What keeps memories separate?

An important part of making memories is that we can recall them on cue, without instead dredging up a slightly different event or fact. To do this, our brains are thought to rely on a process called pattern separation within the hippocampus. The wiring of a specific part of the hippocampus – the dentate gyrus – is thought to allow similar cues to produce outputs that are divergent enough to evoke different memories. What has been unclear is the details of that wiring. Now, three separate groups show that two different types of cell in the dentate gyrus behave in stereotyped ways during the learning process.

The studies focused on the differences between so-called granule cells and mossy cells in the dentate gyrus. Granule cells, due to their high number and very selective activity, are thought to be critical for pattern separation. In these three studies, techniques that allowed mossy cells to be identified show that they are also extremely important. In fact, under certain conditions, mossy cells may even be responsible for making granule cells so selective. These findings help to delineate how the brain correctly accesses the right material from a vast library of potential knowledge.

Danielson et al. (2017) In vivo imaging of dentate gyrus mossy cells in behaving mice. Neuron 93(3):552-559

Goodsmith et al. (2017) Spatial representations of granule cells and mossy cells of the dentate gyrus. Neuron 93(3):677-690

Senzai and Buzsaki (2017) Physiological properties and behavioral correlates of hippocampal granule cells and mossy cells. Neuron 93(3):691-704

Sleep makes room for learning

Sleep is a vital part of life, although its precise roles are still unknown. Considerable evidence suggests it is important for learning, and in particular for consolidating memories formed during previous waking periods. Sleep has also been hypothesized to allow the brain to recalibrate itself after the frenzied activity of waking. In this study, Chiara Cirelli and colleagues show that this recalibration is likely to keep the memory system stable.

The researchers measured the physical size of synapses, the connections between neurons. This serves as a reliable proxy of synaptic strength, as strengthened synapses grow larger and weakened synapses shrink. A theoretical problem exists, however, because strengthened synapses have a natural tendency to grow even stronger, setting up a positive feedback that destablizes the network. To counter this, the brain must have a way of dampening activity.

Using electron microscopy to study the fine anatomical detail of synapses in the mouse cortex, Cirelli and colleagues find that synapses shrink during sleep, equilibrating for the net growth that occurs when the animals are awake and learning. The researchers estimated that ~80% of the synapses they measured decreased in size during sleep, with the largest 20% – those that are the strongest – least likely to change.

De Vivo et al. (2017) Ultrastructural evidence for synaptic scaling across the wake/sleep cycle. Science 355(6324):507-510

The benefits, and perils, of overlearning

When we learn something new, it must undergo a process called consolidation, something that can take hours. In the intervening time, what we just learnt can be disrupted by subsequent incoming information. In this study, researchers from Brown University show that ‘overlearning’ of the first task – that is, continuing to train even once performance is no longer improving – can make it immune to disruption. Unfortunately, as well as stabilizing learning of the first task, overlearning interfered with learning of the second task. The authors show that hyperstabilization is correlated with a switch in brain activity from excitation-dominant to inhibition-dominant.

The findings highlight the benefits and perils of overtraining, and emphasize the importance of consolidation time after learning, with clear implications for classroom learning.

Shibata et al. (2017) Overlearning hyperstabilizes a skill by rapidly making neurochemical processing inhibitory-dominant. Nature Neuroscience 20:470-475

How to improve neuroeducation

In recent years, significant efforts have been made to link the neuroscience of how the brain learns with the practice of classroom education. The benefits to be derived from this emerging field – variously termed science of learning, educational neuroscience, or neuroeducation – are frequently debated, with seemingly as many proponents as there are skeptics. One of the major criticisms of the field is that the intellectual distance between neuroscience and education is too great, a ‘bridge too far’. As such, a primary concern for practitioners is to find the best ways to address this gap.

Here, Ravet and Williams use their own collaborative experience as a case study of transdisciplinary research in neuroeducation. Their specific concern was whether a program of clinical origin, which was aimed at improving the imitation skills of autistic children, could be implemented as a classroom-based intervention. As such, their study involved collaboration between a clinical psychiatrist and an educational researcher. The authors candidly discuss the challenges that arose, the opportunities and benefits created, and the lessons they learned. Their chief conclusion is that transdisciplinary interactions in neuroeducation are difficult but necessary, and less common than they should be. As such it is important for practitioners not just to collaborate across disciplines, but to share their experiences as instruction for others.

Ravet and Williams (2017) What we know now: education, neuroscience and transdisciplinary autism research. Educational Research 59:1-16