In psychological research, a single dissociation is when a manipulation leaves one cognitive function (say, A) intact whilst severing another (say, B). This indicates the functions A and B are at least partially independent. A double dissociation is when there exists, in addition, a manipulation that does the reverse (i.e. leave B intact whilst severing A). On the Wikipedia page for dissociation in neuropsychology/cognitive neuroscience (I'm assuming the "manipulation" is a brain lesion now) it says that "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." This can't really be true, can it? Consider the following Venn diagram, where the (necessary) neural substrate of A and B are represented, including their intersection:

Neural substrates of A and B

A double dissociation could consist of a manipulation in A \ B and another in B \ A. The functions would still be partially dependent, however, and not localized in completely different areas of the brain. So, in effect, the only thing a single dissociation proves is that the circles for A and B are not identical, and the only thing a double dissociation proves (in addition to the fact that they're not identical) is that one is not a subset of the other. Is that about right?

up vote 2 down vote accepted

Double dissociation does not prove the independence of certain cognitive functions and their neural substrates. What it does is provide stronger evidence for which the best explanation is the independence of certain neurocognitive systems. The brain is on many aspects not a marvel of engineering with many areas being responsible for a spectrum of functions and behaviours or for different aspects of a seemingly unitary cognitive faculty. On top of that, it is often difficult to make a proper classification and distinction between certain functions and behaviours (examples are episodic/semantic memory and perceptual/semantic priming) which, in many cases, limits the validity of the conclusions drawn.

So I don't know if a "conclusive demonstration" is possible. However, under properly defined terms, a double dissociation can provide strong support for independence with little possibility of an intersection between neurocognitive systems because if an intersection existed, it would be highly possible for a function to be retained at some level even after the hypothetical brain system responsible for it, suffers a lesion.

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    If I understand you correctly, you are essentially emphasizing that brain functions follow the principle of graceful degredation - such that if there were a (let's say large) overlap between A and B, lesions in either A / B or B / A wouldn't critically degrade A or B, respectively, because the overlapping sections would retain a significant amount of that functionality. In the double dissociation case, they are then usually critically degraded, so one can safely assume that the overlap is minimal to none. Now I'm just wondering whether there are any functions with an "Achilles heel"... – user6682 Sep 16 '14 at 19:31
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    Also, following that logic - shouldn't single dissociation be quite significant evidence as well? If dissociation studies rely mainly on the notion that functions degrade gracefully, then the destruction of one function (and retaining of another) should mean that they can't share any significant neural basis. – user6682 Sep 16 '14 at 19:36
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    So you are stating that a double dissociation is redundant? From what I can tell, a single dissociation is significant evidence that independence between function A and B can exist but a double dissociation is even more significant because it permits safer conclusions. If B is damaged and A is functional we can say that the statement "A only if B" is not true but we can't say the same about "B only if A". This statement will be shown to be false if a double dissociation shows B to be functional when A is damaged. – Lazaros Mitskopoulos Sep 16 '14 at 20:30
  • Not redundant, no - I accept that it's better evidence, in addition to last point you made. I was just wondering whether the way the issue was presented on Wikipedia was accurate. Answer accepted! – user6682 Sep 17 '14 at 5:15
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    A Hofstadter fan.. I slould have noticed from your name. Godel Escher Bach is on my to-read list after reading "the mind's I" which I enjoyed. Thanks for the recommendations, i 'll surely take a look – Lazaros Mitskopoulos Sep 17 '14 at 12:27

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