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Perceptual Awareness in the Reorganizing Brain

Final Report Summary - PERCEPTUAL AWARENESS (Perceptual Awareness in the Reorganizing Brain)

The purpose of our project is to try and understand the neural bases of visual perceptual awareness, i.e. of those brain processes that enable us to consciously perceive a visual shape or object. To do that we have studied patients who, as a result of brain damage, mainly stroke, have lost vision in one half of their visual field. This impairment is clinically known as hemianopia and does not affect the eyes but the visual cortex, i.e. that part of the brain which receives and processes visual information. About fifty years ago it was found in the US and UK and thereafter in many other countries that some patients with hemianopia may show a dissociation between their perceptual awareness of visual stimuli presented to the blind hemifield and the consequent behaviour, a phenomenon known as "blindsight". Oddly enough, if asked about the stimulus shown in their blind hemifield they answer "no idea" but if prompted to guess or to reach for it they do it much better than chance. This seminal finding has led to two avenues of research: i) Basic research attempting to understand the cerebral mechanisms and areas underlying this unconscious but above-chance behaviour and ii) Applied research focused on devising new rehabilitation procedures to try and restore vision in the affected visual field. Clearly, the ambitious general aim of a project like this is to contribute to cast light on the brain mechanisms underlying perceptual awareness, indeed a tantalizing endevour, that, however, has produced some specific contributions such as those of our own project briefly summarized below.
The general strategy we followed is to carry out neuropsychological testing in conjunction with several non-invasive brain imaging techniques such as Electroencephalography (Event Related Potentials), Structural and Functional Magnetic Resonance Imaging, Functional Near Infrared Spectroscopy. A first achievement is to have established that the intact cerebral hemisphere plays an important role in compensating for the visual impairment caused by damage to the other hemisphere. This encourages to devise rehabilitation techniques aimed at reinforcing the activity of the nervous fibers enabling communcation between the two hemispheres. Secondly, another finding is that visual stimuli, especially when moving, evoke strong neural activity when presented to the blind hemifield not only in the intact but also in some cortical areas in the occipital and parietal lobe of the damaged hemisphere. Unfortunately, this activity may not be sufficient to enable the patient to reach full conscious vision but it might do so with appropriate visual training. At any rate, this is an important finding showing that the cortical machinery for vision is still mostly intact despite blindness. A third achievement concerns the role of visual spatial attention that we found in some patients and in some visual tasks could still be at work by facilitating appropriate behavioural and neural responses despite lack of consciousness. Finally, we found that the residual unconscious vision present in some patients has a perceptual organization broadly similar to that of healthy individuals being sensitive to the configuration and numerosity of the visual stimuli presented to the bind field: In other words, blindsight is unlike a misted windshield. This makes us rather optimistic about a positive outcome of new neurally based rehabilitation techniques suggested by our experimental findings.