Finding Consistent Activation Across fMRI Tasks: Using pFDR to Test a Union of Nulls

• Activity Number: 78
• Type: Topic Contributed
• Date/Time: Monday, August 9, 2004 : 8:30 AM to 10:20 AM
• Sponsor: Biometrics Section
• Author(s): Thomas E. Nichols*+
• Companies: University of Michigan
• Keywords: multiple comparisons ; medical imaging ; functional magnetic reasonance imaging ; false discovery rate

Abstract

Psychologists studying memory use Functional Magnetic Resonance Imaging (fMRI) to understand how information is encoded, stored and retrieved in the brain. Short-term, or working memory, can be "probed" in a number of ways, say. By asking a subject to remember a list of words, or a list of digits, or even even a collection of shapes (triangles, squares, etc.). The goal is to identify regions of the brain that support working memory generically, but which are not specialized to words or numbers or shapes. This requires testing a union of nulls (no effect in one or more of the three tasks) versus an intersection of alternatives (effects in all three tasks). Worsley and Friston proposed using the maximum p value to test for a intersection of effects, but their inference is based on the intersection of nulls. We propose using Storey's Positive False Discovery Rate (pFDR) to make inference on the union of nulls. Storey shows that pFDR can be interpreted as the posterior probability of the null given that a statistic lies in a rejection region. Our method is simple, and amounts to making inference on the sum of q values. We demonstrate the method on simulated and real data.