Here, we explored the latter issue by recording the responses of dlPFC neurons of two macaque monkeys during a task that yielded measurable changes in the animals’ behavioral performance at

filtering out a target from a distracter. The experimental design was based on the previous observation that when comparing the ranks of two stimuli within an ordinal scale (e.g., numbers or quantities), humans and monkeys respond faster and more accurately the greater the interstimulus ordinal distance (distance effect; Buckley and Gillman, 1974, Dehaene et al., 1998, Jou and Aldridge, 1999, Moyer and Landauer, 1967 and Nieder et al., 2002). We hypothesized that when monkeys select and sustain attention on a target stimulus that differs

in ordinal rank from a nearby distracter, changes in the animals’ ability to do so would be accompanied by corresponding changes in the selleck compound activity of dlPFC neurons. We found that animals better detected changes in the target as the ordinal distance to the distracter increased (distance effect). More importantly, neurons in the dlPFC better filtered out the target from the distracter through their response rate as ordinal distance between the two stimuli increased. The latter effect was due to a gradual suppression of responses to distracters as a learn more function of ordinal distance to the target. We trained two adult monkeys (Macaca mulatta, Se and Ra) to hold gaze on a fixation spot at the center of a projection screen, and to attend to one of two moving random dot patterns (RDPs) appearing to the left and right of the spot. The dots in the two RDPs moved in the same direction

but differed in their color ( Figure 1). The attended Carnitine palmitoyltransferase II (target) and ignored (distracter) RDPs were defined according to a color/rank-order selection rule (gray < pink < green < blue < red < turquoise). The animals were rewarded for releasing a button after a change in the target’s direction of motion while ignoring similar changes in the distracter (see Figure 1 inset and Experimental Procedures). Within 3–5 months of training, both animals reached stable performances in the task. First, we tested the hypothesis that they did so by learning, from the pattern of hits and errors, the position of the different colors in the ordinal scale according to our color/rank-order selection rule. As an alternative hypothesis, the animals may have learned, for each color pair, which RDP was the target and which one the distracter. The former hypothesis predicts a distance effect in the pattern of reaction times and proportion of correct button releases (hits). The latter predicts no systematic relationship between reaction time and proportion of hits, and rank/ordinal distance between the colors. In animal Se, we found that the hit rate ((number of hits − number of false alarms)/number of trials) increased (p < 0.