Awfully afraid? Dissociating decision- from motor- and sensory-related brain activation during perceptual choices.

We constantly make decisions about how to interpret our current situation and what to do next. Such decisions usually reflect available sensory input and the potential costs and benefits of choosing a particular interpretation or course of action over another. Facial expressions are a particularly informative source of sensory input. For example, the fearful or disgusted facial expression of a conspecific can provide information about the presence of threat or imminent danger of food contamination and poisoning. However, before we can decide on an appropriate course of action in such a situation, we must first perceive and decide whether the observed facial expression is fearful or disgusted.

In a recent study published in The Journal of Neuroscience, Thielscher and Pessoa (2007)Go investigated the neural correlates of perceptual decisions. They based their study on paradigmatic neurophysiological experiments with nonhuman primates (Britten et al., 1992Go). In those experiments, monkeys observed an array of more or less randomly moving dots and reported with an eye movement whether the net motion of the dots was to the left or the right. The difficulty of the decision depends on the proportion of dots moving in the same direction (motion coherence). The activity of single neurons in area MT reflects the degree of motion coherence, and the sensitivity of most neurons corresponds well with the perceptual sensitivity of the animals.

Current models of decision making, such as the race or diffusion model, suggest that a perceptual decision consists of three distinct phases (Fig. 1, top). First, neurons sensitive to motion (or facial expression) represent the sensory input, providing evidence for two alternative percepts. The evidence is then integrated over time, allowing for a more reliable comparison of the two alternatives. In a third phase, the evidence is compared with a criterion and, if the criterion is satisfied, the behavior is initiated. To identify neurons or structures that crucially contribute to this decision-making process, we need to control for their involvement in sensory, perceptual, and motor components (however, see Cisek, 2006Go).