A key aspect of higher cognitive function is the ability to switch rapidly and efficiently between alternative modes of response where this is appropriate behaviourally. Such suppression appears to be highly dependent upon the integrity of the prefrontal cortex, yet other cortical areas are likely to be necessary to implement response switching. Language switching in bilingual speakers is a clear example of a task in which response switching is required. Functional brain imaging studies have demonstrated parietal cortex activation during repeated language switching within a translation task. Here we used event-related dense-sensor EEG recording techniques to examine the time course of language switching during a visually cued naming task in which bilingual participants named digits in either their first or second language. Switch-related modulation of ERP components was evident over parietal and frontal cortices, and in the latter case showed an asymmetry across first and second languages. Correspondence with a frontal ERP component found when suppressing manual responding in a Go/No-Go reaction time task may imply that similar inhibitory mechanisms are involved in both response suppression and language switching.

Transport and grasp kinematics were examined in a task in which subjects selectively reached to grasp a target object in the presence of non-target objects. In a variety of experiments significant interference effects were observed in temporal parameters, such as movement time, and spatial parameters, such as path. In general, the presence of non-targets slowed down the reach. Furthermore, reach paths were affected such that the hand veered away from near non-targets in reaches for far targets, even though the non-targets were not physical obstacles to the reaching hand. In contrast, the hand veered towards far non-targets in near reaches. We conclude that non-targets evoke competing responses, and the inhibitory mechanisms that resolve this competition are revealed in the reach path.

SummaryWithin the medial frontal cortex, the supplementary eye field (SEF), supplementary motor area (SMA), and pre-SMA have been implicated in the control of voluntary action, especially during motor sequences or tasks involving rapid choices between competing response plans. However, the precise roles of these areas remain controversial. Here, we study two extremely rare patients with microlesions of the SEF and SMA to demonstrate that these areas are critically involved in unconscious and involuntary motor control. We employed masked-prime stimuli that evoked automatic inhibition in healthy people and control patients with lateral premotor or pre-SMA damage. In contrast, our SEF/SMA patients showed a complete reversal of the normal inhibitory effect—ocular or manual—corresponding to the functional subregion lesioned. These findings imply that the SEF and SMA mediate automatic effector-specific suppression of motor plans. This automatic mechanism may contribute to the participation of these areas in the voluntary control of action.

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that alters cortical excitability in a polarity specific manner and has been shown to influence learning and memory. tDCS may have both on-line and after-effects on learning and memory, and the latter are thought to be based upon tDCS-induced alterations in neurochemistry and synaptic function. We used ultra-high-field (7 T) magnetic resonance spectroscopy (MRS), together with a robotic force adaptation and de-adaptation task, to investigate whether tDCS-induced alterations in GABA and Glutamate within motor cortex predict motor learning and memory. Note that adaptation to a robot-induced force field has long been considered to be a form of model-based learning that is closely associated with the computation and ‘supervised’ learning of internal ‘forward’ models within the cerebellum. Importantly, previous studies have shown that on-line tDCS to the cerebellum, but not to motor cortex, enhances model-based motor learning. Here we demonstrate that anodal tDCS delivered to the hand area of the left primary motor cortex induces a significant reduction in GABA concentration. This effect was specific to GABA, localised to the left motor cortex, and was polarity specific insofar as it was not observed following either cathodal or sham stimulation. Importantly, we show that the magnitude of tDCS-induced alterations in GABA concentration within motor cortex predicts individual differences in both motor learning and motor memory on the robotic force adaptation and de-adaptation task.

SummaryChildren with neurological disorders may follow unique developmental trajectories whereby they undergo compensatory neuroplastic changes in brain structure and function that help them gain control over their symptoms [1–6]. We used behavioral and brain imaging techniques to investigate this conjecture in children with Tourette syndrome (TS). Using a behavioral task that induces high levels of intermanual conflict, we show that individuals with TS exhibit enhanced control of motor output. Then, using structural (diffusion-weighted imaging) brain imaging techniques, we demonstrate widespread differences in the white matter (WM) microstructure of the TS brain that include alterations in the corpus callosum and forceps minor (FM) WM that significantly predict tic severity in TS. Most importantly, we show that task performance for the TS group (but not for controls) is strongly predicted by the WM microstructure of the FM pathways that lead to the prefrontal cortex and by the functional magnetic resonance imaging blood oxygen level-dependent response in prefrontal areas connected by these tracts. These results provide evidence for compensatory brain reorganization that may underlie the increased self-regulation mechanisms that have been hypothesized to bring about the control of tics during adolescence.