Effects of intra- and extra-maze stimuli and fimbria-fornix lesions on rats' discrimination learning in a pool

Seven fimbria-fornix (FF) lesioned and eight control rats were trained for spatial and cue discrimination tasks in a pool with two visible objects. One object served as the true goal, and the other was the false goal. For the spatial discrimination task, the objects looked the same, but the true goal stayed at one place and the false goal moved in the pool. For the cue discrimination task, the objects looked different, and both moved. Each rat was given three daily trials of both tasks under extra-maze stimuli. After 66 trials, control rats reached 100% and FF-lesioned rats 83.3% of the correct response for the spatial discrimination task. The correct response for both groups remained 75% for the cue discrimination task after 156 trials. The results suggest : the use of extra-maze stimuli is easier for rats compared with intra-maze stimuli, therefore extra-maze stimuli interfere with intra-maze stimuli, resulting in impaired cue discrimination learning. Intra-maze stimuli hardly interrupt spatial discrimination learning because they are weaker than extra-maze stimuli as cues. Furthermore, the results showed that FF lesions impair spatial discrimination, but not cue discrimination.