Into the depths of spatial attention and multisensory integration

During our daily lives our senses are flooded with information. We can see, hear, feel, smell, and taste all at the same time. Luckily, our brain is helping us to make sense of this abundant information by combining information from different senses. The simultaneous presentation of information to different senses often results in behavioral benefits like faster detection and localization as compared to when only a single sense is stimulated. Two processes through which such benefits can occur are crossmodal exogenous spatial attention and multisensory integration (MSI). These two processes are essential for spatial orienting and are central to the studies that are described in the current thesis. In the first part, studies investigating when and how crossmodal exogenous spatial attention and MSI contribute to multisensory improvements in spatial perception are reported. Among others, it was shown that MSI is the main cause of multisensory benefits when sound and light are presented in close spatial and temporal proximity. However, both MSI and crossmodal exogenous spatial attention contribute when the time interval between sound and light increases. At longer intervals MSI does not longer contribute and spatial attention takes over. It was also shown that MSI is reduced when a multisensory stimulus is attended compared to when it is unattended. Several findings from neurophysiological and neuropsychological studies of attention and multisensory perception have revealed that the brain processes information coming from near and far space differently. In the studies that are described in part 2 it was investigated how the behavioral benefits of crossmodal exogenous spatial attention and MSI change as a function of the location of sound and light in three-dimensional space. These studies revealed that crossmodal exogenous spatial attention is distance-specific. Sounds that are presented in far space improve visual perception in far space, but not in near space, and the other way around. Further support for the idea that the attention system takes the distance of information into account comes from a study of spatial attention in stroke patients. It was observed that impairments in visuospatial attention could be distance-specific. Patients could have attention impairments in near space, without visuospatial attention impairments in far space. Patients could also have attention deficits in far space only, or in both near and far space. In a different study, we observed that the benefits of multisensory integration for the detection and localization of sound and light is enhanced in far relative to near space. Not only do these studies add to the understanding of human multisensory perception, they also provide a foundation for the application of these findings to more practical, real-life situations. For example, in situations where responses should be as fast and as accurate as possible (e.g., while driving), it can be very helpful to present a multisensory warning signal to warn for an approaching threat. After all, it was observed that crossmodal exogenous spatial attention and multisensory integration can speed up responses with 10% without decreasing accuracy.