Differential expression of pseudoconditioning and sensitization by siphon responses in Aplysia: novel response selection after training.

Nonassociative training with a noxious unconditioned stimulus (US) applied to the head or tail of freely moving Aplysia caused a qualitative change in siphon responses to midbody test stimulation, so that the midbody test responses came to resemble the unconditioned siphon response (UR) to the US when tested 1 d after exposure to the US. Such a nonassociative, US-induced transformation of test responses into responses resembling the UR has traditionally been termed "pseudoconditioning." Short-term pseudoconditioning was compared to sensitization and to habituation in a reduced preparation that used a photocell to distinguish "head-type" siphon responses from qualitatively different "tail-type" responses. Transformation of test responses (pseudoconditioning) was observed only when the type of preexisting alpha response to the midbody test stimulus was different from the UR. Sensitization, defined as a US-induced enhancement of the alpha response to the test stimulus, was observed when the initial alpha response and the UR were of the same type. General sensory facilitation was excluded as a critical mechanism for pseudoconditioning by the observation that the same midbody test response could be transformed to either a head-type or tail-type response, depending on the site of the US, and by the observation that simply increasing the intensity of the midbody test stimulus in the absence of a head or tail US did not produce similar response transformations. These studies demonstrate pseudoconditioning in a preparation amenable to analysis at the level of identified neurons, and draw attention to a distinctive and widespread form of behavioral modifiability that has been neglected by investigators of learning.