Neuropsychology · Simplified
Network Disconnection Mapper
Two regions can both be perfectly intact and still be unable to do anything together.
The network
Three regions, three pathways, three tasks
Every task needs two regions and the pathway between them. Break something and watch which tasks notice.
Key terms
- Region
- A patch of cortex that does some part of the work. Damage here is a lesion in the usual sense.
- Pathway
- The white-matter tract carrying signals between two regions. Cutting one damages no cortex at all.
- Disconnection
- A deficit caused by regions being unable to communicate rather than by either of them being damaged.
A region drawn with a broken outline and a cross is damaged. A pathway drawn with a gap and a cross is cut. Everything else is working.
What the person can still do
What this shows
The pattern tells you where the break is
Damaging a region and cutting a pathway both produce deficits. They do not produce the same deficits, and the difference is readable.
Key idea: A region takes part in every task that needs it, so damaging one knocks out several tasks at once: the tasks that fail are the ones that share that region. A pathway is used by one pairing only, so cutting one knocks out a single task and leaves everything else untouched, including both of the regions it joins. That is why a very selective deficit, with the obvious candidate regions intact on the scan, is a reason to look at the white matter rather than a reason to doubt the patient. It is also why asking which region does a job can be the wrong question: some jobs are not done anywhere in particular, but in the traffic between places.
This is a cartoon and should be read as one. Real language networks have many more regions, pathways that carry traffic in both directions and serve more than one pairing, and a great deal of redundancy, so a single cut rarely produces the clean single-task failure shown here. The task names are simplified too. What the model is for is the inference, which does hold: the pattern of what fails constrains where the break is, and a deficit is not evidence that any particular region was damaged. Nothing here describes a real patient or a real anatomy.
The longer version adds a fourth region and task, pathways that serve more than one pairing, and a backwards exercise inferring the break from the pattern. It is at Network Disconnection Mapper in the main collection.