Directing attention to locations and to sensory modalities: multiple levels of selective processing revealed with PET.

We used positron emission tomography (PET) to investigate the neural correlates of selective attention in humans. We examined the effects of attending to one side of space versus another (spatial selection) and to one sensory modality versus another (intermodal selection) during bilateral, bimodal stimulation of vision and touch. Attention toward one side resulted in greater activity in several contralateral areas. In somatosensory cortex, these spatial attentional modulations were found only when touch was relevant. In the intraparietal sulcus, spatial attentional effects were multimodal, independent of the modality attended. In occipital areas, spatial modulations were also found during both visual and tactile attention, indicating that tactile attention can affect activity in visual cortex; but occipital areas also showed more activity overall during visual attention. This suggests that while spatial attention can exert multimodal influences on visual areas, these still maintain their specificity for the visual modality. Additionally, irrespective of the attended side, attending to vision activated posterior parietal and superior premotor cortices, while attending to touch activated the parietal operculi. We conclude that attentional selection operates at multiple levels, with attention to locations and attention to modalities showing distinct effects. These jointly contribute to boost processing of stimuli at the attended location in the relevant modality.