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In experiments asking fruit flies to distinguish between ever closer concentrations of an odour, the researchers led by Professor Gero Miesenböck had previously identified a tiny minority of about 200 nerve cells in the brain as critical for decision-making.
In new work, the team found that these nerve cells collect evidence for the alternative choices as minute voltage changes across their surface. These changes build up over time until they reach a hair-trigger point, at which the nerve cell produces a large electrical impulse. This impulse signals that a decision has been reached.