Unimodal and multimodal mechanisms of spatial attention in the human brain

Using positron emission tomography and functional magnetic resonance imaging, I investigated the neural substrate of spatial attention in humans. The aim of the project was to identify brain regions involved in spatial attention independently of the modality of the sensory stimulation (vision and touch) and those involved only when a specific modality was stimulated or attended. The thesis addressed two separate but related issues: attentional modulation of spatial sensory representations, and control structures involved in covert orienting. Spatial attentional modulations were highlighted by directly comparing attention to the left hemifield and attention to the right hemifield, during bilateral stimulation. The results showed that, at relatively early stages of stimulus processing, spatial modulations are mainly modality specific, while higher associative areas are modulated independently of the modality stimulated or attended. However, the same experiments also showed that multimodal effects can affect activity in unimodal areas. Specifically, touch was found to modulate activity in occipital visual areas, in a spatially specific manner. The second part of the project sought to identify structures involved in the control of spatial covert orienting. Orienting attention to peripheral locations resulted in consistent activation of the temporo-parietal junction, independently of which sensory modality was employed. Overall, the results suggest that spatial attention has multiple correlates in the human brain. Controls structures operate supramodally and direct attention toward specific spatial locations. In turn activity in modality specific regions that represents the attended location is increased allowing enhanced processing of stimuli presented at that location. However, because the control structures operate supramodally, regions representing the same portion of space, but in another modality, are also modulated. This integrated system of unimodal and multimodal areas may provide the substrate for a coherent representation of extra-personal space