Blackwell
Publishers, 1996.
Review written for the New Scientist.
Can we find out how a television works by watching what it does when it's
broken?
Our initial response would probably be `No'. However, if it sometimes produces picture without sound, and other times produces sound without picture, we would conclude that different electronic components were responsible for the two different functions. The precise operation of each component could not be deduced, but we would be able to predict what the television would do when either or both of the components is damaged.
Neuropsychologists, who study the behaviour of people with brain damage, work on a similar hypothesis; if we can observe someone who can write but not read, and another person who can read but not write, then it would seem natural to conclude that these two processes are separate. Unfortunately, things are rarely so clear cut in real-life neurological patients. Often, one ability will be more impaired than another in some patients, whilst other patients show the opposite pattern of impairments. However, it would still seem that the two abilities must be separate, as each can be damaged independently of the other. Some patients (Wernicke's aphasics) have difficulty retrieving nouns but have less difficulty generating grammatically correct prose. Other patients (Brocca's aphasics) have little difficulty in retrieving nouns, but are rarely able to string a whole sentence together. From these two types of disorders, neuropsychologists have concluded that language production requires the functioning of (at least) two separate systems in the brain: one which deals with grammar, and one which selects the required nouns.
It is important to realise that this conclusion depends on the assumption that most people's brains work in a roughly similar way. In addition, it takes for granted that a patient with brain damage has a brain like normal people except that some part or parts of it are impaired.
In his book Explorations in Cognitive Neuropsychology, Alan Parkin gives a clear exposition of the principles underlying this field of psychology. He explains why we should think that the brain's functions are split up into several distinct `modules', and how we go about figuring out what these modules do.
Parkin explains how neuropsychology has helped us to understand the mechanisms behind several different mental abilities, including language, memory and action planning. Many examples of real patients are provided, making the disorders easier to understand and providing fascinating light relief in between chunks of structured argument. Particularly fascinating is Parkin's coverage of the phenomenon of `blindsight'. This is the name given to the ability that some partially blind people have to locate objects which they cannot see. Patients are reluctant to say where they think an object is located, saying things like `How can I look at something I have never seen?'. However, if pressed to give a response they perform very accurately, despite insisting that they `couldn't see a darn thing'. This is very counter-intuitive, and many psychologists tried to argue that such performance depended on the object reflecting some light into the seeing part of the visual field. However, this view has been superceded by a theory which is backed up by animal studies and by our knowledge of the anatomy of the brain. Most of the neurons from the eye go to the visual cortex, which is responsible for our conscious visual experiences. It was found that monkeys which had had their entire visual cortex removed were still able to locate objects using visual information, indicating that they were relying on a mechanism which did not use reflected light. A small number of neurons from the eye go to a different part of the brain, and it has been shown that it is this region which allows people to locate objects they cannot (consciously) see. Parkin explains the debate, and shows how these different lines of evidence, when taken together, allow us to understand the intriguing phenomenon of blindsight.
In 1848 Phineas Gage, a railroad engineer, had a tamping iron pass through the front of his brain. The iron bar was 4 cm in diameter, and 1 meter long. Incredible as it may seem, Gage survived, and lived for another 20 years after his accident. However, he went from being a promising worker to being unreliable, disrespectful and lacking in social skills. Many patients who suffer damage to this area of the brain (the frontal lobes), are able to carry on a fairly normal life, but undergo serious personality changes. Parkin describes Gage's incredible experience, and presents the current theories about what exactly this part of the brain is used for. Details aside, this type of evidence shows very, perhaps frighteningly, clearly that our personalities depend upon our brains - change one and you change the other.
The book is aimed primarily at psychology students, but it is written in a way that makes it accessible to all readers. It does use technical vocabulary, particularly when describing the areas of a patient's brain which have been damaged, although it isn't necessary to understand these terms in order to understand the book as a whole.
In writing such a full coverage of the main areas of research, Parkin leads us to believe that we should have a pretty good idea of what's going on in neuropsychology once we've read his book. However, the use of evidence from abnormal people to draw conclusions about normals relies on the fundamental assumption that we all perform mental tasks in roughly the same way. This might seem reasonable, given that we do all have similar physical characteristics. However, it is well-known that left-handed people's brains are organised differently from right-handers' brains, and given that this is the case surely there could be other important differences that have not yet been discovered? This means that different people may perform the same task in fundamentally different ways. We know that televisions are all wired up in roughly the same way, and this is what allows us to draw conclusions about how all televisions work from watching some when they malfunction. But if every television set was made in a different way, we might say that in some the sound was more fragile than the picture, whilst in others the picture was more fragile than the sound. The two functions, (sound and vision), might nonetheless be produced by the same component in yet other televisions. Thus, we would not be able to find out about how televisions work by looking at some which are broken. This may be the situation in humans - with every person wired-up slightly differently. Whilst the success of cognitive neuropsychology might placate this concern, the assumption is still just an assumption, and so should have been highlighted and debated in a book which serves as an introduction to neuropsychology.
This omission aside, Parkin has written a very readable and interesting
exposition of cognitive neuropsychology. It left me feeling fascinated about
the complexities of the human brain, and served as a stark reminder of its
fragility.
Susanna Spears is a postgraduate student in the Department of Cognitive and Computing Science at the University of Sussex.
