Brain - Behaviour relationship

Brain - behaviour relationship refers to the link between brain activity, structure, chemical or genetic profile and performance in cognitive tasks, social or emotional situations.

Recording information about behaviour or brain activity can be measured through a number of different ways. Can be measured electrically (single/multiple-unit recording, evoked potential, EEG), chemically (postmortem analysis, in vivo analysis) or through neuroimaging (MRI, fMRI)

Recording information about behaviour or brain activity once a permanent or reversible change is introduced can be measured through either way. Reversible changes can be measured through TMS and TES. Irreversible changes refer to brain lesions. Since it is unethical to create a brain lesion in a healthy human, this can only be studied in natural cases in humans or through animal studies.

The lesion approach is a way to understand the relationship between the brain and behaviour. It's based on studying the consequences of real or "virtual" (TMS/TES) brain lesions on behaviour.

The key logic is that if a participant shows a change in behaviour e.g being unable to perform a cognitive task following a lesion to a brain area, then the behaviour or execution of the task must depend on this lesioned area. Without a lesion, it may be unclear which part of the brain is related to which cognitive ability.

Neuropsychology is the study of cognitive abilities in patients with acquired brain lesions. An example of this is the discovery of Broca's area. Broca's patient was severely aphasic (difficulties with language) so the area which was lesioned in the patient's brain was deemed as being largely responsible for a certain ability for language (now called Broca's area). Broca's asphasia refers to speech that's slow, laborious and non-fluent. Wernicke's asphasia refers to speech that is fluent but typically repetitive and lacking in meaning.

The lesion approach can tell us:

• to what extent a brain region(s) is critical for a behaviour or cognitive process (can show a causal link)
• how the brain re-adjusts itself following a lesion
• the role of brain regions and biological bases of "normal" (pre-lesioned) cognitive abilities

Advantages

• it allows making inferences on the critical role of a brain area for a behaviour or a cognitive process. If a structure in the brain is damaged and the patient cannot carry out a certain function, it is logical to assume that structure is implicated in that function; can demonstrate one-to-one relations between structure and function
• allows for explanations of behaviour and cognitive processes in the healthy brain
• on the basis of causality, allows for building cognitive models (e.g for language, the Wernicke-Lichtheim model, or more recently the Wernicke-Lichtheim-Geschwind model)

Disadvantages

• Compensatory plasticity cannot be controlled for. Following a brain lesion, patients tend to spotaneously improve or with the help of therapy. Thus, it is unclear what exactly is measured: lesioned systems or residual ones
• Diaschisis: changes in activity and function at sites distant but anatomically connected to the lesioned areas
• Lesions can be widespread, it is unclear which areas within a lesion are actually critical for a cognitive function if the lesion covers a large area in the brain
• Patients may not be easily available; as well as ethical issues with testing
• Lack of comparison - if a brain damaged patient behaves aggressively, it is difficult to know whether this is just how they behaved before brain damage
• Lesion-behaviour relationship can be inconsistent: patients with similar brain lesions may perform differently in cognitive tasks, or those with different brain lesions may perform similarly. The same cognitive functions can be supported by more than one anatomical system. One-to-one correspondence is not always valid

Transcranial magnetic stimulation (TMS) is a method in which a changing magnetic field is used to cause electric current to flow in a small region of the brain via electromagnetic induction. During a TMS procedure, a magnetic field generator or coil is placed near the head. The coil is connected to a pulse generator or stimulator that delivers a changing electric current to the coil.

TMS can be used to study temporary, reversible "virtual" lesions in healthy participants, and may also be used in patients with brain lesions. The key logic is if a reversible and temporary lesion to a brain area causes significant changes in behaviour, then this area is likely to be causally linked to behaviour.

Advantages

• Allows for making inferences on the critical role of a brain area for a cognitive process without confounds related to brain plasticity as in neuropsychology
• Overcomes problems related to large brain lesions in patients, and the scarcity of neuropsychological patients
• Can tell us the precise moment in time the contribution of a brain region to behaviour is critical

Disadvantages of TMS

• Doesn't account for brain plasticity as no time is left for recovery following virtual lesion
• Doesn't explain how the same cognitive functions can be supported by more than one anatomical system

Neuroimaging/brain imaging is the use of various techniques to either directly or indirectly image the structure or function of the nervous system. There are two broad categories

• Structural imaging: deals with the structure of the nervous system and diagnosis of intracranial disease (such as tumours) and injury
• Functional imaging: used to diagnose metabolic diseases and lesions on a finer scale (such as Alzheimer's disease) and also for neurological and cognitive psychology research, and building brain-computer interfaces

Types of neuroimaging techniques

• Computerised Axial Tomography (CAT): X-ray cameras rotate around the head, combining images into a 3D picture of the brain structure. Mainly used to diagnose neurological conditions. Doesn't provide information about brain function
• Positron Emission Tomography (PET): radioactive glucose is taken up by cells in the brain. The PET tracks the brain's consumption of a radioactive glucose injection, providing images of brain function
• Magnetic Resonance Imaging (MRI): takes deatiled pictures of the brain, using a strong magnetic field. Can show functions of the brain

Advantages of the neuroimaging approach

• Contributes to identify the brain area(s) correlated to behaviour/cognitive performance. This association can be made by testing whether behaviour or performance in a cognitive task corresponds to changes in brain activation (fMRI) or in brain structure (MRI)
• Contributes to building cognitive models, often in combition with other neuropsychological evidence
• It helps to identify and quantify brain lesions. Before neuroimaging techniques were available, lesions could only be identified post-mortem. Lesions can now be identified with MRI and CAT. It gives precise localisation of the lesion, thus helps the planning of rehabilitation
• Helps define individual variability in cognitive performance

Main steps in the neuroimaging approach: participants perform a task either while in the scanner, or outside it. Changes in blood oxygenation or in grey/white matter are measured. Behavioural and neuroimaging results are then associated, typically via regression analyses. For example, Dehaene et al (2003) studied maths-related brain regions. Areas in the brain were highlighted where oxygen consumption was at maximal consumption while performing a task, suggesting these areas were involved in the task.

Disadvantages

• It offers information only about correlations between brain and behaviour. However, correlations aren't proof of causality between brain and behaviour
• It's an expensive method
• PET can be quite invasive - the patient must be injected with a radioactive tracker which can be uncomfortable

The best method is to combine neuroimaging with the lesion approach. Neuroimaging in healthy people, neuropsychological studies, and TMS all have advantages and limitations. To make the best use of these methods and overcome the limitations, these approaches need to be used in combination.