Neuropsychology

Double Dissociation Detective

Six fictional case files, two tasks each, and one question: what does this pattern of scores actually establish?

Simulated — invented people, invented tasks, invented control data

Learning objective

By the end you should be able to tell a double dissociation from a single one, and say what each rules out.

About 30 minutes. Nothing you do here is saved or sent anywhere.

These are not patients

Every case file, score, task and control group here was written for teaching. No real person is described and no clinical instrument is used. Nothing on this page diagnoses, assesses or gives a prognosis for anybody.

  1. Commit to an answer to the opening question.
  2. Work through the six case files in order. For each one, read the scores against their own control group, open the task demands, and return a verdict.
  3. Case 5 has a test-length slider. Move it before you commit.

First, a judgement

One fictional person scores 91% on a word-picture matching task and 48% on a picture-naming task. Both tasks use everyday objects.

What does that pattern, on its own, establish?

Challenge - what does a clean crossover buy you?

A fictional report describes two people. One is far below the control range on face recognition and within it on object naming; the other shows the opposite. Both tasks use the same input, the same response and the same number of items, and both differences are well outside measurement error. The report concludes: "face recognition is performed by one brain region and object naming by another."

What does the crossover actually support?

What this demonstrates

A single dissociation has a boring rival

If one person is poor at task B and fine at task A, the obvious explanation is that A and B need different things. The boring rival is that B is simply more demanding, so any general reduction in capacity shows up there first. One person cannot tell those apart, however dramatic the gap.

A double dissociation rules that rival out

If a second person shows the opposite pattern, task difficulty can no longer be the explanation: a task cannot be the harder one for one person and the easier one for another purely by being harder. That is the whole logical work a double dissociation does, and it is real work. It is also the limit of what it does.

Compare each score with its own control group

Raw percentages are not comparable across tasks. A task on which healthy controls average 52% and a task on which they average 95% are different rulers. Case 4 on this page has a 58-point raw gap and no gap at all once both scores are referenced to their own controls. Whenever you see two percentages compared directly, ask what the control groups did.

Precision decides what you are allowed to claim

Case 5 changes verdict as the test gets longer without a single score changing. Short tests produce wide intervals, and a difference inside the interval is a difference you cannot claim. This is not a technicality: single-case dissociations are often built on tasks short enough that the difference would not survive being treated seriously as an estimate.

Two tasks always differ in more than one thing

Task impurity is unavoidable. Two tasks with different names differ in input, response, speed pressure, familiarity, strategy and how much they load on everything else at once. A crossover between two tasks isolates the process you had in mind only if nothing else that differs between them could produce the same crossover. Case 6 is a real crossover produced by two peripheral sensory differences, and nothing about it is a trick - it is what most real cases look like before the follow-up testing.