Object recognition memory and the rodent hippocampus

Abstract

In rodents, the novel object recognition task (NOR) has become a benchmark task for assessing recognition memory. Yet, despite its widespread use, a consensus has not developed about which brain structures are important for task performance. We assessed both the anterograde and retrograde effects of hippocampal lesions on performance in the NOR task. Rats received 12 5-min exposures to two identical objects and then received either bilateral lesions of the hippocampus or sham surgery 1 d, 4 wk, or 8 wk after the final exposure. On a retention test 2 wk after surgery, the 1-d and 4-wk hippocampal lesion groups exhibited impaired object recognition memory. In contrast, the 8-wk hippocampal lesion group performed similarly to controls, and both groups exhibited a preference for the novel object. These same rats were then given four postoperative tests using unique object pairs and a 3-h delay between the exposure phase and the test phase. Hippocampal lesions produced moderate and reliable memory impairment. The results suggest that the hippocampus is important for object recognition memory.

Figure 1.

Reconstructions of coronal sections through the hippocampus showing the smallest (black) and largest (stippled) lesions for each of the three hippocampal lesion groups (1-d, 4-wk, and 8-wk training-surgery intervals). Numbers (right) represent the distance (mm) posterior to bregma.

Figure 2.

Postoperative retention at three training-surgery intervals. Rats received either sham surgeries or bilateral hippocampal lesions 1 d (CON = 16, H = 15), 4 wk (CON = 16, H = 14) or 8 wk (CON = 16, H = 14) after training. (A) The cumulative preference for the novel object after 15 sec of object exploration. The control groups at each training-surgery interval exhibited a preference for the novel object, whereas only the 8-wk hippocampal lesion group exhibited this preference. (B) The cumulative preference for the novel object after 30 sec of object exploration. As in A, the control groups at each training-surgery interval exhibited a preference for the novel object. However, unlike the 15 sec analysis (A), the 8-wk hippocampal lesion group fell just short of performing above chance (P = 0.057). Asterisks indicate group differences (P < 0.05).

Figure 3.

Postoperative training and testing of NOR on four different days in sham operated animals (CON, n = 47) and animals with hippocampal lesions (H, n = 44). Performance was scored over 15 sec of cumulative object exploration (A) and over 30 sec of cumulative object exploration (B). The left side of each panel (line graphs) shows the percent preference for the novel object on four different days for both the hippocampal lesion group (black circles) and the sham group (white circles). The right side of each panel (bar graphs) shows the 4-d mean for the hippocampal lesion group (black bar) and the sham group (white bar). Asterisks indicate group differences (* P = 0.059, ** P < 0.05).

Figure 4.

Cumulative percent preference for the novel objects across 30 sec of object exploration averaged across four different days for the hippocampal lesion group and the control group. The pattern of performance indicates that a group difference emerged by the fourth second of object exploration and this difference was maintained throughout the remainder of the preference test. All points for both groups were above chance. Asterisks indicate group differences (P < 0.05).