Synaptic Pruning and Autism: When the Brain Doesn’t Declutter

Ever wondered how your brain decides what to keep and what to toss out? Deep inside your brain, a quiet but powerful process is shaping who you are. It’s called synaptic pruning—a kind of neurological spring cleaning where unused connections between brain cells are trimmed away, while the ones you use most get stronger. It’s the brain’s way of becoming more efficient, guided by the classic “use it or lose it” rule. And fascinatingly, this process may play a key role in understanding how autistic brains work differently.

Synaptic pruning: Nature and nurture

Genes influence the earliest phase of early synaptic pruning, but later, it’s mostly influenced by experiences ⁽¹⁾. If you have a child who grows up in a highly stimulating environment, with parents who play with them and read to them, they will retain a higher number of synapses than a child who receives little stimulation.

Synaptic pruning is important for brain effectivity 

During normal brain development, from several weeks after conception to about 8 months of age, there is an explosion of synapse formation ⁽²⁾. At 2 or 3 years of age, the number of synapses in the brain is at a peak level. The toddler’s brain has about 15000 synapses per neuron. But shortly thereafter, the brain begins to remove the synapses that aren’t used, or are used infrequently. This is important for the efficient function of the brain.

The most rapid synaptic pruning happens between the ages of 2 and 16. About 50% of the synapses are removed during this time. The pruning continues during adolescence, but at a slower rate. Recent studies ⁽³⁾ have suggested that a second synaptic pruning period occurs during late adolescence, and that this process continues into the third decade of life ⁽⁴⁾, ultimately reaching stable levels of synapses in adulthood.

Autistic brains have less synaptic pruning 

Or, at least, this is what happens in neurotypical brains. In autistic people, the synaptic pruning is slower or incomplete ⁽⁵⁾. By late childhood, when the number of synapses has dropped by 50% in the neurotypical brain, in autistic brains, it drops only by 16%. (This was discovered by Guomei Tang, PhD, who examined brains and measured synaptic density in the brains of 26 autistic people and 22 neurotypical people as a control.) As a result, in autistic brains, there are more connections between neurons and between various brain regions. This may lead to autistic symptoms like sensory oversensitivity or social difficulties.

Our hyperconnected brains have to work harder at processing messages that go through them, and it may take them longer. But there are theories that this may also lead to deep thinking, increased perception of details, and unique creative abilities.

Synesthesia and savant abilities are also a result of the lack of synaptic pruning

Synesthesia, where the activation of one sensory pathway triggers activation of another, unrelated sensory pathway, is another condition that occurs in brains that have had less synaptic pruning. Roughly 4% of the population has synesthesia, but studies have shown ⁽⁶⁾ that it occurs in 18,9% of autistic people.

The lack of synaptic pruning can also contribute to savant abilities. In the general population, the prevalence of savant syndrome is 1%, but in autistic people, it’s about 10%. ⁽⁷⁾

Autism – causation or correlation? 

So is autism caused by an excessive number of synapses in the brain? We can’t say for sure. Research shows only that there is a difference in synaptic density between autistic and neurotypical brains, but it is not clear if this is a cause or an effect of autism or just an association. 

But it’s interesting to note that many of the traits linked to reduced synaptic pruning—like strong senses, noticing patterns, bottom-up thinking, or even things like synesthesia and savant skills—aren’t just problems. They can also be strengths. Having more synapses might mean the brain gets overwhelmed more easily or takes longer to process things, but it can also mean deeper thinking, greater attention to detail, and unique creativity.

Synaptic pruning helps us understand how autistic brains might be wired differently—not wrong, just different. As science learns more, we may begin to see autism less as something that needs to be fixed, and more as a natural part of human diversity.

Sources:

(1) https://www.healthline.com/health/synaptic-pruning#an-indepth-look
(2) https://en.wikipedia.org/wiki/Synaptogenesis
(3) https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2022.840266/full
(4) https://pubmed.ncbi.nlm.nih.gov/21788513/
(5) https://www.cuimc.columbia.edu/news/children-autism-have-extra-synapses-brain
(6) https://pubmed.ncbi.nlm.nih.gov/24252644/
(7) https://embrace-autism.com/synaptic-growth-synesthesia-and-savant-abilities/

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