Hyperexcitability of parietal-motor functional connections in the intact left-hemisphere of patients with neglect

Abstract

Hemispatial neglect is a common and disabling syndrome following unilateral brain damage, particularly to perisylvian right-hemisphere (RH) structures, in which behaviour and awareness appear biased away from the contralesional (typically left) side of space towards the ipsilesional side. Theoretical accounts in terms of hemispheric rivalry have speculated that the intact left hemisphere (LH) may become hyper-excitable after the RH lesion, due to release from rivalry with the damaged RH. We used a novel approach for assessing excitability within the intact LH of neglect patients, applying twin-coil TMS over that hemisphere to study the impact of a conditioning TMS pulse over left posterior parietal cortex (PPC), on motor evoked potentials (MEPs) in response to a left M1 pulse shortly after. We applied this paired-pulse, PPC-M1 TMS paradigm over the LH for 12 RH stroke patients with neglect, plus an age-matched group of 8 RH stroke patients without neglect, and 10 healthy controls. We found that excitability of the LH PPC-M1 system was pathologically increased in neglect patients. A follow-up found that 1 Hz repetitive TMS over the left PPC normalised this over-excitability, and also ameliorated neglect. Our results provide a new form of direct evidence for pathological over-excitability of the LH in the neglect syndrome, specifically here for left PPC influences on left M1, with implications for possible treatment.