Dissecting learning in Aplysia.

Dissecting learning in Aplysia

To understand how a machine works, an engineer might take it apart. One way for a neurobiologist to discover the mechanisms underlying various forms of learning is to trace how these are put together in the nervous system as a young animal matures. Thomas J. Carew at Yale University and his co-workers have applied this developmental approach to the study of nonassociative learning in Aplysia, a marine snail whose adult form became famous in pioneering studies of the molecular basis of memory (SN: 1/22/83, p.58).

Carew's group looked for the onset of three kinds of learning --habituation, dishabituation and sensitization--by testing how readily Aplysia contracts its siphon when this organ is sprayed with a jet of water. After a series of such stimuli, the snail habituates, ignoring the water jets. If the researchers then shock Aplysia's tail, it again becomes responsive to water jets. This is dishabituation. Sensitization occurs when a unstimulated animal first gets a tail shock, prompting its siphon to contract even more readily than usual.

Conventional thinking has held that sensitization and dishabituation are part of the same process. But Carew has now shown that the two are separate because they emerge at different times in development. Moreover, he discovered a fourth kind of learning, called inhibition, which had not been seen in adults until now because it is masked by other processes.

The researchers are now working to uncover the cellular and neural mechanisms that are "turned on' when Aplysia begins to show each kind of learning. Carew notes that the beginning of sensitization is accompanied by an eight-fold increase in the number of cells in the nervous system. "Either there's new circuitry being added [for sensitization],' he says, "and/or the signal that's [triggering the proliferation of new cells] is also turning on sensitization.'