Psychology Ph.D. Dissertations


Reward processing alterations for natural reward in alcohol-preferring (P) rats: Incentive contrast, reward discrimination, and alcohol consumption

Date of Award


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)



First Advisor

Howard C. Cromwell (Advisor)

Second Advisor

William H. O'Brien (Committee Member)

Third Advisor

Mary-Jon Ludy (Committee Member)

Fourth Advisor

Lynne E. Hewitt (Committee Member)

Fifth Advisor

Joshua B. Grubbs (Committee Member)


Relative reward effects highlight the impact of reward value shifts on goal-directed behavior. A popular method used to examine relative reward effects is incentive contrast. Positive contrast is an upshift or increase in behavior toward a particular outcome due to an alternative comparison, while negative contrast is the opposite. The ability to compare rewards and utilize value shifts to make advantageous outcome decisions may be disrupted in substance use disorders such as alcohol use disorder (AUD). We examined the natural reward comparison abilities of Sprague-Dawley (SD) and alcohol-preferring (P) rats in an operant task using 12 sucrose solution pairings to determine 1) the impact of food restriction on contrast; 2) potential line differences in contrast before P rat alcohol exposure; 3) alcohol effects on contrast in P rats; and 4) the impact of reward value shifts on P rat alcohol intake. Animals underwent a repeated-measures design consisting of two single outcome blocks separated by a mixed outcome block. Appetitive and consummatory measures were used to assess positive and negative contrast toward single outcomes relative to alternatives presented during mixed blocks. Restricted naive P rats show generalization and inverse consummatory contrast as well as impaired relative preference and a lack of reward discrimination. Conversely, they show appetitive contrast and discrimination for sucrose outcomes, suggesting that P rats have enhanced reward seeking but upon consumption abandon their valuation of initial outcomes due to an impaired memory-based reward process which limits contrast and enables them to be hyposensitive to natural reward unlike alcohol. Alcohol had no effect on the absence of contrast in unrestricted P rats. Alcohol intake and preference were altered after incentive contrast tests but due to a lack of contrast, it was unclear whether sucrose upshift or downshift impacted drinking. We conclude that P rats show inherent deficits in reward processing and altered reward comparison abilities which could highlight genetic predispositions in AUD. Food restriction was found to be a key mediator of contrast in SD rats and P rats and analysis of both phases of motivation (appetitive and consummatory) is crucial in examining contrast and reward comparison abilities.