Mapping the routes of perception: Hemispheric asymmetries in signal propagation dynamics

The visual system has long been considered equivalent across hemispheres. However, an increasing amount of data shows that functional differences may exist in this regard. We therefore tried to characterize the emergence of visual perception and the spatiotemporal dynamics resulting from the stimulation of visual cortices in order to detect possible interhemispheric asymmetries. Eighteen participants were tested. Each of them received 360 transcranial magnetic stimulation (TMS) pulses at phosphene threshold intensity over left and right early visual areas while electroencephalography was being recorded. After each single pulse, participants had to report the presence or absence of a phosphene. Local mean field power analysis of TMS-evoked potentials showed an effect of both site (left vs. right TMS) of stimulation and hemisphere (ipsilateral vs. contralateral to the TMS): while right TMS determined early stronger activations, left TMS determined later stronger activity in contralateral electrodes. The interhemispheric signal propagation index revealed differences in how TMS-evoked activity spreads: left TMS-induced activity diffused contralaterally more than right stimulation. With regard to phosphenes perception, distinct electrophysiological patterns were found to reflect similar perceptual experiences: left TMS-evoked phosphenes are associated with early occipito-parietal and frontal activity followed by late central activity; right TMS-evoked phosphenes determine only late, fronto-central, and parietal activations. Our results show that left and right occipital TMS elicits differential electrophysiological patterns in the brain, both per se and as a function of phosphene perception. These distinct activation patterns may suggest a different role of the two hemispheres in processing visual information and giving rise to perception.

Cortical thickness is inversely associated with transcranial magnetic stimulation-evoked N45 potential among young adults whose heavy drinking began in adolescence

BACKGROUND

Adolescence is a particularly vulnerable stage of development in terms of the deleterious effects of alcohol. Both lower gray matter (GM) volume and greater GABAergic activity have been associated with chronic alcohol consumption during adolescence. However, the association between these measures has not been investigated.

METHODS

In this exploratory study, we compared 26 young adults with a 10year history of heavy alcohol consumption with 21 controls who used little or no alcohol. Simultaneous transcranial magnetic stimulation and electroencephalography were used to assess transcranial magnetic stimulation-evoked N45 potentials, reflecting a balance between GABAergic inhibition and N-methyl-D-aspartate (NMDA) receptor-mediated glutaminergic excitation in the brain. GM thickness was measured from magnetic resonance images and GM and N45 potentials were then correlated.

RESULTS

Cortical thickness was significantly lower in several brain regions in the heavy-drinking group than the light-drinking group. The N45 amplitude was significantly larger frontally in the heavy-drinking group. Among heavy drinkers, there were several statistically significant correlations between thinner GM and larger frontal N45 amplitudes that were not detectable in the light-drinking group. The strongest correlations were detected in the frontal and parietal lobes, especially in the left superior frontal gyrus and the left supramarginal gyrus, and in both hemispheres in the superior parietal lobes.

CONCLUSIONS

These findings show that a thinner cortex and greater inhibitory neurotransmission are correlated in certain brain regions among young, long-term heavy alcohol users. Studies are needed to explore the possible causal mechanisms underlying these effects.