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.

The spectral dynamics of visual awareness: An interplay of different frequencies?

The goal of this study was to investigate the neurophysiological correlates of visual awareness, with a specific focus on its event-related spectral perturbation (ERSP) features. To this aim, we tried to disentangle the proper neural correlates of consciousness (NCC) from other prerequisite and post-perceptual processing. To do so, we administered an orientation discrimination task, inducing a response bias through task instructions. EEG results showed that different frequency bands are involved in this kind of task, with different spectral and temporal dynamics. In particular, alpha and beta bands seem to be particularly engaged, especially in the aware-unaware contrast, showing a main power suppression for aware trials and replicating previous literature. Moreover, we demonstrated that the process of visual awareness is orchestrated by a complex interaction of different frequencies (i.e., theta, alpha, beta and gamma) being involved as prerequisites and post-perceptual processes.

Olfaction and gustation in blindness: a state of the art of the literature.

To date, there are quite a few studies assessing olfaction and gustation in blindness, with great variability in sample size, participants' age, blindness onset and smell and taste evaluation methods. Indeed, the evaluation of olfactory and gustatory performance can differ depending on several factors, including cultural differences. Therefore, here we analysed through a narrative review, all the works reporting a smell and taste assessment in blind individuals during the last 130 years, trying to summarize and address the knowledge in this field.

Graph Analysis of TMS-EEG Connectivity Reveals Hemispheric Differences following Occipital Stimulation.

(1) Background: Transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) provides a unique opportunity to investigate brain connectivity. However, possible hemispheric asymmetries in signal propagation dynamics following occipital TMS have not been investigated. (2) Methods: Eighteen healthy participants underwent occipital single-pulse TMS at two different EEG sites, corresponding to early visual areas. We used a state-of-the-art Bayesian estimation approach to accurately estimate TMS-evoked potentials (TEPs) from EEG data, which has not been previously used in this context. To capture the rapid dynamics of information flow patterns, we implemented a self-tuning optimized Kalman (STOK) filter in conjunction with the information partial directed coherence (iPDC) measure, enabling us to derive time-varying connectivity matrices. Subsequently, graph analysis was conducted to assess key network properties, providing insight into the overall network organization of the brain network. (3) Results: Our findings revealed distinct lateralized effects on effective brain connectivity and graph networks after TMS stimulation, with left stimulation facilitating enhanced communication between contralateral frontal regions and right stimulation promoting increased intra-hemispheric ipsilateral connectivity, as evidenced by statistical test (p < 0.001). (4) Conclusions: The identified hemispheric differences in terms of connectivity provide novel insights into brain networks involved in visual information processing, revealing the hemispheric specificity of neural responses to occipital stimulation.

Spatiotemporal dynamics of attentional orienting and reorienting revealed by fast optical imaging in occipital and parietal cortices.

The mechanisms of visuospatial attention are mediated by two distinct fronto-parietal networks: a bilateral dorsal network (DAN), involved in the voluntary orientation of visuospatial attention, and a ventral network (VAN), lateralized to the right hemisphere, involved in the reorienting of attention to unexpected, but relevant, stimuli. The present study consisted of two aims: 1) to characterize the spatio-temporal dynamics of attention and 2) to examine the predictive interactions between and within the two attention systems along with visual areas, by using fast optical imaging combined with Granger causality. Data were collected from young healthy participants performing a discrimination task in a Posner-like paradigm. Functional analyses revealed bilateral dorsal parietal (i.e. dorsal regions included in the DAN) and visual recruitment during orienting, highlighting a recursive predictive interplay between specific dorsal parietal regions and visual cortex. Moreover, we found that both attention networks are active during reorienting, together with visual cortex, highlighting a mutual interaction among dorsal and visual areas, which, in turn, predicts subsequent ventral activity. For attentional reorienting our findings indicate that dorsal and visual areas encode disengagement of attention from the attended location and trigger reorientation to the unexpected location. Ventral network activity could instead reflect post-perceptual maintenance of the internal model to generate and keep updated task-related expectations.

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

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 (70ms) 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.

Temporal perception in joint action: This is MY action.

Here we investigated the temporal perception of self- and other-generated actions during sequential joint actions. Participants judged the perceived time of two events, the first triggered by the participant and the second by another agent, during a cooperative or competitive interaction, or by an unspecified mechanical cause. Results showed that participants perceived self-generated events as shifted earlier in time (anticipation temporal judgment bias) and non-self-generated events as shifted later in time (repulsion temporal judgment bias). This latter effect was observed independently from the kind of cause (i.e., agentive or mechanical) or interaction (i.e., cooperative or competitive). We suggest that this might represent a mental process which allows discriminating events that cannot plausibly be linked to one's own action. When an event immediately follows a self-generated one, temporal judgment biases operate as self-serving biases in order to separate self-generated events from events of another physical causality.

Assessing the effects of physical and perceived luminance contrast on RT and TMS-induced percepts.

Simple reaction times (RTs) are inversely related to the luminance of a visual region, with RT increasing as luminance decreases, and decreasing as luminance increases. A potential discrepancy in the link between luminance and RT, however, stems from the perception of luminance itself. Here, we tested whether RT is modulated by a measureable amount of light (luminance) or perceptual amount of light (brightness), as two test regions having the same luminance can be perceived as having different brightness. The current study investigates the effects of brightness using probes and artificial percepts, i.e., transcranial magnetic stimulation (TMS)-induced light and dark percepts. In Experiment 1, participants performed a RT task to light and dark probes presented over two backgrounds, one exhibiting a physical luminance and the other exhibiting perceptual brightness. Experiment 2 tested whether perceptual brightness could influence RT and frequency of artificial percepts. We found that while brightness contrast modulated RT to the dark probes, the frequency of artificial percepts was susceptible to both physical and perceived changes in luminance. These data suggest that some behavioral responses can be influenced by an illusion of brightness, wherein there is no actual change in luminance, as well as the perception of TMS-induced percepts.

Shining new light on dark percepts: visual sensations induced by TMS.

Phosphenes induced by transcranial magnetic stimulation (TMS) are sensations of light, whereas a missing region in the visual field induced by TMS is generally referred to as a scotoma. It is believed that phosphenes are caused by neural excitation, while scotomas are due to neural inhibition. In light of the recent literature it might, however, be surmised that both phenomena are the result of neural noise injected into the cortex by TMS and that the likelihood of perceiving the two kinds of percepts depends on the state of the cortex at the time of stimulation. In the present study, TMS was applied over the left occipital cortex under different background conditions (Experiments 1-2) and using different TMS intensities (Experiment 3). Behavioral responses indicate the visual system processes luminance in a standardized manner, as lighter percepts were reacted to faster than darker percepts; this effect, however, did not extend to percept size. Our results suggest that phenomenological characteristics of artificial visual percepts are in line with the proposed effects of TMS as the induction of random neural noise interfering with the neural dynamics (the state of the cortex) at the time of stimulation.

Blindsight is sensitive to stimulus numerosity and configuration: evidence from the redundant signal effect.

One important, yet relatively unexplored question is whether blindsight, i.e., unconscious visually guided behavior in hemianopic patients, is endowed with basic perceptual properties such as detecting stimulus numerosity and overall configuration. Rather than a forced-choice procedure in which patients are supposed to guess about stimuli presented to the blind hemifield, we used a redundant signal effect paradigm, i.e., the speeding of simple reaction time (RT) when presenting multiple versus single similar stimuli. The presence of an effect of numerosity for the (unseen) stimuli presented to the blind field was indirectly assessed by measuring RT to bilateral versus unilateral stimuli presented to the intact hemifield. Chronic hemianopic patients were tested with unilateral or bilateral black dots, both of which could be either single or quadruple. The latter could either have a fixed spatial configuration representing a diamond or be randomly spatially assembled on every trial. Both configurations covered the same extent of visual field and had the overall same luminance. We found that a numerosity effect as a result of increasing the number of stimuli in the blind field was indeed present but only with the diamond configuration. This is a convincing evidence that this form of blindsight does not depend upon stimulus numerosity per se but is likely to be related to the presence of structured and memorized rather than meaningless changing stimuli.

Speeded manual responses to unseen visual stimuli in hemianopic patients: what kind of blindsight?

Blindsight, i.e., unconscious visually guided behaviour triggered by stimuli presented to a cortically blind hemifield, has been typically found either by using direct (forced choice) or indirect (interhemispheric) methods. However, one would expect to find blindsight also in fast responses to suddenly appearing visual stimuli, a reminiscence of evolutionary ancient adaptive behaviour. In this study we provide preliminary evidence of this form of blindsight by using a conservative method for assessing blindsight based on a comparison between the cumulative probability functions (CPFs) of simple reaction times to blind and intact field stimuli. Furthermore, in two patients with blindsight we provided evidence that their above-chance unconscious responses were likely to be triggered by the intact hemisphere.

Phosphene-guided transcranial magnetic stimulation of occipital but not parietal cortex suppresses stimulus visibility.

Transcranial magnetic stimulation (TMS) applied over the occipital lobe approximately 100 ms after the onset of a stimulus decreases its visibility if it appears in the location of the phosphene. Because phosphenes can also be elicited by stimulation of the parietal regions, we asked if the same procedure that is used to reduce visibility of stimuli with occipital TMS will lead to decreased stimulus visibility when TMS is applied to parietal regions. TMS was randomly applied at 0-130 ms after the onset of the stimulus in steps of 10 ms in occipital and parietal regions. Participants responded to the orientation of the line stimulus and rated its visibility. We replicate previous reports of phosphenes from both occipital and parietal TMS. As previously reported, we also observed visual suppression around the classical 100 ms window both in the objective line orientation and subjective visibility responses with occipital TMS. Parietal stimulation, on the other hand, did not consistently reduce stimulus visibility in any time window.

Coherent activity within and between hemispheres: cortico-cortical connectivity revealed by rTMS of the right posterior parietal cortex.

Introduction: Low frequency (1 Hz) repetitive transcranial stimulation (rTMS) applied over right posterior parietal cortex (rPPC) has been shown to reduce cortical excitability both of the stimulated area and of the interconnected contralateral homologous areas. In the present study, we investigated the whole pattern of intra- and inter-hemispheric cortico-cortical connectivity changes induced by rTMS over rPPC. Methods: To do so, 14 healthy participants underwent resting state EEG recording before and after 30 min of rTMS at 1 Hz or sham stimulation over the rPPC (electrode position P6). Real stimulation was applied at 90% of motor threshold. Coherence values were computed on the electrodes nearby the stimulated site (i.e., P4, P8, and CP6) considering all possible inter- and intra-hemispheric combinations for the following frequency bands: delta (0.5-4 Hz), theta (4-8 Hz), alpha (8-12Hz), low beta (12-20 Hz), high beta (20-30 Hz), and gamma (30-50 Hz). Results and discussion: Results revealed a significant increase in coherence in delta, theta, alpha and beta frequency bands between rPPC and the contralateral homologous sites. Moreover, an increase in coherence in theta, alpha, beta and gamma frequency bands was found between rPPC and right frontal sites, reflecting the activation of the fronto-parietal network within the right hemisphere. Summarizing, subthreshold rTMS over rPPC revealed cortico-cortical inter- and intra-hemispheric connectivity as measured by the increase in coherence among these areas. Moreover, the present results further confirm previous evidence indicating that the increase of coherence values is related to intra- and inter-hemispheric inhibitory effects of rTMS. These results can have implications for devising evidence-based rehabilitation protocols after stroke.

Role of corpus callosum in unconscious vision.

The existence of unconscious visually triggered behavior in patients with cortical blindness (e.g., homonymous hemianopia) has been amply demonstrated and the neural bases of this phenomenon have been thoroughly studied. However, a crosstalk between the two hemispheres as a possible mechanism of unconscious or partially conscious vision has not been so far considered. Thus, the aim of this study was to assess the relationship between structural and functional properties of the corpus callosum (CC), as shown by probabilistic tractography (PT), behavioral detection/discrimination performance and level of perceptual awareness in the blind field of patients with hemianopia. Twelve patients were tested in two tasks with black-and-white visual square-wave gratings, one task of movement and the other of orientation. The stimuli were lateralized to one hemifield either intact or blind. A PT analysis was carried out on MRI data to extract fiber properties along the CC (genu, body, and splenium). Compared with a control group of participants without brain damage, patients showed lower FA values in all three CC sections studied. For the intact hemifield we found a significant correlation between PT values and visual detection/discrimination accuracy. For the blind hemifield the level of perceptual awareness correlated with PT values for all three CC sections in the movement task. Importantly, significant differences in all three CC sections were found also between patients with above-vs. chance detection/discrimination performance while differences in the genu were found between patients with and without perceptual awareness. Overall, our study provides evidence that the properties of CC fibers are related to the presence of unconscious stimulus detection/discrimination and to hints of perceptual awareness for stimulus presentation to the blind hemifield. These results underline the importance of information exchange between the damaged and the healthy hemisphere for possible partial or full recovery from hemianopia.

Bypassing input to V1 in visual awareness: A TMS-EROS investigation.

Early visual cortex (V1-V3) is believed to be critical for normal visual awareness by providing the necessary feedforward input. However, it remains unclear whether visual awareness can occur without further involvement of early visual cortex, such as re-entrant feedback. It has been challenging to determine the importance of feedback activity to these areas because of the difficulties in dissociating this activity from the initial feedforward activity. Here, we applied single-pulse transcranial magnetic stimulation (TMS) over the left posterior parietal cortex to elicit phosphenes in the absence of direct visual input to early visual cortex. Immediate neural activity after the TMS pulse was assessed using the event-related optical signal (EROS), which can measure activity under the TMS coil without artifacts. Our results show that: 1) The activity in posterior parietal cortex 50 ms after TMS was related to phosphene awareness, and 2) Activity related to awareness was observed in a small portion of V1 140 ms after TMS, but in contrast (3) Activity in V2 was a more robust correlate of awareness. Together, these results are consistent with interactive models proposing that sustained and recurrent loops of activity between cortical areas are necessary for visual awareness to emerge. In addition, we observed phosphene-related activations of the anteromedial cuneus and lateral occipital cortex, suggesting a functional network subserving awareness comprising these regions, the parietal cortex and early visual cortex.

Differential impairment of interhemispheric transmission in bipolar disease.

There is abundant evidence that the cerebral white matter and in particular the corpus callosum show several structural abnormalities in both schizophrenia (SCZ) and bipolar disease (BD). However, which cognitive functions are impaired as a result of these anomalies is still unclear. Previous behavioural tests of interhemispheric crosstalk have shown a differential impairment in SCZ with interhemispheric transmission time, as tested with the Poffenberger paradigm, essentially normal but with an abnormally enhanced interhemispheric summation effect, as tested with the redundant signal effect. The present study was inspired by this discrepancy and by the more general question of a possible overlap between the pathophysiology of SCZ and BD. We tested both SCZ and BD patients in the Poffenberger paradigm and redundant signal effect and found a similar dissociation, namely a normal interhemispheric transfer time and an abnormal redundant signal effect. The only difference between the two groups was a selective slowing of speed of response of the dominant right hand in the SCZ group suggesting an impairment of left hemisphere functions. These results cast further light on the question of common and differential impairments of basic psychological functions in the two diseases.

Intraoperative cortical mapping of visuospatial functions in parietal low-grade tumors: changing perspectives of neurophysiological mapping.

OBJECT: The aim of this study was to explore the feasibility of intraoperative visuospatial mapping with the same criteria currently used to define essential language areas. METHODS: The authors compared surgical procedures in 2 patients with similar tumors (Grade II oligodendroglioma in the right parietal lobe) undergoing awake, image-assisted surgery for lesion removal with intraoperative neurophysiological monitoring. The line bisection task was used in both patients but with different criteria. RESULTS: In the first case, the authors respected any area, even within the tumor, where significant interference was found (a stimulation-induced error in 2 of 3 applications defined an essential area). In the second case, they removed 1 essential area located in the tumor and recorded an uneventful clinical response soon thereafter. They continued to monitor the patient without stimulation and stopped the resection when the patient was close to the criteria valid for defining spatial neglect. The signs of spatial neglect were present for 3 days postoperatively and then cleared spontaneously. Subtotal tumor removal was achieved in both cases. CONCLUSIONS: Evidence in the present study reveals that areas for visuospatial functions cannot be assessed with the same criteria used for language functions, since essential areas located in the tumor can be safely removed.

Pre- and post-operative assessment of visuo-spatial functions in right hemisphere tumour patients: a pilot study.

Patients with neglect fail to report, respond to, or orient to stimuli presented on the opposite side of the brain lesion. Spatial neglect is a multi-componential syndrome and many of its symptoms and signs need specific testing to detect their different clinical manifestations. Given that neglect has a dramatic effect on the outcome of these patients, it is of paramount importance for accurate assessment all its manifestations. Thus, the purpose of this study is to assess visuo-spatial functions with a newly-built neuropsychological battery to determine the incidence of visuo-spatial disorders and spatial neglect in patients with right brain tumour. We tested 14 patients undergoing tumour resection, both pre-operatively and post-operatively. Pre-operatively, we found an incidence of 42.86% for visuo-spatial deficits and of 14.29% specifically for spatial neglect. This incidence increases post-operatively to 57.14% for visuo-spatial deficits and to 42.86% specifically for spatial neglect. The present study has important implications for a better knowledge of the spatial disorders in patients with right-brain tumour, thus providing a clearer picture on the everyday life problems faced by these patients, both within the family and in working contexts.

Reaction times and perceptual adjustments are sensitive to the illusory distortion of space.

The Oppel-Kundt illusion (OKI) consists of the perception of a filled space as larger than an empty space of the same size. Here, we used a modified version of that illusion composed of a gradient of vertical lines whose spacing decreased progressively from one side to the other: space is expected to be perceived as larger where the lines are more compressed. We tested the hypothesis that a horizontal stimulus presented in a space perceived as larger will produce faster RTs by asking forty-four healthy subjects to respond as quickly as possible to lateralized stimuli (horizontal bars, vertical bars and circles) presented on different backgrounds (control condition: evenly spaced vertical lines or an empty space; illusory conditions: vertical lines progressively compressed to the right or the left). Subjects' RTs were reliably faster for horizontal stimuli presented on the space perceived as larger than on the space perceived as smaller. To verify that this effect was actually due to a size illusion, the same subjects were asked to adjust the size of the stimuli presented on the same backgrounds as to make them equal to a reference stimulus. For horizontal stimuli, subjects produced adjustments in accordance with the predicted effect of the illusion. Together, these data show that the OKI produces a distortion of space that extends to stimuli placed in front of it and that RTs are influenced by the perceived and not the physical size of the stimuli. Implications for neural bases of illusions and for spatial neglect are discussed.