Cognitive neuropsychology

Cognitive neuropsychology is a branch of neuropsychology that aims to understand how the structure and function of the brain relates to specific psychological processes. It places a particular emphasis on studying the cognitive effects of brain injury or neurological illness with a view to inferring models of normal cognitive functioning.

History
The modern science of cognitive neuropsychology emerged during the 1960s. However there have been a series of influential studies during the last two centuries which have been critical in laying the foundations for studying brain impairment with a view to understanding normal psychological function.

The case of Phineas Gage is one of the earliest examples where a specific brain injury gave clues to the function of a particular brain area. Gage obtained his injury after an accident during the construction of part of a railroad in 1848 in Vermont. Gage was setting an explosive charge when the charge accidentally detonated, firing a 4 foot iron tamping rod through his cheek and out the top of his head. Remarkably Gage survived, but with extensive damage to the ventromedial areas of his frontal lobes. After the accident Gage changed from a responsible, hard working foreman into a brutish and capricious individual. This provided some of the earliest evidence that specific area of the frontal lobes may be involved in the psychology of emotion and personality.

Similarly, Paul Broca's 1861 post mortem study of an aphasic patient, known as 'Tan' after the only word which he could speak, showed that an area of the left frontal lobe (now known as Broca's area) was damaged. As Tan was unable to produce speech but could still understand it, Broca argued that this area might be specialised for speech production and that language skills might be localised to this cortical area.

Clues about the role of the occipital lobes in the visual system were provided by soldiers returning from World War I. The small bore amunition often used in this conflict occasionally caused focal brain injuries. Studies of soldiers with such wounds to the back of their head showed that areas of blindness in the visual field were dependent on which part of the occipital lobe had been damaged, suggesting that specific areas of the brain were responsible for sensation in specific visual areas, known as retinotopy.

Studies on Patient HM are commonly cited as some of the precursors, if not the beginning of modern cognitive neuropsychology. HM had parts of his medial temporal lobes surgically removed to treat intractacable epilepsy in 1953. The treatment proved successful in reducing his dangerous seizures, but left him with a profound but selective amnesia. Because HM's impairment was caused by surgery, the damaged parts of his brain were precisely known, information which was usually not knowable in a time before accurate neuroimaging became widespread. This allowed detailed connections to be made between theories of memory formation and the brain structures removed in HM.

These and similar studies had a number of important implications. The first is that certain cognitive processes (such as language) could be damaged separately from others, and so might be handled by distinct and independent cognitive and neural processes. The second is that such processes might be localised to specific areas of the brain. Whilst both of these claims are still controversial to some degree, the influence led to a focus on brian injury as a potentially fruitful way of understanding the relationship between psychology and neuroscience.

During the 1960s, information processing became the dominant model in psychology for understanding mental processes. This provided an important theoretical basis for cognitive neuropsychology, as it allowed an explanation of what areas of the brain might be doing (i.e. processing information in specific and specialised ways) and also allowed brain injury to be understood in abstract terms as impairment in the information processing abilities of larger cognitive system.

Methods
The 'lesion method' (using brain injury or lesions to infer cognitive function) was probably best described unwittingly by singer-songwriter Joni Mitchell when she wrote "You don't know what you've got 'till it's gone". By understanding what a person can no longer do, and correlating this with a knowledge of exactly which parts of the nervous system are damaged, it is possible to infer previously undiscovered functional relationships.

By using this method, it should also be possible to discover whether a skill is handled by a single cognitive process or a combination of several working together. For example, if a theory states that reading and writing are simply different skills stemming from a single cognitive process, it should not be possible to find a person who, after brain injury, can write but not read or read but not write. This selective breakdown in skills suggests that different parts of the brain are specialised for the different processes and so the cognitive systems are separable.

The philosopher Jerry Fodor has been particularly influential in cognitive neuropsychology, particularly with the idea that the mind, or at least certain parts of it, may be organised into independent modules. Evidence that cognitive skills may be damaged independently seem to support this theory to some degree, although it is clear that some aspects of mind (such as belief for example) are unlikely to be modular. Ironically, Fodor (a strict functionalist) rejects the idea that the neurological properties of the brain have any bearing on its cognitive properties and doubts the whole discipline of cognitive neuropsychology.

Cognitive neuropsychology also uses many of the same techniques and technologies from the wider science of neuropsychology and fields such as cognitive neuroscience. These may include neuroimaging, electrophysiology and neuropsychological tests to measure either brain function or psychological performance.

The principles of cognitive neuropsychology have recently been applied to mental illness, with a view to understanding, for example, what the study of delusions may tell us about the function of normal belief. This relatively young field is known as cognitive neuropsychiatry.