When two sensory stimuli are presented sequentially, discriminating between them requires us to compare the second stimulus with stored information about the first one. A key problem is identifying the brain regions in which such an evaluation is made, and how they shape our subsequent behaviour. Focusing on the somatosensory system, Ranulfo Romo and his colleagues have obtained elegant evidence that a subset of neurons in the secondary somatosensory cortex (S2) participates in the comparison between past and present stimuli, implicating this brain region in the decision-making process.

The authors trained monkeys to discriminate which of two vibrating stimuli that were applied sequentially to their fingertips had the higher frequency, and to report their decision by pressing one of two buttons. Romo et al. recorded the activity of S2 neurons throughout the task, and found that, during the first stimulus, the cells merely encoded its frequency (f1). By contrast, during the second stimulus, the firing of some S2 neurons was not a simple function of f2, but depended on both the remembered and the current stimuli. So, for example, during the first few milliseconds of the second stimulus, the firing rates of some S2 neurons clearly depended on f1, despite the fact that the first stimulus had been presented as many as 3 seconds before. But during the final part of the second stimulus, firing depended on both f1 and f2. More precisely, the main determinant of firing was whether f2 was greater than f1, or f1 was greater than f2. Moreover, the behaviour of these S2 neurons during the final part of the second stimulus correlated with the monkey's choice, indicating that they might be involved in making a decision and reporting it to other parts of the brain for action.

Despite significant variability in the dynamics of the S2 responses during the second stimulus, the results of this study make a strong case for neurons in this area being involved in the comparison of the two stimuli and in the subsequent decision-making process. But as other brain areas, such as the prefrontal and the medial premotor cortices, also seem to have similar properties, it is likely that S2 is part of an extensive decision-related network. Future studies will allow us to disentangle more precisely the corresponding contributions of each of these structures to decision making.