Jan. 27, 1998 Pavlov trained his famous dogs to associate food with the ringing of a bell, so that they came to salivate when they heard one whether or not food was present. In 1968, learning researcher L. J. Kamin noticed a curious phenomenon that occurs with animals trained in this manner.
If, after thorough bell training, another stimulus is added -- a light is flashed at the same time the bell is rung and food is presented -- the animals will not learn to associate the light with the food. Even after extensive training, flashing the light won't produce the salivation response.
This phenomenon, called blocking, is believed to regulate the process by which animals and humans learn from their environment, by preventing them from being distracted by erroneous or redundant signals.
Blocking has now been given a firm physiological basis by experimenters working at the University of Southern California. Neuroscientist Jeansok J. Kim, Ph.D., now at Yale University, collaborating with Richard F. Thompson, Ph.D., a longtime researcher in the field, has identified signals going from the cerebellum to a brain structure known as the inferior olive as the cause of the blocking. A report on the work appears in the Jan. 23 issue of the journal Science.
Instead of the dogs used by Pavlov, the work described in Science used a now-standard conditioning regime in which rabbits learn to associate an external stimulus -- a tone or light -- with a puff of air on an eye. After training, the tone or light causes the rabbit to blink, even without the puff of air.
Years of experimentation, including much previous work done in Dr. Thompson's laboratory, have established that this kind of associative learning takes place in the cerebellum, the round structure at the back of the brain, and specifically involves certain cerebellar cells, called Purkinje cells.
A nearby structure called the inferior olive (named because of its shape) connects to the Purkinje cells. These links are thought to carry information about the unconditioned stimulus (i.e., the air puff as opposed to the tone) to the cerebellum.
The olive is also linked to the cerebellum by projections from the cerebellum, consisting of nerve cells which are inhibited from firing by a specific neurochemical called GABA. Neuroscientists have theorized that these connections could carry an inhibitory message to the olive that prevents the olive from contributing to a new association once learning has taken place.
This theory has now been given strong support by the experiment carried out by Dr. Kim, formerly a post-doctoral researcher at USC, now an assistant professor of psychology at Yale; Dr. Thompson, holder of the William M. Keck Chair in Biological Sciences and Psychology in the USC College of Letters, Arts and Sciences; and USC post-doctoral student David J. Krupa.
By surgically implanting a microsyringe in the olive to inject a specific chemical antagonist which blocks the action of GABA, the researchers were able to selectively turn off the action of the GABA-sensitive cerebellum-olive processes in living animals.
The results of this with regard to blocking were simple and dramatic: animals in which the GABA connection to the olive had been chemically severed showed no blocking effect.
In the experimental procedure, animals were first trained to associate a puff of air with a tone. When this training was established -- so the rabbits blinked whenever they heard the tone -- the animals were trained further by exposing them to a tone and a flash of light, coupled with a puff of air. Half the rabbits received this second training while having the GABA antagonist present in the cerebellar connections to the olive; the other half received neutral, control injections of artificial cerebrospinal fluid (ACSF).
Simultaneously, a separate group of rabbits who had never received tone training were trained with pulses of light only.
In trials, the control group who received only ACSF injections showed classic blocking behavior: they did not learn to associate the light with the puff of air. However, those injected with the GABA blocker did make this association, despite their previous training -- in fact, the performance of these animals was "not statistically different" from that of the never-trained-with-tones rabbit group.
In their paper's conclusion, Kim and Thompson offer a rationale for the existence of the blocking mechanism. "The importance of responding selectively to those stimuli which reliably predict biologically significant events offers a functional explanation for associative learning in an animal's adaptation to its environment. In the interest of efficiency and simplicity, animals should avoid forming associations with other stimuli that provide no new information.... The behavioral phenomenon of blocking ... appears to circumvent such redundant learning."
The research was supported by grants from the National Institute on Aging, the NSF, the Office of Naval Research, and the Sankyo Co.
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