Parietal regions specialized for saccades and reach in the human: a rTMS study

Abstract

Primate neurophysiology and human brain imaging studies have identified effector-related regions in the posterior parietal cortex (PPC). However, this specialization is less clear in human fMRI studies. Here we used fMRI-derived regions of interest to navigate transcranial magnetic stimulation (TMS) and causally determine saccade and reach specificity in three PPC regions. In experiment 1, six subjects performed memory-guided saccades and reaches with their dominant, right-hand to remembered peripheral targets in complete darkness. During the interval between viewing the target and the saccadic eye- or reach-movement, we applied trains of repetitive TMS to anatomically defined regions of interest from individual subjects - 1) superior parieto-occipital cortex (SPOC); 2) the more anterior-lateral medial intraparietal sulcus (mIPS); and 3) a yet more anterior dorsal-lateral PPC region near the angular gyrus (cIPS) - in both hemispheres. Stimulation to left mIPS and cIPS regions increased reach endpoint variability to rightward (contralateral) targets, whereas stimulation of SPOC deviated reach endpoints towards visual fixation. Only rTMS to right mIPS and cIPS disrupted contraversive saccades. We then repeated experiment 1 with the nondominant left-hand to investigate whether rTMS-induced errors remain spatially fixed or reverse (experiment 2). Here we found that stimulation of right mIPS and cIPS caused a significant increase in endpoint variability to leftward (contralateral) targets. In our final third experiment, we investigated reaching with or without visual feedback from the moving hand to determine whether rTMS disrupted the reach goal or the internal estimate of initial hand position needed to calculate the reach vector. In both mIPS and cIPS visual feedback negated rTMS-induced reach errors, whereas rTMS-induced, directional reach biases in SPOC remained. Collectively, these results show that a more medial region centered on parieto-occipital junction is involved only in planning of reach (encodes goal representation), and more lateral regions in human PPC have multiple overlapping maps for saccade and reach planning (encodes reach vector information).

Footnotes

 Canadian Institutes of Health Research, Canada Research Chair Program.