Sunday, 26 June 2005

Choosing between the indistinguishable

Is it possible to have a preference for one taste over another without being able to distinguish between the two? That’s apparently the case with patient B, a 72-year-old man brain-damaged by Herpes simplex encephalitis in 1975. Patient B suffered bilateral damage to the amygdala, the hypocampus, basal forebrain, insular and cingulate cortex, all of which left him severely amnesic, unable to recognise familiar people and objects, with profoundly impaired taste and smell, but with relatively normal language and vision.

Neurologist and best-selling author Antonio Damasio and his colleagues at the University of Iowa asked patient B to taste 38 drinks in succession. In fact, each drink was either a mixture of salt and water (which tastes disgusting), or sugar and water (which tastes much nicer), presented in a random order. Regardless of which drink he was tasting, patient B declared each one to be ‘delicious’, and to taste ‘like pop’ (fizzy drink). In contrast, when healthy control participants and brain-damaged participants (none of whom had damage to the insular) completed the same task, they immediately stopped drinking all of the salt water drinks after the first sip, finding them extremely unpleasant.

So far the evidence simply suggests that patient B has lost his sense of taste. But next, the researchers presented patient B with two drinks at a time. The salt drink was in one beaker, the sugar drink in another, side by side. Patient B was asked to sip each and then to drink the one he preferred. On 18/19 trials of this kind, he chose the sugar drink. Even when urged by the researchers to drink the salty drink, he steadfastly refused.

“The taste comparison likely provides B with an overt feeling that he would rather drink one solution than another, without any overt knowledge of the taste experiences that would normally provide justification for this preference”, the researchers said. “We believe that he is aware of his preference of sucrose over saline, without awareness of the identity of either”.

It’s unlikely B’s other senses underlay his preference. A test with healthy participants showed they were unable to distinguish between the drinks based on smell, and anyway, B’s sense of smell is severely impaired. The researchers also argued it was unlikely at the concentrations they used that B was making his preference based on the ‘feeling’ of the drinks in his mouth.

Adolphs, R., Tranel, D., Koenigs, M. & Damasio, A.R. (2005). Preferring one taste over another without recognising either. Nature Neuroscience, Advance Online Publication DOI: 10.1038/nn1489

Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.

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