Persistence of working memory of rats in an aversively motivated radial maze task

The effects of various within-trial delays on the performance of rats in an 8-arm radial water maze (RWM) were investigated. Rats (n = 13) were trained to escape from 25 degrees C opaque water onto a submerged bench situated at the far end of each channel of the maze. After 20 s the chosen bench was collapsed and the rat had to return to a central platform, 1 cm above water level. After 15 s the platform was lowered and the animal was forced to choose again. This was repeated until all 8 channels had been visited. After 30 daily trials the mean number of correct choices per trial (first 8 choices) was 7.8. In Experiment 1, delays from 40 min up to 21 h were inserted between choices 4 and 5. The number of revisited channels (errors) in the second half of a trial increased rapidly with the duration of the delay, and was not significantly different from chance at 640 min. In Experiment 2, delays of 2.5, 5, 10 or 20 min were inserted between individual choices. Again, performance deteriorated with the duration of the delay. When the incidence of errorless trials was analyzed, performance was not significantly different from random at 5 min inter-choice delays. Comparison with conventional radial maze studies indicates that spatial working memory in the RWM has a slower, but also exponential decay.