Attentional processes constantly filter sensory inputs, and only

a subset of our environment receives fully elaborated perceptual processing. For example, each time that we make an eye movement, the eyes bring another part of our environment into the center of gaze for detailed processing. In addition to these overt shifts of attention, humans can deploy spatial attention without moving the eyes or the head, known as covert shifts of attention (von Helmholtz, 1867). One longstanding metaphor for covert spatial attention is the ‘attentional spotlight’, the notion that attention can only be allocated to one region of space at a time (e.g. Posner, 1980). These models postulate that the attentional spotlight cannot be divided, but that the size of the spotlight can be adapted to task requirements [i.e. the ‘zoom-lens’ model (Eriksen &

St James, 1986)]. In the attended BMS-907351 solubility dmso region of visual space, reaction times are lower and/or detection accuracy is higher than in unattended regions. This notion of a unitary, indivisible spotlight was supported by earlier visual evoked potential (VEP) studies (e.g. Heinze et al., 1994). However, a growing number of studies have challenged the idea of a single, non-divisible attentional spotlight. Behavioral experiments provide evidence that humans can divide attention among multiple non-contiguous spatial locations (e.g. PLX4032 ic50 Castiello

& Umilta, 1992; Awh & Pashler, 2000; Gobell et al., 2004), reporting that reaction time and accuracy are modulated in divided attention designs in the same way as in undivided cued attention paradigms. Another line of evidence for a division of spatial attention has been put forward in steady-state VEP (SSVEP) and functional magnetic resonance imaging studies (e.g. Muller et al., 2003a; McMains & Somers, 2004, 2005). These studies reveal brain activation patterns that clearly fit with a divided spotlight account. In recent years, studies providing evidence for a divided spotlight of attention were called into question, much on the basis that their results can be explained by a unitary attentional spotlight that simply switches very rapidly between to-be-attended locations (e.g. Jans et al., 2010; VanRullen & Dubois, 2011). Correlates of such a periodic sampling of attention have been observed in electrophysiological experiments in non-human primates (Buschman & Miller, 2009) and in psychophysical experiments in humans (VanRullen et al., 2007). The dynamics of how attentional resources are redirected in the visual field are strongly debated, with estimates of latencies for attentional shifts of between approximately 70 ms (Nakayama & Mackeben, 1989) and 300 ms (Duncan et al., 1994).