Neuropsychology · Simplified
Double Dissociation Detective
Evidence arrives one file at a time. Watch what each new file rules out, and what it leaves standing.
Case 1
These are not patients. Every person, task, score and control group is invented for teaching.
Each score is plotted as its distance from that task's own control average, in control standard deviations. The shaded band is the control group range: the scores a control group would usually produce. A filled marker means the whole interval sits below that band; a hollow marker means it does not.
All the values in this plot
| Person and task | Score | Against its control group | Whole interval below the control range? |
|---|
What does this file support?
What the evidence bought
Why a reversal is worth more than a bigger gap
Every file had one rival explanation to beat: that one task is simply harder than the other. Only one shape of evidence beats it.
Key idea: A single dissociation shows that two tasks come apart. It cannot rule out that one task is simply harder, and a second person with the same pattern does not help: that adds severity, not logic. A reversal does rule it out, because a task cannot be more demanding for one person and less demanding for another. That one piece of logic is the whole contribution of a double dissociation.
Two limits on that inference. The first you have just watched: a crossover in the point estimates is not a crossover in the evidence, and test length decides which you have. The second the numbers cannot reach at all: two tasks always differ in more than the process of interest, so a clean reversal on tasks that differ in several ways does not tell you which difference produced it. A double dissociation establishes that at least partly separable processing exists. Where that processing sits is a different question, and none of these files touches it.
The longer version adds six case files, including a large raw gap that is not a dissociation at all and a clean crossover on tasks that differ in too much, plus a challenge on what a crossover buys. It is at Double Dissociation Detective in the main collection.