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 pr oblems at different times prior to surgery. Following surgery, retrograde a mnesia was assessed by measuring retention of the preoperatively learned di scrimination problems in lesioned rats and controls. Rats with rhinal corte x lesions displayed temporally graded retrograde amnesia; retention of obje ct discriminations acquired in the recent past (i.e. 2 or 9 days prior to s urgery) was significantly impaired, whereas retention of object discriminat ions acquired more remotely (i.e. 16, 37, or 58 days prior to surgery) was not. In contrast, rats with mediodorsal thalamic lesions exhibited normal s avings of all discrimination problems. These results suggest that the rhina l cortex, but not the mediodorsal thalamus, plays a time-limited role in th e consolidation of object memory. (C) 1999 Lippincott Williams & Wilkins.