Brain imaging evidence for why we are numbed by numbers

Abstract

We as humans do not value lives consistently. While we are willing to act for one victim, we often become numb as the number of victims increases. The empathic ability to adopt others' perspectives is essential for motivating help. However, the perspective-taking ability in our brains seems limited. Using functional MRI, we demonstrated that the core empathy network including the medial prefrontal cortex (mPFC) was more engaged for events happening to a single person than those happening to many people, no matter whether the events were emotionally neutral or negative. In particular, the perspective-taking-related mPFC showed greater and more extended activations for events about one person than those about many people. The mPFC may be the neural marker of why we feel indifferent to the suffering of large numbers of people in humanitarian disasters.

Figure 1

The core empathy network was more engaged for stories about one person than for stories about many people (the general singularity effect). (A) The medial prefrontal cortex (mPFC) showed greater activation for stories about one person than for stories about many people (One > Many, yellow-red scale). The right anterior insula showed less deactivation for emotionally negative than neural stories (Emot > Neut, green scale). The middle cingulate cortex and middle frontal cortex showed the general singularity effect, but only for emotionally negative stories (interaction, red scale). Colour scales indicate t values. Bar graphs present the mean estimated parameters of the general linear model (betas) for each story type for corresponding regions (purple arrows). Error bars indicate standard errors. R, right. (B) Bar graphs present the mean numbers of voxels that were significantly activated for each story type within the whole empathy network (left) and within the empathy-related mPFC (right). Error bars indicate standard errors. (C) Brain slices illustrate the overlap of individual participants’ activation maps with the core empathy network for each story type, zoomed in over the mPFC, right frontotemporal regions, and right insula. Colorbar indicates the percentage of overlapped participants. All imaging data were visualized with MATLAB (version r2015b, www.mathworks.com) and MRIcron (version 6.6.2013, www.mricro.com).

Figure 2

The regional activation of the perspective-taking mPFC was stronger for stories about one person than for stories about many people in terms of activation amplitude and spatial extension. (A) Bar graphs present the mean estimated model parameters (top) and the mean numbers of significantly activated voxels (bottom) in the perspective-taking mPFC for each story type. Error bars indicate standard errors. (B) Brain slices illustrate the overlap of individual participants’ activation maps with the perspective-taking mPFC. Colorbar indicates the percentage of overlapped participants. All imaging data were visualized with MATLAB (version r2015b, www.mathworks.com) and MRIcron (version 6.6.2013, www.mricro.com).