{{Short description|Neuropsychology term}} {{other uses|Dissociation (disambiguation)}} {{Neuropsychology}} In neuropsychology, '''dissociation''' involves identifying the neural substrate of a particular brain function through identification of case studies, neuroimaging, or neuropsychological testing.
==Dissociation types==
===Single dissociation=== When dissecting complex mental tasks into their subcomponents, a researcher can establish a "single dissociation" between functions. This is done by demonstrating that a lesion to brain structure A disrupts function X but not function Y. Such a demonstration allows one to infer that function X and function Y are independent of each other in some way.{{cn|date=October 2021}}
Dr. Oliver Sacks has described many famous cases of dissociation in his books. Patient D.F., for example, was unable to place a card in a slot, but could do so when told to place it "as if mailing a letter". From this the conclusion was drawn that judging orientation is one ability (which D.F. had lost) and visual control of an action another (which D.F. could still do).<ref name=Goldstein>E. Bruce Goldstein: Sensation and Perception. Wadsworth, Pacific Grove (USA), 2002.</ref>
===Double dissociation=== To strengthen a single dissociation, a researcher can establish a "double dissociation", a term that was introduced by Hans-Lukas Teuber in 1955.<ref>[http://www.accessmylibrary.com/coms2/summary_0286-10954167_ITM What do double dissociations prove? | Cognitive Science (January, 2001)<!--Bot generated title-->]</ref> This is the demonstration that two experimental manipulations each have different effects on two dependent variables; if one manipulation affects the first variable and not the second, the other manipulation affects the second variable and not the first.<ref>Huettel, Scott A.; Song, Allen W.; McCarthy, Gregory: "Functional Magnetic Resonance Imaging" p. 439. Sinauer Associates, Inc., 2004.</ref> If one can demonstrate that a lesion in brain structure A impairs function X but not Y, and further demonstrate that a lesion to brain structure B impairs function Y but spares function X, one can make more specific inferences about brain function and function localization.<ref>{{cite journal |last1=Vaidya |first1=Avinash R. |last2=Pujara |first2=Maia S. |last3=Petrides |first3=Michael |last4=Murray |first4=Elisabeth A. |last5=Fellows |first5=Lesley K. |title=Lesion Studies in Contemporary Neuroscience |journal=Trends in Cognitive Sciences |date=August 2019 |volume=23 |issue=8 |pages=653–671 |doi=10.1016/j.tics.2019.05.009 |pmid=31279672 |pmc=6712987 }}</ref>
In cognitive neuroscience, double dissociation is an experimental technique by which two areas of neocortex are functionally dissociated by two behavioral tests, each test being affected by a lesion in one zone and not the other.<ref>Kolb & Whishaw: ''Fundamentals of Human Neuropsychology,'' 2003</ref> In a series of patients with traumatic brain injury, one might find two patients, A and B. Patient A has difficulty performing cognitive tests for, say auditory memory but has no problem with visual memory. Patient B has the opposite problem. By using neuroimaging (or neuropathology post-mortem) to identify the overlap and dissociation between lesioned areas of the brain, one can infer something about the localization of visual and auditory function in the normal brain.<ref>{{cite journal |last1=Gilmore |first1=John H. |last2=Knickmeyer |first2=Rebecca C. |last3=Gao |first3=Wei |title=Imaging structural and functional brain development in early childhood |journal=Nature Reviews Neuroscience |date=March 2018 |volume=19 |issue=3 |pages=123–137 |doi=10.1038/nrn.2018.1 |pmid=29449712 |pmc=5987539 }}</ref>
Establishing a single dissociation between two functions provides limited and potentially misleading information, whereas a double dissociation can conclusively demonstrate that the two functions are localized in different areas of the brain.
To make the difference between single and double dissociations easier to understand, Parkin<ref name=Parkin>A.J. Parkin: Explorations in Cognitive Neuropsychology. Blackwell, Oxford, 1996.</ref> gives the following example: <blockquote>If your TV set suddenly loses the color you can conclude that picture transmission and color information must be separate processes (single dissociation: they cannot be independent because you cannot lose the picture and still have the color). If on the other hand you have two TV sets, one without sound and one without a picture you can conclude that these must be two independent functions (double dissociation).</blockquote>
=== Examples of double dissociations === Paul Broca and Carl Wernicke were two physicians of the 1800s whose patients were evidence of the double dissociation between generating language (speech) and understanding language. Broca's patients could no longer speak but could understand language (non-fluent aphasia) while Wernicke's patients could no longer understand language but could produce jumbled speech (fluent aphasia). Post-mortems revealed lesions in separate areas of the brain in each case (now referred to as Broca's area and Wernicke's area respectively). Although the neurophysiology of language is now known to be more complicated than described by Broca or Wernicke, this classic double dissociation acted to begin modern neuropsychological investigation of language.<ref>{{Cite journal|title = First-Pass versus Second-Pass Parsing Processes in a Wernicke's and a Broca's Aphasic: Electrophysiological Evidence for a Double Dissociation|journal = Brain and Language|date = 1998-05-01|pages = 311–341|volume = 62|issue = 3|doi = 10.1006/brln.1997.1906|pmid = 9593613|first1 = Angela D.|last1 = Friederici|first2 = Anja|last2 = Hahne|first3 = D. Yves|last3 = von Cramon|s2cid = 23600648}}</ref>
The conditions Capgras delusion and prosopagnosia have also been argued to represent a double dissociation. In the former, a patient is able to recognise a person but does not get the feeling of knowing them.<ref>{{Cite journal|title = Capgras delusion: a window on face recognition|journal = Trends in Cognitive Sciences|date = 2001-04-01|pages = 149–156|volume = 5|issue = 4|doi = 10.1016/S1364-6613(00)01620-X|pmid = 11287268|first1 = Hadyn D.|last1 = Ellis|first2 = Michael B.|last2 = Lewis|s2cid = 14058637}}</ref> In the latter, a patient is unable to recognise a familiar person but sometimes has a feeling of knowing.<ref>{{cite journal |last1=Fiacconi |first1=Chris M. |last2=Barkley |first2=Victoria |last3=Finger |first3=Elizabeth C. |last4=Carson |first4=Nicole |last5=Duke |first5=Devin |last6=Rosenbaum |first6=R. Shayna |last7=Gilboa |first7=Asaf |last8=Köhler |first8=Stefan |title=Nature and extent of person recognition impairments associated with Capgras syndrome in Lewy body dementia |journal=Frontiers in Human Neuroscience |date=24 September 2014 |volume=8 |page=726 |doi=10.3389/fnhum.2014.00726 |pmid=25309399 |pmc=4173644 |doi-access=free }}</ref>
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==References== {{Reflist}}
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{{DEFAULTSORT:Dissociation (Neuropsychology)}} Category:Neurobiological brain disorders Category:Cognitive science Category:Neuropsychology Category:Cognitive neuroscience