Smoothness without Smoothing: Why Gaussian Naive Bayes Is Not Naive for Multi-Subject Searchlight Studies

• Published in: PloS one Vol. 8; no. 7; p. e69566
• Main Authors: Raizada, Rajeev D. S., Lee, Yune-Sang
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 26.07.2013; Public Library of Science (PLoS)
• Subjects: Algorithms; Artificial intelligence; Bayes Theorem; Bayesian analysis; Biology; Blurring; Brain; Brain mapping; Classifiers; Computer Science; Functional magnetic resonance imaging; Humans; Magnetic Resonance Imaging; Mathematics; Medicine; Neighborhoods; Neurosciences; NMR; Normal Distribution; Nuclear magnetic resonance; Pattern recognition; Phonetics; Reproducibility of Results; Smoothing; Smoothness; Studies; Support Vector Machine
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0069566

Spatial smoothness is helpful when averaging fMRI signals across multiple subjects, as it allows different subjects' corresponding brain areas to be pooled together even if they are slightly misaligned. However, smoothing is usually not applied when performing multivoxel pattern-based analyses (MVPA), as it runs the risk of blurring away the information that fine-grained spatial patterns contain. It would therefore be desirable, if possible, to carry out pattern-based analyses which take unsmoothed data as their input but which produce smooth images as output. We show here that the Gaussian Naive Bayes (GNB) classifier does precisely this, when it is used in "searchlight" pattern-based analyses. We explain why this occurs, and illustrate the effect in real fMRI data. Moreover, we show that analyses using GNBs produce results at the multi-subject level which are statistically robust, neurally plausible, and which replicate across two independent data sets. By contrast, SVM classifiers applied to the same data do not generate a replication, even if the SVM-derived searchlight maps have smoothing applied to them. An additional advantage of GNB classifiers for searchlight analyses is that they are orders of magnitude faster to compute than more complex alternatives such as SVMs. Collectively, these results suggest that Gaussian Naive Bayes classifiers may be a highly non-naive choice for multi-subject pattern-based fMRI studies.