Mice and humans evaluate odor stimulus strength using common psychophysical principles

Abstract

Sensory systems translate physical stimuli from the environment-such as light, sound, or chemicals-into signals that the brain can interpret. Across these systems, the amplitude of a stimulus is represented by its perceived intensity. Although previous research has extensively studied how the brain represents physical stimuli, less is known about how it represents perceptual variables such as stimulus intensity. This is primarily due to the difficulty in measuring perceptual responses in animal models, where neural recordings are more accessible. In this study, we use mouse olfaction as a model system to develop a framework for measuring perceived odor intensity. We begin by employing a two-odor concentration classification task to demonstrate that both mice and humans assess stimulus amplitude using a common perceptual scale. We then show that this scale corresponds to intensity. Finally, we apply this method to determine isointense concentrations of different odorants in mice. Our approach offers a powerful tool for testing hypotheses about the neural mechanisms underlying perceived odor intensity, potentially enhancing our understanding of olfactory processing and its neural substrates.

Figure 1.. Mice classify odorant concentrations using a common scale.

A. Task design. Mice (n=4) were trained to categorize eight concentrations of a single odorant by licking the left/right ports for the four lowest/highest concentrations. Concentrations were distributed uniformly on a logarithmic scale, spanning one order of magnitude. Correct responses were rewarded with a drop of water. B. Performance at the final training session with ethyl tiglate (ET, red) and the first session with novel odorants ethyl butyrate (EB, blue) and 2-heptanone (2H, green). The x-axis is centered on the logarithm of odorant concentration in parts per million (ppm) and extends ±0.5 units in logarithmic space. C. Overall performance across all three odorants. Data points show means ± SD across mice. Gray lines and symbols correspond to performance of individual mice. No significant differences were found across odorants (Friedman chi-squared test, p = 0.779). D. Two-odorant task design. Mice were rewarded for licking the left/right ports for the four lowest/highest concentrations of both odorants. E. Mean psychometric curves (n = 4) for the two-odorant task with ET (red) and EB (blue) using the same concentration ranges (midpoint concentration range for both odorants is 10^1.07 ppm). Data points show means ± SEM. F. The same as E, but with a shifted concentration range for EB. G. Schematic of behavioral task performance under a common intensity criterion. If the concentration ranges of the two odorants correspond to different perceived intensity ranges (i), applying a common criterion (ii) would yield performance differences determined by relative differences in intensity perception between stimuli (iii). ${\mathrm{\Delta }}_{\mathit{D}\mathit{B}}$ indicates the relative difference between decision boundaries for individual odorants.

Figure 2.. Humans also apply a shared intensity criterion across odorants.

A. Two-odorant classification performance in human participants (n=17) for a fixed set of eight acetal (AC) concentrations and eight 2-heptanone (2H) concentrations. The midpoints of the concentration ranges were −4.49 (2H) and −4.31 (AC) where values represent the logarithm of odorant concentration (vol/vol). Data points show means ± SEM. B. Same as A, with the 2H concentration range centered at −4.74. C. Same as A and B, with the 2H concentration range centered at −4.99. D. Performance mismatch index (${\mathrm{\Delta }}_{\mathrm{D}\mathrm{B}}$) plotted against the difference between 2H and AC concentration ranges. The 2H range was centered at 15% (EC15) of its maximum perceived intensity. E. Same as D, but with the 2H concentration range centered at 50% (EC50) of its maximum perceived intensity. F. Perceived intensity as a function of concentration. Data points show means ± SD. Violin plots show the mean perceived intensities of concentration pairs that yielded equivalent performance (${\mathrm{\Delta }}_{\mathrm{D}\mathrm{B}}=0$) at the EC15 and EC50 concentration ranges. n.s. indicates p > 0.05, * indicates p < 0.05 (two-sided bootstrap resampling).

Figure 3.. Isointensity is transitive in mice, enabling matched concentration estimates across odorants.

A. Schematic illustrating the expected transitive property of intensity-matched concentrations. B-D. Performance mismatch index (${\mathrm{\Delta }}_{\mathit{D}\mathit{B}}$) as a function of concentration range differences for three odor pairs: (B) ET-EB, (C) ET-2H, and (D) EB-2H. Solid lines show linear fits across mice. Vertical dotted lines show the projection of ${\mathrm{\Delta }}_{\mathrm{D}\mathrm{B}}=0$ from the regression line to the $\mathrm{\Delta }\mathit{l}\mathit{o}\mathit{g}\mathit{C}$ axis. E. Intensity-matched concentration estimates for three odorants at low, medium, and high reference concentrations. Intensity-matched concentrations of 2H were estimated twice—once using ET as reference, and once using EB—to test transitivity.