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 percep tion of stimuli expected to cause widely varying degrees of olfactory and nasal trigeminal stimulation and to directly evaluate the possible role of olfactory nerve stimulation in nasal irritation sensitivity. During each of four identical test sessions, four anosmic and 31 normo smic participants were presented with a range of concentrations extend ing from peri-threshold for normosmics to supra-threshold for anosmics . For each session, odor (O) and nasal irritation (NI) sensitivities w ere summarized in terms of the concentrations required to produce four sensation revels ('iso-response' concentrations). Within-participant variation in these iso-response concentrations was <10-fold for 95% of normosmics, for both O and NI. For O but not NI, these apparent fluct uations in sensitivity were largely accounted for by the uncertainty s urrounding the iso-response concentrations calculated for each session . Anosmics exhibited minimal within-and between-participant Variation in NI and required, for all but the highest perceptual level, a higher concentration than almost all normosmics. Between-participant variati on, expressed in terms of 90% confidence interval widths, was similar to 0.5 log units for both O and NI for the highest perceptual level, b ut increased to similar to 0.8 and 1.8 log units, respectively, for th e lowest (peri-threshold) level. Our findings suggest that: (i) most a pparent variation over time in O sensitivity is actually a reflection of the uncertainty surrounding estimates of sensitivity obtained for e ach session; (ii) within-and between-participant variation in O sensit ivity is far less than is commonly reported; and (iii) low to moderate levels of Nt in normosmics are the result of relatively weak trigemin al stimulation combined with much greater olfactory activation.