Conjunction revisited

The Wellcome Department of Imaging Neuroscience and Institute of Cognitive Neuroscience, University College London, 17 Queen Square, London, WC1N 3AR, UK.
NeuroImage (Impact Factor: 6.36). 04/2005; 25(3):661-7. DOI: 10.1016/j.neuroimage.2005.01.013
Source: PubMed


The aim of this note is to revisit the analysis of conjunctions in imaging data. We review some conceptual issues that have emerged from recent discussion (Nichols, T., Brett, M., Andersson, J., Wager, T., Poline, J.-B., 2004. Valid Conjunction Inference with the Minimum Statistic.) and reformulate the conjunction of null hypotheses as a conjunction of k or more effects. Analyses based on minimum statistics have typically used the null hypothesis that k = 0. This enables inferences about one or more effects (k > 0). However, this does not provide control over false-positive rates (FPR) for inferences about a conjunction of k = n effects, over n tests. This is the key point made by Nichols et al., who suggest a procedure based on supremum P values that provides an upper bound on FPR for k = n. Although valid, this is a very conservative procedure, particularly in the context of multiple comparisons. We suggest that an inference on a conjunction of k = n effects is generally unnecessary and distinguish between congruent contrasts that test for the same treatment and incongruent contrasts of the sort used in cognitive conjunctions. For congruent contrasts, the usual inference, k > 0, is sufficient. With incongruent contrasts it is sufficient to infer a conjunction of k >u effects, where u is the number of contrasts that share some uninteresting effect. The issues highlighted by Nichols et al., have important implications for the design and analysis of cognitive conjunction studies and have motivated a change to the SPM software, that affords a test for the more general hypothesis k >u. This more general conjunction test is described.

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    • "To identify regions of overlapping responses to the three or two different tastes events, we performed conjunction analyses with a conjunction null hypothesis. This statistic identifies voxels that are significantly activated in each of the individual contrasts included in the conjunction (Friston et al., 2005). Furthermore, the three types of the contrast of interest[Capsaicin–AS],[Capsaicin–NaCl], and [2 × Capsaicin–NaCl– AS] were computed to reveal the regions that are selectively orR, right; L, Left.Frontiers in Human Neuroscience | www.frontiersin.orgmore "
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    • "For this study, an AlphaSim Monte Carlo simulation with our particular scanning and analysis parameters – a smoothing kernel of 8 mm and voxel resolution of 3mm – combined with a more conservative criterion with the magnitude statistical threshold of 0.001 and cluster threshold of 25 voxels yielded an FDR of 0.022. To examine regions that showed significant common neural responses to AI and HI as well as to A and AI, conjunction analyses were performed with the contrasts of interest[Friston, Penny, & Glaser, 2005;Nichols, Brett, Andersson, Wager, & Poline, 2005).For visualization purposes, thresholded maps were superimposed on an average, spatially normalized anatomical image obtained from the 18 participants. The locations of neural activity were first classified using the Automated Anatomical Labeling (AAL) map (Tzourio-Mazoyer et al., 2002), and then were further refined with: 1) neuroanatomical atlases (Duvernoy, 1991;Schmahmann et al., 1999); 2) probabilistic maps or profiles for primary auditory cortex (Penhune, Zatorre, MacDonald, & Evans, 1996), planum temporale (Westbury, Zatorre, & Evans, 1999), inferior frontal gyrus pars opercularis (Tomaiuolo et al., 1999), and mouth region of primary motor cortex (Fox et al., 2001); and 3) locations defined by previous reports or reviews on the medial frontal and cingulate areas (Picard & Strick, 1996) and subdivisions of the premotor cortex (Chen, Penhune, & Zatorre, 2008). "
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    • "reas specifically involved in coding peripersonal and extrapersonal space , using the following contrasts : ( WN - WO ) ∩ ( ON - OO ) and ( WO - WN ) ∩ ( OO - ON ) , respectively ( Friston et al . , 1999 ) . To this aim , we performed an SPM ' conjunction null ' analysis ( Nichols et al . , 2005 ) . Given the conservative nature of this analysis ( Friston et al . , 2005 ) , we report data with a p - value < 0 . 001 uncorrected . A threshold of 10 was applied on cluster dimension . For all analyses , location of the activation foci was determined in the stereotaxic space of the MNI coordinates system . Those cerebral regions for which maps are provided were also localized with reference to cytoarchitect"
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