Nevertheless, to rule out this possibility, we examined the time course of the baseline modulations time-locked to cue offset. These time courses were indistinguishable between the three attentional states (Figure S4), demonstrating that the attentional modulations (which are time-locked to stimulus onset) are not due to direct visual responses

to the cue. Second, systematic differences in fixational eye movements between the attentional states could have contributed to the observed variations in V1 responses. This possibility seems unlikely given that the attentional GSK1349572 supplier modulations start before the stimulus-evoked responses (Figure 6). Nevertheless, to rule out this possibility, we compared several eye position statistics in the three attentional states. Our results reveal no significant differences in these statistics depending on the attentional condition (Figure S5), providing further support for the top-down nature of the observed modulations. What is the purpose of the observed attentional modulations? One possible goal of attention is to allocate limited representational resources based on task demands (e.g., Broadbent, 1958). Another possible

goal of attention is to limit the access of task-irrelevant stimuli to circuits that control behavior (e.g., Allport, 1993). If the representation of multiple visual targets in V1 was a limited resource that could be controlled by attention, we would have expected V1 target sensitivity at attended locations to be higher under focal attention than under Crizotinib distributed attention (Figures 1C and 1D). Our finding that V1 population responses at attended locations are indistinguishable under focal and distributed attention suggests that in our task, and at the level of neural populations, target sensitivity in V1 may not be a limited resource that can be enhanced by focal attention. We find that behavioral performance is improved under focal attention relative crotamiton to distributed attention (Figures 2B–2D). As illustrated by our toy example (Figure 1), behavioral improvement

under focal attention is expected even if V1 target sensitivity is not limited and is identical in focal and distributed attention. A simple analysis based on signal detection theory shows that the observed behavioral improvement in accuracy under focal attention is consistent with no changes in neural sensitivity under focal and distributed attention (Suppl. Figure 6; see also Eckstein et al., 2000, Palmer et al., 2000 and Pestilli et al., 2011). This analysis, therefore, provides further support to the hypothesis that in our task, target sensitivity is not a limited resource that can be enhanced by focal attention. While our physiological and behavioral results appear to be inconsistent with attention as a mechanism for allocating limited resources in V1, we cannot rule out the possibility that such a mechanism operates in V1 in other tasks.