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The role of the retino-colliculo-extrastriate pathway in visual awareness and visual field recovery
No full textPatients with visual field defects resulting from post-chiasmatic lesions experience loss of visual function in up to one half of their visual field, with consequent impairments in their daily life activities. Therefore, effective strategies for compensating for the visual field loss are of great clinical relevance. After lesions to the primary visual pathway -which conveys visual information from the retina to the lateral geniculate nucleus, the optic radiations and, then, to the striate cortex-an alternative visual pathway, which projects from the superior colliculus to the extrastriate cortex, is usually spared in patients with visual field defects. In the present review, evidence for spared functioning of this alternative pathway in patients with visual field defects will be presented, both in terms of residual visual abilities, without awareness, for stimuli presented in the blind field, and the ability to integrate unseen visual signals presented in the blind field with concurrent auditory stimuli. Crucially, this review will discuss how the spared retino-colliculo-extrastriate pathway might be a useful tool for compensating for the loss of visual perception. Accordingly, evidence for the compensatory effects of systematic multisensory audio-visual stimulation in patients with visual field defects will be reviewed
Compensatory recovery after multisensory stimulation in hemianopic patients: behavioral and neurophysiological components
Full text linkLateralized post-chiasmatic lesions of the primary visual pathway result in loss of visual perception in the field retinotopically corresponding to the damaged cortical area. However, patients with visual field defects have shown enhanced detection and localization of multisensory audio-visual pairs presented in the blind field. This preserved multisensory integrative ability (i.e., crossmodal blindsight) seems to be subserved by the spared retino-colliculo-dorsal pathway. According to this view, audio-visual integrative mechanisms could be used to increase the functionality of the spared circuit and, as a consequence, might represent an important tool for the rehabilitation of visual field defects. The present study tested this hypothesis, investigating whether exposure to systematic multisensory audio-visual stimulation could induce long-lasting improvements in the visual performance of patients with visual field defects. A group of 10 patients with chronic visual field defects were exposed to audio-visual training for 4 hours daily, over a period of 2 weeks. Behavioral, oculomotor and electroencephalography (EEG) measures were recorded during several visual tasks before and after audio-visual training. After audio-visual training, improvements in visual search abilities, visual detection, self-perceived disability in daily life activities and oculomotor parameters were found, suggesting the implementation of more effective visual exploration strategies. At the electrophysiological level, after training, patients showed a significant reduction of the P3 amplitude in response to stimuli presented in the intact field, reflecting a reduction in attentional resources allocated to the intact field, which might co-occur with a shift of spatial attention towards the blind field. More interestingly, both the behavioral improvements and the electrophysiological changes observed after training were found to be stable at a follow-up session (on average, 8 months after training), suggesting long-term effects of multisensory audio-visual training. These long-lasting effects seem to be subserved by the activation of the spared retino-colliculo-dorsal pathway, which promotes orienting responses towards the blind field, able to both compensate for the visual field loss and concurrently attenuate visual attention towards the intact field. These results add to previous findings the knowledge that audio-visual multisensory stimulation promote long-term plastic changes in hemianopics, resulting in stable and long-lasting ameliorations in behavioural and electrophysiological measures
Patterns of spontaneous recovery of neglect and associated disorders in acute right brain-damaged patients.
No full textPlasticity in body and peripersonal space representations
No full textA successful interaction with objects in the environment requires integrating
information concerning object-location with the shape, dimension and position of body parts in space. The former information is coded in a multisensory representation of the space around the body, i.e. peripersonal space (PPS), whereas the latter is enabled by an online, constantly updated, action-orientated multisensory representation of the body (BR) that is critical for action. One of the critical features of these representations is that both PPS and BR are not fixed, but they dynamically change depending on different types of experience. In a series of experiment, I studied plastic properties of PPS and BR in humans. I have developed a series of methods to measure the boundaries of PPS representation (Chapter 4), to study its neural correlates (Chapter 3) and to assess BRs. These tasks have been used to study changes in PPS and BR following tool-use (Chapter 5), multisensory stimulation (Chapter 6), amputation and prosthesis implantation (Chapter 7) or social interaction (Chapter 8). I found that changes in the function (tool-use) and the structure (amputation and prosthesis implantation) of the physical body elongate or shrink both PPS and BR. Social context and social interaction also shape PPS representation. Such high degree of plasticity suggests that our sense of body in space is not given at once, but it is constantly constructed and adapted through experience.Allo scopo di interagire con oggetti presenti nell’ambiente esterno è necessario integrare le informazioni sulla posizione degli oggetti nello spazio con informazioni riguardanti la forma, dimensione e posizione delle singole parti del corpo rispetto all’oggetto stesso. Due diverse rappresentazioni supportano la codifica di tali informazioni: da una parte, la rappresentazione dello Spazio Peripersonale, una rappresentazione multisensoriale dello spazio intorno al corpo, e dall’altra una rappresentazione multisensoriale del corpo, costantemente aggiornata e orientata all’azione. Una caratteristica critica di queste rappresentazioni è rappresentata dalle loro proprietà plastiche, cioè dalla possibilità di modificarsi in seguito a diversi tipi di esperienza. In questa tesi mi sono focalizzata sullo studio delle proprietà plastiche delle rappresentazioni del corpo e dello spazio peripersonale. Ho sviluppato una serie di metodi per valutare il confine dello spazio peripersonale (Capitolo 4), per studiare i suoi correlati neurali (Capitolo 3) e per valutare le rappresentazioni multisensoriali del corpo. Questi compiti sono stati usati per studiare modificazioni plastiche del corpo e dello spazio peripersonale in seguito all’utilizzo di uno strumento (Capitolo 5), in seguito a una stimolazione multisensoriale (Capitolo 6), amputazione e impianto di protesi (Capitolo 7) e nell’ambito delle interazioni sociali. I risultati ottenuti hanno mostrato come la modificazione nella funzione (in seguito all’utilizzo di uno strumento) o della struttura fisica (in seguito ad amputazione ed impianto di protesi) del corpo determinano una estensione o una contrazione sia della rappresentazione dello spazio peripersonale che della rappresentazione del corpo. Inoltre, i risultati ottenuti hanno dimostrato che la rappresentazione dello spazio peripersonale viene plasmata anche dalle interazioni sociali. Tale livello di plasticità suggerisce che l’esperienza del nostro corpo viene continuata costruita e aggiornata tramite le diverse esperienze
Effetti dell'integrazione visuo-acustica in pazienti con disturbo di campo visivo
No full textHuman brain is provided with a flexible audio-visual system, which interprets and guides responses to external events according to spatial alignment, temporal synchronization and effectiveness of unimodal signals. The aim of the present thesis was to explore the possibility that such a system might represent the neural correlate of sensory compensation after a damage to one sensory pathway. To this purpose, three experimental studies have been conducted, which addressed the immediate, short-term and long-term effects of audio-visual integration on patients with Visual Field Defect (VFD).
Experiment 1 investigated whether the integration of stimuli from different modalities (cross-modal) and from the same modality (within-modal) have a different, immediate effect on localization behaviour. Patients had to localize modality-specific stimuli (visual or auditory), cross-modal stimulus pairs (visual-auditory) and within-modal stimulus pairs (visual-visual). Results showed that cross-modal stimuli evoked a greater improvement than within modal stimuli, consistent with a Bayesian explanation. Moreover, even when visual processing was impaired, cross-modal stimuli improved performance in an optimal fashion. These findings support the hypothesis that the improvement derived from multisensory integration is not attributable to simple target redundancy, and prove that optimal integration of cross-modal signals occurs in processing stage which are not consciously accessible.
Experiment 2 examined the possibility to induce a short term improvement of localization performance without an explicit knowledge of visual stimulus. Patients with VFD and patients with neglect had to localize weak sounds before and after a brief exposure to a passive cross-modal stimulation, which comprised spatially disparate or spatially coincident audio-visual stimuli. After exposure to spatially disparate stimuli in the affected field, only patients with neglect exhibited a shifts of auditory localization toward the visual attractor (the so called Ventriloquism After-Effect). In contrast, after adaptation to spatially coincident stimuli, both neglect and hemianopic patients exhibited a significant improvement of auditory localization, proving the occurrence of After Effect for multisensory enhancement. These results suggest the presence of two distinct recalibration mechanisms, each mediated by a different neural route: a geniculo-striate circuit and a colliculus-extrastriate circuit respectively.
Finally, Experiment 3 verified whether a systematic audio-visual stimulation could exert a long-lasting effect on patients’ oculomotor behaviour. Eye movements responses during a visual search task and a reading task were studied before and after visual (control) or audio-visual (experimental) training, in a group of twelve patients with VFD and twelve controls subjects. Results showed that prior to treatment, patients’ performance was significantly different from that of controls in relation to fixations and saccade parameters; after audiovisual training, all patients reported an improvement in ocular exploration characterized by fewer fixations and refixations, quicker and larger saccades, and reduced scanpath length. Similarly, reading parameters were significantly affected by the training, with respect to specific impairments observed in left and right hemisphere–damaged patients. The present findings provide evidence that a systematic audio-visual stimulation may encourage a more organized pattern of visual exploration with long lasting effects.
In conclusion, results from these studies clearly demonstrate that the beneficial effects of audio-visual integration can be retained in absence of explicit processing of visual stimulus. Surprisingly, an improvement of spatial orienting can be obtained not only when a on-line response is required, but also after either a brief or a long adaptation to audio-visual stimulus pairs, so suggesting the maintenance of mechanisms subserving cross-modal perceptual learning after a damage to geniculo-striate pathway. The colliculus-extrastriate pathway, which is spared in patients with VFD, seems to play a pivotal role in this sensory compensation
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