Our results are consistent with those from previous fMRI

experiments (Buracas and Boynton, 2007 and Murray, 2008) reporting additive offsets with attention as well as a voltage-sensitive dye experiment that reached a similar conclusion about selection (E. Seidemann, personal communication). Equal increases in responses at all contrasts may result when responses are averaged across populations of neurons for at least two reasons. First, if some neurons show enhancement primarily at low and intermediate contrasts (contrast-gain like changes) and other neurons show enhancement primarily at high contrasts (response-gain like changes), then the overall sum of activity (and, consequently, any population readout that depends on this sum) would be check details expected to show enhancement at all contrasts (i.e., an additive offset). Indeed, an electrophysiological study has reported that some Selleckchem PD-332991 neurons exhibit contrast-gain, others response-gain, and yet

others exhibit additive changes in the same experiment (Williford and Maunsell, 2006). Moreover, the normalization model of attention (Reynolds and Heeger, 2009) can yield contrast-gain or response-gain like changes in different neurons dependent on the locations and sizes of their receptive fields. These effects in individual neurons can appear as an additive offset change when averaged across neurons (unpublished simulations). Second, the majority

of single-unit electrophysiology experiments used stimulus parameters that were matched to the tuning properties of the individual units being recorded. But in fact, any either stimulus that is the target of attention will give rise to activity in many neurons whose receptive fields and tuning properties may only partially match with the stimulus. Small baseline shifts with attention (Luck et al., 1997, Reynolds et al., 2000 and Williford and Maunsell, 2006) in each of many neurons may sum to a large effect in the overall population output, evident in the fMRI responses. The behavioral performance improvements with attention may, for some stimuli and tasks, depend primarily on this component of the population responses that is correlated across neurons (not the response- and/or contrast-gain changes evident in each individual neuron’s responses). Our max-pooling selection rule exemplifies how such a baseline shift can lead to improved behavioral performance. Hence, it is possible to reconcile the attentional modulation effects that have been measured with fMRI with those measured electrophysiologically.