These results also provide a constraint on a general attentional mechanism: it must be able to modulate the responses of specific and arbitrary subgroups of neurons, even when they are located far apart in cortex. We trained two rhesus monkeys (Macaca

mulatta) to perform a change detection task in which we simultaneously manipulated spatial and a form of task or feature attention ( Figure 1A). On each trial, two achromatic Gabor stimuli flashed synchronously. At an unsignaled and randomized time, either the orientation or the spatial frequency of one of the stimuli changed. The monkey was rewarded for making an eye movement to the stimulus that changed within 500 ms. We manipulated attention by cueing the monkey in blocks as to which of the two stimuli was more likely to change (left or right: spatial attention) and which stimulus feature would change (orientation or spatial frequency: feature attention; see Experimental Procedures). We Adriamycin nmr only included data sets in which the monkey completed at least four blocks of each spatial and feature attention condition. Spatial attention alternated on successive blocks and feature attention alternated every four blocks (Figure 1B). We attempted to choose ranges of orientation and Tariquidar cell line spatial frequencies so that the animals’ average performance in the two tasks was equivalent (overall performance

for the two animals on the orientation task was 64% correct, 8% standard deviation [SD]; 92% correct, 2% SD at the largest change; overall performance on the spatial frequency task was 68% correct, 11% SD; 95% correct, 4% SD at the largest change). While animals performed this change detection task, we recorded simultaneously from all the extracellular microelectrodes in a 6 × 8 array in V4 in each cerebral hemisphere. The data presented here are from 9 days of recording. We recorded from a total of 68 single units and 588 multiunits. We did not find any significant differences

in the effect of attention on single and multiunits (see also Cohen and Maunsell, 2009) and many of the analyses presented the here required large simultaneously recorded neuronal populations, so single and multiunits are combined for all analyses. The type of task-based feature attention that we used differs from previous studies that manipulated feature attention by changing the visual stimulus outside the neuron’s receptive field (Hayden and Gallant, 2009, Martinez-Trujillo and Treue, 2004 and Treue and Martinez Trujillo, 1999). We directed the animals to pay attention to either orientation or spatial frequency, rather than one orientation versus another. Also, in our task, there were no visual differences between attention conditions during the period in which we analyzed responses. We focused all analyses on the stimulus presentation immediately before the change, when the stimuli were identical in every trial. The only difference between attention conditions was the location and type of stimulus change the animal was expecting.