Statistical parametric mapping (SPM) is a

statistical Statistics (from German language, German: ', "description of a State (polity), state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a s ...

technique for examining differences in

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

activity recorded during

functional neuroimaging Functional neuroimaging is the use of neuroimaging technology to measure an aspect of brain function, often with a view to understanding the relationship between activity in certain brain areas and specific mental functions. It is primarily used a ...

experiments. It was created by Karl Friston. It may alternatively refer to software created by the Wellcome Department of Imaging Neuroscience at

University College London University College London (Trade name, branded as UCL) is a Public university, public research university in London, England. It is a Member institutions of the University of London, member institution of the Federal university, federal Uni ...

to carry out such analyses.

Approach

Unit of measurement

Functional neuroimaging is one type of 'brain scanning'. It involves the measurement of brain activity. The measurement technique depends on the imaging technology (e.g.,

fMRI Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area o ...

and

PET A pet, or companion animal, is an animal kept primarily for a person's company or entertainment rather than as a working animal, livestock, or a laboratory animal. Popular pets are often considered to have attractive/ cute appearances, inte ...

). The scanner produces a 'map' of the area that is represented as

voxel In computing, a voxel is a representation of a value on a three-dimensional regular grid, akin to the two-dimensional pixel. Voxels are frequently used in the Data visualization, visualization and analysis of medical imaging, medical and scient ...

s. Each voxel represents the activity of a specific volume in three-dimensional space. The exact size of a voxel varies depending on the technology. fMRI voxels typically represent a volume of 27 mm³ in an equilateral cuboid.

Experimental design

Researchers examine brain activity linked to a specific mental process or processes. One approach involves asking 'which areas of the brain are significantly more active when doing task A compared to task B?'. Although the tasks might be designed to be identical, except for the behaviour under investigation, the brain is still likely to show changes in activity between tasks due to factors other than task differences (as the brain coordinates many parallel functions unrelated to the task). Further, the signal may contain noise from the imaging process itself. To filter out these random effects, and to highlight the areas of activity linked specifically to the process under investigation, statistics look for the most significant differences. This involves a multi-stage process to prepare the data, and to analyse it using a

general linear model The general linear model or general multivariate regression model is a compact way of simultaneously writing several multiple linear regression models. In that sense it is not a separate statistical linear model. The various multiple linear regre ...

Image pre-processing

Images from the scanner may be pre-processed to remove noise or correct for sampling errors. A study usually scans a subject several times. To account for the motion of the head between scans, the images are typically adjusted so voxels in each image correspond (approximately) to the same site in the brain. This is referred to as ''realignment'' or ''motion correction'', see image realignment. Functional neuroimaging studies usually involve multiple participants, each of whom have differently shaped brains. All are likely to have the same gross anatomy, saving minor differences in overall brain size, individual variation in topography of the

gyri In neuroanatomy, a gyrus (: gyri) is a ridge on the cerebral cortex. It is generally surrounded by one or more sulcus (neuroanatomy), sulci (depressions or furrows; : sulcus). Gyri and sulci create the folded appearance of the brain in huma ...

and

sulci Sulci or Sulki (in Greek , Stephanus of Byzantium, Steph. B., Ptolemy, Ptol.; , Strabo; , Pausanias (geographer), Paus.), was one of the most considerable cities of ancient Sardinia, situated in the southwest corner of the island, on a small isla ...

of the

cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. It is the largest site of Neuron, neural integration in the central nervous system, and plays ...

, and morphological differences in deep structures such as the

corpus callosum The corpus callosum (Latin for "tough body"), also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental ...

. To aid comparisons, the 3D image of each brain is transformed so that superficial structures line up, via '' spatial normalization''. Such normalization typically involves translation, rotation and scaling and nonlinear warping of the brain surface to match a standard template. Standard brain maps such as the Talairach-Tournoux or templates from the Montréal Neurological Institute (MNI) allow researchers from across the world to compare their results. Images can be smoothed to make the data less noisy (similar to the 'blur' effect used in some image-editing software) by which voxels are averaged with their neighbours, typically using a Gaussian filter or by

wavelet A wavelet is a wave-like oscillation with an amplitude that begins at zero, increases or decreases, and then returns to zero one or more times. Wavelets are termed a "brief oscillation". A taxonomy of wavelets has been established, based on the n ...

transformation.

Statistical comparison

Parametric statistical models are assumed at each voxel, using the general linear model to describe the data variability in terms of experimental and confounding effects, with residual variability. Hypotheses expressed in terms of the model parameters are assessed at each voxel with univariate statistics. Analyses may examine differences over

time Time is the continuous progression of existence that occurs in an apparently irreversible process, irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequ ...

(i.e. correlations between a task variable and brain activity in a certain area) using linear

convolution In mathematics (in particular, functional analysis), convolution is a operation (mathematics), mathematical operation on two function (mathematics), functions f and g that produces a third function f*g, as the integral of the product of the two ...

models of how the measured signal is caused by underlying changes in neural activity. Because many statistical tests are conducted, adjustments have to be made to control for

type I error Type I error, or a false positive, is the erroneous rejection of a true null hypothesis in statistical hypothesis testing. A type II error, or a false negative, is the erroneous failure in bringing about appropriate rejection of a false null hy ...

s (false positives) potentially caused by the comparison of levels of activity over many voxels. A type I error would result in falsely assessing background brain activity as related to the task. Adjustments are made based on the number of resels in the image and the theory of continuous

random field In physics and mathematics, a random field is a random function over an arbitrary domain (usually a multi-dimensional space such as \mathbb^n). That is, it is a function f(x) that takes on a random value at each point x \in \mathbb^n(or some other ...

s in order to set a new criterion for statistical significance that adjusts for the problem of

multiple comparisons Multiple comparisons, multiplicity or multiple testing problem occurs in statistics when one considers a set of statistical inferences simultaneously or estimates a subset of parameters selected based on the observed values. The larger the numbe ...

Graphical representations

Differences in measured brain activity can be represented in various ways. They can be presented as a table, displaying coordinates that show the most significant differences in activity between tasks. Alternatively, differences in brain activity can be shown as patches of colour on a brain 'slice', with the colours representing the location of voxels with statistically significant differences between conditions. The color gradient is mapped to statistical values, such as t-values or z-scores. This creates an intuitive and visually appealing map of the relative statistical strength of a given area. Differences in activity can be represented as a 'glass brain', a representation of three outline views of the brain as if it were transparent. Only the patches of activation are visible as areas of shading. This is useful as a means of summarizing the total area of significant change in a given statistical comparison.

Software

SPM is software written by the Wellcome Department of Imaging Neuroscience at

to aid in the analysis of functional neuroimaging data. It is written using

MATLAB MATLAB (an abbreviation of "MATrix LABoratory") is a proprietary multi-paradigm programming language and numeric computing environment developed by MathWorks. MATLAB allows matrix manipulations, plotting of functions and data, implementat ...

and is distributed as

free software Free software, libre software, libreware sometimes known as freedom-respecting software is computer software distributed open-source license, under terms that allow users to run the software for any purpose as well as to study, change, distribut ...

References

{{reflist

External links

* Wikibooks SPM Wikibook.
fMRI guide by Chris Rorden

Introduction to fMRI: experimental design and data analysis

Cambridge Imagers
- Neuroimaging information and tutorials.
Buttons in SPM5
PowerPoint presentation from the SPM for dummies course
ISAS (Ictal-Interictal SPECT Analysis by SPM)
- Yale University

Biostatistics Computing in medical imaging Neuroimaging Neuroimaging software