Non-invasive induction and cancellation of visuo-spatial deficits by repetitive Transcranial Magnetic Stimulation (rTMS): Lessons on “surgery-less” manipulation of complex visuo-spatial brain networks

Repetitive Magnetic Stimulation (rTMS) has shown to modify the excitability of targeted cortical regions in animals and humans, thus transiently altering the efficiency of neural projections within extended brain networks. Adequate processing and behavioral output depend on a given ‘state’ of functional interactions between cortical and subcortical nodes within this network. We applied rTMS trains targeted at the visuoparietal (VP) cortex, which is a crucial cortical node of an extended visuo-spatial neural network, in both, intact (n=2) and injured cats (n=2) with unilateral ablation of the VP region. All four intact cats were intensively trained in a set of visuo-spatial tasks consisting in the detection and localization of moving or static targets. In two of these cats, a 50 mm circular coil was centered on the left VP cortex and Sham or real rTMS was delivered during 20 minutes at 1 Hz. Real but not Sham rTMS significantly increased the number of errors in orienting responses towards static but not moving targets, presented at the contralateral visual hemifield (38±4%; and 48±3% p<0.05 vs. pre rTMS), whereas no increase respect to baseline was observed for ipsilateral targets (5±2%; 2±1%; n.s). Performance went back to baseline error levels 45 minutes after the end of the stimulation (4±2; 6±1%). In 2 other animals, the right or left parietal and primary visual cortex was surgically removed, generating a Daily stimulation with 1 Hz rTMS on the intact VP region resulted in a progressive reduction of detection¬orienting mistakes to moving but not static stimuli (down to 34±5% and 28±4% errors; p<0.05). We conclude that rTMS is able to interact with brain networks in both ways, transiently disrupting visuo-spatial processing in normal animals, and also canceling spatial neglect generated by lesions of the same areas. It constitutes, thus, a non-invasive ‘surgery-less’ method to manipulate brain activity and promote recovery after injuries.

Dynamical complexity and temporal plasticity in pancreatic beta-cells.

We discuss some of the biological and mathematical issues involved in understanding and modelling the bursting electrical activity in pancreatic beta-cells. These issues include single-cell versus islet behaviour, parameter heterogeneity, channel noise, the effects of hormones, neurotransmitters, and ions, and multiple slow biophysical processes. Some of the key experimental and modelling studies are described, and some of the major open questions are discussed.