The latest refrain in popular science is that ‘your brain is plastic’, that experience has the potential to ‘rewire’ your brain, and that many previous mysteries in cognitive science can be explained by ‘neuroplasticity’. What they don’t tell you is that these phrases are virtually meaningless.
Neuroplasticity sounds very technical, but there is no accepted scientific definition for the term and, in its broad sense, it means nothing more than ‘something in the brain has changed’. As your brain is always changing the term is empty on its own.
This is from the introduction to the influential scientific book Toward a Theory of Neuroplasticity:
Given the central important of neuroplasticity, an outsider would be forgiven for assuming that it was a well defined and that a basic and universal framework served to direct current and future hypotheses and experimentation. Sadly, however, this is not the case. While many neuroscientists use the word neuroplasticity as an umbrella term it means different things to different researchers in different subfields… In brief, a mutually agreed upon framework does not appear to exist.
It’s currently popular to solemnly declare that a particular experience must be taken seriously because it ‘rewires the brain’ despite the fact that everything we experience ‘rewires the brain’.
It’s like a reporter from a crime scene saying there was ‘movement’ during the incident. We have learnt nothing we didn’t already know.
Neuroplasticity is common in popular culture at this point in time because mentioning the brain makes a claim about human nature seem more scientific, even if it is irrelevant (a tendency called ‘neuroessentialism‘).
Clearly this is rubbish and every time you hear anyone, scientist or journalist, refer to neuroplasticity, ask yourself what specifically they are talking about. If they don’t specify or can’t tell you, they are blowing hot air. In fact, if we banned the word, we would be no worse off.
As every change in the brain can be referred to as ‘neuroplasticity’ you need to look out for what is actually meant. As we are constantly learning more about the brain, the possible list is endless, but here are some of the most common processes associated with the term:
Structural changes in the brain
Synaptic plasticity refers to changes in the strength of connections between synapses, the chemical or electrical connection points between brain cells. Synaptic plasticity is an umbrella term in itself, and means nothing except something has changed at the synapse, but may include many specific processes such as long-term potentiation (LTP) or depression (LTD), changes in the number of receptors for specific neurotransmitters, and changes in which proteins are expressed inside the cell, among many others known and unknown. As a rule of thumb, nothing changes in the brain without changes in the synapses.
Synaptogenesis and synaptic pruning refers to the creation and removal of whole synapses or groups of synapses which build or destroy connection between neurons.
Neuronal migration is the process where neurons extend from their ‘place of birth’ to connect to far reaching areas across the brain.
Neurogenesis is the creation of new neurons. It largely occurs in the developing brain although over the last decade or so we’ve realised that limited neurogenesis occurs in the adult brain.
Neural cell death is literally where neurons die. This can happen through damage, over-excitation or disease, but also as a natural ‘programmed’ process including apoptosis. When this programmed cell death fails, it can sometimes lead to cancer.
Other forms of ‘neuroplasticity’ may be inferred from structural changes in the brain that do not involve direct measurement of individual neurons.
These usually come from brain scans and can involve changes in the density of white matter or grey matter on structural MRI scans, or to how densely radioactively labelled markers bind to specific receptors in parts of the brain.
Functional reorganisation – changes in how tasks are organised in the brain
As we develop, brain areas becomes specialised for specific tasks and ways of making sense of the world. For example, the very back of your brain is labelled the visual cortex, because it deals with sight.
If experience changes dramatically or parts of the brain are damaged, areas previously specialised for a certain function can ‘take on’ some of the work of other areas, without necessarily detectably changing in structure. For example, the ‘visual cortex’ in blind people can be used to perceive touch.
Functional reorganisation is often inferred without directly measuring the brain. For example, immediately after brain injury, someone might not be able to speak because the areas previously used for language are damaged. However, speech may be regained or it might improve, depending on the extent of damage, as the brain has a limited ability to reorganise the share of work to undamaged areas.
Learning or habit
This is the loosest and most problematic use of ‘neuroplasticity’. By definition if we learn something, acquire a habit or tendency, good or bad, something has changed in the brain. Without specifying what the brain is doing, we know nothing more.
UPDATE: You might also be interested in a subsequent post that tackles the myths that neuroplasticity is a new idea and, until quite recently, we thought the brain was ‘fixed’.