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The Simon effect based on allocentric and egocentric reference frame: common and specific neural correlates

An object’s location can be represented either relative to an observer’s body effectors (egocentric reference frame) or relative to another external object (allocentric reference frame). In non-spatial tasks, an object’s task-irrelevant egocentric position conflicts with the side of a task-relevant manual response, which defines the classical Simon effect. Growing evidence suggests that the Simon effect occurs not only based on conflicting positions within the egocentric but also within the allocentric reference frame.
Although neural mechanisms underlying the egocentric Simon effect have been extensively researched, neural mechanisms underlying the allocentric Simon effect and their potential interaction with those underlying its egocentric variant remain to be explored. In this fMRI study, spatial congruency between the task-irrelevant egocentric and allocentric target positions and the task-relevant response hand was orthogonally manipulated (Fig. 1 A-B).
Behaviorally, a significant Simon effect was observed for both reference frames (Fig. 1 C).


Li 2019

Figure 1. (A) Experimental stimuli. The target stimulus is the fork, with two levels of luminance. The gray dotted line represents the invisible egocentric mid-sagittal plane. The four egocentric locations (−2.67°, −1.7°, 1.7°, 2.67°) of the fork are marked on the top of the figure, and the four allocentric locations (−3.6°, −2°, 2°, 3.6°) of the fork (with reference to the mid-sagittal plane of the plate) are marked on the left and right side of the figure. For the examples of the four experimental conditions defined here, participants were asked to respond to the light grey via their left hand, and to the dark grey via their right hand. The bottom-up stimuli are fully counter-balanced for the three critical contrasts in the present experiment, i.e., the main effect of egocentric congruency “EgoC (AlloC + AlloIC) vs. EgoIC (AlloC + AlloIC)”, the main effect of allocentric congruency “AlloC (EgoC + EgoIC) vs. AlloIC (EgoC + EgoIC)”, and the interaction contrast “EgoC (AlloC > AlloIC) vs. EgoIC (AlloC > AlloIC)”. (B) Two hypothetical models of the neural interaction between the egocentric and allocentric Simon effect.
(C) Behavioral results. Mean RTs (ms) and mean error rates (%) with standard errors in the four experimental conditions.

Neurally, three sub-regions in the frontoparietal network were involved in different aspects of the Simon effect, depending on the source of the task-irrelevant object locations. The right precentral gyrus, extending to the right SMA, was generally activated by Simon conflicts, irrespective of the spatial reference frame involved, and showed no additive activity to Simon conflicts. In contrast, the right postcentral gyrus was specifically involved in Simon conflicts induced by task-irrelevant allocentric, rather than egocentric, representations. Furthermore, a right lateral frontoparietal network showed increased neural activity whenever the egocentric and allocentric target locations were incongruent, indicating its functional role as a mismatch detector that monitors the discrepancy concerning allocentric and egocentric object locations (Fig. 2).

Li 2019

Figure 2. (A) Common neural correlates underlying the single-source and the double-source incongruent conditions. The right precentral gyrus was activated by the conjunction analysis between the single-source allocentric conflict, i.e., EgoC (AlloIC > AlloC), the single-source egocentric conflict, i.e., AlloC (EgoIC > EgoC), and the double-source conflict, i.e., EgoIC_AlloIC > EgoC_AlloC. Mean parameter estimates extracted from the activated cluster are shown as a function of the four experimental conditions. (B) Specific neural correlates underlying the allocentric Simon conflict. Parameter estimates were extracted from the activated clusters in the right PosCG, and are shown as a function of the four experimental conditions. Error bars indicate SEs.
(C) Overlay of the activated brain regions in (A) (green) and (B) (orange).



Li, H., Liu, N., Li, Y., Weidner, R., Fink, G. R., & Chen, Q. (2019). The Simon Effect Based on Allocentric and Egocentric Reference Frame: Common and Specific Neural Correlates. Scientific Reports, 9(1), 1-16.