FUNCTIONAL AND NEURAL BASES OF SPATIAL-TEMPORAL DISORDERS OF VISUAL AWARENESS

Visual extinction is a spatial-temporal disorder of visual awareness. Right-hemisphere patients with visual extinction frequently miss the stimulus presented in their contralesional field on trials with bilateral presentation. They can also need the left stimulus to be presented with a temporal lead in order to be perceived as simultaneous to the right one. In the present work, three experiments were carried out in order to investigate functional and neural bases of spatial-temporal disorders of visual awareness. The first one focused on neural correlates of spatial-temporal disorders of visual awareness in patients with visual extinction at a chronic stage. An impairment of visual awareness is likely to depend upon unbalanced top-down modulation from dorsal fronto-parietal on occipital areas, typically intact in the ipsilesional hemisphere, biasing spatial-temporal processing of visual information towards the ipsilesional side. However, it is not clear yet whether the effect of a dysfunctional top-down modulation can only affect responses or, rather, induce neural changes in targeted visual areas. In this experiment, we addressed this issue by using position emission tomography (PET) at rest to measure possible differences between the cortical metabolism of one patient showing visual extinction at a chronic stage, and two patients who recovered from extinction, one at a chronic and another at a sub-acute stage. We found the structurally intact visual cortex of the extinction patient being hypometabolic in the right as compared to the left hemisphere, whereas no hemispheric asymmetry in the metabolism of visual cortex was found in the patients who recovered from extinction. Our data suggest that neural changes in structurally intact occipital areas might be crucial to explain the lack of visual awareness for contralesional stimuli in chronic extinction patients. In the second and third experiment we investigated the possible contribution of mechanisms of contingent attentional capture to spatial-temporal processing of visual information both in patients with attention disorders (experiment 2) and in healthy individuals (experiment 3). It was suggested that impairments of right parietal patients can be accounted for by a deficit in disengaging exogenous attention from ipsilesional stimuli and reorienting it towards contralesional events. It has been shown that the disengagement deficit depending on their behavioural relevance of ipsilesional stimuli (task demands and expectation of the patient). Also the results in the healthy subjects suggest that attending to a sensory modality can speed up the relative perception of stimulus presented in that rather than a different modality, reducing the time necessary for the stimulus to be perceived (“prior entry” hypothesis). The second experiment focused on functional basis of spatial-temporal disorders of visual awareness in patients with visual extinction, investigating whether task-set related factors can affect spatial-temporal processing of visual information in these patients. Specifically, we examined the performance of a visual extinction patient on a temporal order judgment (TOJ) task under conditions with different attentional set and compared his performance to that of one right-brain-damaged patient without visual extinction and four healthy subjects age matched. In the not focused condition the extinction patient shown spatial-temporal bias (PSS=-255), while he shown an important reduction of this bias under focused condition (PSS=-44). These findings indicate a clear modulation of patient’s performance with the attentional set, particularly for the PSS. On the contrary, no such a modulation was found both in the brain-damaged control patient (not focused condition: PSS=-4, focused condition: PSS=-2) and in healthy subjects (not focused condition: mean PSS=-16; focused condition: mean PSS=3). Furthermore the extinction patient showed higher accuracy of temporal discrimination for left-sided stimuli, but not for right-sides stimuli, in the focused (62,5%) as compared to the not focused (46,7%) condition. The aim of the third experiment was to investigate whether mechanisms of contingent attentional capture would also affect spatial-temporal processing of visual information in healthy subjects. Eighteen participants were tested on a similar TOJ paradigm as that used in experiment 2. In the not focused task-set condition subjects showed no bias in temporal order judgment (PSS=-3.6). On the contrary focusing subject’s attentional set onto one stimulus dimension yielded a clear-cut bias in temporal order judgment (PSS=-11). Present findings support evidence indicating that exogenous attention would affect sensitivity in a TOJ task and also extend this evidence, showing that in a TOJ task, as in other visual orienting tasks, capture of attention by external stimuli is contingent to the current attentional set.