No abstract

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.

Time processing is important in several cognitive and motor functions, but it is still unclear how the human brain perceives time intervals of different durations. Processing of time in millisecond and second intervals may depend on different neural networks and there is now considerable evidence to suggest that these intervals are possibly measured by independent brain mechanisms. Using repetitive transcranial magnetic stimulation (rTMS), we determined that the cerebellum is essential in explicit temporal processing of millisecond time intervals. In the first experiment, subjects' performance in a time reproduction task of short (400-600 ms) and long (1,600-2,400 ms) intervals, were evaluated immediately after application of inhibitory rTMS trains over the left and right lateral cerebellum (Cb) and the right dorsolateral prefrontal cortex (DLPFC). We found that rTMS over the lateral cerebellum impaired time perception in the short interval (millisecond range) only; for the second range intervals, impaired timing was found selectively for stimulation of the right DLPFC. In the second experiment, we observed that cerebellar involvement in millisecond time processing was evident when the time intervals were encoded but not when they were retrieved from memory. Our results are consistent with the hypothesis that the cerebellum can be considered as an internal timing system, deputed to assess millisecond time intervals.