Functional imaging studies on the neural representations of associative information during object memory retrieval

Abstract

To successfully identify and respond to objects, the human brain utilizes previously stored information related to the objects. Understanding this object memory retrieval process is important in that it provides a basis for interpreting the human cognitive process of interacting with the outside world. To understand how the learned information is processed in the human brain during retrieval, we conducted two different experiments on automatic and active retrieval using object-value memory and object-object memory, respectively. In the study on automatic retrieval of object-value memory, we focused on how the long-term value memory is maintained and processed efficiently in order to induce automatic retrieval. Using long-term value learning with fractal objects and functional magnetic resonance imaging (fMRI), we found that the ventral striatum (VS) retains the learned values even in the absence of any immediate outcome. Neural responses in the VS showed significant value discrimination and the similarity of neural representations for good-valued objects increased after learning. This suggests that the VS is involved in storing and efficient processing of positive values associated with an object. In the study on active retrieval of object-object memory, we focused on the information processing in the hippocampus, a well-known core region for declarative memory. Using a simple object-object association paradigm with fMRI and representational similarity analysis (RSA), we successfully confirmed that the hippocampus is involved in cortical reinstatement during active retrieval. Moreover, we found that there exists an additional information processing beyond reinstatement in the hippocampal subregion CA3/DG during retrieval, and suggest that this information processing in CA3/DG may be related to memory strengthening. Taken together, the obtained results show how information is processed during automatic and active retrieval of object memory and provide insights to better understand the process of memory retrieval.