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According to my textbook Cognitive Psychology by E. Bruce Goldstein,

a double dissociation occurs if damage to one area of the brain causes a function A to be absent while function B is present, and damage to another area causes function B to be absent while function A is present. To demonstrate a double dissociation, it is necessary to find two people with brain damage that satisfy the above conditions.

I don't understand how this is any different from merely having a control group and experimental group. Suppose I take a group of people who destroyed hippocampuses and observe they cannot store episodic memories. I then take a group of people who have hippocampus and observe they can store episodic memories. This tells me the function of the hippocampus must be in part to store episodic memories.

My Question:

How does a double dissociation yield information any different from this?

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In brain damage/lesion studies, a double dissociation gives evidence that function A and B are, to some degree, implemented in different regions of the brain. In general, a double dissociation shows evidence that A and B are independent of each other.

This is a different kind of claim than saying that function A is implemented in brain region X. For this kind of claim, performing an experiment that lesioned (removed) a brain region in some subjects but not others (obviously not an ethical experiment, but let's ignore that) would be a reasonable way to test the claim.

However, this kind of experiment wouldn't be able to provide evidence that functions A and B are implemented in independent regions. To illustrate why, let's consider the possible outcomes of the experiment:

  • Functions A and B both remain in tact or Functions A and B are both disrupted. Clearly this outcome doesn't provide any evidence that A and B are implemented in different regions, since both functions are affected equally by the lesion.
  • Only function A is disrupted, function B remains in tact. The most that this outcome shows is that region X (the lesioned area) is crucial for function A but not B. However, what is doesn't show is that function A is independent of function B. It could be the case that function A can be disrupted without disrupting function B, but that any disruption of function B will disrupt function A.
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It actually has nothing to do with having an experimental and control group.

A double dissociation refers to documenting two distinct patterns of impairment in two different groups or individuals, proving that two functions are neurologically distinct.

For example, someone with hippocampal damage will have trouble forming declarative memories (memories you are consciously aware of). If you repeatedly present them with a tone followed by an electric shock, and then play the tone and ask them what's about to happen, they won't be able to answer correctly. However, they are still capable of forming a conditioned aversion, which means that the tone will induce a feeling of fear in them even though they have no conscious memory of getting shocked. In fact, it will elicit just as much fear in a patient with hippocampal damage as in a normal control.

In contrast, someone with amygdala damage will have no trouble learning the tone-shock association - they'll figure it out as quickly as a normal control would, in fact. However, they won't be afraid of the tone. Even though they know full well that tone=shock (declaratively), and the shock is just as painful to them as it would be for a normal control, they can't form the unconscious association that the tone should be feared.

A control group is important to determine which functions are intact (as opposed to just 'less impaired'). But for a double dissociation, you need two kinds of patients with opposite impairments - not just one patient group with one function impaired.

To illustrate why one patient group wouldn't be enough, imagine you have a patient who fails second-order theory of mind tests ('I know you don't know that I know where you hid the candy') but passes first-order theory of mind ('I know you don't know where I hid the candy'). Does that mean that first-order and second-order theory of mind are distinct skills? No, because we have yet to find anyone who fails first-order theory of mind tests who doesn't also fail second-order theory of mind. Instead, it makes more sense to think of the two tests as measuring the same thing, with the first-order tests just being easier than the second-order tests.

If you were to damage the hippocampus in one group and leave the other group undamaged, you'd know the hippocampal damage caused their memory problems. But when they successfully develop a phobia of a tone paired with a shock, you wouldn't know if that's just an easier kind of memory to form, or if it's truly a separate memory system. You need the patient with the opposite pattern of deficits to prove that you really do have two separate systems in play.

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