Olfactory blocking and odorant similarity in the honeybee

Abstract

Blocking occurs when previous training with a stimulus A reduces (blocks) subsequent learning about a stimulus B, when A and B are trained in compound. The question of whether blocking exists in olfactory conditioning of proboscis extension reflex (PER) in honeybees is under debate. The last published accounts on blocking in honeybees state that blocking occurs when odors A and B are similar (the "similarity hypothesis"). We have tested this hypothesis using four odors (1-octanol, 1-nonanol, eugenol, and limonene) chosen on the basis of their chemical and physiological similarity (experiment 1). We established a generalization matrix that measured perceptual similarity. Bees in the "block group" were first trained with an odor A and, in the second phase, with the mixture AB. Bees in the "novel group" (control group) were first trained with an odor N and, in the second phase, with the mixture AB. After conditioning, bees in both groups were tested for their response to B. We assayed all 24 possible combinations for the four odors standing for A, B, and N. We found blocking in four cases, augmentation in two cases, and no difference in 18 cases; odor similarity could not account for these results. We also repeated the experiments with those six odor combinations that gave rise to the similarity hypothesis (experiment 2: 1-hexanol, 1-octanol, geraniol) and found augmentation in one and no effect in five cases. Thus, blocking is not a consistent phenomenon, nor does it depend on odor similarity.

Figure 1.

The four odors used in experiment 1: (1) 1-octanol, (2) 1-nonanol, (3) limonene, and (4) eugenol. (A) Molecular structures. (B) Glomerular activation patterns (neural olfactory coding) at the level of the antennal lobe, the primary olfactory neuropile of the bee brain (courtesy of G. Galizia) (for explanations about the optophysiological measurements of antennal lobe activity, see Galizia and Menzel 2000). (C) Molecular structures of the odors used in experiment 2 (same as in Hosler and Smith 2000).

Figure 2.

Experiment 1: Behavioral generalization matrix. The percentage of PER in the single extinction test is represented for the four odors tested as a function of the trained odor. In the four cases in which the odor tested corresponded to the odor trained (e.g., training with 1-octanol, test of 1-octanol), the number of bees per column is 60. In the other cases, the number of bees per column is 20.

Figure 3.

Experiment 1 (A) Generalization between odor pairs. Bars which do not differ at 0.05 are indicated with same letter: O, 1-octanol; N, 1-nonanol; L, limonene; and E, eugenol. (B) The four triads of odors used in the blocking experiments and the generalization levels between odors within each triad. A double-headed arrow indicates symmetric generalization. A single-headed arrow indicates asymmetric generalization. Thick line indicates higher generalization; thin line, intermediate generalization; and dashed line, low generalization.

Figure 4.

Experiment 1. Global analysis of blocking (pooled data for training and test). (Left) Percentage of PER in the six conditioning trials of the first training phase (preconditioning with a single odor, A or N). (Middle) Percentage of PER in the six conditioning trials of the second training phase (compound conditioning with AB). §Significant difference between sixth preconditioning trial and the first compound-conditioning trial. (Right) Percentage of PER in the extinction test. Response to B was significantly higher in the novel than in the block group.

Figure 5.

Experiment 1: Blocking and odor similarity. Odor combinations were pooled according to their similarity (high, intermediate, and low) as established through the generalization matrix. (Left) Percentage of PER in the six conditioning trials of the first training phase (preconditioning with a single odor, A or N). (Middle) Percentage of PER in the six conditioning trials of the second training phase (compound conditioning with AB). (Right) Percentage of PER in the extinction test.