A sniff cue (red cross-hair) was then displayed for 667 ms, and recurred with a stimulus-onset asynchrony of 2 s to prompt additional sniffs, as necessary. On the open-sniff trials, subjects made a binary choice with the left or right keyboard arrow once they had accumulated sufficient evidence that clove or lemon was dominating the mixture. Subjects were instructed to emphasize accuracy, ensuring that a decision would be made only when sufficient evidence had been accumulated to the

criterion threshold. This was the primary instruction Afatinib chemical structure given to the subjects. They were incidentally reminded that upon reaching their decision, they should respond by button press as quickly as possible, so that recorded decision times closely reflected the time that they reached their decision. At the end of each trial, subjects also made a perceptual rating on a visual analog scale ranging from pure clove to pure lemon, by moving a cursor from the midpoint of this continuum (representing

equal proportions of the two odors). For the fixed-sniff trials, this estimate yielded binary choice measures according to which side of the midpoint the rating fell on. The next odor was presented 18 s after the end of the previous odor presentation, to minimize olfactory habituation. Binary decisions, analog ratings, and odor presentation times were recorded for each trial. Olfactory and visual stimuli presentations were controlled using Cogent2000 (http://www.vislab.ucl.ac.uk/cogent.php). This was the same as Experiment 1, except that all trials were of the open-sniff www.selleckchem.com/products/MLN-2238.html type. Because this experiment the took place in an MRI scanner, subjects responded using one of two button boxes held in either hand, one representing clove, the other lemon (hand side counter-balanced across runs). These buttons were also used to make the perceptual rating along a visual analog scale. Subjects were not told the outcomes of their decision, to prevent cognitive feedback or reward processing from confounding the neuroimaging findings. Sniffs were visually cued, as before, but were back-projected from a computer monitor

onto a tilted mirror that was affixed to the MRI headbox in front of the subject’s eyes. The letters “L” and “C” (lemon and clove) were presented on opposite sides of the screen to indicate which side represented which odor, and this was counterbalanced across subjects and sessions. Sniff rate was again set at two seconds in order to time-lock this to the data-acquisition rate of the MRI scanner (2,000 ms; see below). Subjects completed two runs of 36 trials on 2 consecutive days (four runs total) to minimize subject fatigue and odor habituation. Each of the nine mixtures was presented eight times each day (144 trials in total over 2 days), and trials were arranged in pseudorandom order such that every mixture preceded every other mixture one time to minimize effects of mixture sequence.