Metrics of odorant dissimilarity. Labeled magnitude scale vs magnitude estimation

Magnitude estimation (ME) has long been the standard rating tool for stimulus intensity because of the simplicity and generalizability of the resultant power function relationships. However, the choice of a scaling procedure for rating complex stimuli (odorant) dissimilarity must be based on a different set of criteria due to an inability to evaluate relationships against a physical continuum. Recently, chemosensory labeled magnitude scales (LMS) have been developed, which produce ratings with many of the same properties as ME. The current study sought compare LMS to ME as metrics for describing the magnitude of perceived odorant dissimilarity. Forty subjects rated pair-wise odorant dissimilarity with either a LMS or ME. Each subject evaluated two sets of odorants with differing ranges of dissimilarity (high contrast and low contrast) in order to compare the sensitivity of the two measures to possible contrast convergence. The ME ratings were normalized such that the two metrics produced equal means. The scales produced similar ratings of odorant dissimilarity (r = 0.92) and showed small but similar effects of contrast convergence. However, the coefficient of variation of dissimilarities rated with ME was 2.8 times that of dissimilarities rated with the LMS. Subjects accepted the LMS without the usual anxiety that accompanies first time users of ME. LMS provide stable ratings of odorant dissimilarity while preserving the inferred ratio scale properties of ME.