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Cognitive neuropsychology is a branch of cognitive psychology that aims to understand how the structure and function of the brain relates to specific psychological processes. Cognitive psychology is the science that looks at how the brain's mental processes are responsible for our cognitive abilities to store and produce new memories, produce language, recognize people and objects, as well as our ability to reason and problem solve. Cognitive neuropsychology 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. Evidence is based on case studies of individual brain damaged patients who show deficits in brain areas and from patients who exhibit double dissociations. Double dissociations involve two patients and two tasks. One patient is impaired at one task but normal on the other, while the other patient is normal on the first task and impaired on the other. For example, patient A would be poor at reading printed words while still being normal at understanding spoken words, while the patient B would be normal at understanding written words and be poor at understanding spoken words. Scientists can interpret this information to explain how there is a single cognitive module for word comprehension. From studies like these, researchers infer that different areas of the brain are highly specialised. Cognitive neuropsychology can be distinguished from cognitive neuroscience, which is also interested in brain damaged patients, but is particularly focused on uncovering the neural mechanisms underlying cognitive processes.
The modern science of cognitive neuropsychology emerged during the 1960s as a reaction to behaviorism. Scientists realized that there were other sources of data and consciousness became a major area of interest. A particular area of interest for cognitive psychologists was memory. By studying patients with amnesia, which was caused by injuries to the medial temporal cortex, scientists were able to determine the affected areas of the brain. A patient with amnesia will not be able to remember events of the previous day (episodic memory,) but they will still remember how to tie their shoes (procedural memory,) remember a series of numbers for a few seconds (working memory) and be able to recall historical events they have learned in school (semantic memory.) Many other studies like this have been done in the field of neuropsychology examining lesions and the effect they have on certain areas of the brain and their functions. 
The case of Phineas Gage was one of the earliest in which a brain injury provided clues to the function of a particular brain area. Gage survived an 1848 accident in which an iron rod 1¼ inches in diameter was driven through his head, destroying most or all of his left frontal lobe. Though he suffered no loss of sensory or motor function, Gage's consequent personality changes prevented his return to his position as a railway construction foreman (though most presentations of Gage greatly exaggerate his psychological changes—see Phineas Gage).
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 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 localized to this cortical area.. Broca did a similar study on another patient, Lelong, a few weeks later. Lelong, like Tan, could understand speech but could only repeat the same 5 words. After examining his brain, Broca noticed that Lelong had a lesion in approximately the same area as his patient Tan.  He also noticed that in the more than 25 patients he examined with aphasia, they all had lesions to the left frontal lobe but there was no damage to the right hemisphere of the brain. From this he concluded that the function of speech was probably localized in the inferior frontal gyrus of the left hemisphere of the brain, an area now known as Broca's 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 ammunition 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 Henry Molaison, formerly known as patient H.M., are commonly cited as some of the precursors, if not the beginning of modern cognitive neuropsychology. Molaison had parts of his medial temporal lobes surgically removed to treat intractable epilepsy in 1953. Much of the hippocampus was also removed along with the medial temporal lobes. The treatment proved successful in reducing his dangerous seizures, but left him with a profound but selective amnesia.After the surgery, Molaison's long term memory was still intact, but he no longer had short term memory. He was still able to remember big events from before the surgery, such as the stock market crash in 1929, but he could no longer form new memories. This accidental experiment showed scientists how the brain processes different types of memory. Because Molaison'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. Scientists concluded that while the hippocampus is needed in the creation of new memories, it is not needed in the retrieval of old ones; they are two separate processes. They also realized that the hippocampus and the medial temporal lobes, both of the areas removed from Molaison, are the areas responsible for converting short term memory to long term memory. 
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. (For more on the cognitive neuropsychological approach to language, see Eleanor Saffran, among others.) The second is that such processes might be localized to specific areas of the brain. Whilst both of these claims are still controversial to some degree, the influence led to a focus on brain 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 systems.
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. This is called the lesion method.
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. Useful technology in cognitive neurospychology includes positron-emission tomography (PET) and functional magnetic resonance imaging (fMRI). These techniques make it possible to identify the areas of the brain responsible for performing certain cognitive tasks by measuring blood flow in the brain. PET scans sense the low-level radiation in the brain and produce a 3-D images, whereas an fMRI works on a magnetic signal and is used to “map the brain”. Electroencephalography (EEG) records the brain’s electrical activity and can identify changes that occur over milliseconds. EEG is often used in patients with epilepsy to detect seizure activity. 
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.
See also 
- Capgras delusion
- CDR Computerized Assessment System
- Clive Wearing
- Cognitive bias
- Cognitive neuropsychiatry
- Cotard delusion
- Emotion and memory
- Face perception
- Fregoli delusion
- HM (patient)
- List of topics related to brain mapping
- Neuropsychological test
- Phineas Gage
- Primary sensory cortex
- "The term cognitive neuropsychology often connotes a purely functional approach to patients with cognitive deficits that does not make use of, or encourage interest in, evidence and ideas about brain systems and processes"-- Daniel L. Schater (2000) Understanding Implicit memory: A cognitive neuroscience approach in Michael S. Gazzaniga (ed.) Cognitive neuroscience: a reader. Wiley-Blackwell.
- Harlow (1868)