Peer-Reviewed
Manipulating reward value modifies the perception of temporal cues
T. Galtress
University of York
K. Kirkpatrick
University of York
Background
As early as 1981, Seth Roberts demonstrated that reducing the value of a reward through pre-feeding prior to an experimental session resulted in a rightward shift in the temporal function during a peak interval (PI) procedure. Recent research in our laboratory has investigated the nature of this shift in pre-fed animals by using two different temporal durations for comparison as well as exploring other methods of devaluation such as pairing the food pellet reward with the nausea inducing chemical Lithium Chloride (LiCl).
Methods
Rats were trained on a discrete trials instrumental PI procedure. Rewarded fixed interval (FI) 60 s training trials were intermixed with a small proportion of non rewarded PI 180 s probe trials. The food pellet reward was then devalued by either pre-feeding the animal with the food pellets immediately before the beginning of the test session, or giving the animal prior exposure to the food pellets paired with LiCl. The pre-fed animals were then retrained with an FI 30 s, PI 90 s procedure and subjected to the same pre-feeding devaluation as experienced previously.
Results
In the pre-fed rats repeated measures ANOVA on the mid points of the PI response functions revealed a main effect of phase (hungry v pre-fed), a main effect of temporal duration (FI 60 s v FI 30 s) and no interaction. Also repeated measures ANOVA on the proportion of shift in both phases revealed no difference between the FI 60 s and FI 30 s. In the LiCl paired rats repeated measures ANOVA on the LiCl test session compared to baseline PI response functions revealed an effect of devaluation.
Conclusions
Pre-feeding affected the perceived delay to reward with pre-fed rats producing a rightward shift in the mid point of the temporal response function compared to non pre-fed baseline levels. The results also demonstrated that the pre-feeding induced shifts in the PI temporal function were proportional and therefore scalar in nature, suggesting that the internal clock speed was dependent on the relative value of the reward. Other methods of reward devaluation had a similar effect. Pairing food pellets with the nausea inducing chemical Lithium Chloride reduces the appetitive value and also produces a rightward shift in the PI temporal function in a subsequent test phase. This suggests that the effect is present both when the hedonic value of the reward is dependent on the present physical state of the rat and also as a result of past devaluation experience.
