Defining the face processing network: Optimization of the functional localizer in fMRI

Abstract

Functional localizers that contrast brain signal when viewing faces versus objects are commonly used in functional magnetic resonance imaging studies of face processing. However, current protocols do not reliably show all regions of the core system for face processing in all subjects when conservative statistical thresholds are used, which is problematic in the study of single subjects. Furthermore, arbitrary variations in the applied thresholds are associated with inconsistent estimates of the size of face-selective regions-of-interest (ROIs). We hypothesized that the use of more natural dynamic facial images in localizers might increase the likelihood of identifying face-selective ROIs in individual subjects, and we also investigated the use of a method to derive the statistically optimal ROI cluster size independent of thresholds. We found that dynamic facial stimuli were more effective than static stimuli, identifying 98% (versus 72% for static) of ROIs in the core face processing system and 69% (versus 39% for static) of ROIs in the extended face processing system. We then determined for each core face processing ROI, the cluster size associated with maximum statistical face-selectivity, which on average was approximately 50 mm(3) for the fusiform face area, the occipital face area, and the posterior superior temporal sulcus. We suggest that the combination of (a) more robust face-related activity induced by a dynamic face localizer and (b) a cluster-size determination based on maximum face-selectivity increases both the sensitivity and the specificity of the characterization of face-related ROIs in individual subjects.

Quantifying interindividual variability and asymmetry of face-selective regions: A probabilistic functional atlas

“…Empirical evidence indicates that the task could elicit reliable activations of FSRs across subjects (Fox et al, 2009;Pitcher et al, 2011a). Specifically, the dynamic face localizer data were acquired over three blocked-design functional runs, each of which lasted 198 s. Each run contained two block sets, intermixed with three 18-s rest blocks at the beginning, middle and end of the run.…”

“…For the spatial features, to our knowledge, only a few studies have provided quantitative estimates of intersubject variability of the FSRs, mostly focusing on the variability of the location of peak activation. The interindividual variability of peak location of FSRs reportedly ranged from several millimeters to centimeters (Fox et al, 2009;Kanwisher et al, 1997;Pinsk et al, 2009). Moreover, the interindividual variability in the volume of FSRs was also examined either in subjects' native space (Kanwisher et al, 1997;Pinsk et al, 2009) or by fixing the size of the FFA at 200 mm 3 (Rossion et al, 2012), resulting in a finding that the interindividual variability of the volume (i.e., the SD of the volume measurements) is approximately equal to or greater than the mean volume of each FSR.…”

“…Substantial evidence demonstrates that a dedicated network of the FSRs is recruited for face processing Ishai, 2008;Rossion et al, 2012;Zhen et al, 2013;Zhu et al, 2011). These FSRs could be reliably localized across individuals even with a short period of functional localizer scans (Fox et al, 2009;Kawabata Duncan and Devlin, 2011). Among them, the most reliable regions are located in the occipitotemporal cortex.…”

“…The interindividual variability of the FSRs is rarely investigated, although the variability has commonly been observed in previous studies (Fox et al, 2009;Goffaux et al, 2011;Kanwisher et al, 1997;Pinsk et al, 2009). Only recently have four groups begun to develop probabilistic atlases (or maps) to characterize the variability of face-selective activation (Engell and McCarthy, 2013;Frost and Goebel, 2012;Julian et al, 2012;Tahmasebi et al, 2012).…”

“…Specifically, to capture reliably the variability of the FSRs, 202 subjects were scanned with a robust dynamic face localizer (Fox et al, 2009;Pitcher et al, 2011a) in the same scanner and with the same MR protocol. First, the six well-studied FSRs, the OFA, pFFA, aFFA, pcSTS, pSTS, and aSTS, were delineated in each individual using a semi-automated procedure.…”