Synaptic Pruning and Neural Networks

Synaptic pruning (aka neural pruning)¹ helps the formation of neural networks. In this post we’ll look to understand the terms “synaptic pruning” and “neural networks.” We’ll also look at how synaptic pruning can help the formation of neural networks. 

What is a “neural network?”

A “neural network” is a series of connected neurons. Information travels along these networks that enable us to do things. For example, when you see a ball thrown to you and you try to catch it, sensory neurons in your eyes send a signal along a network that connects to your visual and motor cortices in your brain that then send signals to the neurons connected to your arm, hand and finger muscles so you can lift your hands and catch the ball. The more times you’ve thrown and caught a ball, the more developed this neural network will be and the better baseball player you’ll become.

This is why bad habits are hard to break – you literally have to rewire the networks in your brain and allow the old habits (and their networks) to decay.

What is “synaptic pruning?” (aka “neural pruning)

Neurons have dendrites, which are the branch like things that connect with other neurons. The connection between two dendrites is called a synapse and the connections between the synapses is how information travels around our brain (as we saw in the above baseball example).

Synaptic pruning happens when we lose these synaptic connections because we do not need them. For instance, if you learned a few words of Spanish in 9th grade but never studied it again, those connections you made have probably been pruned by now which is why you can’t speak Spanish anymore. Think of it like pruning a tree – if you don’t need the branches anymore, you cut them off. This is what happens to connections in our brain. You literally have to use it or lose it!

Interestingly, this process of synaptic pruning is most observable in adolescents.

Grey Matter is made up of neuronal cell bodies, dendrites and synapses. Most grey matter in the brain is in the cerebral cortex (the outer layer).

White Matter is made up of mostly axons and is found mostly in the interior of the brain. The white matter (axons) connect neurons in deep regions of the brain (e.g. hippocampus and amygdala) with the cortex (e.g. prefrontal cortex).

How Synaptic Pruning Forms Neural Networks

Unused synapses are pruned, which increases the effectiveness of the more commonly used neural circuits. This is how synaptic pruning helps the formation and connections of other more commonly used neural networks. It’s a little bit like throwing out old clothes in your closet that you never wear so you’ve got more space for your favourite clothes you wear all the time. Or, it’s like deleting old apps from your phone so the ones you do use run faster.

Synaptic pruning happens most during adolescence, especially the thinning of grey matter in the outer layer of the brain (the cortex). Grey matter is made from the synaptic connections, cell bodies and dendrites – the stuff that makes the neural networks. The pruning helps to improve our cognition and maybe even our IQ because it strengthens other networks. In fact, one study found that children with a faster development of frontal lobe grey matter (compared with other kids) had higher IQs and this grey matter was also pruned faster than other kids (Shaw et al. 2006 link).

Synaptic pruning is like deleting old apps from your phone so the ones you do use can run faster.

Synaptic pruning can be studied by observing changes in grey matter in the brain using MRI scanners. Grey matter in the brain consists of cell bodies, dendrites and synapses. The more grey matter, the more synaptic connections and neural networks. The less grey matter, the fewer synapses. Therefore, measuring changes in grey matter can show the synaptic pruning process. This is especially true for adolescents (teenagers) because synaptic pruning occurs mostly during this stage. In later adult life, there may be other explanations for decreases in grey matter (e.g. diseases like Alzheimer’s).

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Key Study: Squeglia et al. (2013)

Aims: The aim of this study was to see how “neurostructural redevelopment” (e.g. synaptic pruning) correlates with cognition and behaviour.

Methods: This correlational study used brain scans to observe the grey matter in the brains of 185 kids between 12-14 years old from San Diego, USA. The cortical thickness (e.g. amount of grey matter in the outer layer of the brain) was measured using MRIs. As well as brain scanning, the researchers also gathered data from the kids on a range of cognitive abilities, including problem solving, working memory and learning. For example, the digit-span task was used to assess working memory capacity.

To control for extraneous variables and to make sure the sample was representative of a “healthy population,” kids were excluded if they had experienced things like drug, alcohol or cigarette use; if they psychological disorders or learning disabilities or if they weren’t fluent in English.

Results – Synaptic Pruning

One correlation found in the study was a negative correlation between age and cortical thickness in 10 areas of the brain (7 of these being in the frontal regions). In other words, the older the kids were, the less grey matter they had. This is consistent with other evidence that shows synaptic pruning is taking place between the ages of 12-14. Boys showed more “cortical thinning” (synaptic pruning) than girls.

Results – Neural Networks & Synaptic Pruning

Another important correlation was that cortical thickness in the parietal lobe was negatively correlated with performance on cognitive tasks, including learning, memory and problem solving. In other words, the thinner the parietal lobe cortex (less grey matter – more synaptic pruning) the better the kids were at tests of learning, memory and problem solving. This is similar to other studies that suggests the synaptic pruning helps to improve cognitive function through “…the elimination of unnecessary synaptic connections.” That is to say, the process of synaptic pruning helps develop the neural networks associated with learning and memory.

Interestingly, no significant correlation was found for cortical thickness and working memory or attention.

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IB Psychology Exam Tips

• The terms “neural network” or “neural pruning” might be a Biological Approach SAQ in Paper One.
• You’ll need to know the definition of the key terms and at least one study.
• The formation of neural networks by neural pruning is an example of neuroplasticity, so you could use the information in this post to explain neuroplasticity.

Comprehension Questions

See how well you understood what you just read by trying to answer these questions without re-reading the article. 

1. Define “synaptic pruning.”
2. Define a “neural network.” 
3. How does synaptic pruning help the formation of neural networks?
4. Explain how Squeglia et al.’s study demonstrates the process of synaptic pruning and the formation of neural networks?

References

Squeglia, L. M., Jacobus, J., Sorg, S. F., Jernigan, T. L., & Tapert, S. F. (2013). Early adolescent cortical thinning is related to better neuropsychological performance. Journal of the International Neuropsychological Society : JINS, 19(9), 962–970. doi:10.1017/S1355617713000878 (Link)

Kharitonova, M., Martin, R. E., Gabrieli, J. D., & Sheridan, M. A. (2013). Cortical gray-matter thinning is associated with age-related improvements on executive function tasks. Developmental cognitive neuroscience, 6, 61–71. doi:10.1016/j.dcn.2013.07.002 (Link)

¹The IB calls is neural pruning, but the far more commonly used term in psychology is synaptic pruning – both mean the same thing.