The nervous system has long been recognized as the locus of the control of human action, but the nature of its contribution and the mechanisms by which this is achieved are still a matter of active debate. The main thrust of investigation has been empirical, and has been initiated from two fields within psychology—physiological psychology and human neuropsychology.
Physiological psychology has investigated the influence of general physiological systems upon the fundamental aspects of behaviour, and has concentrated on affective and conative mechanisms rather than cognitive processes. Because the site of these mechanisms is in the central subcortical parts of the head, and in lower brain systems, they have generally been studied in animal preparations, for the survival of human cases with damage to these areas is relatively poor. Research has identified three major functional systems. First, the limbic system, which includes the cingulate gyrus, the septal region, the fornix, the hippocampus and the amygdala, is involved in the evaluation of experience as punishing or rewarding. It also maintains a memory of these evaluations, so that behaviour can be adaptive and appropriate to its context. Rage and fear, taming, flight and attack are all associated with this region, as is the regulation of psychological mood. Second, the medial forebrain bundle, grouped around the hypothalamus, is involved in the basic motivational systems for hunger, thirst and sexual behaviour. It can be convenient to think of subsystems within the hypothalamus which turn such drives on or off, although the system is in reality more complex. Related structures subserve the effects of reward and punishment, and also exert control on the endocrine system of hormones, and on the autonomic nervous system involved in emotion and anxiety. If pleasure is generated anywhere within the brain, it is here. Third, the system in the brainstem governs the operation of reflex responses and maintains the general level of alertness and attention within the rest of the nervous system.
The other, and more recently prominent, field is that of human neuropsychology. This has mainly investigated cognitive functions, and in the cerebral cortex of human subjects. Its origins are in clinical neuropsychology, the study of patients with damage to the central nervous system. From the latter half of the nineteenth century, investigators recognized that fairly discrete behavioural defects could be associated with relatively localized injuries to the surface of the brain. This study of focal brain lesions, promoted by the observation of those injured in both World Wars, laid the basis of modern neuropsychology as both a research area and an applied clinical discipline. The most widely adopted functional model derived from this work is that of regional equipotentiality within an interactionist theory. Interactionist theory, originating with Hughlings Jackson and developed by Luria and Geschwind, proposes that higher abilities are built up from a number of more basic component skills which are themselves relatively localized. Regional equipotentiality argues for localization only within certain rather loosely defined regions. That higher functions appear incompletely localized in the brain may be due to the flexibility of cognitive systems in employing basic components in complex performance.
Three main approaches are adopted in modern clinical neuropsychology. First, behavioural neurology, derived from the work of Luria and most widely practised in the former Soviet Union, is individual-centred and aims at a qualitative analysis and description of the patient’s problems rather than a quantitative assessment. The focus of interest is not only the level of performance, but also the way in which a given task is performed. Second, an approach, popular in the USA, concentrates on the use of test batteries. The two currently most important are the Halstead-Reitan and the Luria-Nebraska Neuropsychological Batteries. Such batteries, composed of a large number of standard tests, seek to give a complete description of the patient’s level of performance across the whole spectrum of abilities, and use statistical methods. Diagnostic indicators are also usually a feature of the results. Third, the individual-centred, normative approach is most commonly practised in Britain. It relies to some extent upon formal psychometric assessment, but emphasizes the need to tailor the assessment to the nature of a particular patient’s difficulties. The aim is an accurate description of the dysfunction being investigated, going beyond a simple diagnostic classification to an understanding in cognitive psychological terms. This approach is more efficient in terms of time and resources, but makes greater demands upon professional skill and clinical insight. The approaches are, of course, rather less distinct in clinical practice.
The clinical tradition in neuropsychology has developed since the mid-1960s through important contributions from cognitive experimental psychology. The stimulus for this development was undoubtedly the study of the commissurotomy or ‘split-brain’ patients by Sperry and Gazzaniga in the early 1960s. These patients, in whom the two lateral hemispheres of the cortex had been surgically separated for the treatment of epilepsy, provided a unique opportunity to study the functions of each hemisphere operating alone. It was demonstrated that each was capable of perceiving, learning and remembering, and that there were in addition relative specializations characteristic of each hemisphere: the left subserved speech and verbal, symbolic, logical and serial operations, while the right undertook spatial-perceptual, holistic and parallel processes. The split-brain patients also provided a milieu for the empirical investigation of the seat of consciousness, although the conclusions to be drawn are still very much a matter of debate. However, apart from the research findings directly derived from the split-brain patients which are sometimes difficult to interpret, this work demonstrated that methods already employed in a different context in experimental psychology could be used to investigate brain organization in normal intact human adults. An enormous literature has built up around these techniques, and the results, although far from unanimous, broadly support the conclusion derived from the split-brain patients. This is that the hemispheres possess relative specializations for cognitive function. While it was at one time thought that this might relate to the type of material processed, or the response mechanisms employed, it is now thought that the nature of the processing determines the relative proficiency of each hemisphere. No one specification of the relevant processing characteristics has yet been widely accepted.
Since about 1980, the specialism of cognitive neuropsychology has also become established out of a fruitful interchange between cognitive psychology and human neuropsychology. Data derived from the deficits exhibited by those with neuropsychological impairments are used to refine and test models of normal cognitive processes; cognitive models developed with normal subjects are applied to understand the cognitive deficits of those with injury or disease of the brain. This approach has been particularly successful in extending our understanding of the higher intellectual functions, notably including reading, writing, spelling and calculation. Clinical application of the approach permits a precise analysis of a patient’s disability in neuropsychological terms by reference to the processing elements which are dysfunctional. Allied to the cognitive neuropsychology approach are developments in cognitive neuroscience which have resulted in the creation of a new generation of models of brain function, variously referred to as ‘neural networks’, ‘parallel distributed processing (PDP)’ or ‘connectionist’ models. These models, which relate to advances in computer science and artificial intelligence, employ the concept of a connectionist matrix which has the ability to learn and to acquire intelligent functions, and to respond to malfunction, in a way which provides a persuasive analogy for human brain function. These models are likely to have an increasingly important role in advancing our understanding of higher functions.
Alongside these developments in experimental neuropsychology has been a renewed interest in electrophysiological processes. The technology of averaged evoked response recording, and different ways of looking at the ongoing electrical activity of the brain (EEG), have both produced significant advances in directly linking cognitive events in the psychological domain to observable concurrent events in the physiological domain. While this research is difficult in technological and methodological terms, it holds the promise of being able to identify accurately, with good temporal resolution, the concomitants of cognitive processes within the physiological activity of the brain. Despite inventive research, this promise is some way from being fulfilled. At the same time, psychophysiological studies, which have a longer history, have continued into the psychological correlates of autonomic nervous system functions. Much has been learned of the peripheral changes in heart rate, electrodermal response, respiration, blood pressure and vascular changes which accompany changes in emotion and mood, but the problems of individual and situational variability, and the poor temporal association between mental and physiological states because of the slow response of the autonomic processes, have led to a decline in interest.
Dramatic developments in medical imaging of brain structures—by nuclear magnetic resonance imaging (NMRI) and magnetic resonance imaging (MRI)—and of physiological brain processes—by positron-emission tomography (PET scan) and single photon emission computed tomography (SPECT)—are, however, providing new opportunities for the study of the association between neurological structures and psychological processes.
A number of fundamental problems face the apparently successful study of brain-behaviour relationships: First, the philosophical issue of the mind-body problem. Most neuroscientists adopt a position of psychoneural monism, assuming that some identity can be established between mental and physiological events. This, of course, may be a conceptual error. It is possible that developments in electrophysiology may provide a means for the empirical investigation of this issue, till now primarily the domain of philosophers. Second, much of experimental neuropsychology proceeds by inference to, rather than direct observation of, physiological processes, so placing great importance upon methodological rigour. Third, it has to be admitted that we still have no real idea of how the brain operates to produce high-level cognition. A rather vague cybernetic-electronic model is often assumed, though this is being modified by the introduction of connectionist models, but there is no real certainty that this in any way reflects the actual principles of operation within the brain. Fourth, the nervous system is a very complex set of highly integrated subsystems. It is unlikely that significant progress will be made in our understanding of it until more adequate models can be developed, both of the physiological performance of large neural systems and of the psychological structure of cognitive abilities.
