Blocking can occur without losses in attention in rats with selective removal of hippocampal cholinergic input.

Prior studies showed that 192 IgG-saporin lesions of cholinergic input to the hippocampus disrupted reductions in processing of uninformative stimuli. In 2 experiments in this study, the performance of rats with these lesions was examined in blocking procedures. In both lesioned and normal rats, previous pairing of one conditioned stimulus (CS) with food blocked conditioning of a 2nd CS when a compound of both CSs was paired with food. However, in subsequent savings tests, lesioned rats showed faster learning than did normal rats when the blocked CS was established as a signal for either reinforcement or nonreinforcement. Thus, the reduced attention to the blocked CS found in normal but not lesioned rats was not essential for the occurrence of blocking. Although rats with selective removal of hippocampal cholinergic input may be unable to reduce attention to redundant stimuli, other mechanisms of stimulus selection remain available to them.