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Social Cognitive and Affective Neuroscience 2009 4(2):208-213; doi:10.1093/scan/nsp011
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© The Author (2009). Published by Oxford University Press. For Permissions, please email: journals.permissions@oxfordjournals.org

Independence in ROI analysis: where is the voodoo?

Russell A. Poldrack1,2 and Jeanette A. Mumford1

1Department of Psychology and 2Department of Psychiatry & Biobehavioral Sciences, University of California, Los Angeles, CA 90095, USA

We discuss the effects of non-independence on region of interest (ROI) analysis of functional magnetic resonance imaging data, which has recently been raised in a prominent article by Vul et al. We outline the problem of non-independence, and use a previously published dataset to examine the effects of non-independence. These analyses show that very strong correlations (exceeding 0.8) can occur even when the ROI is completely independent of the data being analyzed, suggesting that the claims of Vul et al. regarding the implausibility of these high correlations are incorrect. We conclude with some recommendations to help limit the potential problems caused by non-independence.

Keywords: functional magnetic resonance imaging; region of interest analysis; bias; statistics; multiple comparisons

Correspondence should be addressed to Russell A. Poldrack, UCLA Department of Psychology, Wendell Jefferey and Bernice Wenzel Team Chair in Behavioral Neuroscience, Franz Hall, Box 951563, Los Angeles, CA 90095-1563, USA. E-mail: poldrack{at}ucla.edu

Received February 28, 2009. Accepted March 6, 2009.


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