Silvia Convento

Improving ideomotor limb apraxia by electrical stimulation of the left posterior parietal cortex

Limb apraxia, a deficit of planning voluntary gestures, is most frequently caused by damage to the left hemisphere, where, according to an influential neurofunctional model, gestures are planned, before being executed through the motor cortex of the hemisphere contralateral to the acting hand. We used anodal transcranial direct current stimulation delivered to the left posterior parietal cortex (PPC), the right motor cortex (M1), and a sham stimulation condition, to modulate the ability of six left-brain-damaged patients with ideomotor apraxia, and six healthy control subjects, to imitate hand gestures, and to perform skilled hand movements using the left hand. Transcranial direct current stimulation delivered to the left PPC reduced the time required to perform skilled movements, and planning, but not execution, times in imitating gestures, in both patients and controls. In patients, the amount of decrease of planning times brought about by left PPC transcranial direct current stimulation was influenced by the size of the parietal lobe damage, with a larger parietal damage being associated with a smaller improvement. Of interest from a clinical perspective, left PPC stimulation also ameliorated accuracy in imitating hand gestures in patients. Instead, transcranial direct current stimulation to the right M1 diminished execution, but not planning, times in both patients and healthy controls. In conclusion, by using a transcranial stimulation approach, we temporarily improved ideomotor apraxia in the left hand of left-brain-damaged patients, showing a role of the left PPC in planning gestures. This evidence opens up novel perspectives for the use of transcranial direct current stimulation in the rehabilitation of limb apraxia.

Multisensory processing after a brain damage: clues on post-injury crossmodal plasticity from neuropsychology

Current neuropsychological evidence demonstrates that damage to sensory-specific and heteromodal areas of the brain not only disrupts the ability of combining sensory information from multiple sources, but can also cause altered multisensory experiences. On the other hand, there is also evidence of behavioural benefits induced by spared multisensory mechanisms. Thus, crossmodal plasticity can be viewed in both an adaptive and maladaptive context. The emerging view is that different crossmodal plastic changes can result following damage to sensory-specific and heteromodal areas, with post-injury crossmodal plasticity representing an attempt of a multisensory system to reconnect the various senses and by-pass injured areas. Changes can be considered adaptive when there is compensation for the lesion-induced sensory impairment. Conversely, it may prove maladaptive when atypical or even illusory multisensory experiences are generated as a result of re-arranged multisensory networks. This theoretical framework posits new intriguing questions for neuropsychological research and places greater emphasis on the study of multisensory phenomena within the context of damage to large-scale brain networks, rather than just focal damage alone.

Neuromodulation of early multisensory interactions in the visual cortex

Merging information derived from different sensory channels allows the brain to amplify minimal signals to reduce their ambiguity, thereby improving the ability of orienting to, detecting, and identifying environmental events. Although multisensory interactions have been mostly ascribed to the activity of higher-order heteromodal areas, multisensory convergence may arise even in primary sensory-specific areas located very early along the cortical processing stream. In three experiments, we investigated early multisensory interactions in lower-level visual areas, by using a novel approach, based on the coupling of behavioral stimulation with two noninvasive brain stimulation techniques, namely, TMS and transcranial direct current stimulation (tDCS). First, we showed that redundant multisensory stimuli can increase visual cortical excitability, as measured by means of phosphene induction by occipital TMS; such physiological enhancement is followed by a behavioral facilitation through the amplification of signal intensity in sensory-specific visual areas. The more sensory inputs are combined (i.e., trimodal vs. bimodal stimuli), the greater are the benefits on phosphene perception. Second, neuroelectrical activity changes induced by tDCS in the temporal and in the parietal cortices, but not in the occipital cortex, can further boost the multisensory enhancement of visual cortical excitability, by increasing the auditory and tactile inputs from temporal and parietal regions, respectively, to lower-level visual areas.

Roles of the right temporo-parietal and premotor cortices in self-location and body ownership

In the rubber hand illusion (RHI), the feeling that a fake hand belongs to oneself can be induced by the simultaneous, congruent touch of the fake visible hand and ones own hidden hand. This condition is also associated with a recalibration of the perceived location of the real hand. A cortical network, including premotor and temporo‐parietal areas, has been proposed as the basis of the RHI. However, the causal contribution of these areas to the discrete illusory components remains unclear. We used transcranial direct current stimulation (tDCS) to assess the contribution of the right premotor cortex (rPMc) and the right temporo‐parietal junction (rTPJ) to the RHI and explored the role of these areas in modulating the subjective experience of embodiment and the misperception of the hand position. We found that anodal tDCS of both rPMc and rTPJ increased the misjudgement of the real hand location towards the fake hand. Crucially, the difference in proprioceptive displacement evoked by the congruent and incongruent visuo‐tactile stroking was minimised when tDCS was applied over the rPMc, while it was amplified when the rTPJ was targeted. The parietal effects of tDCS also extended to the self‐report components of the RHI. These findings suggest that the tDCS of rTPJ modulates the RHI depending on the temporal congruency of the visuo‐tactile stimulation, while the tDCS of rPMc induces a general recalibration of hand coordinates, regardless of the visuo‐tactile congruency. The present results are discussed in the view of a multicomponent model of the RHI.

Selective attention gates the interactive crossmodal coupling between cortical sensory systems

Cortical sensory systems often activate in parallel, even when stimulation is experienced through a single sensory modality1–3. Critically, the functional relationship between coactivated cortical systems is unclear: Co-activations may reflect the interactive coupling between information-linked cortical systems or merely parallel but independent sensory processing. Here, we report causal evidence that human somatosensory cortex (S1), which co-activates with auditory cortex during the processing of vibrations and textures4–9, interactively couples to cortical systems that support auditory perception. Acute manipulation of S1 activity using transcranial magnetic stimulation (TMS) impairs auditory frequency perception when subjects simultaneously attend to auditory and tactile frequency, but not when attention is directed to audition alone. Auditory frequency perception is unaffected by TMS over visual cortex thus confirming the privileged coupling between the somatosensory and auditory cortical systems in temporal frequency processing10–13. Our results provide a key demonstration that selective attention can enhance the functional coupling between cortical systems that support different sensory modalities. The gating of crossmodal coupling by selective attention may critically support multisensory interactions and feature-specific perception. Author Contributions S.C. and J.M.Y. designed the study and wrote the manuscript. S.C. and M.S.R. performed the experiments and analyzed the data. All authors contributed to the final revisions of the manuscript.