Shared experiences mean much of our communication is short-hand. If I told you the world will never be the same again after 9/11, you would know exactly what I was talking about, even though my statement contained little explicit information. Psychologists call this shared information “common ground”.
Now Melissa Duff and colleagues at the Beckman Institute have made the surprising discovery that, despite their severe memory deficits, amnesic patients are capable of using this kind of shared information to succeed as well as controls at a challenging task which, on the face of it, depends on memory ability.
Each amnesic participant was paired with a researcher and given a set of 12 Chinese tangrams (similar to those pictured). The researcher also had their own identical set, and without being able to see each other’s tangrams, the researcher and patient had to work together in conversation to arrange their tangrams in the same order as fast as possible. The patient-researcher pairs repeated the task several times over two days, and just as when healthy controls took part, they gradually became quicker and quicker at the task, as together they developed labels for the different icons. Amazingly, six months later, the amnesic patients still remembered 80 per cent of the tangram labels they had developed during the task.
In contrast, during a control task, the amnesic patients were woefully impaired at remembering arbitrary labels allocated to a different set of 12 tangrams. After several learning sessions over two days, the patients could only remember an average of 5 out of 12 of the labels. Yet, healthy controls had learned all 12 labels early on the first day.
So how is it that the amnesic patients – most of whom had brain damage to their hippocampus caused by asphyxiation – were so capable of learning labels for the icons during the collaborative task, and yet so impaired at more straightforward learning? The researchers said it was because in the collaborative task, labels for the icons evolved gradually and had personal meaning to the amnesics, rather than being arbitrarily allocated by the researchers. They also said: “It may be that the learning, and the common ground, occurred on linguistic, conceptual or semantic, and perceptual levels”, and that this multidimensional learning was likely based more on ‘procedural’ mechanisms (like learning to ride a bike), presumably preserved in the patients because such mechanisms do not depend on a fully functional hippocampus (the brain’s usual repository for newly learned facts).
Duff, M.C., Hengst, J., Tranel, D. & Cohen, N.J. (2006). Development of shared information in communication despite hippocampal amnesia. Nature Neuroscience, 9, 140-146.