P 131. Repetitive TMS over the left posterior parietal cortex affects the regulation of body sway

Abstract

During upright stance, skin contact with an earth-fixed referent provides tactile feedback about own body sway relative to the contact location. In this situation body sway variability is reduced compared to standing without contact. Recent studies on the time course of sway before, during and after periods of intermittent touch contact indicated that the stabilisation of sway is a time-consuming integrative process. In addition, it has been suggested that dorsolateral prefrontal cortex is involved in the processing of tactile feedback for the control of body sway. Using repetitive TMS, we aimed to investigate the influence of activity inhibition within the left posterior parietal cortex (PPC) as well as the left lateral prefrontal cortex (PFC) on the time course of sway stabilisation after the onset of finger tip contact as well as sway destabilisation after contact removal. We expected that PPC inhibition would disrupt the processing of tactile feedback for sway control indicated by more delayed and less strong sway reduction compared to PFC inhibition. In two sessions, separated by a week, 5 adult participants received 20 min of 1 Hz repetitive TMS stimulation at 110% passive motor threshold over the left PPC (CP3) and lateral PFC (F3) respectively. Before and after each stimulation interval, blind-folded and ear-plugged participants stood quietly on a force plate with their right forearm as well as fingers of the right hand reaching forward. In this posture, the fingers were held at hip level slightly above a contact plate of which its vertical position was controlled by a linear motor. Body sway was assessed in terms of Centre-of-Pressure (CoP) motion and trunk kinematics in the antero-posterior direction. Within each of six trials of 120 s duration, five pairs of touch onset and removal were timed at random intervals by driving the contact plate upwards until contact was established, respectively downwards until contact was safely removed. The minimum time period between subsequent onsets and removals was 7 s. Time course of sway was evaluated across 3 s before and after each contact event. As expected, sway reduction was attenuated as well as rate of sway stabilisation and destabilisation were lower after PPC inhibition. In addition, body sway was generally increased. In contrast, sway control with or without tactile feedback remained unaffected compared to both pre-stimulation baselines after lateral PFC stimulation. 1 Hz repetitive TMS over the left PPC disrupts the sensorimotor control of body balance as well as the sensory reorganization following onset and removal of tactile body sway-related feedback from the finger tips of the right hand.