Nonassociative learning processes affecting swimming probability in the seaslug Tritonia diomedea: habituation, sensitization and inhibition

Abstract

The role of nonassociative learning processes in determining whether or not a chemical stimulus will elicit the Tritonia diomedea swimming response was examined in a variety of conditioning experiments. Iterative presentation of a chemical stimulus resulted in a reduced swimming probability (SP). By the criteria of Thompson and Spencer (Thompson RF, Spencer WA. Psychol. Rev. 1966;73:16–43) and others, this iterative reduction of SP was concluded to be the result of habituation. Site-specificity and a below zero effect implicated sensory pathways in habituation memory storage. The iterative reduction in SP was reversible, confirming that a sensitization-like process can also influence SP. It was further concluded that a short-term decrement in swimming cycle number was most likely due to a constraint in the effector pathway. Experience with a tactile stimulus had a long-lasting decremental effect on SP. This heterostimic reduction of SP was amplified in a multistimic paradigm that included both chemical and tactile stimuli during training. The chemical stimuli alone did not alter SP in this experiment. Multistimic reduction lasted for a week and was reversed temporarily by an excitatory chemical stimulus. The long-lasting reduction of SP by tactile stimulation appears to be the result of a novel nonassociative inhibitory process, which was distinguished from other learning processes by its duration and specificity. A total of three distinct learning processes are postulated to account for the role of simple types of experience in determining SP in Tritonia: habituation, sensitization, and nonassociative inhibition.