The effects of attention on visual perception depend on the level of perceptual load in the task (Lavie,
2005). Due to the capacity limits of visual perception, tasks involving higher perceptual load (e.g., search tasks involving many similar items or tasks requiring complex perceptual discriminations; e.g., Lavie,
1995; Lavie & Cox,
1997) result in reduced visual cortex responses to unattended stimuli (e.g., Rees, Frith, & Lavie,
1997; Schwartz et al.,
2005; Yi, Woodman, Widders, Marois, & Chun,
2004; see Lavie,
2005,
2010, for reviews) and lead to the experience of
inattentional blindness (Carmel, Thorne, Rees, & Lavie,
2011; Cartwright-Finch & Lavie,
2007; Macdonald & Lavie,
2008; Simons & Chabris,
1999). The effects of perceptual load on neural responses are established across different load manipulations and in a variety of tasks. These effects are found to extend throughout the visual cortical hierarchy, from occipital cortex including primary visual cortex area V1, the superior colliculus, and lateral geniculate nucleus (LGN) (e.g., Bahrami, Lavie, & Rees,
2007; O'Connor, Fukui, Pinsk, & Kastner,
2002; Rees, Frith, & Lavie,
1997; Schwartz et al.,
2005), through to cortical areas involved in the perception and recognition of complex images, meaningful objects, and scenes (e.g., Pinsk, Doniger, & Kastner,
2003; Yi et al.,
2004). Together, the previous behavioral and neuroimaging studies form a large body of research in support of the resolution offered by perceptual load theory for the enduring controversy on the extent to which perception depends on attention (e.g., Lavie,
1995; Lavie & Tsal,
1994). According to perceptual load theory, when the task involves a high enough level of perceptual load to exhaust all available capacity (e.g., a large number of search stimuli that are all highly similar to the target or complex discriminations of conjunctions of features) there is simply none left to process any task-irrelevant stimuli. Visual cortex responses to task-irrelevant unattended stimuli are therefore reduced, resulting in their reduced perception. On the other hand, when the task involves only a low level of perceptual load (e.g., a few search stimuli or pop-out feature detection) any spare capacity that the attended task does not use “spills over” to the processing of irrelevant stimuli so that perception of distractors and accompanying neural responses remain unaffected by inattention (see Lavie,
2005,
2010, for reviews).