Short- and Long-Term Habituation of Photonegative and Exploratory Responses in the Flatworm Planaria (Dugesia)

Two experiments address the habituation of photonegative and exploratory responses in the flatworm planaria (Dugesia). Planarians possess a well-documented photonegative response; Experiment 1 showed that repeated exposures to a bright light source with short inter trial intervals (ITIs) within 1 experimental session gradually weakens the unconditioned photonegative response. In addition, it was found that presentation of an unexpected arousal-increasing stimulus (dropped water or a shock) temporarily re-establishes the photonegative response. Experiment 2 addressed the development of long-term habituation; we recorded the locomotor activity of the animals exposed to an inescapable bright light. Experiments 2A and 2B showed that planarians develop long-term habituation but only when they were trained in relatively novel contexts; when they were trained in familiar contexts (in surfaces similar to the ones in the home) the development of habituation was severely impaired. The results are discussed by reference to the theory of short- and long-term habituation put forward by Allan R. Wagner (Wagner, 1976), highlighting the impact that this theory has had in the research of invertebrate learning.