed right hemisphere.

The hormonal theory may also apply to the fact that:

Left handedness is more common in males than in females as also are allergies, stuttering and certain immune disorders.

In terms of the differences in right and left handedness (left and right cerebral dominance):

Left-handers are more likely to experience neuronal abnormalities in their left hemisphere; this may include:

Dyslexia

Childhood allergies

Migraine headaches (in adulthood)

Disorders of the immune system

Increased likelihood of stuttering – debatable issue

However, many left-handed individuals are found to excel in areas such as:

Mathematics

Aspects of science

Architecture and design, due to their enhanced visual and spatial orientation.