Specificity of Human Parietal Saccade and Reach Regions during Transcranial Magnetic Stimulation

Abstract

Single-unit recordings in macaque monkeys have identified effector-specific regions in posterior parietal cortex (PPC), but functional neuroimaging in the human has yielded controversial results. Here we used on-line repetitive transcranial magnetic stimulation (rTMS) to determine saccade and reach specificity in human PPC. A short train of three TMS pulses (separated by an interval of 100 ms) was delivered to superior parieto-occipital cortex (SPOC), a region over the midposterior intraparietal sulcus (mIPS), and a site close to caudal IPS situated over the angular gyrus (AG) during a brief memory interval while subjects planned either a saccade or reach with the left or right hand. Behavioral measures then were compared to controls without rTMS. Stimulation of mIPS and AG produced similar patterns: increased end-point variability for reaches and decreased saccade accuracy for contralateral targets. In contrast, stimulation of SPOC deviated reach end points toward visual fixation and had no effect on saccades. Contralateral-limb specificity was highest for AG and lowest for SPOC. Visual feedback of the hand negated rTMS-induced disruptions of the reach plan for mIPS and AG, but not SPOC. These results suggest that human SPOC is specialized for encoding retinally peripheral reach goals, whereas more anterior-lateral regions (mIPS and AG) along the IPS possess overlapping maps for saccade and reach planning and are more closely involved in motor details (i.e., planning the reach vector for a specific hand). This work provides the first causal evidence for functional specificity of these parietal regions in healthy humans.

A key region in the human parietal cortex for processing proprioceptive hand feedback during reaching movements

“…The combined results of this and other studies (Chib et al, 2009;Della-Maggiore et al, 2004;Vesia et al, 2010) identify this region as a key area for processing proprioceptive information for motor control. Furthermore, this region can be clearly distinguished from an area around the anterior IPS, which is crucial for online sensorimotor processing in the presence of visual feedback about the hand position.…”

“…Moreover, since the temporal resolution of fMRI is sub-optimal for studying fast online control processes, fMRI studies usually report BOLD activation resulting from a temporal mixture of sensorimotor processes. Other studies administering TMS over the mIPS to study reaching related processes have applied TMS during the planning phase, before the actual movement started (Busan et al, 2009(Busan et al, , 2012Davare et al, 2012;Vesia and Crawford, 2012;Vesia et al, 2006Vesia et al, , 2008Vesia et al, , 2010. The only exception is a study by Striemer et al (2011).…”

“…The tested sites constitute a compromise between feasibility of the study with respect to the testing time and covering the portions of the PPC that might be involved in online processing of sensory information about the reaching hand/ arm. We did not include more medial-posterior areas as they have mainly been associated with processing information about the target of reach (Bernier and Grafton, 2010;Diedrichsen et al, 2005;Gallivan et al, 2009;Vesia et al, 2010 (Reichenbach et al, 2011) previous TMS studies investigating online control or proprioceptive processing during reaching (Chib et al, 2009;Della-Maggiore et al, 2004;Reichenbach et al, 2011) (Table 1). With these sites, we covered a broad region of the left PPC for which sensorimotor processing related to our questions had been localized previously.…”

“…Recent fMRI studies have assessed the relative contribution of different PPC areas to the processing of visual and proprioceptive information during motor control, mainly by investigating brain activity during the whole movement (Bernier and Grafton, 2010;Blangero et al, 2009;Filimon et al, 2009;Pellijeff et al, 2006), in which different temporal stages of movement control can hardly be disentangled. TMS studies have extended our current knowledge on this topic (Vesia and Crawford, 2012) to the temporal domain by focusing on the movement preparation phase (Vesia et al, 2008(Vesia et al, , 2010. As different mechanisms have been suggested for planning vs. execution of a goal-directed movement (Desmurget and Grafton, 2000;Glover, 2002;Sabes, 2000), their neural implementation may also rely on distinct cortical regions.…”

Tracking post-error adaptation in the motor system by transcranial magnetic stimulation

“…A second alternative explanation would suggest that the modulation of the excitability of the CST might be triggered by the output of a conflict monitoring system involving the DLPFC and the ACC (Botvinick et al, 2001). Previous studies administered single or brief burst of high-frequency magnetic pulses to different regions of the brain in specific time intervals respect to an event in order to momentarily disrupt the corresponding neural activity at specific stages of the task (Desmurget et al, 1999;Pascual-Leone et al, 2000;Vesia et al, 2010). Particularly, applying single pulses to these regions at specific time intervals respect to the commission of mistaken responses might help to disentangle i) their functional role by measuring the behavioral effect of the suppression of its activity and (ii)the exact time-course of their activation.…”