Dissociation of the Perirhinal Cortex and Hippocampus During Discriminative Learning of Similar Objects.

Discriminative learning is a paradigm that has been used in animal studies, in which memory of a stimulus is enhanced when it is presented with a similar stimulus rather than with a different one. Human studies have shown that through discriminative learning of similar objects, both item memory and contextual memories are enhanced. However, the underlying neural mechanisms for it are unclear. The hippocampus and perirhinal cortex (PRC) are two possible regions involved in discriminating similar stimuli and forming distinctive memory representations. In this study, 28 participants (15 males) were scanned using high-resolution fMRI when a picture (e.g., a dog) was paired with the same picture, with a similar picture of the same concept (e.g., another dog), or with a picture of a different concept (e.g., a cat). Then, after intervals of 20 min and 1 week, the participants were asked to perform an old/new recognition task, followed by a contextual judgment. The results showed that during encoding, there was stronger activation in the PRC for the "similar" than for the "same" and "different" conditions and it predicted subsequent item memory for the "similar" condition. The hippocampal activation decreased for the "same" versus the "different" condition and the DG/CA3 activation predicted subsequent contextual memory for the "similar" condition. These results suggested that the PRC and hippocampus are functionally dissociated in encoding simultaneously presented objects and predicting subsequent item and contextual memories after discriminative learning.