As we get older, when we suffer memory problems, we often laughingly talk about our brain being ‘full up’, with no room for more information. A new study suggests that in some sense (but not the direct one!) that’s true.

To make new memories, we need to recognize that they are new memories. That means we need to be able to distinguish between events, or objects, or people. We need to distinguish between them and representations already in our database.

We are all familiar with the experience of wondering if we’ve done something. Is it that we remember ourselves doing it today, or are we remembering a previous occasion? We go looking for the car in the wrong place because the memory of an earlier occasion has taken precedence over today’s event. As we age, we do get much more of this interference from older memories.

In a new study, the brains of 40 college students and older adults (60-80) were scanned while they viewed pictures of everyday objects and classified them as either "indoor" or "outdoor." Some of the pictures were similar but not identical, and others were very different. It was found that while the hippocampus of young students treated all the similar pictures as new, the hippocampus of older adults had more difficulty with this, requiring much more distinctiveness for a picture to be classified as new.

Later, the participants were presented with completely new pictures to classify, and then, only a few minutes later, shown another set of pictures and asked whether each item was "old," "new" or "similar." Older adults tended to have fewer 'similar' responses and more 'old' responses instead, indicating that they could not distinguish between similar items.

The inability to recognize information as "similar" to something seen recently is associated with “representational rigidity” in two areas of the hippocampus: the dentate gyrus and CA3 region. The brain scans from this study confirm this, and find that this rigidity is associated with changes in the dendrites of neurons in the dentate/CA3 areas, and impaired integrity of the perforant pathway — the main input path into the hippocampus, from the entorhinal cortex. The more degraded the pathway, the less likely the hippocampus is to store similar memories as distinct from old memories.

Apart from helping us understand the mechanisms of age-related cognitive decline, the findings also have implications for the treatment of Alzheimer’s. The hippocampus is one of the first brain regions to be affected by the disease. The researchers plan to conduct clinical trials in early Alzheimer's disease patients to investigate the effect of a drug on hippocampal function and pathway integrity.