Proboscis extension reflex

Proboscis extension reflex (PER) is the extension by an insect with an extendable proboscis (e.g. a bee or fly) of her proboscis (sticking out of her tongue) as a reflex to antennal stimulation. It is evoked when a sugar solution is touched to a bee's antenna.

The proboscis extension reflex is part of an insect's feeding behavior. When the antenna is stimulated by sugar water, the proboscis automatically sticks out to drink. This reflex response can be used to study insect learning and memory in the context of foraging. The PER paradigm is most commonly used in associative learning experiments in honeybees and bumblebees because of the ease in using PER in simple Pavlovian conditioning.

There are two steps in a PER experiment. The first step trains the individual to associate a conditioned stimulus (CS), such as an odor with an unconditioned stimulus (US) such as a sugar. For example, the bee is presented with an odor (CS) and an application of the sugar (US) solution to its antennae. She reflexively extends her proboscis, and she is immediately fed with sugar to reinforce her response. After some number of pairings of the CS and US, the second step in the PER paradigm tests whether or not the association has been learned. The odor (CS) is presented to the bee in the absence of the sugar solution (US), and the association is confirmed if the bee extends her proboscis to this CS alone. The procedure combines classical and operant methods: the pairing of odor (CS) with sugar to antenna (US) is the classical segment; the subsequent reinforcement by feeding with sugar is the operant segment.

The PER paradigm has been successfully used to investigate olfactory learning in honeybees. Experiments by Bitterman, used first-order classical conditioning to associate an odor with a sugar reward. Individual bees were placed in a tube with their head sticking out. Then a stream of odorant blown towards the bee's head was immediately followed by touching the antenna with a sugar droplet. After only three such trials, the odor alone caused the bee to extend its proboscis approximately 90% of the time. Bees also showed second-order conditioning, learning to associate a second odor with the original odor. The PER paradigm has also been used in honeybees to study motion learning, thermal learning,habituation, and reversal learning.

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