Studying hemispheric asymmetries in visually responsive areas: a TMS-EEG study

The last decades of neuroscientific research have seen a gradual flourishing of studies regarding the neural correlates of human consciousness, with evidence from perceptual studies and theoretical models progressively trying to elucidate the brain dynamics responsible for awareness to emerge. However, despite of the everincreasing number of studies in the field, many aspects are still waiting for clarification. One example of this, in the field of visual awareness, regards the possible hemispheric asymmetry in the neural mechanisms giving rise to visual experiences. In fact, it is known by now that areas located along both the classically defined ventral stream (associated with “vision for perception”) and along the dorsal stream (the “vision for action” stream) can elicit visual percepts – in the form of phosphenes – when stimulated via transcranial magnetic stimulation. However, until now a direct comparison between the two hemispheres in the neural dynamics giving rise to these visual percepts has never been done. With this work, therefore, we tried to shed light on possible differences between the two hemispheres in two cortical areas associated with either one of the two streams: we stimulated the early visual cortex (Experiment 1) and the posterior parietal cortex (Experiment 2) of both hemispheres to elicit phosphenes and compare the associated EEG activity. In both cases we found a clear hemispheric difference, with a left hemisphere showing an early local activation, followed by a more widespread ignition of neural activity; the right hemisphere, on the other side, displayed a later activation mainly localized over central electrodes. These results, consistent across the two experiments, point to the existence of distinct neural mechanisms in the two hemispheres for perceptual awareness. The last part of this work is dedicated to better understand the functioning of transcranial magnetic stimulation, a stimulation technique commonly used in cognitive neuroscience. In spite of its widespread diffusion, the specific influence of some stimulation parameters is not completely understood. To shed some light on this aspect, we stimulated three premotor cortical targets in close proximity, each at three different coil orientation (0°, 45° and 90° respect to stimulated site). Our 4 aim was to disentangle the effect of coil orientation and slight coil transitions on the elicited TEP response. Our preliminary results seem to suggest that both factors have an influence, with orientation being the most influential factor: specifically, an orientation perpendicular to that of the stimulated gyrus seems to be able to elicit the strongest and most reliable response.