Haloperidol can increase responding to both discrete and contextual cues in trace conditioned rats

Haloperidol has been shown to enhance attentional selectivity in conditioning procedures. For example, in latent inhibition (LI) it improves animals' ability to treat as irrelevant, stimuli that have previously been presented without consequence. The present study tested whether this finding would generalize to other procedures that present animals with weak predictors. We therefore used a trace conditioning procedure to present rats with a conditioned stimulus (CS) weakened through temporal discontiguity (rather than preexposure in LI) and a flashing light background provided an alternative experimental stimulus. In Experiment 1, a noise CS was paired contiguously (at '0 s') with food or at a 10 s trace interval. In Experiment 2, the trace interval was lengthened to 20 s. In both experiments, haloperidol treatment generally reduced responding in 0 s contiguous groups. By contrast, 0.03 mg/kg haloperidol enhanced conditioning, selectively, to the weakly predictive trace CS, though it was without effect on responding within the trace interval. In addition, again at 0.03 mg/kg, haloperidol significantly increased excitatory conditioning to contextual stimuli in trace groups relative to contiguous groups. At the shorter (10 s) Experiment 1 trace, this result was shown in the extinction test of conditioning to the background stimulus. At the longer (20 s) Experiment 2 trace, this result was shown in the acquisition of responding to the box context in the inter-trial-interval. The demonstration that low dose haloperidol can increase conditioning is novel. This increase was seen selectively with stimuli (both trace-conditioned and contextual) that should have been treated as weak predictors so these results are contrary to what was expected on the basis of haloperidol effects on stimuli weakened through pre-exposure. The possibility that increased contextual conditioning could be relevant to the interpretation of haloperidol-induced enhancement of LI is discounted. However, it is suggested that this result could nonetheless reflect cognitive enhancement.