New connections may be memorable.

New connections may be memorable

It seems logical that the learning process would involve the construction of new circuitry inside the brain. Scientists have been hard pressed, however, to pinpoint learning-related anatomical changes in neural systems.

New research by Craig Bailey and his colleagues at Columbia University's College of Physicians and Surgeons in New York City strengthens the case for the nerve-circuit model to learning. Bailey last year presented the first solid evidence of learning-associated circuit formation in the slug-like "sea hare,' Aplysia. This creature is a popular model for neurobehavioral experiments because its neural circuitry is well understood and because researchers can train it in ways that allow testing for short- and long-term memory.

The new research, presented last week in New Orleans at the meeting of the Society for Neuroscience, confirms previous evidence that the number of synapses, or neural interconnections, increases as Aplysia undergo training to respond to particular stimuli. Moreover, these synapses demonstrate an increased ability to secrete transmitter substances that are critical to nerve firing. The latest research shows for the first time that a gradual decrease in the number of synapses parallels the behavioral changes observed as an Aplysia eventually "forgets' how to respond to conditioned stimuli. The study suggests that nerve-circuit patterns are constantly changing as learning, remembering and forgetting occur.

"Learning,' Bailey concludes, "may resemble a process of growth and differentation,' involving gene regulation, nerve protein synthesis and consequent rerouting of circuitry.

Meanwhile, William Greenough, of the University of Illinois College of Medicine in Urbana-Champaign, has been performing experiments to see if such neural changes are in face due to learning. Some scientists have argued that the increased number of synapses might simply be due to a general increase in hormone levels or metabolism caused by the extra stimulation and handling that memory training involves. Working with rats, Greenough showed that given equivalent amounts of handling and exercise, rats that had to learn more complicated tasks developed more complicated neural connections.

The precise cellular processes involved in learning and memory are still not known. However, Greenough says, "The [learning-associated] mechanisms brain cells use may not be unique to memory. Natural processes that the cells use for other things may be coopted for use in memory.'