Abnormal moral reasoning in complete and partial callosotomy patients

Abstract

Recent neuroimaging studies suggest lateralized cerebral mechanisms in the right temporal parietal junction are involved in complex social and moral reasoning, such as ascribing beliefs to others. Based on this evidence, we tested 3 anterior-resected and 3 complete callosotomy patients along with 22 normal subjects on a reasoning task that required verbal moral judgments. All 6 patients based their judgments primarily on the outcome of the actions, disregarding the beliefs of the agents. The similarity in performance between complete and partial callosotomy patients suggests that normal judgments of morality require full interhemispheric integration of information critically supported by the right temporal parietal junction and right frontal processes.

Figure 1

The top panel shows MR images of midsagittal brain slices from the 6 patients with either full (J.W., V.P., and D.D.V) or partial (O.T., A.P., and P.F.) corpus callosum resections. The bottom panel is from Hofer & Frahm (2006) showing fractional anisotropy maps of the midsagittal corpus callosum from 4 female (on the left side) and 4 male (on the right side) subjects and their classification scheme for originating brain regions of white matter projections.

Figure 2

An example of the moral scenarios read to each patient and subject. Each scenario included either a negative or neutral belief and a negative or neutral outcome and foreground. The table at the bottom of the figure represents the four possible outcomes (adapted from Young & Saxe, 2008).

Figure 3

A plot of accidental harms (where the agent falsely believed that harm would not occur, but the harmful outcome did occur) and failed attempts (where the agent falsely believed that harm would occur but the harmful outcome did not occur). All the split-brain patients fall below the 95% confidence interval (the dotted line).