The operant task is another well-known method for studying drug addiction; that can also be used to assess the reinforcing effects of foods (Elmer et al., 2002; Hayward et al., 2002; Ward and Dykstra, 2005). Progressive ratio (PR) schedules in the operant task provide the reinforcing properties of gustatory stimuli as the absolute value for each tastant (Reilly, 1999). The break point in the PR test, which is defined as the last number of lever presses to get the reinforcer,