Learning and attention were examined in rats after injections of one of several molecules related to adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (MSH). The initial phase of the learning process was linearly related to the length of the peptide with the smallest fragment (MSH/ACTH 4-10) improving learning the most and the largest molecule (ACTH 1-24) exerting no effect. Later stages of the learning problem, which were sensitive to the attentional state of the organism, resulted in U-shaped relations with the length of the same peptides. Enhancement of attention was significant only for the MSH compounds. These data indicate that some behaviors may be influenced as a function of the redundant information contained in the molecule while other behaviors may be discretely related to the specific conformation of the molecule
