Effects of Changing Reward on Performance of the Delayed Spatial Win-shift Radial Maze Task in Pedunculopontine Tegmental Nucleus Lesioned Rats

Because it was designed to assess working memory, the delayed spatial win-shift (DSWS) radial maze task has been used to investigate the involvement of corticostriatal structures in executive processing. Excitotoxic lesions of the pedunculopontine tegmental nucleus (PPTg) produce profound deficits in performance of this task that are not accounted for by motor impairment. Thus, PPTg DSWS deficits are hypothesized to support a role for PPTg in complex cognitive processing. However, other studies indicate that the behaviour of PPTg lesioned rats varies depending on level of motivational excitement, assessed by the presence or absence of deprivation, or by manipulations of reward value. Since DSWS performance may also be affected by motivational dysfunction, the present experiment was conducted to examine the effects of post-surgical presentation of a more positive food reward (chocolate drops) on the DSWS retention performance of PPTg lesioned rats. Results confirmed a PPTg lesion deficit: lesioned rats made significantly more errors in both training and test phases, and made errors significantly earlier in their choice sequence in the test phase. Main effects of phase on number of errors indicated that the PPTg test phase deficit was not simply the result of a carry-over impairment from the training phase. PPTg rats receiving chocolate made significantly fewer errors than PPTg rats receiving food pellets. Results suggest that PPTg DSWS deficits are not the result of altered motivation or hedonic appreciation of reward value (or reward change) and therefore support the hypothesis of executive cognitive deficits in PPTg lesioned rats.