Do we have a system to form memories and another to recall them or are both processes done by the same system in the brain? This has been a long standing question about the hippocampus. ScienceDaily has a report (here) on a paper answering this question. (Nozomu H. Nakamura, Magdalena M. Sauvage. Encoding and reactivation patterns predictive of successful memory performance are topographically organized along the longitudinal axis of the hippocampus. Hippocampus, 2015; DOI: 10.1002/hipo.22491).
The researchers used tags for molecules known to be involved in memory formation and also tags for the ones used in retrieval, they found that the same cells did both jobs. This is not really surprising because the patterns of cortical activity had been shown to be very similar for a particular memory through its formation, strengthening and recalling. These patterns seemed to come from activity in the hippocampus. A single system for formation and recall also makes it easier to understand the ways that memories are changed when they are recalled and used.
“For their studies with rats, researchers adapted a standardised word-based memory test for humans, using however scents instead of words. The researchers hid small treats in sand-filled cups. In addition, each cup also contained a different scent, such as thyme or coriander which could be smelled by the rats when searching for the treats. Each training unit consisted of three phases. During the learning phase, researchers presented several scents to the animals. A pause followed, and subsequently a recognition phase. In the latter, the animals were presented the scents from the learning phase as well as other smells. The animals demonstrated that they recognised a scent from the learning phase by running to the back wall of their cage, where they were rewarded with food for the correct response. If, on the other hand, they recognised that a scent had not been presented during the learning phase, they demonstrated it by digging in the sand with their front paws.”
Here is the abstract: “An ongoing debate in human memory research is whether the encoding and the retrieval of memory engage the same part of the hippocampus and the same cells, or whether encoding preferentially involves the anterior part of the hippocampus and retrieval its posterior part. Here, we used a human to rat translational behavioural approach combined to high-resolution molecular imaging to address this issue. We showed that successful memory performance is predicted by encoding and reactivation patterns only in the dorsal part of the rat hippocampus (posterior part in humans), but not in the ventral part (anterior part in humans). Our findings support the view that the encoding and the retrieval processes per-se are not segregated along the longitudinal axis of the hippocampus, but that activity predictive of successful memory is and concerns specifically the dorsal part of the hippocampus. In addition, we found evidence that these processes are likely to be mediated by the activation/reactivation of the same cells at this level. Given the translational character of the task, our results suggest that both the encoding and the retrieval processes take place in the same cells of the posterior part of the human hippocampus.”