Eyeblink conditioning given in the explicit presence of hippocampal ? results in accelerated learning and enhanced multiple-unit responses, with slower learning and suppression of unit activity under non-? conditions. Recordings from putative pyramidal cells during ?-contingent training show that pretrial ?-state is linked to the probability of firing increases versus decreases rather than to the magnitude of such responses. These findings suggest that the learning facilitation during ? may be due to the recruitment of additional neurons that increase their firing rate during trials.