Frequency discrimination by young infants.

The nonnutritive sucking of groups of 1-month-old infants was conjugately reinforced with either a 200-H or a 500-H pure tone. Following response acquisition and satiation, the sucking of the experimental groups was reinforced with a new pure tone (either 500 or 200 H) to a criterion of response decrement, after which sucking was reinforced with the original tone. Control groups were reinforced with a single tone (200 or 500 H) throughout the experiment. Frequency of nonnutritive sucking increased significantly in the experimental groups following a shift in frequency of the reinforcing tone but remained unchanged in the control groups, indicating discrimination between 200- and 500-H tones.