Empirical comparison of maximal voxel and non-isotropic adjusted cluster extent results in a voxel-based morphometry study of comorbid learning disability with schizophrenia
Introduction
Our understanding of brain abnormalities in schizophrenia has been greatly advanced through the use of neuroimaging techniques. Since the first computed tomography study (Johnstone et al., 1976) demonstrated enlarged ventricular size, the ability of neuroimaging to inform researchers in this field has been transformed by the development of magnetic resonance imaging (MRI) with its excellent spatial resolution and, more recently, the ability to combine and contrast large numbers of images through analysis techniques such as voxel-based morphometry (VBM).
Under Statistical Parametric Map (SPM) tests with Random Fields (RF) correction for multiple comparisons, we have the facility to quote maximal voxel, cluster extent and set level corrected P values. The set level results have the most power but minimal localization and are limited to reporting whole brain hypothesis tests. The maximal voxel results provide voxel-wise localization with minimal power for rejection of the null hypothesis. The cluster extent results provide regional rejection of the null hypothesis and reporting power less than that expected for the set level results but greater than that expected for maximal voxel results (Friston et al., 1994, Friston et al., 1995). In structural VBM analysis, it is the accepted practice to limit the reported results to maximal voxel returns, and it is taken that these maximal voxels point to regions where differences exist in the contrasted groups (Job et al., 2002, Keller et al., 2002, Suzuki et al., 2002, Woermann et al., 1999).
The assumption of uniform smoothness in the SPM{t} image, as required for RF multiple comparison correction, cannot be assured in structural VBM analysis. Thus, without adjustment for non-isotropic smoothness, these cluster localization tests are not used (Ashburner and Friston, 2000). It has been shown that the deviation from uniform smoothness in the SPM{t} can be corrected for by evaluating cluster extents in terms of an adjusted cluster size. The adjusted size in resels is evaluated from the cluster coverage in the resel per voxel (RPV) image (Worsley et al., 1999, Cao, 1999, Hayasaka et al., 2004). A comparison of adjusted cluster results RF corrected for multiple comparisons and permutation methods (Nichols and Holmes, 2001) was made by Hayasaka et al. (2004). In this, it was shown that adjusted cluster results are valid under RF correction when the full width half maximum (FWHM) smoothness estimate of the SPM{t} is greater than three times the voxel sampling resolution, and the degrees of freedom (df) exceeds 30. We report on cluster extents from a VBM study of comorbid learning disability and schizophrenia, previously reported on the basis of maximal voxel VBM (Moorhead et al., 2004). In this empirical report, we compare the maximal voxel results with adjusted cluster extent results, and we contrast these adjusted cluster extent results with those given when no correction is made for the underlying non-isotropic smoothness. In this, the FWHM is more than seven times the sampling resolution and df > 42. Thus, the requirements for the use of RF correction of multiple comparisons are met (Hayasaka et al., 2004).
Section snippets
Participants
The MRI scans for this report were obtained from individuals who took part in a clinical comparison of four matched subject groups. The subject groups were comorbid learning disability and schizophrenia, schizophrenia alone, learning disability alone and a normal control group (Moorhead et al., 2004, Sanderson et al., 1999, Sanderson et al., 2001). The patients with comorbid learning disability and schizophrenia meet research diagnostic criteria for schizophrenia (Spitzer et al., 1975) and have
Results
The contrasts between the normal controls and the schizophrenia, comorbid and learning disability groups were examined to assess the specificity that could be obtained in the cluster results for differing primary thresholds. The primary threshold was set to uncorrected t values of 3.3, 3.7, 4.07, 4.45 and 4.79. At each of these settings, the number of significant suprathreshold clusters (unadjusted) was noted and the results tabulated in Table 1. These results show that the maximum specificity
Discussion
Structural VBM implemented under the SPM99 package involves the formation of statistical parametric maps that give a voxel-wise perspective from which rejection of the null hypothesis can be assessed. After corrections for multiple comparisons, localized rejection is made on the basis of maximal voxel significance. Corrections for multiple comparisons in the statistical map are determined in random fields analysis by computing the likely number of resolution elements given the smoothness of
Conclusion
We have implemented an empirical comparison of unadjusted and adjusted cluster extent, tested for null hypothesis rejection and corrected for multiple comparisons by random fields methods in SPM structural analysis. Due to non-isotropic smoothness in the SPM{t}, the validity of unadjusted cluster results is uncertain. Adjusting the cluster size to take account of deviation in the underlying SPM{t} from the image mean allows valid corrected P values to be computed for cluster extents. Our
Acknowledgment
This study was funded by a program grant from the Medical Research Council of Great Britain.
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