Learning and Memory in Aplysia

Aplysia is a marine snail. Aplysiids were given the common name of 'sea hare' by the Ancient Greeks because of their supposed resemblance to the European hare (the "ears" are certainly reminiscent, see picture).

juvenile Aplysia

The question is, why should one study this animal for learning and memory? It sure would have problems to do pattern matching...

Aplysia's orienting and homing capabilities seem rather underdeveloped as well. But in any case, with a snail's speed, it wouldn't really be suited to study maze navigation anyway...

So what Aplysia behavior could one use to train it? The main focus on Aplysia neurobiology has been on classical conditioning of the defensive withdrawal reflex. A light tactile stimulus that would normally not produce any defensive reflex in the animal, is repeatedly paired with a strong noxious stimulus, usually an electric shock. After a few pairings, the animal shows the defensive reflex to the light tactile stimulus. Still, classical conditioning has been studied in a variety of animals, why Aplysia? The nice feature that makes Aplysia so attractive for neurobiologists is its large brain cells (neurons). The cell body of one neuron can measure up to 1mm in diameter, which makes it relatively easy to study the physiology of these cells to find out how they accomplish learning.

For operant conditioning, on the other hand, one needs a spontaneous behavior that can be modified by reinforcement, so a reflex is not suitable. Well, about the only thing these animals do in their natural environment is to eat and reproduce. Naturally, their feeding behavior has received some attention from neurobiologists (including me) interested in operant conditioning (see videos of the Aplysia biting behaviour).

The following documents describe our research on operant conditioning of Aplysia's feeding behavior on various levels:

Operant conditioning of biting behavior in Aplysia. - my personal account of the development of an operant conditoning experiment using Aplysia biting behavior.
Operant Reward Learning in Aplysia: Neuronal Correlates and Mechanisms. - a technical paper published 2002 in the journal Science, describing the biting paradigm and its neuronal underpinnings. Also available as PDF offprint.
Catharine Rankin wrote a very nice news article on our 2002 Science paper: A Bite to Remember. (in PDF)
The University of Texas' press release for our 2002 Science paper: Scientists Connect Brain Cell Changes to Reward Learning.
Even the journal Nature stepped in and had Thomas Carew write a 'News and Views' article on our 2002 Science paper: Understanding the consequences. (in PDF)
In the wake of the Science article, I've been invited to write two review articles. The first one was written along the lines of this page on Operant Reward Learning in Aplysia (in PDF). The second one was more general and encompassed Operant Conditioning in Invertebrates (in PDF).
"Extending In Vitro Conditioning in Aplysia to Analyze Operant and Classical Processes in the Same Preparation" was the logical next step to further increase the usefullness of this paradigm (PDF offprint).