Cognitive neuropsychological evidence

How do cognitive neuropsychologists set about the task of understanding how the cognitive system functions? A crucial goal is the discovery of dissociations, which occur when a patient performs normally on one task but is impaired on a second task. In the case of KF, a dissociation was found between performance on short-term memory tasks and on long-term memory tasks. Such evidence can be used to argue that normal individuals possess at least two separate memory systems.

There is a potential problem in drawing sweeping conclusions from single dissociations. A patient may perform poorly on one task and well on a second task simply because the first task is more complex than the second, rather than because the first task involves specific skills that have been affected by brain damage. The solution to this problem is to look for double dissociations. A double dissociation between two tasks (1 and 2) is shown when one patient performs normally on task 1 and at an impaired level on task 2, and another patient performs normally on task 2 and at an impaired level on task 1. If a double dissociation can be shown, then the results cannot be explained in terms of one task being harder than the other.

In the case of short-term and long-term memory, such a double dissocation has been shown. KF had impaired short-term memory but intact long-term memory, whereas amnesic patients have severely deficient long-term memory but intact short-term memory (see Chapter 7). These findings suggest there are two distinct memory systems which can suffer damage separately from each other.

If brain damage were usually very limited in scope, and affected only a single cognitive process or mechanism, then cognitive neuropsychology would be a fairly simple enterprise. In fact, brain damage is often rather extensive, so that several cognitive systems are all impaired to a greater or lesser extent. This means that much ingenuity is needed to make sense of the tantalising glimpses of human cognition provided by brain-damaged patients.

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