Comparing and validating different methods for olfactory threshold measurement in dogs

Sensory thresholds- the limits at which an organism can detect the presence of a given stimulus- can provide insight into the evolution of sensory abilities and the relative salience of different stimuli to a species. Understanding dog olfactory thresholds can further benefit the training of detection dogs by identifying odors that are salient and detectable. Current methods to measure olfactory thresholds vary widely and often lack controls or external validation. We aimed to (a) compare three commonly used procedures to measure olfactory thresholds in dogs, (b) evaluate how cognitive fatigue and/or sensory adaptation could be potential confounds, and (c) validate threshold methods by comparing them to a reference odor and empirical measurements of our odor delivery system. We first compared three differing threshold measurement methods using 1-bromooctane, a common training odorant for detection dogs: (1) random presentation of different concentrations; (2) a blocked descending procedure where an initial odor concentration is presented in a block of 10 trials with decreasing half-log concentrations presented in subsequent blocks until dogs reach chance performance; and (3) the descending staircase procedure. We found that a blocked procedure provided the lowest threshold values, with a mean dog threshold for 1-bromooctane equivalent to a ∼10^-5 dilution in mineral oil. Control tests indicated our findings were not driven by fatigue or sensory adaptation. We also found that the blocked procedure generated olfactory thresholds for amyl acetate- a volatile with multiple published olfactory thresholds for dogs- at 129 ppt, at the lower range of prior work, validating the descending block approach. Instrumental analysis found that our olfactometer delivered odor in decreasing concentrations as predicted. We identified a valid method for assessing dog olfactory thresholds that can guide future research on this topic.