Perceptual load theory

The perceptual load theory was originated by Nilli Lavie in the mid-nineties[1][2] in order to resolve the debate in attention research on the role of attention in information processing.[3][4][5] The question of the debate was whether attention affects information processing at early stages of perception (the ‘early selection’ view) or only at later stages such as memory or response selection (‘the late selection’ view). Accordingly the debate is often called ‘the early and late selection’ debate. Perceptual load theory stipulates that perception has limited capacity but operates in automated, involuntary manner on all the information within its capacity. In other words all the information that can be perceived (within the brain’s limited capacity) 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 attended information, resulting in no perception of unattended information (‘early selection’). In contrast, in tasks of low perceptual load, since perception cannot be voluntarily stopped, spare capacity from processing the information in the attended task will inevitably spill over, resulting in the perception of task-irrelevant information that people intended to ignore (‘late selection’).

The theory resolves the early and late selection attention debate by explaining that tasks of low perceptual load result in late selection effects of attention, 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.[3][4][5][6][7] 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.[3][4][5][6][7][8] 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.[7][8][9]

‘High’ versus ‘Low’ Load

The distinction between ‘low’ and ‘high’ load displays is relative, rather than absolute.[1][3][4] 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.[3]

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.[1][3][4][5][6][7] This ‘spillover’ under low-load conditions is seen as automatic and inevitable, thus not under voluntary control.[1] 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.[1][3][4][5] 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.[1][3] According to the theory, this results in the effective rejection of task-irrelevant distractors in high-load displays.[2]

Perceptual load research

Perceptual load theory received support from many studies that varied perceptual load in the task and found that distractor processing[3][5][9][10][11] and measures of perception and awareness[11][12][13] 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[14][15][16][17][18][19][20] Even the brain response related to novelty was found to be modulated by the level of perceptual load in the task.[21]

Criticisms

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There have been several criticisms of perceptual load as a working theory, Tsal and Benoni noted that a large number of studies in favour of perceptual load had measured the level of perceptual load only from display size. They claimed that the large number of stimuli diluted the distractor under high load.[22] In their experiments, they separated the effects of load and dilution using dilution displays, where the neutral stimuli were large in number but clearly distinguished from the target resulting in a low perceptual processing load. It was found that when dilution was controlled for, the high load condition showed the greater distractor interference, directly in contrast to Lavie’s theory. Lavie wrote a response, arguing that the irrelevant distractor competes with the search non-targets for remaining capacity.[23] Thus, distractor processing is reduced under conditions in which the search non-targets receive the spill over of capacity instead of the irrelevant distractor. Tsal and Benoni then responded,claiming that Lavie’s experiment did not take into account for the majority of their experiments and did not actually show the dilution theory to be incorrect.[24]

Perceptual load theory has been coined as a hybrid theory between early and late selection theories. However, it has been shown that this is not the case.[25] According to perceptual load theory under high load conditions early selection occurs, since processing a large amount of information exhausts all of the available mental resources. However, in low load displays the relevant stimuli do not require all existing attentional resources and spare resources are inadvertently assigned to irrelevant stimuli, enabling them to be processed. This means that attention selects relevant information early in processing, as it automatically identifies the target, a hallmark of early selection theories. Hence, the theory does not provide a hybrid of both early and late selection but an early selection theory with flexible adaptations regarding the efficiency of selection.

Other criticisms of perceptual load have also been made, with researchers claiming that the distractor effect came from other sources. Eltiti et al., 2005 argued that this the distractor effect was due to target distractor Salience (neuroscience) (how much they stand out from the other items in the display).[26] Target-distractor salience was manipulated by the varying the onset and offset of both targets and distractors. Interference was not observed in high load conditions when there was distractor offset, as the offset served as a less salient event. But interference effects were obtained in high load conditions when both the target and distractor appeared as onsets. It was found that perceptual load did not have a significant interaction with distractor presentation, compatibility or target presentation, contrary to the position of perceptual load theory. It has also been found that distractor interference on a high perceptual load trial was more likely to occur when it was preceded by a low perceptual load trial rather than by a high perceptual load trial.[27][28]

It has also been found that changing attentional focus will produce results that perceptual load does not predict. In an experiment where a target stimulus was cued with a 100% valid precue (participants always had the target location indicated to them), the perceptual load effect was eliminated.[29] Research by Chen and Cave indicated that the effect of dilution was influenced by the extent of attentional focus. By using cueing they were able to limit the range of attentional focus to a small number of stimuli in a high load display.[30] Only those items inside the zoom of attention affected the processing of the distractor, contrary to perceptual load theory.

It has also been found that working memory load is influential on distractor processing. In Lavie, Hirst, de Fockert, & Viding, it was found interference effects from irrelevant distractors were larger when working memory load was high.[31] Other studies have also shown that when attention is targeted at a local level (small area of focus), distractors at the global level (larger area of focus) produced more interference under high than under low working memory load.[32] The opposite effect of improving selective attention during attention to the global level also occurred showing that the effect of working memory load on attention is dependent on the level of the to-be-attended information. High working memory has also been shown to either increase or decrease distractibility, contingent on whether the contents of working memory overlap with the processing of the target or the distractor respectively.[33][34][35]

In 2008 Wenger analysed the perceptual load theory by testing the assumption regarding changes in processing capacity, the assumptions regarding sensory mechanisms using signal detection theory and applying these analyses at the level of the individual observer to see how individual differences vary processing.[36] They found that there are considerable individual differences in perceptual load capacity along with significant violations of some of the foundational assumptions of perceptual load theory, such as the assumption that capacity limitations are independent of sensory limitations.

Research has shown that distractors under low load do not always capture attention.[37] In an Eriksen flanker task, low load displays were employed. Subjects were then pre-cued, regarding the positions of the target. They found that a large difference in response times (flanker effect) for near flankers for the pre-cue condition but this effect did not occur when the flankers appeared at a large distance from each other. In the non-cued condition, again there was a flanker effect observed for near targets but not for far, regardless of whether the target was a letter or a number, leading to the conclusion that distractors do not always lead to the automatic capture of attention in low load displays, again contrary of perceptual load theory.

In other cases it has been found that the varying the amount of perceptual load in a task has no effect on observed effects. In Chen (2003) participants were asked to perform the Stroop task (This is where a word description of a colour is presented in another colour, i.e. the word green written in red ink.[38] The Stroop effect is when the subjects take longer to respond to words written this way as opposed to words written in the colour they represent i.e. the word green written in green ink). This stroop task was broken into two conditions, where participants either saw a single feature (low load) or a combination of features (high load). The results showed that the regardless of the perceptual load of the display, the stroop effect remained constant when both the distracting and the target stimuli were a part of the same object. Under the perceptual load hypothesis there should have been greater Stroop interference in the high load than the low load condition which did not occur.

The major problem with perceptual load theory is that perceptual load itself is never truly defined. While increases in perceptual load reduces irrelevant interference, increasing cognitive and sensory load produce more interference.[39][40] This requires perceptual load tasks to be clearly defined as to not combine with cognitive load tasks to rule out this confounding variable and bias. Researchers in the field believe that is may be an improbable occurrence as the distinction between the various types of ‘loads’ as not concrete. Lavie et al. (2004) defined cognitive load as a form of control which "depends on/ higher cognitive functions, such as working memory, that are required for actively maintaining current processing priorities to ensure that low-priority stimuli do not gain control of behaviour".[39] Since the majority of perceptual load tasks employ visual search paradigms it is not possible to tease these loads apart because search tasks contain a cognitive component.

See also

References

  1. 1 2 3 4 5 6 Lavie N. (1994) Perceptual load and physical distinctiveness as determinants of the locus of attentional selection. PhD Thesis, (Tel Aviv).
  2. 1 2 Lavie, Nilli (2011) Q&A. Current Biology, Volume 21, Issue 17, R645 - R647
  3. 1 2 3 4 5 6 7 8 9 Lavie, N. (1995). Perceptual load as a necessary condition for selective attention. Journal of Experimental Psychology: Human Perception and Performance, 21, 451-468.
  4. 1 2 3 4 5 6 Lavie, N. & Tsal, Y. (1994). Perceptual load as a major determinant of the locus of selection in visual attention. Perception & Psychophysics, 56, 183-197.
  5. 1 2 3 4 5 6 Lavie, N. & Cox, S. (1997). On the efficiency of attentional selection: Efficient visual search results in inefficient rejection of distraction. Psychological Science, 8, 395-398.
  6. 1 2 3 Lavie, N. (2001). The role of capacity limits in selective attention: Behavioural evidence and implications for neural activity. In J. Braun & C. Koch (Eds.). Visual Attention and Cortical Circuits. pp. 49-68. Cambridge, Massachusetts: MIT press.
  7. 1 2 3 4 Lavie, N. (2000). Selective attention and cognitive control: dissociating attentional functions through different types of load. In S. Monsell & J. Driver (Eds.). Attention and performance XVIII, pp. 175-194. Cambridge, Massachusetts: MIT press.
  8. 1 2 Lavie, N., Hirst, A., De Fockert, J. W. & Viding, E. (2004) Load theory of selective attention and cognitive control. Journal of Experimental Psychology: General, 133, 339-354.
  9. 1 2 Lavie, N. (2005) Distracted and confused?: selective attention under load. Trends in Cognitive Sciences, 9, 75-82.
  10. ↑ Lavie, N. (2010) Attention, Distraction and Cognitive Control under Load. Current Directions in Psychological Science, 19(3), 143-158
  11. 1 2 Forster, S. & Lavie, N. (2008). Failures to Ignore Entirely Irrelevant Distractors: The Role of Load. Journal of Experimental Psychology: Applied, 14, 73-83.
  12. ↑ Lavie, N. Lin, Z. Zokai, N. & Thoma, V (2009). The role of perceptual load in object recognition. Journal of Experimental Psychology: Human Perception and performance 21(1), 42-57.
  13. ↑ Macdonald J. & Lavie, N. (2008). Load induced blindness. Journal of Experimental Psychology: Human Perception and performance. 34(5), 1078-1091.
  14. ↑ Cartwright-Finch, U. and Lavie, N. (2007). The role of perceptual load in Inattentional Blindness. Cognition. 102(3), 321-340
  15. ↑ Lavie, N., Beck, D. M. & Konstantinou, N. (2014). Blinded by the load: attention, awareness and the role of perceptual load. Philosophical Transactions of the Royal Society B: Biological Sciences, 369 (1641).
  16. ↑ Rees, G., Frith, C., & Lavie, N. (1997). Modulating irrelevant motion perception by varying attentional load in an unrelated task. Science, 278, 1616-1619.
  17. ↑ Schwartz, S., et al. (2005). Attentional load and sensory competition in human vision: modulation of fMRI responses by load at fixation during task-irrelevant stimulation in the peripheral visual field. Cerebral Cortex, 15, 770–786.
  18. ↑ O’Connor, D. H., Fukui, M. M., Pinsk, M. A., & Kastner, S. (2002). Attention modulates responses in the human lateral geniculate nucleus. Nature Neuroscience, 5, 1203–1209.
  19. ↑ Pinsk, M. A., Doniger, G. M., & Kastner, S. (2004). Push-pull mechanism of selective attention in human extrastriate cortex. Journal of Neurophysiology, 92, 622-629.
  20. ↑ Bishop, S. J., Jenkins, R., & Lawrence, A. (2007) The neural processing of task-irrelevant fearful faces: Effects of perceptual load and individual differences in trait and state anxiety. Cerebral Cortex, 17, 1595–1603.
  21. ↑ Yi, D-J., Woodman, G. F., Widders, D., Marois, R., & Chun, M. M. (2004). Neural fate of ignored stimuli: Dissociable effects of perceptual and working memory load. Nature Neuroscience, 7, 992–996.
  22. ↑ Tsal, Y., & Benoni, H. (2010). Diluting the burden of load: perceptual load effects are simply dilution effects. Journal of Experimental Psychology: Human Perception and Performance, 36(6), 1645
  23. ↑ Lavie, N., & Torralbo, A. (2010). Dilution: A theoretical burden or just load? A reply to Tsal and Benoni (2010)
  24. ↑ Tsal, Y., & Benoni, H. (2010). Much dilution little load in Lavie and Torralbo's (2010) response: A reply
  25. ↑ Benoni, H., & Tsal, Y. (2013) Conceptual and methodological concerns in the theory of perceptual load. Frontiers in psychology, 4
  26. ↑ Eltiti, S., Wallace, D., & Fox, E. (2005). Selective target processing: Perceptual load or distractor salience?. Perception & Psychophysics, 67(5), 876-885.
  27. ↑ Theeuwes, J. (2004). Top-down search strategies cannot override attentional capture. Psychonomic bulletin & review, 11(1), 65-70.
  28. ↑ Biggs, A. T., & Gibson, B. S. (2010). Competition between color salience and perceptual load during visual selection can be biased by top-down set. Attention, Perception, & Psychophysics, 72(1), 53-64.
  29. ↑ Linster, C., Johnson, B. A., Morse, A., Yue, E., & Leon, M. (2002). Spontaneous versus reinforced olfactory discriminations. The Journal of neuroscience, 22(16), 6842-6845
  30. ↑ Chen, Z., & Cave, K. R. (2013). Perceptual load vs. dilution: the roles of attentional focus, stimulus category, and target predictability. Frontiers in psychology, 4
  31. ↑ Lavie, N., Hirst, A., de Fockert, J. W., & Viding, E. (2004). Load theory of selective attention and cognitive control. Journal of Experimental Psychology: General, 133(3), 339
  32. ↑ Ahmed, L., & De Fockert, J. W. (2012). Focusing on attention: the effects of working memory capacity and load on selective attention. PloS one, 7(8)
  33. ↑ De Fockert, J. W. (2013). Beyond perceptual load and dilution: a review of the role of working memory in selective attention. Frontiers in psychology, 4
  34. ↑ Park, S., Kim, M. S., & Chun, M. M. (2007). Concurrent working memory load can facilitate selective attention: evidence for specialized load. Journal of Experimental Psychology: Human Perception and Performance, 33(5), 1062
  35. ↑ de Liaño, B. G. G., Umiltà, C., Stablum, F., Tebaldi, F., & Cantagallo, A. (2010). Attentional distractor interference may be diminished by concurrent working memory load in normal participants and traumatic brain injury patients. Brain and cognition, 74(3), 298-305
  36. ↑ Wenger, M., & Fitousi, D. (2008). Testing Lavie's (1995) perceptual load theory. Journal of Vision, 8(6), 988-988
  37. ↑ Paquet, L., & Craig, G. L. (1997). Evidence for selective target processing with a low perceptual load flankers task. Memory & Cognition, 25(2), 182-189.
  38. ↑ Chen, Z. (2003). Attentional focus, processing load, and Stroop interference. Perception & Psychophysics, 65(6), 888-900.
  39. 1 2 Lavie, N., Hirst, A., de Fockert, J. W., & Viding, E. (2004) Load theory of selective attention and cognitive control. Journal of Experimental Psychology: General, 133(3), 339
  40. ↑ Lavie, N., & De Fockert, J. W. (2003) Contrasting effects of sensory limits and capacity limits in visual selective attention. Perception & Psychophysics, 65(2), 202-212
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