Contrast effects in the rat: a developmental study

Two experiments investigated and development of positive and negative contrast effects in the consummatory behavior of juvenile and adult rats. Over a 4-day session, animals were given alternating 30-sec presentations of 2 bottles containing either 32 or 4% sucrose solutions. In Experiment I, different groups of rats were tested both cross-sectionally and longitudinally using a cohort-sequential design. Cross-sectionally, animals at all ages exhibited both types of contrast and demonstrated increasing positive contrast effects with greater age. Discrepancies between the longitudinal and cross-sectional samples were found to be due to the repeated testing procedure of the former design. Experiment II replicated the positive and negative contrast effects in the juvenile animals, indicating that the processes responsible for contrast effects are operating in the rat at a very early age.