Posterior Parietal Cortex Encodes Memory Engrams

Post by: Amanda McFarlan

What's the science?

The hippocampus is a neural structure that is famously implicated in the study of learning and memory, and is known to be required for the rapid encoding of memory. The neocortex, however, has been traditionally thought to store memories more slowly, over time. Recent studies have shown that the posterior parietal cortex (a neocortical region also implicated in memory) may also be capable of rapidly acquiring memory representations known as ‘engrams’. A memory engram (i.e. memory trace) is hypothesized to be the way in which memories are represented and stored in the brain. So far, it has been unclear whether the posterior parietal cortex can encode true memory engrams. This week in Science, Brodt and colleagues used brain imaging to investigate dynamic changes in neocortical structures (including the posterior parietal cortex) during learning and memory.

How did they do it?

The authors used functional magnetic resonance imaging (MRI) to measure dynamic changes in neocortical brain activity in healthy participants (male and female) during an ‘encoding and recall’ memory task. The participants were tested in two conditions: experimental (39 participants) and control (33 participants). The experimental condition consisted of two sessions, 13 hours apart. In the first session, participants received diffusion-weighted MRI scans (as a baseline to measure later changes in brain microstructures) followed by a functional MRI (fMRI) scan during which they performed an object-location association task. The object-location association task was made up of two parts: encoding and cued recall. During encoding, participants were shown pairs of cards that appeared in their own unique spot on an 8x5 grid. Each card had an image of an item belonging to one of three categories: fruits and vegetables, animals or inanimate objects. Pairs of cards were shown one after the other for 2 seconds each, followed by a brief delay before the next pair. During cued-recall, participants were shown the first card in a pair and had to indicate the location of the second card in the 8x5 grid. Importantly, participants went through 8 repetitions of these encoding-recall runs, 4 in each session. After a short break, the participants received a second set of diffusion-weighted MRI scans followed by two high-resolution T1- and T2-weighted scans (which show brain anatomy). The second session (13 hours after the first session) consisted of a third set of diffusion-weighted MRI scans and an additional task and fMRI session. The object-location association task in the second session started with an initial recall to determine the retention memory from the first session. The control condition was almost identical to the experimental condition, except that participants did not undergo the object-location association task and fMRI scan.   

What did they find?

The authors used whole-brain analyses to identify active brain areas during memory recall in the first and second sessions. They found an experience-dependent increase in brain activity in the bilateral precuneus (part of the posterior parietal cortex), the dorsal visual stream, the cerebellum, thalamus, and motor areas with subsequent repetitions of the object-location association task. Moreover, increased activation in the bilateral precuneus persisted after 12 hours and was positively correlated with memory performance. These findings suggest that the posterior parietal cortex, especially the precuneus, may encode memory engrams. To be considered an engram, however, there needs to be evidence of structural plasticity in the brain. The authors analyzed diffusion-weighted MRI scans to investigate whether there were changes in mean diffusivity, a measure of water diffusion in the brain that can indirectly reflect learning-dependent plasticity. They found evidence of microstructural changes, as indicated by a decrease in mean diffusivity, in bilateral precuneus and areas along the dorsal and ventral visual streams. These changes were compared to diffusion-weighted scans in the control group to determine whether they were learning-specific. Indeed, the authors found evidence of learning-specific changes in the left precuneus, the left middle occipital gyrus and left fusiform gyrus that persisted for 12 hours and were positively correlated with memory performance. Altogether, these findings suggest that the posterior parietal cortex, especially the precuneus, may be an important structure for acquiring and storing memory engrams.

What's the impact?

This is the first study to provide evidence of a rapid neocortical engram in the posterior parietal cortex using a combination of fMRI and diffusion imaging. The precuneus has been shown to exemplify all the criteria for a memory engram: functional responses corresponding to memory recall that persist over time, with evidence of local structural plasticity.

Brodt et al. Fast track to the neocortex: A memory engram in the posterior parietal cortex. Science (2018). Access to the original scientific publication here.