The finding that retention functions decrease more rapidly on trials initiated by a food sample than on trials initiated by a no-food sample was replicated. This result was also obtained for samples that do not contrast hedonically, but that are characterized by the presence versus absence of an event. These results are consistent with an asymmetrical-coding model in which it is held that (a) a no-food sample is not coded, (b) responding to the comparison associated with a no-food sample occurs as a default, and (c) the default response is overridden by the presence in working memory of a code activated by the presentation of food. Sample-to-comparison mapping arrangements that are less favorable for the operation of a default response strategy resulted in symmetrical, and probably prospective, coding. These results suggest that coding processes in pigeons are flexible, controlled processes.