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Removing facial features from structural MRI images biases visual quality assessment

Fig 2

Defacing biases human assessment of image quality, particularly when image quality is low.

We examined biases with an “optimized” version of the BA plot, in which the x-axis represents the rating assigned to the nondefaced version of an image. Corresponding “standard” BA plots—in which the x-axis shows the average of the two ratings [37]—are reported in Fig B in S1 Text. Rating pairs where the defaced image’s quality was underestimated with respect to the nondefaced ( > 0) are represented in yellow. Conversely, pairs where the defaced image’s quality was overestimated () are in purple. Pairs within the 95% LoA are represented with dim colors, and the LoA boundaries are indicated with dashed colored lines annotated with their value. For example, the LoA for all raters pooled together was [−0.91, 0.81] (left panel). Finally, the bias is represented by a gray or black dashed line with a label reporting value and their corresponding 95% CI interval (parametric estimation). All raters displayed 95% LoA exceeding one unit, indicating that defacing introduces large variability in human assessments. All raters had negative—albeit small—biases, indicating that they systematically rated defaced images higher. However, these biases were statistically significant only for Raters 1 and 3, as well as the four raters aggregated together—indicated by the bias label and line colored in black. Relevant statistics (bias, LoA, 95% CI) are reported in Table B in S1 Text, and the full report of statistics, including 95% CI intervals calculated both by parametric and non-parametric means are distributed within the S6 Data file. Source tabular data for the BA analysis and results in Table B in S1 Text are found within the S1 Data file.

doi: https://doi.org/10.1371/journal.pbio.3003149.g002