Social distance evaluation in human parietal cortex

Abstract

Across cultures, social relationships are often thought of, described, and acted out in terms of physical space (e.g. "close friends" "high lord"). Does this cognitive mapping of social concepts arise from shared brain resources for processing social and physical relationships? Using fMRI, we found that the tasks of evaluating social compatibility and of evaluating physical distances engage a common brain substrate in the parietal cortex. The present study shows the possibility of an analytic brain mechanism to process and represent complex networks of social relationships. Given parietal cortex's known role in constructing egocentric maps of physical space, our present findings may help to explain the linguistic, psychological and behavioural links between social and physical space.

Figure 1. Results of a doll-arrangement task.

(A) A typical arrangement by a participant, with compatible dolls placed close to the self doll. (B) Average distances between the self and incompatible dolls for all participants. The average distance between the self and compatible dolls across participants was significantly (physically) shorter than that between the self and incompatible dolls. Error bars indicate the standard deviation.

Figure 2. Examples of stimuli used in the PD and SD tasks and the average results.

(A) Stimulus for PD task. Texture and lighting cues on the pair of panels reflect a physical arrangement. Trials varied in terms of the angle of the arrangement. Participants indicated which panel was closer. (B) Stimulus for SD task. Participants indicated which person they would be more willing to interact and cooperate with in real life. Outside the scanner, participants rated face pairs for the level of difference between the faces in terms of the task above. (C) Results from PD task outside the scanner. Mean reaction times and standard deviations were calculated across all participants (n = 24). (D) Results from SD task outside the scanner.

Figure 3. Brain activations during PD and SD, and a representative result from correlation analysis.

(A) Blue sections indicate parietal activations for the PD task, identified by the contrast PD vs. PC (control task). These are the only regions identified by this contrast in the entire brain. Red regions correspond to parietal activations for the SD task (SD vs. SC). This panel illustrates only the parietal activations for the SD task. (B) In addition to areas active during the SD (red) and PD (blue) tasks, the green section in the left parietal cortex corresponds to the region whose activity was negatively correlated with the behavioural ratings in the SD task. Horizontal section z = +46 is displayed. (C) Negative correlation between the left intraparietal activity and the ratings in a representative participant (N = 12, r = −0.58, p<0.05). X-axis indicates mean ratings from 12 blocks in the SD task, and Y-axis indicates the corresponding mean level of intraparietal activity (ratio to baseline).

Figure 4. Brain activations during SD and SF (A), and results from PPI analysis (B, C).

(A) Pink sections indicate brain regions active in common between in the SD and SF tasks, [Boolean intersection (SD vs. SC) ∩ (SF vs. SC) for display purpose]. Red sections indicate brain regions active during the SD task (SD vs. SC). While many regions active during the SD task were also active during the SF task, parietal activations were observed only in the SD task. The right hemisphere is displayed in the right panel. (B) The yellow section is a parietal region whose activity enhanced its coupling with that in the left fusiform region only in the SD task. Horizontal section z = +44 is displayed. C, Relationship between activities of the left fusiform gyrus and the left intraparietal cortex in a representative participant [N = 60, r = 0.33, p<0.01 for the SD task (yellow dots); r = 0.01, p>0.93 for the SF task (grey dots)]. X-axis indicates the degree of activity (mean adjusted) in each trial of the fusiform gyrus, and Y-axis indicates that of the intraparietal cortex.