“Justification of Effort” in Rats: Effects of Physical and Discriminative Effort on Reward Value

The justification of effort is the tendency for humans to justify the amount of work they have put into an achievement by increasing the perceived value of that achievement after the effort has been expended. A positive effect of effort on reward value has also been reported for pigeons and starlings. The present study examined the effects of different amounts of required physical and discriminative effort on primary reward value in rats. On each day of training, rats underwent one high-effort training session and one low-effort training session in an operant chamber to earn either a grape-flavored reward pellet or a bacon-flavored reward pellet. Half of the rats exerted high effort to earn the grape pellets and low effort to earn the bacon pellets, with the arrangement reversed for the other half. On each test trial, each rat had the opportunity to consume 3 pellets of each flavor at the choice point in a T-maze. Analysis indicated no significant difference between the frequency of high-effort flavor choices and low-effort flavor choices. A positive effect of effort on reward value was not demonstrated in this experiment.

Effect of exposure to high pressure on subsequent spatial learning and memory in rats

The effects of high helium pressure on the subsequent acquisition of spatial memory were studied in male rats. Thirty-two rats were exposed to 65 ATA helium-oxygen pressure for 4.2 days, decompressed (total time in chamber 5 days), and then tested in an eight-arm radial maze. Thirty-two control rats were exposed in the chamber to 1 ATA air. Each rat had 20 sessions in the maze (2 sessions/day for 10 days), and the number of correct (visiting an arm not previously visited to obtain the reward pellet) and incorrect choices (visiting a previously visited arm) were recorded. Statistical analysis showed that the rats exposed to 65 ATA performed significantly better than 1-ATA controls during the first 8 of 20 sessions. This effect was most pronounced in sessions 5-8. Results for sessions 9-20 showed that the pressure-treated rats still made more correct choices but to an extent that did not always reach statistical significance. Possible explanations include the pressure-treated rats performing better because of hunger after a lower food consumption at pressure. Alternatively, pressure itself may enhance proposed mechanisms of spatial memory such as long-term potentiation.