Proboscis extension reflex

Proboscis extension reflex (PER) is when a bee extends her proboscis (sticks out her tongue) as a reflex to antennal stimulation. It is evoked when a sugar solution is touched to a bee's antenna.

Use of PER
The proboscis extension reflex is part of a bee'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 bee 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.

How the PER learning paradigm works
As in classical conditioning, there are two steps in a PER experiment. The first step trains the individual to associate an unconditioned stimulus (US), such as a sugar reward, with a conditioned stimulus (CS), such as an odor. The two stimuli are paired in such a fashion that the bee associates the presentation of the US with the CS. 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 rewarded with the sugar to reinforce her response. After some number of reinforcements, the bee should have made the association between the odor and the sugar. The second step in the PER paradigm tests whether or not the association is learned. If the association of the US and CS has been learned, then a conditioned response (CR) should be elicited in the presence of the CS, even if the US is absent. This time, the odor (CS) is presented to the bee in the absence of the sugar solution (US). If the bee has learned the association, then she will extend her proboscis (CR) regardless of whether the sugar solution (US) is applied to her antennae.

PER in honeybees
The PER paradigm has been successfully used to investigate olfactory learning in honeybees. Honeybees show first-order conditioning (aka classical conditioning) by associating an odor with a sugar reward. Individual bees were placed securely to a tube with their head sticking out. Then, each bee was trained to associate odor with sugar water. Training took place by blowing a stream of odorant air towards the bee's face and immediately touching the antenna with a sugar droplet. Learning takes place when only the odorant air is blown, and the bee extends her proboscis even in the absence of a reward. After learning, honeybees can evoke the PER response approximately 90% of the time on the third trial. In addition, honeybees are also capable of second-order conditioning by 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.

PER in bumblebees
Although the majority of PER studies are performed on honeybees, there is at least one successful study of using PER on bumblebees. Although bumblebees are slower than honeybees at learning through a PER paradigm, they are still able to associate odor with a food reward by eliciting the proboscis extension even in the absence of a food reward. Once the association is learned, the bumblebees are able to evoke the PER response 85% of the time after 10 trials. Although difficult, successful PER learning can be studied in bumblebees.

PER and learning laterality
Recently, interesting findings in PER studies show laterality in olfactory learning in the two antennae i.e., one antenna is better at associative learning than the other antenna. In honeybees, individuals had either their right or their left antenna covered with a silicone sleeve, leaving the other antenna exposed. The bees that had their right antenna exposed were better at associating an odor with a food reward than bees that had their left antenna exposed. The same study also found that the right antenna has more olfactory receptors than the left antenna, a possible cause for this laterilized PER learning. However, other causes such as internal differences in the actual olfactory pathway or the central nervous system must not be ruled out just yet.