When is the hippocampus involved in recognition memory?

Abstract

The role of the hippocampus in recognition memory is controversial. Recognition memory judgments may be made using different types of information, including object familiarity, an object's spatial location, or when an object was encountered. Experiment 1 examined the role of the hippocampus in recognition memory tasks that required the animals to use these different types of mnemonic information. Rats with bilateral cytotoxic lesions in the hippocampus or perirhinal or prefrontal cortex were tested on a battery of spontaneous object recognition tasks requiring the animals to make recognition memory judgments using familiarity (novel object preference); object-place information (object-in-place memory), or recency information (temporal order memory). Experiment 2 examined whether, when using different types of recognition memory information, the hippocampus interacts with either the perirhinal or prefrontal cortex. Thus, groups of rats were prepared with a unilateral cytotoxic lesion in the hippocampus combined with a lesion in either the contralateral perirhinal or prefrontal cortex. Rats were then tested in a series of object recognition memory tasks. Experiment 1 revealed that the hippocampus was crucial for object location, object-in-place, and recency recognition memory, but not for the novel object preference task. Experiment 2 revealed that object-in-place and recency recognition memory performance depended on a functional interaction between the hippocampus and either the perirhinal or medial prefrontal cortices. Thus, the hippocampus plays a role in recognition memory when such memory involves remembering that a particular stimulus occurred in a particular place or when the memory contains a temporal or object recency component.

Figure 1.

A–D, Diagrams of the four object recognition memory tasks: novel object preference task (A), object location task (B), object-in-place task (C), and temporal order task (D).

Figure 2.

Diagrammatic reconstructions showing the cases with the largest (gray) and smallest (black) lesions in the mPFC (A), PRH (B), and HPC (C) groups. The numbers correspond to the approximate position relative to bregma (Swanson, 1998).

Figure 3.

Diagrammatic reconstructions showing the cases with the largest (gray) and smallest (black) lesions in the PRH + HPC Contra group (A) and PRH + HPC Ipsi group (B). The numbers correspond to the approximate position from bregma (Swanson, 1998).

Figure 4.

Diagrammatic reconstructions showing the cases with the largest (gray) and smallest (black) lesions in the mPFC + HPC Contra group (A) and mPFC + HPC Ipsi group (B). The numbers correspond to the approximate position from bregma (Swanson (1998)).

Figure 5.

Performance of the experimental groups in the novel object preference task. A, B, Performance of the bilateral hippocampal (HPC), perirhinal (PRH), or medial prefrontal cortex (mPFC) lesion groups following a 5 min (A) or a 3 h (B) delay between the sample and test phases. C, D, Performance of the medial prefrontal cortex–hippocampal ipsilateral (mPFC + HPC Ipsi) or contralateral (mPFC + HPC Contra) lesions, perirhinal–hippocampal ipsilateral (PRH + HPC Ipsi) or contralateral (PRH + HPC Contra) lesion groups, following a 5 min (C) or a 3 h (D) delay. Illustrated for each group is the mean (+SEM) discrimination ratio. ***p < 0.001.

Figure 6.

Performance of the experimental groups in the object location task. A, Performance of the sham lesioned animals (SHAM), bilateral hippocampal (HPC), perirhinal (PRH), or medial prefrontal cortex (mPFC) lesion groups. B, Performance of animals with combined lesions in the hippocampus and medial prefrontal cortex in the same hemisphere (mPFC + HPC Ipsi) or opposite hemispheres (mPFC + HPC Contra) and animals with combined lesions in the hippocampus and perirhinal cortex in the same hemisphere (PRH + HPC Ipsi) or opposite hemispheres (PRH + HPC Contra) groups. Illustrated for each group is the mean (+SEM) discrimination ratio. **p < 0.01, ***p < 0.001.

Figure 7.

Performance of the experimental groups in the object-in-place task. Left, Performance of the sham lesioned animals (SHAM), bilateral hippocampal (HPC), perirhinal (PRH), or medial prefrontal cortex (mPFC) lesion groups. Right, Performance of animals with combined lesions in the hippocampus and medial prefrontal cortex in the same hemisphere (mPFC + HPC Ipsi) or opposite hemispheres (mPFC + HPC Contra) and animals with combined lesions in the hippocampus and perirhinal cortex in the same hemisphere (HPC + PRH Ipsi) or opposite hemispheres (PRH + HPC Contra) groups. Illustrated for each group is the mean (+SEM) discrimination ratio. ***p < 0.001.

Figure 8.

Performance of the experimental groups in the temporal order memory task. A, Performance of the sham lesioned animals (SHAM), bilateral hippocampal (HPC), perirhinal (PRH), and medial prefrontal cortex (mPFC) lesion groups. B, Performance of animals with combined lesions in the hippocampus and medial prefrontal cortex in the same hemisphere (mPFC + HPC Ipsi) or opposite hemispheres (mPFC + HPC Contra) and animals with combined lesions in the hippocampus and perirhinal cortex in the same hemisphere (PRH + HPC Ipsi) or opposite hemispheres (PRH + HPC Contra) groups. Illustrated for each group is the mean (+SEM) discrimination ratio. ***p < 0.001.