Statistical parametric mapping
Statistical parametric mapping or SPM is a statistical technique for examining differences in brain activation recorded during functional neuroimaging experiments using brain imaging technologies such as fMRI or PET. It may also refer to a specific piece of software created by the Wellcome Department of Imaging Neuroscience (part of University College London) to carry-out such analyses.
The statistical parametric mapping approach
Unit of measurement
Functional neuroimaging, one type of 'brain scanning', involves the measurement of brain activity. The technique used to measure brain activation varies depending on the technology being used (see fMRI and PET for examples) although regardless of the particular method used, the scanner will produce a 'map' of the area being scanned represented as voxels. Each voxel typically represents the activation of a particular co-ordinate in three dimensional space. The exact size of a voxel will vary depending on the technology used, although fMRI voxels typically represent a volume of 1mm3.
Experimental design
Researchers are often interested in examining brain activation linked to a specific psychological process or processes. An experimental approach to this problem might involve asking the question 'which areas of the brain are significantly more active when a person is doing task A compared to task B ?'. Although each task might be designed to be identical except for the aspect of behaviour under investigation, the brain is still likely to show changes in activation between tasks due to factors other than task differences, as it is involved with co-ordinating a whole range of parallel functions.
To control for the activation caused by these additional functions, and to highlight the areas of activation linked specifically to the process under investigation, statistics are used to look for the most significant difference above and beyond and other brain activity.
This involves multi-stage process to prepare the data, and then to analyse it using a general linear model.
Image pre-processing
Images from the brain scanner may be pre-processed before any statistical comparison takes place to remove noise or correct for sampling errors.
Functional neuroimaging studies usually involve a number of participants, all of which will have slightly differently shaped brains. All are likely to have the same gross anatomy, although minor differences will be apparent due to differences in (for example) skull shape and size. To aide comparisons, the 3D image of each individual's brain is transformed so the structures 'line up', a process known as normalisation. Standard brain maps such as the Talairach-Tournoux or MNI map are often used to allow researchers from across the world to compare their results in a common format.
Images are often smoothed (similar to the 'blur' effect used in some desktop image editing software) by which voxels are averaged with their neighbours, typically using a gaussian filter or by wavelet transformation, to make the data less noisy.
Statistical comparison
Parametric statistical models are assummed at each voxel, using the general linear model to describe the variability in the data in terms of experimental and confounding effects, and residual variability. Hypotheses expressed in terms of the model parameters are assessed at each voxel with univariate statistics.
Analyses may also be conducted to examine differences over a time series, or correlations between a task variable and brain activation in a certain area.
Because many statistical tests are being conducted, adjustments have to be made to control for Type II errors potentially caused by the number of multiple comparisons. Adjustments are made based on the theory of continuous random fields and a new criteria for statistical significance is set.
Graphical Representations
Brain activation can be represented in a number of ways, including tables of co-ordinates of statistically significant difference in activation. However, brain activation is commonly shown as patches of colour on MRI brain 'slice', that represent voxels which show statistically significant differences between conditions.
It may also be represented as a 'glass brain', a representation of three outline views of the brain, as if it were made of glass and only the patches of activation visible as areas of shading.
|
|
SPM software
SPM is software written by the Wellcome Department of Imaging Neuroscience to aide in the analysis of functional neuroimaging data. It is written using Matlab and is distributed as free software.
See also
External links
- SPM software and documentation from the Wellcome Department of Imaging Neuroscience.
- Cambridge Imagers - Neuroimaging information and tutorials.