The perceptual load theory was originated by Nilli Lavie in the mid-nineties in order to resolve the debate in attention research on the role of attention in information processing. The question of the debate was whether early selection, the prevention of perception, controls selective attention, or if all information is sensed and then late selectional processes control what information reaches one's consciousness. Accordingly, the debate is often called ‘the early and late selection’ debate.
Contents
- Key assumptions
- High versus Low Load
- Perceptual load research
- Visual Distractors
- Cross Modal Distractors
- Faces
- The Development of Early and Late Selection
- Criticisms
- References
Perceptual load theory stipulates that both early and late selection occur, depending on the processing demands of the task. It states that perception has limited capacity but operates in an automated, involuntary manner. If information doesn't exceed the brains processing capacity, it will be perceived. In tasks involving a large amounts of information, in other words high perceptual load, capacity is fully exhausted by the processing of the task-relevant stimuli. This results in no perception of task-irrelevant information (‘early selection’). In contrast, in tasks of low perceptual load, not all the processing capacity is taken up by the task. This results in the spare processing capacity 'spilling-over', resulting in the perception of task-irrelevant information (‘late selection’) and increased distractor interference.
The theory resolves the early and late selection attention debate by explaining that tasks of low perceptual load result in late selection, whereas tasks of high perceptual load result in early selection attention effects.
Key assumptions
Perceptual load theory is a hybrid model, combining a limited capacity approach with a parallel simultaneous processing approach where perception proceeds in parallel on all information within its limited capacity until capacity runs out. Voluntary control is limited in the theory to setting up priorities so that processing of stimuli that are relevant to the current task is prioritized over those that are irrelevant. However, what dictates whether a stimulus is processed or not is the level of load in the task. Irrelevant stimuli are still perceived in conditions of low perceptual load, despite their low priority. From 2000 onwards Load theory was expanded to explain the interaction between perceptual load and load on cognitive control processes that actively maintain task priorities.
‘High’ versus ‘Low’ Load
The distinction between ‘low’ and ‘high’ load displays is relative, rather than absolute. The level of perceptual load can be raised by increasing the number of task units (for example the number of words in a word search task or the number of letters in a letter search task, or increasing the tasks demands for the processing each of these items. For example, a low-load task may involve searching for a target that has a distinguishing feature (such as colour), whereas a high-load task may involve a conjunction search, where the target is defined by a combination of features (such as colour and shape) which makes it harder to detect it.
Under conditions of ‘low’ perceptual load, the theory predicts that any remaining capacity that has not been allocated to the processing of relevant stimuli will ‘spill over’ to task-irrelevant stimuli. This ‘spillover’ under low-load conditions is seen as automatic and inevitable, thus not under voluntary control. The allocation of attention and subsequent perceptual processing is prioritised so that stimuli designated as task-relevant are attended before task-irrelevant stimuli, continuing in this order until the capacity is exhausted. Therefore, the theory asserts that irrelevant items such as distractors will only be perceived, and cause interference, under conditions of low load, when perceptual capacity has not been used up in the processing of relevant items.
Conversely, ‘high’ perceptual load displays involve either a larger set of relevant items to search, or require more information to process each item. These increased processing demands prevent irrelevant, low-priority items from consuming scarce processing capacity, which results in less distractor interference, as there is no remaining processing capacity for them to be perceived. According to the theory, this results in the effective rejection of task-irrelevant distractors in high-load displays.
Perceptual load research
Perceptual load theory received support from many studies that varied perceptual load in the task and found that distractor processing and measures of perception and awareness all depend on the level of perceptual load in the task. Brain imaging studies also found much evidence that brain response to a variety of unattended stimuli (ranging from motion to emotion) is modulated by the level of perceptual load in the attended task Even the brain response related to novelty was found to be modulated by the level of perceptual load in the task.
Visual Distractors
A lot of research has gone into the effects of visual distractors on visually orientated tasks. It's been shown using both neuroimaging and behavioural studies that increased perceptual load on a visual task causes the decreased perception, and therefore interference, of visual distractors. In contrast, in visually orientated tasks of low perceptual load, there is increased perception and awareness of visual distractors, and therefore, more interference.
Cross-Modal Distractors
Whilst much is known about the effect of visual perceptual load on distractor interference, little has been discovered about the link between cross-modal distraction and perceptual load, e.g. auditory distractor interference on visual tasks. There exists a debate over whether there is a single shared sensory resource, or if there are separate distinct perceptual capacities for each of the senses. If there exists distinct capacities, it would imply that auditory distractors would operate independently of the perceptual load of visual tasks. There exists evidence for this side of the argument; it was shown that in a response competition paradigm for a letter search task, auditory, unlike visual, distractor interference was not reduced, but increased under high load. However, there also exists support for the shared sensory resource argument. In a more recent study, auditory evoked responses were measured under high and low visual load. It was shown that with increasing perceptual load on a visual task, noise distraction caused smaller P3 responses (which signify awareness to auditory stimuli). This signified decreased auditory distractor interference, consistent with the mechanism seen in unimodal visual distraction.
Faces
A strong body of support exists for the theory that face processing and recognition occurs in an encapsulated 'module', unique solely to faces. From this stemmed the proposition that faces would operate independently of the perceptual load of tasks that don't involve faces. There is evidence for this; it has been shown that face distractors operate independently of the perceptual load in a name categorisation task, however, in a face categorisation task, their perception is significantly reduced in high load. This is in line with the classical perceptual load theory. Further research is needed to prove the existence of a face-specific perceptual capacity.
The Development of Early and Late Selection
Late selectional abilities, and therefore distractor rejection under low perceptual load, are dependent on executive functions, which are in turn controlled by the frontal lobe. The frontal lobe is the last area of the brain to reach full maturity, and therefore, noticeable differences are seen in children compared to adults in distractor rejection tasks. Multiple studies have shown that, in comparison to adults, there is decreased distractor rejection in children at low loads, but no difference in distractor interference when carrying out tasks at high perceptual loads. This suggests that late selection uses slower developing neural mechanisms than early selection. However, what remains unknown is when these selectional abilities reach full maturity.
Criticisms
The bulk of the evidence that accumulated since the mid nineties provides strong support for the perceptual load theory. The few criticisms that were made recently typically target just one of the tasks termed the Eriksen Flanker task that was used to research load theory.