title: Can IPS reach visual awareness without V1? Evidence from TMS in healthy subjects and hemianopic patients.
abstract: 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.
authors.affiliation: University of Verona and National Institute of Neuroscience-Italy.
authors.forename: Chiara
authors.initials: C
authors.lastname: Mazzi
authors.affiliation: University of Verona and National Institute of Neuroscience-Italy; Rehabilitation Unit, Zevio Hospital, Zevio, Verona, Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Mancini
authors.affiliation: University of Verona and National Institute of Neuroscience-Italy. Electronic address: silvia.savazzi@univr.it.
authors.forename: Silvia
authors.initials: S
authors.lastname: Savazzi
mesh_terms: D000328:Adult; D001364:Awareness / Q000502:physiology; D005260:Female; D007839:Functional Laterality / Q000502:physiology; D064368:Healthy Volunteers; D006423:Hemianopsia / Q000000981:diagnostic imaging / Q000503:physiopathology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D010296:Parietal Lobe / Q000000981:diagnostic imaging / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation; D014793:Visual Cortex / Q000000981:diagnostic imaging / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology*; D055815:Young Adult
keywords: Parietal cortex; Phosphene; Primary visual cortex; Vision-for-action stream; Visual awareness