Chiara Mazzi

Can IPS reach visual awareness without V1? Evidence from TMS in healthy subjects and hemianopic patients.

TMS of the occipital cortex can elicit conscious visual percepts, the so-called phosphenes, i.e. the experience of flashes of light in the absence of an external stimulus. Previous evidence has shown that phosphenes can be elicited by TMS of the intraparietal sulcus (IPS). Here, we tested whether IPS-phosphenes are generated without any contribution from the occipital cortex. In Experiment 1, healthy subjects showed that parietal phosphenes were reliable different in terms of threshold, highly variable in size and, in contrast to the occipital ones, they did not change in lateralization within the visual field. Moreover, to strengthen these results and to ascertain whether feedback to primary visual cortex is necessary for visual awareness to emerge, in Experiment 2, we induced phosphenes by applying TMS over the parietal cortex of the lesioned hemisphere in two hemianopic patients with a complete destruction of primary visual cortex. For both patients we found that they could perceive phosphenes, that a psychophysical threshold function could be created and that this function was not different from that obtained with healthy subjects. In addition, they could reliably draw the phosphenes and position them within their blind field and they could reliably score the phenomenical characteristics of their phosphenes both in terms of vividness and brightness in a manner similar to healthy subjects. The two brain-damaged patients have thus shown that stimulation of visual areas within the intraparietal sulcus of their lesioned hemisphere, can, in fact, elicit conscious visual percepts. These results seem to demonstrate that TMS-induced visual phenomenology over IPS can generate conscious visual percepts and, more importantly, that it does so independently of any contributions from the primary visual cortex.

Waves of awareness for occipital and parietal phosphenes perception.

Transcranial magnetic stimulation (TMS) of the occipital cortex is known to induce visual sensations, i.e. phosphenes, which appear as flashes of light in the absence of an external stimulus. Recent studies have shown that TMS can produce phosphenes also when the intraparietal sulcus (IPS) is stimulated. The main question addressed in this paper is whether parietal phosphenes are generated directly by local mechanisms or emerge through indirect activation of other visual areas. Electroencephalographic (EEG) signals were recorded while stimulating left occipital or parietal cortices inducing phosphene perception in healthy participants and in a hemianopic patient who suffered from complete destruction of the early visual cortex of the left hemisphere. Results in healthy participants showed that the onset of phosphene perception induced by occipital TMS correlated with differential cortical activity in temporal sites while the onset of phosphene perception induced by parietal TMS correlated with differential cortical activity in the stimulated parietal site. Moreover, IPS-TMS of the lesioned hemisphere of the hemianopic patient with a complete lesion to V1 showed again that the onset of phosphene perception correlated with differential cortical activity in the stimulated parietal site. The present data seem thus to suggest that temporal and parietal cortices can serve as different local early gatekeepers of perceptual awareness and that activity in the occipital cortex, although being relevant for perception in general, is not part of the neural bases of the perceptual awareness of phosphenes.

Early Local Activity in Temporal Areas Reflects Graded Content of Visual Perception

In visual cognitive neuroscience the debate on consciousness is focused on two major topics: the search for the neural correlates of the different properties of visual awareness and the controversy on the graded versus dichotomous nature of visual conscious experience. The aim of this study is to search for the possible neural correlates of different grades of visual awareness investigating the Event Related Potentials to reduced contrast visual stimuli whose perceptual clarity was rated on the four-point Perceptual Awareness Scale. Results revealed a left centro-parietal negative deflection (Visual Awareness Negativity; VAN) peaking at 280–320 ms from stimulus onset, related to the perceptual content of the stimulus, followed by a bilateral positive deflection (Late Positivity; LP) peaking at 510–550 ms over almost all electrodes, reflecting post-perceptual processes performed on such content. Interestingly, the amplitude of both deflections gradually increased as a function of visual awareness. Moreover, the intracranial generators of the phenomenal content (VAN) were found to be located in the left temporal lobe. The present data thus seem to suggest (1) that visual conscious experience is characterized by a gradual increase of perceived clarity at both behavioral and neural level and (2) that the actual content of perceptual experiences emerges from early local activation in temporal areas, without the need of later widespread frontal engagement.

Blind-Sight vs. Degraded-Sight: Different Measures Tell a Different Story

Blindsight patients can detect, localize, and discriminate visual stimuli in their blind field, despite denying being able to see the stimuli. However, the literature documents the cases of blindsight patients who demonstrated a preserved degree of awareness in their impaired visual field. The aim of this study is to investigate the nature of visual processing within the impaired visual field and to ask whether it reflects pure unconscious behavior or conscious, yet degraded, vision. A hemianopic patient (SL) with a complete lesion to the left primary visual cortex was tested. SL was asked to discriminate several stimulus features (orientation, color, contrast, and motion) presented in her impaired visual field in a two-alternative forced-choice task. SL had to report her subjective experience: in the first experiment as “seen” or “guessed,” whereas in the second experiment as the degree of clarity of her experience according to the perceptual awareness scale. In the first experiment, SL demonstrated a performance above-chance in the discrimination task for “guessed” trials, thus showing type 1 blindsight. In the second experiment, however, SL showed above-chance performance only when she reported a certain degree of awareness, thus showing that SL’s preserved discrimination ability relies on conscious vision. These data show that graded measures to assess awareness, which can better tap on the complexity of conscious experience, need to be used in order to differentiate genuine forms of blindsight from degraded conscious vision.

The influence of posterior parietal cortex on extrastriate visual activity: A concurrent TMS and fast optical imaging study

The posterior parietal cortex (PPC) is a critical node in attentional and saccadic eye movement networks of the cerebral cortex, exerting top-down control over activity in visual cortex. Here, we sought to further elucidate the properties of PPC feedback by providing a time-resolved map of functional connectivity between parietal and occipital cortex using single-pulse TMS to stimulate the left PPC while concurrently recording fast optical imaging data from bilateral occipital cortex. Magnetic stimulation of the PPC induced transient ipsilateral occipital activations (BA 18) 24-48ms post-TMS. Concurrent TMS and fast optical imaging results demonstrate a clear influence of PPC stimulation on activity within human extrastriate visual cortex and further extend this time- and space-resolved method for examining functional connectivity.

On the “blindness” of blindsight: What is the evidence for phenomenal awareness in the absence of primary visual cortex (V1)?

Blindsight has been central to theories of phenomenal awareness; that a lesion to primary visual cortex (V1) abolishes all phenomenal awareness while unconscious visual functions can remain has led to the view that this region plays a crucial role in generating visual consciousness. However, since the early 20th century, there have been reports, many of which controversial, of phenomenal awareness in patients with V1 lesions. These reports include selective sparing of motion awareness, hemianopic completion and visual aftereffects. More recently, there have been successful attempts of inducing visual qualia with noninvasive brain stimulation. Here we critically review this evidence and discuss their implications to theoretical understanding of phenomenal awareness.

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.

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.

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.

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.