Retention of a Brightness Discrimination Task in Paramecia, P. caudatum

Previous research into the possibility of learning in paramecia in this laboratory has shown that these organisms can learn to go to and remain in a specific location based on cathode shock reinforcement. The present experiment was designed to determine whether paramecia could retain (remember) the learned brightness discrimination task. The results indicate that the retention interval for this task in paramecia is shorter than 1 minute. It is possible that paramecia can remember this task for longer than a second but shorter than the 1-minute interval that was used during test. It is also possible that remembering for more than a few seconds requires a nervous system, which paramecia do not have. Previous investigations into the learning capabilities of the singled-celled organism Paramecium caudatum have been inconclusive at best. Although some researchers have found support for tube-escape behavior and discrimination learning, other studies have refuted these results. Day and Bentley (1911) studied the swimming behavior of paramecia in a restrictive capillary tube to determine if the paramecia would change their swimming behavior to avoid the possibly aversive sides of the tube. An increase in the frequency in reversal of the paramecia‟s position, paired with a decrease in time between reversals, suggested the paramecia were learning to avoid the sides of the capillary tubes. Although the researchers thought their findings lent support to learned behavior modification, Buytendijk (1919, as cited in French, 1940) suggested the change in behavior could be a result of increased “flexibility” of the paramecium. Buytendijk (above) suggested that the paramecia simply became more efficient at escaping the walls of the capillary tube due to some sort of sensitization or other mechanism, rather than a learned behavior. French (1940) used capillary tubes that opened into a pool of medium, which was thought to be reinforcing, to study tube-escape behavior in paramecia. Over the course of successive trials he found that the time it took for a subject to escape the tube into the medium decreased. French thought this was a clear example of trial and error learning in paramecia, however, Applewhite and Gardner (1973) thought there could be an alternate explanation. French did not clean the capillary tube between trials, which lead Applewhite and Gardner to believe there was some sort of change in the medium inside the tube that was influencing the paramecia‟s ciliary action and swimming behavior. When they attempted to replicate French‟s findings with clean tubes before each trial they were unable to do so. Gelber (1952, 1956, 1957, 1958) completed a series of investigations into paramecium behavior using bacteria as a food reinforcer. Multiple subjects in a pool of medium were trained to approach a platinum wire that was coated in bacteria. A non-reinforced control group of subjects was trained with a bare wire, and a second control group did not have any wire approach training. This procedure allowed Gelber to test for the attraction of the wire vs. food reinforcement. The wire was presented multiple times and, between each presentation the medium pool was gently „swirled‟ to mix the liquid and redistribute the paramecia throughout the medium. During testing, each group of subjects was exposed to a bare wire, and counts of