Cue distinctiveness and response-to-change in scopolamine injected or hippocampal rats.

Our previous findings indicated that hippocampal lesions (H) or scopolamine' injections (Sc) affected stimulus information acquired by distant observation of the white-black T-maze arms (the passive test), but left the information intact when it was gained by T-maze exploration (the active test). Because this difference might reflect the attentional deficit in H or Sc rats, in the present experiment we attempted to investigate the effect of lowered distinctiveness of cues (dark grey vs black T-maze arms) on the performance of rats in the active test. A total of 75 rats were assigned to four groups: (i) damaged in the dorsal hippocampus (H); (ii) sham operated (C); (iii) scopolamine (Sc) injected (1.0 mg/kg i. p.) and (iv) saline injected (NaCL). Each group showed a significant preponderance of choices of the arm which was changed in brightness between the two consecutive trials, separated by 1 min break. The groups did not differ markedly among themselves in the percentage of changed arm choices (H group, 85 percent; C, 74 percent; Sc -72 percent; NaCL, 72 percent). This result indicates that H or Sc rats are able to perceive a slight difference of brightness and to retain it over a period of 1 min. Therefore, the different performance of H as well as Sc rats in the passive and active test, observed previously, cannot be accounted for by the attentional deficit hypothesis.