This study aims at understanding the basis of an apparent learning phenomenon reported by Gelber (1952). A platinum wire is lowered into a depression slide with paramecia for 15 seconds, then raised for 25 seconds, and this is repeated 40 times. Every third trial, the wire is baited with immobile bacteria. At the end, the wire is sterilized and lowered again for 3 min, and paramecia attached to the wire are counted. Many more paramecia are found attached after training than in the control. It looks as if paramecia have developed an attraction to the platinum wire. This phenomenon was studied in more detail by Gelber in a series of papers between 1952 and 1962.

However, Jensen (1957) argued that the wire introduces bacteria in the medium, in the center of the slide, which are still present during the test. Thus, according to Jensen, the results can be simply explained by the fact that paramecia are attracted to bacteria, and no learning is involved. Katz and Deterline successfully repeated Gelber’s experiment. But then they stirred the medium before the final test: the “learning” phenomenon disappeared.

One possible explanation could be that the final stirring disrupts the learned behavior. Indeed, paramecia are highly sensitive to mechanical disturbances. But a question remains: if paramecia do indeed develop an attraction to the platinum wire, then what is the sensory signal? Platinum is chemically neutral, so it would not be chemical. A possibility would be light: the wire may cast a shadow or reflect light, and perhaps the training impacts the photophobic response of paramecia. Indeed, Gelber (1956) indicates that when paramecia are trained in light but tested in dark, the phenomenon disappears. But Katz and Deterline did both training and testing in dark, and the “learning” phenomenon occurs, unless the medium is stirred. Thus, there is no light stimulus that paramecia could use.

Thus, learning has not been convincingly demonstrated in this series of experiments. A simple experiment to do would be to lower the wire at different positions in the slide during testing and training.