Evaluating functional localizers: the case of the FFA

Abstract

Functional localizers are routinely used in neuroimaging studies to test hypotheses about the function of specific brain areas. The specific tasks and stimuli used to localize particular regions vary widely from study to study even when the same cortical region is targeted. Thus, it is important to ask whether task and stimulus changes lead to differences in localization or whether localization procedures are largely immune to differences in tasks and contrasting stimuli. We present two experiments and a literature review that explore whether face localizer tasks yield differential localization in the fusiform gyrus as a function of task and contrasting stimuli. We tested standard localization tasks-passive viewing, 1-back, and 2-back memory tests--and did not find differences in localization based on task. We did, however, find differences in the extent, strength and patterns/reliabilities of the activation in the fusiform gyrus based on comparison stimuli (faces vs. houses compared to faces vs. scrambled stimuli).

Figure 1

Peaks for different localizer task types (left). 1-back are in green, passive-viewing in blue. On the right are the peaks for the different contrasting stimuli. Peaks for different contrasting stimuli (right). Multi-objects are in blue, scrambles in red, and single-objects in green.

Figure 2

3-Dimensional 68% confidence intervals for passive viewing (blue) and 1-back (green) tasks. The coordinates represent the dimensions in Talairach Space in millimeter units.

Figure 3

3-Dimensional 68% confidence interval for multiple object (blue), single object (green), and scrambled images (red) as comparison stimuli. The coordinates represent the dimensions in Talairach Space in millimeter units

Figure 4

Plotted Peaks for each participant for each run of the passive viewing, 1-back and 2-back memory tests for Faces > Houses. Blue denotes passive viewing, green denotes 1-back, and red denotes 2-back. Spheres denote the first iteration of the task, and cubes denote the second iteration. From the figure one can see no discernable pattern of changes in peak location based on task, or the first or second iteration of performing the same task.

Figure 5

Mean (± 1 standard error) within-task Consistency Ratios across all subjects, as a function of increasing the threshold for what constitutes an active voxel in our right fusiform ROI.

Figure 6

Mean (± 1 standard error) within-task Consistency Ratios across all subjects, as a function of increasing the threshold for what constitutes an active voxel in our left fusiform ROI.

Figure 7

Mean (± 1 standard error) Consistency Calculations for both within- and between-task for the left and right Fusiform ROIs.

Figure 8

Consistency results for Experiment 2 for the left and right ROIs for passive viewing and one-back for different comparison stimuli (houses and scrambles)

Figure 9

Pearson correlations between- and within-task for Experiment 2