Functional magnetic resonance imaging (fMRI) is a brain-scanning technique that measures blood flow in the brain when a person performs a task. fMRI works on the premise that neurons in the brain that are the most active during a task use the most energy.
Energy requires glucose and oxygen. Oxygen is carried in the bloodstream attached to haemoglobin (found in red blood cells) and is released for use by these active neurons, at which point the haemoglobin becomes deoxygenated.
Deoxygenated haemoglobin has a different magnetic quality from oxygenated haemoglobin. An fMRI can detect these different magnetic qualities and can be used to create a dynamic (moving) 3D map of the brain, highlighting which areas are involved in different neural activities. fMRI images show activity approximately 1-4 seconds after it occurs and are thought to be accurate within 1-2 mm. An increase in blood flow is a response to the need for more oxygen in that area of the brain when it becomes active, suggesting an increase in neural activity.
Evaluation of fMRI
Invasive or Non-Invasive: An advantage of fMRI is that is non-invasive. Unlike other scanning techniques, for example Positron Emission Tomography (PET), fMRI does not use radiation or involve inserting instruments directly into the brain, and is therefore virtually risk-free. Consequently, this should allow more patients/participants to undertake fMRI scans which could help psychologists to gather further data on the functioning human brain and therefore develop our understanding of localisation of function.
Spatial Resolution: fMRI scans have good spatial resolution. Spatial resolution refers to the smallest feature (or measurement) that a scanner can detect, and is an important feature of brain scanning techniques. Greater spatial resolution allows psychologists to discriminate between different brain regions with greater accuracy. fMRI scans have a spatial resolution of approximately 1-2 mm which is significantly greater than the other techniques (EEG, ERP, etc.) Consequently, psychologists can determine the activity of different brain regions with greater accuracy when using fMRI, in comparison to when using EEG and/or ERP.
Temporal Resolution: fMRI scans have poor temporal resolution. Temporal resolution refers to the accuracy of the scanner in relation of time: or how quickly the scanner can detect changes in brain activity. fMRI scans have a temporal resolution of 1-4 seconds which is worse than other techniques (e.g. EEG/ERP which have a temporal resolution of 1-10 milliseconds). Consequently, psychologists are unable to predict with a high degree of accuracy the onset of brain activity.
Causation: fMRI scans do not provide a direct measure of neural activity. fMRI scans simply measure changes in blood flow and therefore it is impossible to infer causation (at a neural level). While any change in blood flow may indicate activity within a certain brain area, psychologists are unable to conclude whether this brain region is associated with a particular function.
In addition, some psychologists argue that fMRI scans can only show localisation of function within a particular area of the brain, but are limited in showing the communication that takes place among the different areas of the brain, which might be critical to neural functioning.
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