Slight changes in the direction of motion occurred at all three locations, but the trial ended without reward if the animal responded to a change at an uncued location. To measure the effect of normalization

for each neuron (Figure 1A), we collected data while the animal was cued to attend to the location outside of the receptive field, so that spatial attention did not modulate the neuron’s rate of firing. To prevent feature attention from modulating the response, the Gabors presented at the cued location always drifted in the same direction, which was intermediate between the preferred and null directions of the neuron. While attention was directed outside the receptive field, series of Gabors were presented at the two locations within the receptive field. Whenever a pair of Gabors appeared in the receptive field, one drifted in the preferred direction of the DNA Damage inhibitor neuron and the other drifted in the null direction, PCI 32765 but the locations of the preferred and null stimuli were pseudorandomly selected on each presentation. Additionally, each receptive field stimulus had a pseudorandomly selected contrast of 0%, 50%, or 100%. Using 0% contrast meant that stimuli sometimes briefly

appeared alone in the receptive field. The stimulus presentations were short (200 ms; Figure 1C) so that the animal did not have time to adjust its attention based on the contrast or number of Gabors that appeared from (Williford and Maunsell, 2006, Lee and Maunsell, 2009 and Lee and Maunsell, 2010). To measure the effect of spatial attention for each neuron (Figure 1B),

the animal’s attention was directed to one of the two locations within the receptive field. The drifting Gabors within the receptive field were independently and pseudorandomly set to a contrast of 0% or 100% on each presentation. One Gabor within the receptive field drifted in the preferred direction and the other drifted in the null direction. For most neurons (72 of 117) drift direction was pseudorandomly assigned to the receptive field locations for each short stimulus presentation. If the animal responded to a direction change from preferred to null or vice versa (i.e., 180° direction change) the trial was terminated without reward. For the remaining neurons (45/117) the locations of the preferred and null directions were fixed, but results from those neurons were not significantly different. By presenting the Gabors at 0% or 100% contrast, we could measure attention with one or two stimuli in the receptive field. Different MT neurons showed different degrees of normalization. Figure 2A shows responses from a neuron with pronounced normalization. The average response to a preferred direction alone (in either receptive field location; thick black line) was substantially reduced when a null stimulus was added to the other receptive field location (dashed line).