Rhinal cortex, but not medial thalamic, lesions cause retrograde amnesia for objects in rats

Male Long-Evans rats were trained on five separate object discrimination problems at different times prior to surgery. Following surgery, retrograde amnesia was assessed by measuring retention of the preoperatively learned discrimination problems in lesioned rats and controls. Rats with rhinal cortex lesions displayed temporally graded retrograde amnesia; retention of object discriminations acquired in the recent past (i.e. 2 or 9 days prior to surgery) was significantly impaired, whereas retention of object discriminations acquired more remotely (i.e. 16, 37, or 58 days prior to surgery) was not. In contrast, rats with mediodorsal thalamic lesions exhibited normal savings of all discrimination problems. These results suggest that the rhinal cortex, but not the mediodorsal thalamus, plays a time-limited role in the consolidation of object memory.