Brain Advance Access published online on October 22, 2008
Brain, doi:10.1093/brain/awn273
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Hyperexcitability of parietal-motor functional connections in the intact left-hemisphere of patients with neglect
1Laboratorio di Neurologia Clinica e Comportamentale, Fondazione Santa Lucia IRCCS, Via Ardeatina, 306, 00179 Rome, 2Clinica Neurologica, Dipartimento di Neuroscienze, Università di Roma Tor Vergata, Via Montpellier 1, 00133 Rome, 3Dipartimento di Psicologia, Università di Palermo, Palermo, Italy, 4Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, Queen Square, London WC1N 3BG and 5UCL Institute of Cognitive Neuroscience, University College London, 17 Queen Square, London WC1N 3AR, UK
Correspondence to:
Dr Giacomo Koch, Laboratorio di Neurologia Clinica e Comportamentale, Fondazione Santa Lucia IRCCS, Via Ardeatina, 306, 00179 Rome, Italy E-mail: g.koch{at}hsantalucia.it
Hemispatial neglect is common after unilateral brain damage, particularly to perisylvian structures in the right-hemisphere (RH). In this disabling syndrome, behaviour and awareness are biased away from the contralesional side of space towards the ipsilesional side. Theoretical accounts of this in terms of hemispheric rivalry have speculated that the intact left-hemisphere (LH) may become hyper-excitable after a RH lesion, due to release of inhibition from the damaged hemisphere. We tested this directly using a novel twin-coil transcranial magnetic stimulation (TMS) approach to measure excitability within the intact LH of neglect patients. This involved applying a conditioning TMS pulse over left posterior parietal cortex (PPC), in order to test its effect on the amplitude of motor evoked potentials (MEPs) produced by a subsequent test pulse over left motor cortex (M1). Twelve RH stroke patients with neglect, an age-matched group of eight RH stroke patients without neglect, and 10 healthy controls were examined. We found that excitability of left PPC-M1 circuits was higher in neglect patients than the other groups, and related to the degree of neglect on clinical cancellation tests. A follow-up found that 1 Hz repetitive TMS over left PPC normalized this over-excitability, and also ameliorated visual neglect on an experimental measure with chimeric objects. Our results provide ‘direct’ evidence for pathological over-excitability of the LH in the neglect syndrome, as quantified by left PPC influences on left M1, with implications for possible treatment.
Key Words: neglect syndrome; transcranial magnetic stimulation; connectivity; rTMS; parietal cortex; cortical excitability
Abbreviations: ISI, inter-stimulus interval; LH, left hemisphere; MEPs, motor evoked potentials; PPC, posterior parietal cortex; RH, right hemisphere; rTMS, repetitive TMS; TMS, transcranial magnetic stimulation
Received May 10, 2008. Revised September 9, 2008. Accepted September 22, 2008.
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