Discrimination of Pairs of Chemosensory Stimuli in Relation to Respiration

Are paired chemosensory stimuli with very short interstimulus-intervals (ISIs) more often perceived as distinct events when delivered synchronously with inspiration than when delivered asynchronously?

Twin stimuli (H₂S, phenylethyl alcohol [PEA], CO₂) were repeatedly administered intranasally to 40 participants using a constant-flow olfactometer, synchronized with inspiration, at varying ISIs. In the first part of the study, participants indicated whether they perceived one or two stimuli. In the second part, event-related potentials (CSERP) were recorded from 20 participants using only H₂S stimuli with very short ISIs (0.6-1.6 s). The CSERPs were then qualitatively and quantitatively evaluated based on whether stimuli were perceived or not.

The perception rate of twin stimuli increased with increasing ISI. A plateau in the discrimination of separate twin stimuli, averaging 90%, was observed with CO₂ or H₂S stimulation at ISIs between 1.6 and 3.0 s, respectively, up to 9.0 s. For PEA stimulation, a plateau of approximately 70% was observed between 3.0 and 9.0 s. CSERP detection rates were 100% for both perceived first and second twin stimuli, while non-perceived second stimuli showed a 70% rate. The latencies of the CSERPs did not differ across the three conditions. The amplitudes were larger for non-perceived second stimuli than for perceived first and second stimuli.

Predominantly inspiration-synchronous stimulation allows higher detection of responses at both psychophysical and electrophysiological levels, even at very short ISIs. These findings may help to reduce the duration of clinical experiments and electrophysiological assessments of olfactory function.