Having distracters in locations where they are not very distracting or in locations that are not defined a priori probably affects the demand of the attentional system to suppress them. Attentional resources in humans are limited in terms of the number of objects or locations that can be processed simultaneously (e.g. Trick & Pylyshyn, 1993); for a review, see Cavanagh & Alvarez (2005). In the current study, there might be a neurophysiological

correlate of this limitation. We find that the peak alpha amplitude in the divided attention condition is about half the amplitude in the undivided condition. PLX4032 in vitro The divided spotlight of attention account predicts that the number of to-be-ignored locations increases from one in the undivided condition click here to two in the divided attention condition. Our data therefore indicate that there is a relationship between an increase in the number of suppressed locations and reduction in the amplitude of the measure of attentional suppression. Such a relationship would logically result in a limit on the number of locations/objects that can be suppressed, because, at some point, the amplitude of suppressive alpha oscillations might become too small to be effective. Because, in many circumstances, the enhancing

and suppressive effects of attention are closely related (Pinsk et al., 2004; Frey et al., 2010), this decrease in suppressive alpha amplitude might directly affect the number of objects that can be processed simultaneously. Given this reasoning, it seems reasonable that the brain is able to employ a divided spotlight of attention for only a limited number of stimuli/objects. Whenever the threshold for is crossed, the attentional system might settle into a blinking mode (VanRullen et al., 2007) or settle into a serial search. We therefore hypothesise that a divided spotlight of attention can only be achieved with a limited number of stimuli and distracters, which forces the attentional system to suppress them on the basis of their location and nature. It may even be that attentional suppression is a necessary prerequisite

for having a divided spotlight of attention. This idea is somewhat at odds with the hypothesis of Cave et al. (2010), who proposed a model with four different modes of attention, with selection of non-contiguous regions of space and inhibition of distracter locations as separate modes. Examining the limits of divided attention and its relationship with suppression is therefore an interesting avenue for future research. In their review on attention to multiple stimulus locations, Jans et al. (2010) introduce several lines of evidence for their argument that divided attention is unlikely to be a standard feature of the attentional system. For example, they point out that the saliency map (Koch & Ullman, 1985), an influential model for visual attention, encodes relevance in a single spatial location. However, Jans et al.