Odor and blank air stimuli were presented in 8 blocks of 20 trials each (160 total trials), with a variable stimulus onset asynchrony (SOA 14.5–20.5 s). A trial was initiated with the foveal presentation of digits counting down in 1-s intervals from 3 to 1. This was followed by the display of a smell icon which remained on the screen for 5.5 s, with an odor or blank air presented within 2 and 4 s. The smell icon was replaced by the question ‘Did you smell anything?’ above a picture of two foot pedals labeled ‘Yes’ and ‘No,’ prompting participants for 2.5 s to indicate whether or not they had detected an odor by pressing the corresponding foot pedal with the left or right foot. A variable delay interval warranted an average SOA of 17.5 s. Each of the three H2S concentrations and blank air were presented 40 times, and for any given block, stimuli were delivered to either the right or left nostril (4 blocks each in a counterbalanced order). Stimuli were presented in a pseudorandomized sequence based on
