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

Reliability in reporting perceptual experience: behaviour and electrophysiology in hemianopic patients

Patients with hemianopia can present with the so called blindsight phenomenon: the ability to perform above chance in the absence of acknowledged awareness. Proper awareness reports are, thus, crucial to distinguish pure forms of blindsight from forms of conscious, yet degraded, vision. It has in fact been recently shown that 1) dichotomous and graded measures to assess awareness can lead to different behavioural results in patients with hemianopia and that 2) different grades of perceptual clarity show different electrophysiological correlates in healthy participants. Here, in hemianopic patients, we assessed awareness by means of the four-point Perceptual Awareness Scale (PAS) and investigated its neural correlates with Event Related Potentials (ERPs). Results showed that patients, in most of the cases, can rate the clarity of their perceptual experience in a graded manner. Moreover, graded perceptual experiences correlated with the amplitude of deflections in ERPs. These results call for the need to assess perceptual awareness with graded measures and for the importance to use electrophysiological data to correlate behaviour with neural processing

Late positivity does not meet the criteria to be considered a proper neural correlate of perceptual awareness

Contrastive analysis has been widely employed in the search for the electrophysiological neural correlates of consciousness. However, despite its clear logic, it has been argued that it may not succeed in isolating neural processes solely involved in the emergence of perceptual awareness. In fact, data from contrastive analysis would be contaminated by potential confounding factors reflecting distinct, though related, processes either preceding or following the conscious perception. At present, the ERP components representing the proper correlates of perceptual awareness still remain to be identified among those correlating with awareness (i.e., Visual Awareness Negativity, VAN and Late Positivity, LP). In order to dissociate visual awareness from post-perceptual confounds specifically related to decision making, we manipulated the response criterion, which affects how a percept is translated into a decision. In particular, while performing an orientation discrimination task, participants were asked to shift their response criterion across sessions. As a consequence, the resulting modulation should concern the ERP component(s) not exclusively reflecting mechanisms regulating the subjective conscious experience itself but rather the processes accompanying it. Electrophysiological results showed that N1 and P3 were sensitive to the response criterion adopted by participants. Additionally, the more the participants shifted their response criterion, the bigger the ERP modulation was; this was consequently indicative of the critical role of these components in the decision-making processes regardless of awareness level. When considering data independently from the response criterion, the aware vs. unaware contrast showed that both VAN and LP were significant. Crucially, the LP component was also modulated by the interaction of awareness and response criterion, while VAN results to be unaffected. In agreement with previous literature, these findings provide evidence supporting the hypothesis that VAN tracks the emergence of visual awareness by encoding the conscious percept, whereas LP reflects the contribution from post-perceptual processes related to response requirements. This excludes a direct functional role of this later component in giving rise to perceptual awareness

Markers of TMS-evoked visual conscious experience in a patient with altitudinal hemianopia

Abstract Transcranial magnetic stimulation (TMS) of the occipital and parietal cortices can induce phosphenes, i.e. visual sensations of light without light entering the eyes. In this paper, we adopted a TMS-EEG interactive co-registration approach with a patient (AM) showing altitudinal hemianopia. Occipital and parietal cortices in both hemispheres were stimulated while concurrently recording EEG signal. Results showed that, for all sites, neural activity differentially encoding for the presence vs. absence of a conscious experience could be found in a cluster of electrodes close to the stimulation site at an early (70 ms) time-period after TMS. The present data indicate that both occipital and parietal sites are independent early gatekeepers of perceptual awareness, thus, in line with evidence in favor of early correlates of perceptual awareness. Moreover, these data support the valuable contribution of the TMS-EEG approach in patients with visual field defects to investigate the neural processes responsible for perceptual awareness

The glamor of old-style single-case studies in the neuroimaging era: insights from a patient with hemianopia

[no abstract available