Participants were seated in an IAC sound–attenuated booth using a chin and forehead rest, with a video camera monitoring participants’ compliance and behavior. While focusing on a fixation cross and breathing normally through the nose,2 H2S stimuli (10 ppm, Scott Speciality Gases, Plumsteadville, PA) at high (undiluted) and low (diluted to 50%) concentrations were delivered to the left or right nostril by a constant-flow olfactometer (OM2s, Heinrich Burghart GmbH, Wedel, Germany) through a Teflon tube inserted approximately 1 cm into the naris. Stimulus duration was 200 ms (not more than 50 ms rise time according to manufacturer specification). For any given session, the air stream at the exit of the olfactometer had a constant flow rate (about 8 l/min), temperature (the measured range was 381–39°C at the olfactometers head to approximate 37°C body temperature in the nasal cavity), and relative humidity (above 80%). Odors were presented in four blocks of 24 trials each, with a variable interstimulus interval (15–25 s). White noise of approximately 75 dB SPL was presented binaurally via Telephonics TDH–49P earphones to preclude hearing the switching valves.
