Brain Structures To Functional Processes example essay topic

This essay will discuss whether the specific human functions are confined to specific areas in the outer layer of the two hemispheres of the brain (cerebral cortex). The evidence and research methods employed to support or dispute this statement and current understanding of this subject will be discussed. A conclusion will summarise the material covered and offer a clear statement as to whether there is evidence of localisation of function in the cerebral cortex. The search for localisation of function is, historically, one of the major pursuits of human neuropsychology. A large percentage of neurological investigation in the late nineteenth and first half of the twentieth century has been based on an assumption, that psychological and behavioural functions are localised within specific areas in the brain. This idea was initially put forward by the phrenologists Franz Gall and Casper Spurzheim in the early nineteenth century.

Although details of their theory (each bump on the skull represented a character trait) were incorrect, the concept that the cerebral cortex is subdivided into functional areas, each responsible for a different mental faculty was the springboard for most future neurological research. In 1861 Paul Broca reported that a series of patients with speech defects all had damage to the inferior prefrontal cortex of the left hemisphere. Broca concluded that this area was responsible for production of speech and it became known as Broca's area (Zimbardo, 1997). In 1874, Carl Wernicke, based on ten clinical cases, claimed the area located in the posterior and upper portion of the left temporal lobe was responsible for language comprehension and this became known as Wernicke's area, (Pinel, 1999). These findings were collected during post mortem's on brain damaged patients. This was not scientific or reliable, due to little knowledge of neuroanatomy it was impossible to ensure that all cases had damage to exactly the same area of the cortex.

This was highlighted by the case of Phineas Gage, who in 1848 sustained a brain injury apparently to Broca's area, but did not suffer any language impairment. This was used as evidence by those against the concept of localisation of function. It was discovered some years later, that the damage sustained by Gage was actually to the prefrontal cortices in the ventral and inner surfaces of both hemispheres. This area, it is now understood, has influence on personality functioning, this would concur with reports that Gage experienced extreme personality changes (Dam asio, 1994).

Researchers developed more 'scientific' methods of exploring cortical activity. The use of animals in experiments meant more control over the location and extent of cortical damage. In 1870 Fritsch and Hit zig demonstrated that when an electrical current was applied to an area of the cerebral cortex on a slightly anaesthetised dog, the limbs on the opposite side of its body moved, suggesting this area could be the motor cortex. In 1873 Ferrier claimed to have localised the auditory centre in animals as being in the temporal lobe. These claims launched a concerted effort amongst neuro physiologists to explore the cerebral cortex of animals and discover where the various functions were located, for example Pavlov worked on conditioned reflexes, and Sherrington on reflex action, (Pinel, 1999). However the question remained as to whether it was possible to extend the results from animal experiments and apply them to humans.

In the 1930's Wilder Penfield brought the search for localisation of function into the human arena. Penfield with the use of electrical interference began to map the speech cortex in patients, this was in order to carry out accurate surgical excisions of the cortex as a treatment for seizures and to avoid damaging speech areas, (Penfield and Roberts, 1959). Penfield discovered that if a certain area of cortex was electrically excited it could cause movement in a particular part of the body, or stimulate the senses, emotion or a memory (Zimbardo, 1997). These findings appeared to provide further proof of functional localisation.

However during the early twentieth century Karl Lashley carried out research on memory using rats, which appeared to contradict this assumption. Lashley maintained that even when areas of the brain which had been specified as responsible for memory were removed rats were still able to perform complex tasks previously learned (Franz and Lashley, 1917). Lashley suggested that the memories for complex tasks were stored throughout the cortex (mass action) and that all parts of the cortex played an equal role in their retention (equipotentiality) (Pinel, 2000). Subsequently the findings of Lashley have been reassessed in the light of a greater understanding of the complexities of the brain and it would appear that they are consistent with localisation of function (Rose, 2001). However Lashley was concerned that this 'search for centres' was limiting and holding back a greater understanding of a very complex system (Lashley, 1930), this argument and his ideas about equipotentiality have some relevance in today's understanding of the functioning brain. Documented cases of brain surgery patients have contributed greatly to present understanding of cortex function.

Such as H.M., who had the medial portions of both his temporal lobes removed to treat epilepsy and as a result suffers from severe anterograde amnesia. H.M. cannot form new long-term memories, he has retained memories from before his operation and can retain information in the short term, however once he has stopped thinking about something it is gone and has no recall of it, (Pinel, 2000). It has been noted that H.M. can improve his performance in a practiced task even though he has no conscious memory (explicit) of carrying out the task before. It would appear that there is another type of memory storage at work and this has been called implicit memory, (Pinel, 2000). Pinel states that cases such as H.M. have reawakened an interest in relating brain structures to functional processes, but now with a greater awareness of the complexities of function.

Caution is needed when conclusions about localisation of function are based on functional deficit as a result of lesions in the brain, whether they are caused from injury, surgery, drugs to block neural pathways or genetic abnormalities. The structure of the brain is highly convoluted and the accuracy of making a lesion or interpreting it's position correctly is extremely difficult, (Pinel, 2000). Damage can also spread from the lesion to other areas and this can lead to a misinterpretation of which area is responsible for the function deficit. A more scientific approach would be to assess the function of the system that remains, (To ates, 1986). Modern functional brain imaging techniques (functional MRI), monitor and display increases in oxygen to the blood supply of any area in the brain, this technology has provided the clearest picture so far of how the brain functions.

It is evident that the idea of the brain being divided into centres of specific functions is far too simplistic. MRI has shown that when a specific mental task is being carried out, no single area of the brain is activated exclusively, many different areas are involved and the combination of areas changes depending on the task involved (Greenfield, 2000). It would appear that localisation of function does exist but that it is not one area but several which are involved, and that differences between functions however subtle will correspond to a different combination of areas in the brain being stimulated. Research such as Peterson et al (1989) has demonstrated this, for example PET scans revealed that different areas became active for saying a heard noun, than for saying a verb associated with a heard noun. Although there is evidence for localisation of function in the cerebral cortex, it is far more complex than originally conceived and it is necessary to incorporate into this picture of brain function some other issues which influence it. Cerebral asymmetry has been used as further evidence of functional differentiation of the cerebral cortex, for example, it has long been accepted that the left hemisphere performs better on verbal tasks and the right on situations which require non-verbal skills (Gazzaniga & LeDoux, 1978).

However there has been evidence that following early damage to the left hemisphere, the right hemisphere is capable of linguistic development (Dennis & Whitaker, 1976). This lead to a review of split-brain studies by Gazzaniga and LeDoux (1977) and they concluded that neither hemisphere is responsible for a function in isolation and that either hemisphere can control functions which were originally located in the other hemisphere (Gazzaniga & Le Doux, 1977). They did not dispute that there is evidence for localisation, but highlighted that the brain has the ability to adapt. This introduces the concept of plasticity. There is great flexibility in the organization of the brain, the neural connections appear to be able to change and adapt.

Initially this mechanism is at work during the stages of development as the organism adjusts to its particular environment (Goldman-Raki c, 1980), and then during adulthood this process has the ability to compensate for any losses of input or output, (Legg, 1989). However it is in younger patients that more dramatic plasticity is observed, such as the case of Harrison, it was found that following severe damage to his left hemisphere his right hemisphere had taken over the left side's functions. This became evident when Harrison had most of the left side of his brain removed and could still walk and talk, (Greenfield, 2000). The brain is able to make new neural connections to adapt to the individuals requirements, and these connections can be made and strengthened through appropriate stimulation (environmental enrichment), (Legg, 1989). However plasticity has limits, age slows down cell regeneration and the brain is not immune from this.

The size of the lesion also affects chances of recovery, the smaller the lesion the more able the brain is to forge new neural paths to overcome the deficit, or for other areas involved in the same function to compensate for it, (Legg, 1989). The relevance of position of the lesion and how this relates to loss and recovery of function needs to be explained within the context of the hierarchical organisation of the cortex. There is evidence that the sensory structures within the brain are organised in a hierarchy, and that their position within this is based on the specificity and complexity of the structure's function, (Pinel, 1999). Starting with the primary cortex through the secondary cortex and finally the association cortex these structures perform progressively more complex functions. Therefore a lesion in a higher level structure causes a more specific and complex deficit (Pinel, 1999). This hierarchy is demonstrated in the visual system.

The visual cortex processes information collected by the eye and provides the picture that we 'see'. This information is sent to the retina from the eye and is converted into neural signals which are then sent to the primary visual cortex. It is here the brain begins to make sense of what the eye has 'seen'. The neural signals are then relayed to the secondary visual cortex and finally to the association visual cortex. As the neural signals progress through the levels of cortex the function of processing: - integrating, giving meaning and creating a visual image, becomes more and more complicated until a picture emerges.

Damage to higher levels of the visual cortex produces more complex and subtle deficits, such as visual agnosia, when stimuli can be seen but their meaning is lost, (Sacks, 1985). In conclusion, the search for localisation of function has been a recurrent theme throughout the recent history of neuropsychology. Although there has been evidence to dispute this view of cerebral organisation, it has tended to demonstrate an oversimplification of the functional localisation rather than disprove the theory completely. There is evidence for localisation of function within the cerebral cortex, however it is far more complex than this suggests. Many areas are involved in even basic functions and even small subtle differences in the function being performed can lead to a dramatic change in the areas involved with the cortex. It has been suggested that perhaps humans have a natural drive built into their brains to discover generally useful laws of nature.