Remembering and imagining appear to be very different functions, one recovering true information from the past, the other considering the unreal or exploring the future. And yet many patients with damage to the hippocampus (a structure in the temporal lobes) – and resultant memory impairment – struggle in imagining the future. Moreover, neuroimaging data show the hippocampus is involved in both tasks. Taken together, this evidence suggests that memory for the past and imagination for the future may depend on shared neural processes.
A new imaging study by Brock Kirwan and his colleagues confirms at a broad anatomical level that both memory and future imagination call on similar regions of the hippocampus. But the research also shows how these two mental functions do depend on distinct neural processes after all.
Fourteen study participants were invited into a scanner where they were presented with photographs in a series of runs. One run contained only personal photographs that the participant had taken in the last five years; another presented photos of unfamiliar situations, known to be novel to each participant thanks to a biographical survey they completed earlier.
After each image, the participants had to either recall what they’d just seen (in the case of the personal photos), or imagine the presented situation (in the case of the unfamiliar photos), as vividly as possible, for 8.5 seconds. It was during these contrasting mental tasks that the key scanning data were collected. Participants then immediately rated the vividness of each memory or imagined situation, and only trials that met a threshold of vividness were included in analysis (this disqualified the bulk of the “imagine” trials).
The two tasks activated different brain areas: a range of frontal areas for the imagining task; cingulate and parahippocampal areas for the remembering task. But both tasks were also associated with activation within the hippocampus itself – in the left anterior and (more weakly) posterior regions, specifically. Next, the researchers used a “classifier accuracy analysis”, which essentially gives a computer a bunch of information unfolding over time (in this case, patterns of activation across hippocampal voxels, each measuring about 2 cubic millimetres) and asks it to identify from the patterns of activity, which mental process was ongoing at the time – remembering or imagination.
The computer was far from perfect at this task, but it did significantly better than chance. This shows that if we zoom in enough, we find there are some consistent differences in how the neural resources of the hippocampus are put to work in remembering and imagining. This raises the question – what role is the hippocampus serving when we imagine the future? One possibility is that it makes available raw materials (memories?) that are then recombined to create an imagining. However it does it, the new research confirms that the hippocampus appears to be crucial to the creation or recreation of realities beyond our current sensory inputs: a key component of mental time travel.