Size and emotion averaging: costs of dividing attention after all.

Perceptual averaging is a process by which sets of similar items are represented by summary statistics such as their average size, luminance, or orientation. Researchers have argued that this process is automatic, able to be carried out without interference from concurrent processing. Here, we challenge this conclusion and demonstrate a reliable cost of computing the mean size of circles distinguished by colour (Experiments 1 and 2) and the mean emotionality of faces distinguished by sex (Experiment 3). We also test the viability of two strategies that could have allowed observers to guess the correct response without computing the average size or emotionality of both sets concurrently. We conclude that although two means can be computed concurrently, doing so incurs a cost of dividing attention.