title: Effects of thresholding on correlation-based image similarity metrics.
abstract: The computation of image similarity is important for a wide range of analyses in neuroimaging, from decoding to meta-analysis. In many cases the images being compared have empty voxels, but the effects of such empty voxels on image similarity metrics are poorly understood. We present a detailed investigation of the influence of different degrees of image thresholding on the outcome of pairwise image comparison. Given a pair of brain maps for which one of the maps is thresholded, we show that an analysis using the intersection of non-zero voxels across images at a threshold of Z = ±1.0 maximizes accuracy for retrieval of a list of maps of the same contrast, and thresholding up to Z = ±2.0 can increase accuracy as compared to comparison using unthresholded maps. Finally, maps can be thresholded up to to Z = ±3.0 (corresponding to 25% of voxels non-empty within a standard brain mask) and still maintain a lower bound of 90% accuracy. Our results suggest that a small degree of thresholding may improve the accuracy of image similarity computations, and that robust meta-analytic image similarity comparisons can be obtained using thresholded images.
authors.affiliation: Poldrack Lab, Department of Psychology, Stanford University Stanford, CA, USA ; Program in Biomedical Informatics, Stanford University Stanford, CA, USA.
authors.forename: Vanessa V
authors.initials: VV
authors.lastname: Sochat
authors.affiliation: Poldrack Lab, Department of Psychology, Stanford University Stanford, CA, USA.
authors.forename: Krzysztof J
authors.initials: KJ
authors.lastname: Gorgolewski
authors.affiliation: Poldrack Lab, Department of Psychology, Stanford University Stanford, CA, USA.
authors.forename: Oluwasanmi
authors.initials: O
authors.lastname: Koyejo
authors.affiliation: Poldrack Lab, Department of Psychology, Stanford University Stanford, CA, USA ; Department of Data Analysis, Ghent University Ghent, Belgium.
authors.forename: Joke
authors.initials: J
authors.lastname: Durnez
authors.affiliation: Poldrack Lab, Department of Psychology, Stanford University Stanford, CA, USA.
authors.forename: Russell A
authors.initials: RA
authors.lastname: Poldrack
keywords: functional magnetic resonance imaging; human connectome project; image classification; image comparison; neuroimaging; thresholding