What brain imaging allows us to investigate

Brain imaging techniques produce representations of brain structure or indicators of brain activity. Researchers use them to investigate relationships between the brain and human behaviour without directly observing exposed brain tissue.

The key distinction is between structure and function. A structural image shows anatomy—for example, the size or shape of a brain region. By contrast, a functional image shows a measured indicator of activity during a task or resting state. Neither explains behaviour on its own. Researchers still have to connect the measurement to what participants actually do.

MRI: investigating brain structure

Magnetic resonance imaging (MRI) uses strong magnetic fields and radio waves to generate detailed images of brain anatomy. This brain imaging technique can reveal differences in brain tissue, helping researchers examine how those differences relate to behaviour or cognition.

Suppose researchers want to investigate whether sustained practice of a complex movement skill is associated with structural changes in the brain. They could obtain MRI scans before and after a training period, then assess any changes alongside participants’ performance. The scan measures anatomy. The performance measure establishes whether the person’s skill has changed; one cannot substitute for the other.

MRI provides detailed structural images without ionizing radiation. Yet an anatomical difference doesn't tell us exactly what a region is doing. A relationship between anatomy and performance doesn't establish which caused which, either. Pre-existing differences might influence who becomes skilled, rather than skill practice being the sole cause of those differences.

fMRI: investigating activity during behaviour

Functional magnetic resonance imaging (fMRI) uses changes in blood oxygenation as an indirect indicator of neural activity. Neural activity is the electrical and chemical signalling carried out by nerve cells. Active tissue generally receives increased blood flow, producing an oxygenation signal that researchers can measure with this brain imaging technique.

Structural MRI and fMRI answer different questions: where is the tissue, and where does the measured activity signal change? A functional image is not a photograph of a thought.

Suppose participants alternate between making a planned sequence of finger movements and resting their hands. By comparing the fMRI signals between these conditions, researchers can identify regions associated with performing the movement task. That comparison matters. Activity in a region during a task is not informative unless researchers consider activity in an appropriate comparison condition.

fMRI can locate activity changes relatively precisely, but the blood-oxygenation signal is delayed relative to neural signalling. Movement in the scanner can distort measurements too. And lying still in a noisy scanner may differ substantially from performing the same behaviour in everyday life.

What the evidence can—and cannot—show

When a region’s signal increases during a task, this supports an association between that region and the task. It does not automatically show that the region causes the behaviour or is necessary for it. It doesn't show that the region works alone, either. Nor can we confidently infer a particular mental state from activation of a region that participates in many different processes.

Researchers need clearly defined tasks and suitable comparison conditions to investigate behaviour convincingly. They also need behavioural measurements alongside the images. Evidence is stronger when different methods support the same explanation. Participants need appropriate safety screening and informed consent, with procedures for protecting sensitive scan information and handling unexpected findings.

Particular regions make particular contributions

Localization of function is the organization of brain functions in which particular regions make specialized contributions to particular behaviours or cognitive processes. It helps explain why damage to one part of the brain can affect a specific ability while leaving other abilities relatively intact.

This doesn't mean that every complex behaviour has one isolated brain location. Think of specialized contributors working together, not separate boxes that each contain a complete ability.

For example, the primary motor cortex contributes to the control of voluntary movement, while the primary visual cortex contributes to the initial processing of visual information. To reach for an object, a person needs both kinds of processing, along with contributions from other regions. Several localized functions can therefore support a single behaviour.

Explaining behaviour and cognition

To explain an observable outcome through localization, we need to connect it to a region’s contribution. Damage to tissue important for controlling a particular movement can make that movement difficult to produce: part of the system supporting it has been disrupted.

The same reasoning applies to cognition, the set of mental processes involved in acquiring, processing, storing and using information. Language, for instance, requires regions to coordinate their contributions to understanding speech, planning speech and controlling its production. Damage may affect these components differently. It needn't remove language as a single, indivisible ability.

How evidence supports localization

Brain imaging can identify regions whose activity changes during a task. Studying people with brain damage can show that disruption of a region is associated with disruption of a function. The two approaches tell us different things. Activity suggests involvement, whereas the loss of an ability following damage may support the importance of the damaged tissue for that ability.

Neither approach is conclusive on its own. A task usually contains several processes, so activity might reflect attention, movement or task difficulty rather than the process of interest. Damage can also affect several regions or interrupt the connections between them. We can't always attribute a deficit neatly to one location.

Specialization within a connected brain

A brain network is a set of interconnected brain regions whose coordinated activity supports a function. Networks often explain complex behaviours and cognitive processes better than a single region working alone.

Localization and network explanations are compatible. A region can have a specialized role and still depend on communication with other regions. The strongest explanation identifies what a region contributes without treating that contribution as the complete explanation of the person’s behaviour.

The brain can change with experience

Neuroplasticity is the capacity of the nervous system to change its structure or functioning in response to experience, learning or injury. The brain isn't simply a fixed arrangement that produces behaviour. What a person does can, in turn, influence how that arrangement works.

A neuron is a nerve cell specialized for receiving and transmitting information. A synapse is a junction at which a neuron communicates with another cell. Plasticity can change the strength of synaptic communication or involve the formation or removal of connections. It can also change how brain networks are recruited.

Repeated use can strengthen relevant connections; those used less often may weaken or be removed. Treat this as a useful general pattern, not a rule that every repeated action must produce a visible anatomical change.

How plasticity helps explain learning

Imagine learning to coordinate both hands to operate an unfamiliar control panel. At first, the task is slow and effortful. Practice makes performance more accurate and coordinated. Changes in how the networks supporting perception and movement function provide a biological explanation for this improvement.

Keep the relationship precise. Practice supplies experience, which can alter neural connections and network activity. Those changes can then support improved performance. Saying only that “the brain changes” leaves the explanation unfinished.

Plasticity occurs across the lifespan, but its extent and form vary with age. They also depend on the brain system involved and the conditions of learning. Don't treat plasticity as unlimited adaptability or a guarantee that every person will improve equally.

Recovery and changing function

After injury, changes in surviving connections and in the recruitment of brain regions can contribute to recovery. Repeated rehabilitation activities may help a person practise functions supported by remaining tissue.

Recovery doesn't necessarily mean that damaged tissue returns to its original state. A person may regain some ability through altered networks, or develop a different strategy for achieving the same goal. The location and extent of damage can limit improvement, as can the opportunities for rehabilitation.

Nor is plasticity automatically beneficial. Repeated experience can reinforce unhelpful behavioural patterns as well as useful skills. The term describes a capacity for change, not a promise of a positive outcome.

Investigating plasticity rather than assuming it

To build stronger evidence for plasticity, researchers can measure the same participants before and after an experience. They should also assess behaviour alongside the brain measures and include a suitable comparison group. Without these steps, differences between experienced and inexperienced people may reflect pre-existing characteristics rather than effects of experience.

Structural MRI may reveal anatomical change, while fMRI may reveal altered task-related activity. Neither directly reveals every change at individual synapses. An fMRI signal change also needs careful interpretation: reduced activity after practice could reflect more efficient performance rather than deterioration.

Plasticity qualifies localization rather than disproving it. Brain regions can make specialized contributions even as their connections and patterns of participation remain capable of change.