Human responses to propionic acid. I. Quantification of within- and between-participant variation in perception by normosmics and anosmics

The objective of this study was to fully characterize normosmic perceptionof stimuli expected to cause widely varying degrees of olfactory and nasaltrigeminal stimulation and to directly evaluate the possible role ofolfactory nerve stimulation in nasal irritation sensitivity. During each offour identical test sessions, four anosmic and 31 normosmic participantswere presented with a range of concentrations extending from peri-thresholdfor normosmics to supra- threshold for anosmics. For each session, odor (O)and nasal irritation (NI) sensitivities were summarized in terms of theconcentrations required to produce four sensation levels ('iso-response'concentrations). Within-participant variation in these iso-responseconcentrations was < 10-fold for 95% of normosmics, for both O and NI.For O but not NI, these apparent fluctuations in sensitivity were largelyaccounted for by the uncertainty surrounding the iso-responseconcentrations calculated for each session. Anosmics exhibited minimalwithin- and between-participant variation in NI and required, for all butthe highest perceptual level, a higher concentration than almost allnormosmics. Between-participant variation, expressed in terms of 90%confidence interval widths, was approximately 0.5 log units for both O andNI for the highest perceptual level, but increased to approximately 0.8 and1.8 log units, respectively, for the lowest (peri- threshold) level. Ourfindings suggest that: (i) most apparent variation over time in Osensitivity is actually a reflection of the uncertainty surroundingestimates of sensitivity obtained for each session; (ii) within- andbetween-participant variation in O sensitivity is far less than is commonlyreported; and (iii) low to moderate levels of NI in normosmics are theresult of relatively weak trigeminal stimulation combined with much greaterolfactory activation.