Elsevier

Brain Research

Volume 1080, Issue 1, 29 March 2006, Pages 91-100
Brain Research

Research Report
The role of perceptual load in visual awareness

https://doi.org/10.1016/j.brainres.2005.10.023Get rights and content

Abstract

Does awareness depend on attention? This is a fundamental issue for understanding the relationship of attention and awareness, yet previous research provided mixed results. Here, I describe new research that shows that the effects of attention on awareness depend on the level of perceptual load in the attended task. Awareness reports in both the inattentional blindness and change blindness paradigms were found to depend on the extent to which an attended primary task loads attention. Neuroimaging results revealed the involvement of frontoparietal attention network in awareness and transcranial magnetic stimulation experiments confirmed a causal role for frontoparietal activity in awareness. These results clarify the role of attention and associated frontoparietal activity in visual awareness within the framework of load theory of attention.

Introduction

Would focusing attention on a current task prevent intrusions of task-irrelevant stimuli into awareness? This fundamental issue has intrigued psychologists for many years and has led to an enduring controversy between early selection views suggesting that attention can prevent irrelevant distractors from reaching awareness (Broadbent, 1958, Neisser and Becklen, 1975) and late selection views suggesting that attention affects later processes such as response selection and memory but does not affect perceptual awareness (Deutsch and Deutsch, 1963, Tipper, 1985).

A possible resolution to this debate has been offered within a hybrid perceptual load model (Lavie, 1995, Lavie et al., 2004). According to this model, focusing attention on a task at hand can prevent perception of task-irrelevant stimuli (early selection) when the processing of task-relevant stimuli involves a high level of perceptual load that consumes all available capacity. By contrast, when processing of the task-relevant stimuli places lower demands on the perceptual system, any spare capacity from the task-relevant processing spills over involuntarily, resulting in the perception of irrelevant stimuli (late selection).

Although perceptual load model has the clear implication that intrusions of irrelevant distractors into awareness should not occur in situations of high perceptual load in the relevant task, previous tests of this theory (see Lavie, 2005 for review) have mainly used either indirect measures of distractor effects on RTs or assessed neural activity in sensory cortices related to the distractor perception and awareness but with one exception (Rees et al., 1997) have typically not included measures of the extent to which distractors have in fact entered visual awareness.

In the present article I describe new research on the effects of perceptual load on visual awareness in studies that assess awareness within the “Inattentional Blindness” (Mack and Rock, 1998) and “Change Blindness” (Rensink et al., 1997) paradigms. I then describe research on the implications of these behavioral findings for the neural mechanisms of awareness. I begin with a short review of the existing behavioral and neuroimaging evidence for perceptual load theory.

Increased perceptual load means that either the number of items that need to be perceived is increased, or that for the same number of items, perceptual identification is more demanding on attention with high load. The role of perceptual load in determining distractor processing was first established in behavioral experiments (Lavie, 1995, Lavie and Cox, 1997) using the response competition paradigm (Eriksen and Eriksen, 1974). The response competition paradigm was chosen for the first behavioral load experiments because it has been widely accepted as a conventional measure of distractor perception within modern research of early and late selection debate (Miller, 1991, Yantis and Johnston, 1990, for review, see Lavie and Tsal, 1994). However, as I discuss below, this paradigm does not allow direct conclusions about conscious awareness. In a typical response-competition task subjects make speeded responses indicating whether a central target letter is one of two pre-specified letters (e.g. ‘X’ or ‘N’) while attempting to ignore a peripheral distractor letter. Slower responses in the presence of a distractor with an incongruent identity (distractor ‘X’ for target ‘N’) compared with a congruent distractor (distractor ‘X’ for target ‘X’) or a neutral distractor (e.g. a distractor ‘L’ that was not associated with any of the task responses) indicate that the distractor identity was perceived at least to the extent of recognizing the association of distractor identity and (incongruent or congruent) response.

Perceptual load experiments showed that response competition effects from distractors with incongruent (vs. congruent) identity were found on target RTs when the target task involved low perceptual load (e.g., just one target letter was present, any additional task requiring only simple presence/absence detection) but were eliminated when the target task involved high perceptual load (search for the target among many similar letters (Lavie and Cox, 1997), or having to discriminate precise location and size for an adjacent shape, (Lavie, 1995)). These results clearly show that high perceptual load reduces processing of the distractor identity. However, since they are based on measures of the distractor congruency effects on target RTs, they do not specifically address the effects of perceptual load on subjective conscious perception. It remains entirely possible, for example, that subjects were not aware of the distractors' identity not only under conditions of high perceptual load, where target RTs were unaffected by the distractor identity but also under conditions of low perceptual load, where target RTs varied as a function of the distractor identity, because the identity effects on RTs in the conditions of low perceptual load might just reflect implicit unconscious recognition of the response association for the distractor instead of conscious representation of its identity. Conversely, it is also logically possible that the distractor reached awareness under both conditions of low and high perceptual load. The elimination of distractor congruency effects on target RTs with high load on this alternative account could be due to effects of load on processes other than conscious awareness, such as response selection. Although there is evidence against two specific suggestions (a) that reduced distractor congruency effects with high load are merely due to the associated slowing of responses with load (Lavie and DeFockert, 2003) and (b) that perceptual load increases active suppression of the distractor responses under high load (Lavie and Fox, 2000), the RT load studies leave open the general possibility that perceptual load effects are on responses rather than on conscious awareness. The new research I describe on the effects of perceptual load on direct measures of conscious awareness (in the sections on the role of perceptual load in inattentional blindness and change blindness) can provide more conclusive evidence on this issue.

Converging results from neuroimaging tests of perceptual load theory (Mesulam, 1981, Miller, 1991, Milner and Goodale, 1995, Mitchell et al., 2004, Most et al., 2001, Nobre et al., 2003, Neisser and Becklen, 1975, O'Connor et al., 2002, Ooi and He, 1999, Pessoa et al., 2002, Pinsk et al., 2003, Pisella et al., 2004, Rees et al., 1997, Rees et al., 1999, Schwartz et al., 2005, Yi et al., 2004) clearly show that load effects on distractor processing cannot be merely due to effects on RTs. These studies demonstrated that high load eliminates visual cortex activity related to task-irrelevant distractors. For example, Rees et al. (1997) found that neural activity related to motion (vs. stationary) distractors in visual cortex (e.g., MT) was found in conditions of low load in a relevant task on words at fixation (detection of the letter case) but was eliminated by high load in the relevant task (involving more complex word discrimination). Other studies found that visual cortex activity related to a task-irrelevant checkerboard dependant on the level of load in a relevant task, decreasing as load was increased (O'Connor et al., 2002, Tong et al., 1998). Rees et al. (1999) showed that fixated words did not elicit greater activity than letter strings when ignored during performance of a high load task of monitoring a rapid superimposed picture stream for repetitions. Yi et al. (2004) similarly showed that when subjects attempt to ignore pictures of places presented in the background, while monitoring for face repetitions at fixation, parahippocampal activity related to the place backgrounds is substantially reduced by increasing the load in the face identification task.

These results convincingly show that neural activity in areas of visual cortex that are related to perception of the task-irrelevant stimuli is determined by the level of load in task-relevant processing. However, apart from a study by Rees et al. (1997), this research typically has not assessed the effects of distracters on visual awareness. Rees et al. (1997) did accompany their neuroimaging experiment with assessment of the subjective duration of motion after-effects from the motion distractors presented either during low load or high load tasks. They found that the subjective duration of the motion after effect was significantly reduced by high load for each subject (all t's > 1.9; all P values ≤ 0.05, Fig. 1). This result is encouraging for the suggestion that perceptual load determines awareness at least in the case of visual motion.

To establish the role of perceptual load in visual awareness, it is important to examine whether perceptual load effects on awareness can generalize across various measures of visual awareness. Below I describe more recent experiments that test the effects of perceptual load on explicit reports about subjective awareness in the inattentional blindness and in the change blindness paradigms.

Section snippets

The role of perceptual load in “inattentional blindness”

In order to test whether conscious awareness of task-irrelevant stimuli depends on the level of perceptual load in task-relevant processing we ran a series of experiments that modified the typical “inattentional blindness” paradigm (Mack and Rock, 1998) to include manipulations of perceptual load in the relevant task (Cartwright-Finch and Lavie, in press, Cartwright-Finch and Lavie, 2005).

Like in a typical inattentional blindness experiment, participants performed a task for a few (typically

The role of perceptual load in “change blindness”

In the change blindness paradigm subjects are instructed in advance that their task is to detect whether a change occurred between two successive images and report about it immediately following the images. Thus, unlike inattentional blindness the event for which awareness is reported is expected and awareness reports are given immediately following the images. Indeed, under normal circumstances awareness reports will show very little failure to detect any change between the images. However,

Neuroimaging of change blindness

The finding that awareness is determined by the level of perceptual load in a primary task has implications for the neural correlates of awareness. These should include neural structures associated with effects of attentional load in parietal and frontal cortex (Wojciulik and Kanwisher, 1999, Schwartz et al., 2005). To examine the neural correlates of awareness within the change blindness task, we have conducted an event-related fMRI study comparing activity during consciously detected changes

Summary and conclusions

The extent to which a current task engages full attention under conditions of high perceptual load has been shown to determine neural processing of task-irrelevant stimuli as well as their interference effects on behavior. The present research shows that the level of perceptual load in a current task also determines the extent to which other task-irrelevant stimuli reach visual awareness. High load in a current task was found to increase the level of both “inattentional blindness” and “change

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