Learning and memory in the American cockroach, Periplaneta americana: New behavioral paradigms for associative learning

Abstract

Although there is much information about insect associative learning, less is known about the underlying neural mechanisms. This is partly due to the lack of behavioral paradigms providing a suitable model for studying learning mechanisms at the level of individual neurons. This thesis describes the background to, and the demonstration of, two associative learning paradigms: visual associative learning and spatial learning. Both have been developed on the restrained cockroach so that later these methods can be employed in conjunction with electrophysiology. By projecting their antennae intermittently towards a position of potential food sources, cockroaches sample salient information. Here, this antennal behavior, called an "antennal projection response (APR)," is used to demonstrate long-term memory where an APR is elicited by a conditioning stimulus (CS: green light) paired with a spatially coincident odor (unconditioned stimulus: US). The acquired APR to the green light cue persists for up to 72 hours. Spatial learning is also a vitally important behavior in most animals that must remember locations of food and landmarks and that must navigate. Spatial learning abilities were here tested by observing APRs towards a cue, where the cockroach learns the position of a visual cue (CS) associated with a food odor (US), relative to the position of another visual stimulus in the contralateral visual field (the contralateral visual reference stimulus: ConRS). Memory of positional information, tested by altering the relative positions of the CS and ConRS, was investigated. Cockroaches showed significant APRs to visual cues not only when a position of the visual cue and spatial reference cue were exactly matched during training trials, but also during tests when the relative angles between the visual cue and spatial reference cue were matched but rotated around the head's vertical axis. When these angles were not the same as the angle used for training, the CS was not recognized. These results suggest that cockroaches employ two different mechanisms to find a food source: retinotopic matching and recognition of angular relationships between a source and landmark. The application of these paradigms to studies that could investigate possible neural mechanisms of these behaviors is discussed.