Spatial Constraints in Multisensory Attention

Our sensory organs continuously receive a large amount of input from the external world; some of these are important for a successful interaction with the environment, whereas others can be ignored. The operation of selecting relevant signals and filtering out irrelevant information is a key task of the attentional system (Desimone and Duncan 1995; Kastner and Ungerleider 2001). Attentional selection can occur on the basis of many different criteria, with a main distinction between endogenous control (i.e., selection based on voluntary attention, current aims, and knowledge) and stimulus-driven control (i.e., selection based on the intrinsic features of the sensory input). Accordingly, we can decide to pay attention to the face of one person in a crowded room (i.e., attending to subtle details in a rich and complex environment), or attention can be captured by a loud sound in a quiet room (i.e., attention captured by a salient stimulus).

Many different constraints can guide endogenous and stimulus-driven attention. We can voluntarily decide to attend to a specific visual feature, such as color or motion, but the very same features can guide stimulus-driven attention if they stand out from the surrounding environment (“pop-out” item, e.g., a single red stimulus presented among many green stimuli). Here, I will focus on processes related to attentional selection based on spatial location. The investigation of mechanisms of spatial attention control is appealing for many reasons. Spatial selectivity is one of the most important characteristics of single neurons (i.e., the neuron’s receptive field) and well-organized maps of space can be found throughout the brain (Gross and Graziano 1995). These include sensory areas (e.g., striate and extrastriate occipital regions, for retinotopic representations of the visual world; Tootell et al. 1982), subcortical regions [e.g., the superior colliculus (SC); Wallace et al. 1997], and higher-level associative areas in frontal and parietal cortex (e.g., Ben Hamed et al. 2001; Sommer and Wurtz 2000). This widespread selectivity for spatial locations opens the question about how/ whether these anatomically segregated representations contribute to the formation of an integrated representation of external space. Indeed, from a subjective point of view, signals about different visual features (e.g., shape/color) as well as motor commands seem to all merge effortlessly, giving rise to a coherent and unified perception–action system that allows us to interact spatially with the external environment.