Two orienting mechanisms in posterior parietal lobule: An rTMS study of the Simon and SNARC effects

Abstract

The aim of the present study was to investigate, with repetitive transcranial magnetic stimulation (rTMS), the involvement of posterior parietal lobule (PPL) circuits in producing the Simon effect and the spatial-numerical association of response codes (SNARC) effect. The Simon effect is indexed by faster responses to left-side stimuli with left- than with right-side key-presses and faster responses to right-side stimuli with right- than with left-side key-presses. The SNARC effect is indexed by faster responses to smaller numbers with left- than with right-side key-presses and faster responses to larger numbers with right- than with left-side key-presses. Participants were required to perform a parity judgement on numbers ranging from 1 to 9 (without 5), by pressing a left or a right response key. The numbers were presented to either the left or the right side of fixation. Two bilateral PPL areas lying along the intraparietal sulcus (IPS) were stimulated with rTMS. Results suggested a causal role of the anterior portion of PPL of both hemispheres in the Simon effect and of the posterior portion of PPL of both hemispheres in the Simon effect and the SNARC effect.

This research was supported in part by grants from European Commission (RTN Grant HPRN-CT-2000–00076) and MURST to CU. We are grateful to Giovanni Galfano, Stefania Bon, and Elena Betta for their help and assistance. A special thank to Vincent Walsh for suggestions that motivated the research, Giorgio Fuggetta for fruitful interactions, and Giuseppe Vallar and three anonymous reviewers for useful comments on a previous version of the present manuscript.

Notes

¹ Motor attention is here defined as the preparation and/or the priming of a movement within a specific effector (e.g., the index vs. the middle finger on the right hand, like in Rushworth et al., 2001a).

² No evidence for the presence of a MARC effect was found between participants (i.e., between those who received a MARC-compatible and those who received a MARC-incompatible mapping) in the present study, and therefore this factor was not included in subsequent analyses.

³ The gaze effect consists of a correspondence effect between the direction of a perceived eye gaze and the side of a manual key-press response. The correspondence between gaze direction and response side affects performance even though gaze direction is task irrelevant (Ansorge, 2003; Zorzi, Mapelli, Rusconi, & Umiltà, 2003).