Trial spacing effects in pavlovian conditioning: A role for local context

Two conditioned lick-suppression experiments with rats were conducted in order to replicate and extend findings by Ewing, Larew, and Wagner (1985). Ewing et al. observed that excitatory responding to a CS paired with a footshock US was attenuated when the ITIs thatpreceded each CS-US trial were short (60 sec) relative to when they were long (600 sec). This effect was isolated in the influence of the preceding ITI because the preceding ITI was consistently short for one CS and consistently long for a different CS, while the following ITIs were equally often short and long for both CSs. Ewing et al. interpreted this finding in the framework of Wagner’s (1981) SOP model. Experiment 1 replicated this trial-spacing effect and demonstrated a similar effect under conditions in which thefollowing ITI was consistently short for one CS and consistently long for a different CS, while the durations of preceding ITIs were equally often short and long for both CSs. Experiment 2 revealed that the detrimental effect of a short preceding or a short following ITI could be alleviated by extinguishing the conditioning context after CS-US training. The latter observation indicates that the trial-spacing effect is not mediated by a failure of a CS trained with a short ITI to enter into excitatory associations with the US, a conclusion that is not wholly consistent with the SOP model. Finally, we suggest that short pretrial and short posttrial ITIs may enhance the excitatory value of local context cues that modulate responding to a CS.