Inhibition of the primary motor cortex and the upgoing thumb sign.

BACKGROUND: The upgoing thumb sign has been frequently observed in patients with minor strokes and transient ischemic attacks as an indicator of brain involvement. We assessed the effect of primary motor cortex (M1) inhibition in the development of the upgoing thumb sign. METHODS: Used repetitive Transcranial Magnetic Stimulation (rTMS, 1 Hz frequency for 15 min, 1s ISI, 900 pulses) at 60% of resting motor threshold to inhibit the right or left primary motor cortex of 10 healthy individuals. Participants were examined before and after rTMS by a neurologist who was blind to the site of motor cortex inhibition. RESULTS: 10 neurological intact participants (5 women/5 men) were recruited for this study. 2 cases were excluded due to pre-existing possible thumb signs. After the inhibition of the primary motor cortex, in 6 subjects out of 8, we observed a thumb sign contralateral to the site of primary motor cortex inhibition. In one subject an ipsilateral thumbs sign was noted. In another case, we did not find an upgoing thumb sign. CONCLUSION: The upgoing thumb sign is a subtle neurological finding that may be related to the primary motor cortex or corticospinal pathways involvements.

The cortical representation of foveal stimuli: evidence from quadrantanopia and TMS-induced suppression.

To address the extent to which the visual foveal representation is split, we examined a 29-year-old patient with a lower right quadrantanopia following surgical removal of the left occipital cortex above the calcarine sulcus and compared her performance with subjects receiving transcranial magnetic stimulation (TMS) over the occipital lobes. In a letter/digit classification task, the patient responded accurately to targets presented in the upper visual field, for all horizontal eccentricities. In the lower visual field, she failed to discriminate letters from digits when targets were presented in the right, but not the left visual field (RVF and LVF, respectively). This pattern was also true for the foveal targets, with poor performance to foveal-RVF (0.5 degrees to the right of fixation) but not foveal-LVF (0.5 degrees to the left of fixation) targets. Similar patterns of normal performance to LVF but not RVF or foveal-RVF targets were observed in a group of nine normal observers when TMS was applied over their left occipital cortex. Complementary impairments to LVF and foveal-LVF target classification were induced with TMS over the right occipital cortex. Thus, we have induced an hemianopic pattern in normal observers contralateral to the magnetically stimulated hemisphere. This correspondence between real and TMS-induced visual field defects is further evidence, in neurologically intact subjects, that the cortical representation of the fovea is split between the two hemispheres along the vertical meridian.

Stimulation in the dorsolateral prefrontal cortex changes subjective evaluation of percepts.

Nelson and Narens have proposed a metacognition model that dissociates the objective processing of information (object-level) and the subjective evaluation of the performance (i.e., the metalevel). Neurophysiological evidence also indicates that the prefrontal cortices (PFC) are the brain areas which perform the metalevel function [1]-[3]. A corresponding neural mechanism of Nelson and Narens's model, called dynamic filtering theory [4], [5], indicates that object-level processing is distributed in the posterior cortices and regulated by the prefrontal cortices with a filtering or gating mechanism to select appropriate signals and suppress inappropriate signals and noise. Based on this model, a hypothesis can be developed that, in the case of uncertainty or overloading of object-level processing, the prefrontal cortices will become more active in order to modulate signals and noise. This hypothesis is supported by a recent fMRI study [6] showing that the PFC (Brodmann area 9, BA9) was activated when subjects were overloaded in a bimodal attentional task, compared to a unimodal task. Here, we report a study showing that applying repetitive transmagnetic stimulation (rTMS) over the BA9 in order to interfere with its functional activity resulted in significant increas in guessed responses, compared to three other control conditions (i.e., no-TMS, sham TMS on BA9, and rTMS on Cz). The results are compatible with the dynamic filtering theory and suggest that a malfunction of the PFC would weaken the quality of meta-cognitive percepts and increase the number of guessed responses.

Brain deactivation in the outperformance in bimodal tasks: an FMRI study.

While it is known that some individuals can effectively perform two tasks simultaneously, other individuals cannot. How the brain deals with performing simultaneous tasks remains unclear. In the present study, we aimed to assess which brain areas corresponded to various phenomena in task performance. Nineteen subjects were requested to sequentially perform three blocks of tasks, including two unimodal tasks and one bimodal task. The unimodal tasks measured either visual feature binding or auditory pitch comparison, while the bimodal task required performance of the two tasks simultaneously. The functional magnetic resonance imaging (fMRI) results are compatible with previous studies showing that distinct brain areas, such as the visual cortices, frontal eye field (FEF), lateral parietal lobe (BA7), and medial and inferior frontal lobe, are involved in processing of visual unimodal tasks. In addition, the temporal lobes and Brodmann area 43 (BA43) were involved in processing of auditory unimodal tasks. These results lend support to concepts of modality-specific attention. Compared to the unimodal tasks, bimodal tasks required activation of additional brain areas. Furthermore, while deactivated brain areas were related to good performance in the bimodal task, these areas were not deactivated where the subject performed well in only one of the two simultaneous tasks. These results indicate that efficient information processing does not require some brain areas to be overly active; rather, the specific brain areas need to be relatively deactivated to remain alert and perform well on two tasks simultaneously. Meanwhile, it can also offer a neural basis for biofeedback in training courses, such as courses in how to perform multiple tasks simultaneously.

Elevated haemoglobin levels in the motor cortex following 1 Hz transcranial magnetic stimulation: a preliminary study.

One hertz transcranial magnetic stimulation (TMS) over the motor cortex has been reported to increase activity in the motor cortex contralateral to stimulation, as evidenced by the elevated motor evoked potential on the corresponding hand muscle. Little research, however, has assessed concomitant changes in the haemoglobin level in the unstimulated motor cortex. An aim of this study was to measure the change of oxy- and deoxy-haemoglobin levels in the left motor cortex after 20 min of 1 Hz TMS over the right motor cortex. Subjects carried out a finger to thumb tapping task sequentially with six blocks of ten cycles (30 s on and 60 s off). One block was performed before TMS and five after TMS. The results show that the level of oxyhaemoglobin in the unstimulated cortex increased after TMS over the contralateral hemisphere and that the increase lasted 40 min after 1 Hz stimulation. Deoxy-haemoglobin was slightly decreased during the first 15 min after stimulation. The results identify long term physiological changes resulting from 1 Hz stimulation and help to inform our understanding of interhemispheric interactions in TMS studies.

Magnetic stimulation and the crossed-uncrossed difference (CUD) paradigm: selective effects in the ipsilateral and contralateral hemispheres.

When a visual target is presented to one hemifield, manual responses made to the target using the ipsilateral hand (uncrossed responses) are faster than responses using the contralateral hand (crossed response), because there is no need for visuomotor information to be transferred between the hemispheres. This difference in response times is termed the crossed-uncrossed difference (CUD) and is a valuable means of estimating interhemispheric transfer time. We aimed to investigate the CUD by applying repetitive transcranial magnetic stimulation (rTMS) over the left and right occipital cortex during a lateralized target-detection task. Eleven neurologically healthy subjects, all right-handed, participated in the study. Relative to sham TMS we increased the CUD, by inhibiting the crossed latencies, but only when rTMS was applied to the hemisphere receiving visual information. These results replicate and extend previous findings and suggest the inhibitory rTMS effect under the crossed condition might be because the weak visual output is unable to activate the crossed pathway.