Reply to Balan and Gottlieb

REPLY: Several important questions are raised in the recent comparison by Balan and Gottlieb (2009) between the description by Balan et al. (2008) of lateral intraparietal (LIP) neuron activity in monkeys performing visual search with manual responses and our recent report of frontal eye field (FEF) neuron activity in monkeys performing visual search with saccade responses (Cohen et al. 2009). Both studies manipulated the number of distractors in the search display. Balan et al. 2008 reported decreased firing rate with increasing set size, but they reported no delay in the time at which activity for the target exceeded activity for the distractors, referred to as target selection time. Cohen and colleagues (2009) also reported decreased firing rate with increasing set size, but also observed delayed target selection time with increasing set size. How should we interpret the different observations across laboratories, effectors, cortical areas, and task designs? First, we emphasize that this is not a case of one observation being correct and the other incorrect. The results of both studies are valid within their respective contexts. However, the question remains: which difference between studies explains the different outcomes? We argue that task demands are the major factors. In other words, measured under the same task demands, we predict that the same pattern of results would be obtained across effector (manual response vs. saccade) and cortical area (LIP vs. FEF). The first step of the argument is to appreciate that categorical statements about the relationship between target selection time and response time are not possible because these times vary for at least three reasons. First, neuron type matters. Multiple