We use classical conditioning of the honeybee (Apis mellifera) proboscis extension reflex with a visual (A) and an olfactory (X) conditioned stimulus in a blocking paradigm. Typically, learning about one element (X) of a compound (AX) is decreased (blocked) if the other component (A) has previously been rewarded alone. Our results show that visual pretraining did not produce blocking in honeybees: instead, forward pairings of A with a reward increased subsequent learning about X relative to a backward pairing control. This finding violates the independence assumption, which holds that elements of inter-modal compound stimuli change associative strength independently of each other. Furthermore, it is at odds with common theories of conditioning that predict blocking and assume that the elements of a compound stimulus rely on one common internal reinforcing signal. Taking the functional anatomy of the honeybee brain into account, we suggest that vision and olfaction may not rely on the same internal reinforcing signal; compound interactions might thus reflect the wiring of the honeybee nervous system and the biological significance of different sensory modalities during natural behaviour. [References: 29]
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