The search for the neural mechanisms of the set size effect

The set size effect in visual search refers to the linear increase in response time (RT) or decrease in accuracy as the number of distractors increases. Previous human and monkey studies have reported a correlation between set size and neural activity in the frontal eye field (FEF) and intraparietal sulcus (IPS). In a recent functional magnetic resonance imaging study, we did not observe a set size effect in the superior precentral sulcus (sPCS, thought to be the human homolog of the FEF) and IPS in an oculomotor visual search task (Ikkai et al., 2011). Our task used placeholders in the search array, along with the target and distractors, in order to equate the amount of retinal stimulation for each set size. We here attempted to reconcile these differences with the results from a follow-up experiment in which the same oculomotor visual search task was used, but without placeholders. A strong behavioral set size effect was observed in both studies, with very similar saccadic RTs and slopes between RT and set size. However, a set size effect was now observed in the sPCS and IPS. We comment on this finding and discuss the role of these neural areas in visual search.