Generalization of habituation and intrinsic sensitization in the leech

Using the shortening reflex of the medicinal leech Hirudo medicinalis we ex amined stimulus generalization of habituation learning. Preparations receiv ed mechanosensory stimulus at two positions on the leech body wall, one sit e used to carry out habituation training and a second novel site to test fo r generalization of habituation. After training, the specific mechanosensor y neurons activated by each stimulus were assessed using intracellular reco rdings. As expected, the closer the two sites were to each other, the great er the degree of generalization of habituation at the novel site and the mo re sensory cells were shared. However, a form of behavioral facilitation wa s observed at the trained site that resembled behavioral sensitization, but differed from the standard sensitization process in several respects. (1) Facilitation was induced by stimulation of the novel site before habituatio n training at the trained site, although the stimulus intensity at the nove l site was equivalent to the training stimuli and was not the strong, noxio us stimuli that normally induce sensitization. (2) The magnitude of the fac ilitating effect was proportional to the proximity of the novel and trained stimulation sites. (3) Although behavior at the trained site was facilitat ed, behavior at the novel site was habituated, indicating that the induced behavioral facilitation did not generalize throughout the animal, as normal ly occurs during sensitization, but was Limited to a single stimulus-respon se pathway.