Odour concentration affects odour identity in honeybees

Abstract

The fact that most types of sensory stimuli occur naturally over a large range of intensities is a challenge to early sensory processing. Sensory mechanisms appear to be optimized to extract perceptually significant stimulus fluctuations that can be analysed in a manner largely independent of the absolute stimulus intensity. This general principle may not, however, extend to olfaction; many studies have suggested that olfactory stimuli are not perceptually invariant with respect to odour intensity. For many animals, absolute odour intensity may be a feature in itself, such that it forms a part of odour identity and thus plays an important role in discrimination alongside other odour properties such as the molecular identity of the odorant. The experiments with honeybees reported here show a departure from odour-concentration invariance and are consistent with a lower-concentration regime in which odour concentration contributes to overall odour identity and a higher-concentration regime in which it may not. We argue that this could be a natural consequence of odour coding and suggest how an 'intensity feature' might be useful to the honeybee in natural odour detection and discrimination.

Figure 1

Over a range of concentration of four orders of magnitude, odorants of the same concentration appear more similar than odorants of the same molecular identity. Data were pooled for three conditioning odorants (1-hexanol, 2-octanone and geraniol). (a) At the low conditioning concentration (0.0002 M), honeybees do not appear to differentiate the conditioned odorant (CS) (grey bar) from a novel low-concentration odorant (p=0.704). However, they respond significantly less to the high concentration (2.0 M) of the same conditioning odorant type as the CS; they also respond less to a novel odorant at the high concentration (2.0 M). (b) When the high concentration was used as the CS, honeybees appear to be able differentiate a novel, high-concentration odorant from the CS, but they respond more to the high concentration of a novel odorant than to the low concentration of the same odorant type as the CS or the low-concentration novel odorant. (Bars labelled with the same letter in each graph were not significantly different at α=0.05).

Figure 2

The intermediate concentration was most similar to the high concentration of molecularly identical odorants; the extent of the difference between intermediate and high concentrations was dependent on odorant type (p=0.050). Each graph represents the response to the intermediate concentration of the conditioned odorant (CS) (grey bar) versus the response to the high and low concentration of odorants molecularly identical to the conditioned odorant and versus the response to the intermediate concentration of a molecularly novel odorant. The conditioned odorant was: (a) 1-hexanol; (b) 2-octanone; (c) geraniol. For 1-hexanol and 2-octanone, the response to the intermediate concentration was significantly different from all test odorants. For geraniol, the response to the intermediate concentration was not significantly different from the high concentration (see table 1; bars labelled with the same letter in each graph were not significantly different at α=0.05).