Measuring motivation: using the cross point of two demand functions as an assessment of the substitutability of two reinforcers

Behavioural demand functions are recognised as a way to assess the strength of animal motivation quantitatively. The purpose of the present experiment was to develop a method to evaluate the substitutability of two reinforcer s by use of the degree of diversion of the cross point of two demand functi ons. Two strains of rats, Long Evans and Wistar Kyoto, worked in a closed e conomy with two partly substitutable reinforcers available for 2 h each day in a one-compartment chamber. Three kinds of waters were used in the exper iments, namely, distilled water, acidified water and saccharine water. Four combinations of water were tested. The cost of the most attractive reinfor cer was set using a fixed ratio (FR) reinforcement schedule. The schedule v aried from FRIO to FR100 over 5 consecutive days and each schedule was repe ated three times. The cost of the substitutable reinforcer was kept constan t at FR55 throughout the experiment. Demand functions were generated on sca les with FR-values on the x-axis and number of reinforcements on the y-axis . The cross points of the two demand functions in the four conditions were calculated. The results showed no significant difference between the two st rains with regard to the cross point at a 5% level of significance. For dis tilled water versus distilled water which acted as control condition, the d emand functions crossed at FR67.34 +/- 3.75. Testing saccharine water on th e variable side and distilled water on the constant side resulted in cross point at FR69.84 +/- 3.89. The cross point for acidified water versus disti lled water was FR84.0 +/- 4.68. The cross point for saccharine water tested against acidified water was FR88.31 +/- 4.92. No significant difference be tween the cross points in conditions I and 2 was found; neither was there s ignificant difference between conditions 3 and 4. Comparing condition I or 2 with condition 3 or 4 showed a significant difference between the cross p oints, indicating that the demand for distilled water is higher than that f or acidified water. Further experiments will reveal, if this method provide s a way to enhance the sensitivity of traditional behavioural economics. (C ) 2001 Elsevier Science B.V. All rights reserved.