Spatiotemporal Analysis of Visuospatial Attention and Modulation by Repetitive Transcranial Magnetic Stimulation (rTMS)

Introduction: Previous studies showed that repetitive TMS produces temporary virtual lesions. If performed over the right posterior parietal cortex (PPC) visuospatial tasks are impaired due to the effect of the stimulation [1]. The aim of the present study was to deconvolute the temporal dynamics of the neural processes involved in line bisection judgement tasks and their dependency on the function of the PPC. Materials and Methods: We studied twelve right-handed healthy subjects. The performance on a computerized visuo-spatial line-bisection judgement task was examined before and after the application of rTMS over the right and left PPC at P3 and P4 locations. We used a Magstim magnetic stimulator with a figure-of eight coil. Each stimulus train consists of 900 stimuli delivered with a frequency of 1Hz. Visual stimuli consisted of black vertically centered lines presented for 450 ms with an interstimulus interval of 1050 ms. Continuous EEG was recorded with a 64-channel system. Spatiotemporal analysis was performed within the temporal coupled current density approach within a standardized head model. Results: Six regions were identified to contribute significant source activity related to the line bisection judgement task. Right middle occipital gyrus (BA 18), bilateral inferior occipital gyrus (BA 19), right superior PPC (BA 7), right inferior PPC (BA 40) and right superior temporal cortex (BA 41). Differences in the time course of the source activation depending on the side of stimulation were evaluated. After left side stimulation there was an increase of source acitivty within the right superior and inferior PPC compared to a reduction of source activity within the PPC after right side stimulation in the time range of the N1. Noteworthy, after right side stimulation there was an increase of activity at the right-sided temporal source. Conclusion: The present study demonstrates the effects of virtual lesions to the PPC on the processing of visuospatial perception on a source-activation level. The increased activation of the right PPC after left side stimulation might reflect transcortical disinhibition [2]. In addition the results suggest that inhibition of the right PPC induces an activation of the right temporal cortex, supporting the hypothesis of an alternative visuospatial processing. References: [1] Fierro B et al. Neuroreport 2000; 14: 1519–1521. [2] Di Lazzaro V et al. Experimental Brain Research 1999; 124: 520–524.