Perceptual Load Affects Change Blindness in Real-World Interactions (PDF)

Received: 17 October 2017 Revised: 13 June 2018 Accepted: 25 June 2018 DOI: 10.1002/acp.3441 SHORT PAPER ‐ 5000 WORDS OR LESS Perceptual load affects change blindness in a real‐world interaction Gillian Murphy | Lisa Murphy School of Applied Psychology, University College Cork, Cork, Ireland...

Received: 17 October 2017 Revised: 13 June 2018 Accepted: 25 June 2018 DOI: 10.1002/acp.3441 SHORT PAPER ‐ 5000 WORDS OR LESS Perceptual load affects change blindness in a real‐world interaction Gillian Murphy | Lisa Murphy School of Applied Psychology, University College Cork, Cork, Ireland Correspondence Gillian Murphy, School of Applied Psychology, University College Cork, Cork, Ireland. Email: [email protected] Summary Change blindness is the striking inability to detect seemingly obvious changes that occur between views of a scene. The current study assessed perceptual load as a factor that may affect change blindness for human faces. The study had participants (n = 103) interact with a researcher in a testing room that imposed low or high perceptual load. Midway through the conversation, the researcher was replaced by another person. Thirty‐nine percent of participants failed to detect the change. There was a significant effect of perceptual load, with greater change detection under low load (71%) than high load (52%). This research suggests that the perceptual load imposed by a task may have a significant effect on the likelihood of change blindness and ought to be considered in future research. KEY W ORDS attention, change blindness, perceptual load 1 | I N T RO D U CT I O N unexpected object (Mack & Rock, 1998). Although these are two distinct effects with their own theoretical underpinnings, both are more Imagine yourself in a common social situation—paying for an item in a likely in novel, complex, or difficult situations (Jensen et al., 2011). clothing store, ordering a drink from a bartender, or attending a uni- In an early example of change blindness research, participants versity lecture. If the person you were looking at ducked out of sight scanned photographs while having their eye movements tracked for just a moment (behind the register, bar, or lectern), would you (Grimes, 1996). During a saccade, the original image was replaced by notice if an entirely different person returned in their place? Research an altered one, containing a conspicuous change, such as two cowboys has shown that many of us would not notice such a change (termed switching heads. Rather than relying on eye‐tracking equipment to change blindness; Simons & Ambinder, 2005), and indeed, most of us detect saccades, researchers soon developed the “flicker paradigm,” underestimate how likely we would be to suffer from such a cognitive where a blank screen is briefly flashed to conceal a change to the stimuli failure (termed change blindness blindness; Levin, Momen, Drivdahl IV, being viewed (Rensink, O'Regan, & Clark, 1997). Although the change is & Simons, 2000). Change blindness research has progressed from arti- clearly visible and easy to detect in the absence of the flicker, change ficial computer‐based paradigms to more ecologically valid real‐world blindness is likely to occur due to the disruption (Noë, Pessoa, & methods, providing insight into visual attention in everyday settings. Thompson, 2000). Even when informed of the change, subjects often Change blindness can be defined as “the surprising difficulty require multiple viewings of the original and altered stimuli to detect observers have in noticing large changes to visual scenes” (Simons & the change (Rensink et al., 1997). A real‐world example of this effect is Rensink, 2005, p. 16). Change blindness differs from other examples continuity errors in film (Levin & Simons, 2000), with one study finding of failed change detection in that the extent of detection failure is just 33% of viewers detected the replacement of an actor by another counter‐intuitive or surprising (Jensen, Yao, Street, & Simons, 2011). man during a camera cut in a brief film sequence, despite paying close Change blindness is similar to another common failure of visual enough attention to the film to provide rich descriptions of the actor's awareness, inattentional blindness. The key difference is that change movements and clothing (Levin & Simons, 1997). blindness is the failure to notice a change that occurs, whereas Change blindness research has also extended to real‐world inter- inattentional blindness is the failure to notice the presence of an actions. In one well‐known study by Simons and Levin (1998), Appl Cognit Psychol. 2018;32:655–660. wileyonlinelibrary.com/journal/acp © 2018 John Wiley & Sons, Ltd. 655 656 MURPHY AND MURPHY unwitting participants stopped to give directions to a confederate crucially, reduces interference from distractors (unlike task difficulty; carrying a map on a college campus. During their conversation, two Lavie & De Fockert, 2003). This has been further supported by neuro- people carrying a door walk between the confederate and the partic- imaging studies that have shown a modulated neural response to ipant, allowing the confederate to swap places with one of the door distractors under high perceptual load (see Murphy et al., 2016, for a carriers. The change happens out of sight (behind the door), and the review). second confederate continues the conversation with the participant. High perceptual load also reduces awareness of irrelevant stimuli, There were 15 participants in this study, seven of whom failed to resulting in inattentional blindness for easily visible objects (see Lavie, detect this change in their conversation partner (47%). Beck, & Konstantinou, 2014, for a review). In one study, participants This example of real‐world change blindness was replicated by were presented with a cross on screen and asked to identify which Levin, Simons, Angelone, and Chabris (2002). Participants approached arm (horizontal or vertical) was blue (imposing low perceptual load) a counter to participate in a research experiment. After signing a con- or which arm was marginally longer (imposing high perceptual load; sent form, participants handed the form to a man behind the counter Cartwright‐Finch & Lavie, 2007). On a final critical trial, an unexpected who ducked out of sight to put away the form. A second man then grey shape was presented alongside the cross. Under low perceptual stood up in place of the first and gave the participants directions to load, 55% of participants detected the change, whereas under high the interviewing room; 15/20 participants failed to notice this change perceptual load, that figure was reduced to just 10%. The role of per- (75% suffered from change blindness). In a second study, Levin and ceptual load on awareness has also been demonstrated across other colleagues approached participants and asked them to take a photo- modalities, with studies demonstrating increased inattentional deaf- graph. As the participant looked through the lens preparing to take ness (Macdonald & Lavie, 2011) and inattentional numbness (Murphy the shot, confederates walked between the participant and the exper- & Dalton, 2016) under high perceptual load. Reduced awareness imenter carrying a large piece of cardboard, allowing the experimenter under load has been demonstrated in driving, with increased percep- to switch places with another man; 19/36 participants missed the tual load resulting in inattentional blindness for a large roadside ele- change (53% suffered from change blindness). Levin and colleagues phant (Murphy & Greene, 2017a) and a pedestrian about to cross then replicated the photo‐taking study with a larger sample and using the road (Murphy & Greene, 2016a), as well as inattentional deafness female experimenters, finding that 19/67 (28% of participants) missed for driving and non‐driving related sounds (Murphy & Greene, 2017b). the change (Levin et al., 2002). It is clear then that change blindness There is some evidence that perceptual load also influences rates can and does occur in real‐life settings, with many participants failing of change blindness. As discussed by Lavie (2006), rates of change to notice when a conversation partner is replaced by another. It is also blindness in the flicker paradigm drop to practically zero when the clear that there can be substantial variance in the observed rate of object that changes is cued (Rensink et al., 1997). If focused attention change blindness, with the studies described above ranging from allows for successful detection of change, this suggests that the failure 28% to 75%. Many studies have small sample sizes that makes the to detect changes in the uncued condition is due to attention being assessment of individual factors difficult. The current study will loaded by other objects in the scene. Indeed, Lavie and colleagues pre- explore a factor that may influence the likelihood of change blindness sented participants with a standard perceptual load letter search task, in a social interaction; perceptual load. flanked by two images. They found that the level of search load in the Perceptual load theory (Lavie, 2005, 2010; Murphy, Groeger, & central task affected rates of change blindness for the images (69% Greene, 2016) is a model of selective attention that makes specific detection under low load, 58% detection under high load; Lavie, predictions about the role of perceptual load in conscious awareness. 2006). It is unclear if real‐world manipulations of perceptual load will Perceptual load can be defined as “the amount of information involved have the same impact on change blindness in an everyday setting. in the processing of the task stimuli” (Macdonald & Lavie, 2011, Demonstrating such an effect would be significant for perceptual load p. 1780). This can be manipulated by either increasing the amount of theory as the model is lacking in real‐world evidence. Despite load material to be processed (Lavie & De Fockert, 2003;e.g., detect a theory making predictions that are relevant for many everyday tasks, target letter surrounded by one neutral letter vs. six neutral letters), there has been very little testing of the model in ecologically valid set- by increasing the complexity of the material to be processed (Beck & tings, with most studies employing classic computer‐based flanker par- Lavie, 2005;e.g., detect a target letter surrounded by six target‐dissim- adigms (Murphy et al., 2016). Identifying factors that predict rates of ilar letters vs. six target‐similar letters), or by presenting the same awareness is also important for advancing our understanding of material, but increasing the complexity of the judgement to be made change blindness, as we move from demonstrating the existence of (Cartwright‐Finch & Lavie, 2007;e.g., determine which arm of a cross the effect, to trying to understand the causes and solutions (Riggs & is a particular color vs. determine which arm of a cross is fractionally Sarter, 2016; Taylor, Witt, & Pratt, 2017). Converging evidence sug- longer). As perceptual capacity is limited, the load model states that gests that perceptual load may affect change blindness by consuming when a task imposes high perceptual load, irrelevant distractors are attentional resources rather than influencing search patterns. A recent less likely to be processed, resulting in less behavioral interference eye‐tracking study illustrated that eye movements do not differ under by distractors (Lavie, 1995; Forster & Lavie, 2008; Lavie & De Fockert, low and high perceptual load when watching a film clip, suggesting 2003; Murphy & Greene, 2016a; Murphy & Greene, 2017a). Percep- that the effect of load on awareness is underpinned by limits in per- tual load effects are distinct from effects of task difficulty or general ceptual capacity rather than altered visual search patterns (Greene, clutter in a scene, as high perceptual load results in longer reaction Murphy, & Januszewski, 2017). The same is true for change blindness times and increased errors (as does greater task difficulty) but, effects, where fixation patterns have been shown to have no effect 657 MURPHY AND MURPHY (Beck, Rees, Frith, & Lavie, 2001). The current study will draw from the perceptual load has been used in previous research (Greene et al., perceptual load model and assess whether perceptual load affects 2017; Murphy & Greene, 2016b). Forty‐nine participants were awareness for a clearly visible change in a real‐world interaction. assigned to the low load condition, with 54 participants in the high In the current study, participants greeted a researcher in a testing room that imposed low or high perceptual load. During their interac- load condition. Each room contained one pair of researchers (see Figure 2). tion, the researcher moved out of sight, and a different researcher Upon entering the testing room, a researcher greeted the partici- returned in their place. Participants' awareness for the change was pant with a smile and received their completed consent form. The then assessed verbally. We hypothesized that in line with previous researcher said “just one second” before going behind the divider, findings, a substantial number of participants would suffer from where the second researcher was hidden. The second researcher then change blindness for a human face in a real‐world interaction. We also emerged and presented the participant with a “personality test” (a page expected a significant effect of perceptual load, with increased rates featuring a Rorschach inkblot) before instructing the participant to go of change blindness under high perceptual load, relative to the low next door to the interviewing room to answer questions about the pic- perceptual load condition. ture. Pilot testing suggested that some participants could hear movement behind the divider, so for the main experiment, both rooms had 2 METHOD | a radio playing the same radio station at a low volume. Two participants entered the room before the second researcher was fully concealed behind the divider. One further participant followed the first researcher 2.1 | Participants One hundred six participants took part in this study, but three partic- behind the divider and saw the second researcher. All three were removed from the analysis, and their data are not included here. ipants were removed from the analysis because of errors in the testing In the interviewing room, participants met with one of two procedure (e.g., they entered the room and saw the two researchers as researchers who followed the same interview protocol. First, partici- they changed place). One hundred three remaining participants com- pants were asked if they noticed anything unusual in the previous room. pleted this study. Sixty were female, and the mean age was 35.1 years If participants described a change in the experimenter, they were (SD = 12.6), with a range between 17 and 69. Participants were mem- recorded as demonstrating no change blindness and debriefed. If they bers of the public visiting the university on Culture Night 2016, an reported seeing nothing unusual or described something other than annual event in the Republic of Ireland where places of interest pro- the experimenter (e.g., “I saw lots of clutter in the room”), they were vide free tours and entertainment. Visitors to the School of Applied pressed further and asked if they noticed anything unusual about the Psychology in University College Cork could tour the building and experimenter in particular and to describe what was unusual specifi- attend talks on different areas in psychology as well as partaking in cally. Although some participants answered yes to noticing something experiments and testing out the labs and equipment. odd about the experimenter, none of these participants described the experimenter switching places with another person, they instead 2.2 | Materials and procedure described unremarkable aspects of the interaction (“she gave me this sheet” or “she smiled at me”). They were questioned further and asked, Participants were invited to partake in a “personality test” as part of “did the experimenter change places with another person at any point?” their Culture Night visit. Upon approaching the experimenter and and all participants said they had not noticed that happen. Therefore, all completing an informed consent form, participants were instructed participants who reported noticing the change answered yes to the very to bring their consent form to one of two small testing rooms (impos- first question, where they were asked about anything unusual in the ing low or high perceptual load, as illustrated in Figure 1). The low load previous room. For those that did report a change, they were also asked room was bare and contained only a desk and a divider. The high per- to describe how they came to notice it. This was to ensure that partici- ceptual load room was identical in size and shape, but the walls and pants had genuinely detected a change and that no errors had occurred divider were covered in posters, and the desk was filled with colorful in the testing procedure. No participants were removed due to their office clutter (paper, pens, folders, etc.). This manipulation of answers as all gave responses that indicated they had genuinely FIGURE 1 The low perceptual load (left) and high perceptual load (right) testing rooms used in the current study [Colour figure can be viewed at wileyonlinelibrary.com] 658 MURPHY AND MURPHY FIGURE 2 The pairs of researchers who participants met in the testing room—Pair 1 (above) and Pair 2 (below). These researchers were student volunteers who were asked to wear dark clothes and simple make up, with their hair tied back. This was to minimize the effect of distinctive clothing, hair, or make‐up in assisting participants to detect the change. All researchers were female and in their early twenties [Colour figure can be viewed at wileyonlinelibrary.com] detected a change (rather than say, seeing the other researcher behind with was replaced by another individual. This is in line with previous the divider). At the end of the interview, participants were debriefed studies that have demonstrated this rather unintuitive result and rep- about the real purpose of the study. licates the finding that change blindness occurs in real‐world human Participants were randomly assigned to the low or high perceptual interactions when there is a brief disruption to an individual's view. load testing room based on the order in which they approached the Perceptual load was found to have a significant effect on rates of experimenter. To ensure any experimenter effects were evenly distrib- change detection. In line with our second hypothesis, participants uted, the pairs of researchers in the testing room alternated their role were less likely to notice the change under high perceptual load. as either the greeter or the person hiding behind the divider. The pairs Although many studies have shown that high perceptual load of researchers also swapped rooms during the evening, so half of the increases inattentional blindness (Cartwright‐Finch & Lavie, 2007; participants met Pair 1 under low load and half met Pair 2 under low Murphy & Greene, 2017a) and impairs eyewitness memory and ability load, and vice versa. Likewise, the researchers in charge of debriefing to pick out an individual from a line‐up (Greene et al., 2017; Murphy & also swapped rooms frequently, so they debriefed half of the low load Greene, 2016b), the current study is just the second to demonstrate participants and half of the high load participants. increased change blindness under perceptual load, the first using human faces. 3 | RESULTS The change was detected by 63/103 participants (63%). The effect of perceptual load was assessed with a chi‐square test. There was a significant main effect of perceptual load, with greater detection under low load (71% detected) than high load (52% detected), χ2(1) = 4.14, p < 0.05. It is important to note that due to methodological constraints, perceptual load was not directly assessed in this study. Although the perceptual load manipulation was designed to increase the amount of information to be processed, it cannot be definitively classed as a perceptual load manipulation without verifying that the high perceptual load condition reduced interference by task‐irrelevant distractors (Murphy et al., 2016; Roper, Cosman, & Vecera, 2013). Although this is easily achieved in a computer‐based letter search task (i.e., by comparing response times in the presence of a distractor that is either 4 | DISCUSSION congruent or incongruent with the target letter, as in Lavie & De Fockert, 2003; Murphy & Greene, 2017c), it is almost impossible to As expected based on previous research, a significant number of par- directly assess in an everyday interaction that still retains some eco- ticipants (39%) failed to notice when the person they were interacting logical validity. Related to this is the fact that participants in the 659 MURPHY AND MURPHY current paradigm were not explicitly told what was task‐relevant and When trying to assess change detection across different paradigms, task‐irrelevant. This is markedly different from letter search tasks researchers should, as much as is practically possible, match the com- where participants are told to focus on the target letters and ignore plexity of irrelevant background stimuli. Such a consideration might the distractor letter (Lavie & De Fockert, 2003). Although this makes also extend beyond the visual modality as there is evidence that per- the current paradigm more ecologically valid (everyday selective atten- ceptual load has cross‐modal effects on awareness. For example, tion is rarely so clear‐cut), it also meant that participants were not auditory perceptual load has been shown to increase inattentional forced to encode any aspect of the scene. We would therefore urge blindness for an unexpected object (Murphy & Greene, 2017a), and caution in interpreting these findings as the paradigm is so different visual load has been shown to increase inattentional deafness from traditional perceptual load experiments. The current study does (Macdonald & Lavie, 2011; Murphy & Greene, 2017b) and suggest that increasing the amount of information to be processed inattentional numbness (Murphy & Dalton, 2016). Likewise, there is in a scene can reduce change detection for human faces, though fur- an auditory analogue of change blindness, “change deafness” ther research is required to understand the pathway involved. (Vitevitch, 2003). Future research might examine cross‐modal effects This move towards more ecologically valid paradigms, even at the of perceptual load on change detection, for example, change blindness expense of experimental control, is important as the perceptual load under auditory load or change deafness under visual load. This would model is lacking in applied evidence (Murphy et al., 2016). The current be an interesting avenue of research with both theoretical and applied study is a novel demonstration of everyday perceptual load effects, implications, given that our everyday experiences are rich and using a paradigm that to our knowledge has never been applied to multi‐modal. load theory. The use of human faces as the source of change also The current study demonstrated a high rate of change blindness makes a significant contribution as there is mixed evidence within for human faces in a real‐world interaction. We have illustrated for the literature concerning the effect of perceptual load on the process- the first time that high perceptual load increases the likelihood of ing of human faces (Murphy et al., 2016). Although some research has change blindness in everyday life. As the perceptual load manipulation suggested that faces are a “special case” in the load model, with no was simple and commonly encountered (a sparse office vs. a cluttered, effect of perceptual load on face processing due to a face‐specific per- messy office), this suggests that change blindness may be affected by ceptual capacity (Neumann, Mohamed, & Schweinberger, 2011), scene complexity during numerous everyday tasks. Future studies may others have found that high perceptual load does reduce processing examine change blindness in different settings, using different load of distractor faces, for example, in the case of eyewitness memory manipulations. One example is that continuity errors in film may be (Murphy & Greene, 2016b). The current study provides further evi- less likely to be detected by the audience if the scene imposes high dence that perceptual load may be an important factor in eyewitness visual perceptual load. Future research should control for perceptual memory for human faces, with witnesses perhaps less sensitive to load (in all modalities) across change blindness paradigms. changes in a high perceptual load scene. A limitation of the current study, and indeed most perceptual load ACKNOWLEDGMENTS studies, is the rather arbitrary notion of “low” and “high” perceptual We would like to thank our fantastic student actors Louise Foley, load. Perceptual load is not precisely defined and so can be difficult Sarah Daly‐Butz, Laura O'Connor, and Eadaoinn Doherty. We would to operationalize outside of traditional paradigms (Murphy et al., also like to thank Dr. Samantha Dockray for her assistance with the 2016). When manipulating load in a new paradigm, such as the change data collection. blindness paradigm used in the current study, researchers cannot conclusively state that one condition is objectively “high” load. For example, if in the current study, the testing room contained 30 items of clutter versus 50 items of clutter, which condition should be termed “high perceptual load”? All that can be said with certainty is that one condition is “higher load” than another (Murphy & Greene, 2017c). Given that the purpose of the current study was to assess perceptual load effects on change blindness, these arbitrarily selected low and high load conditions are suitable and allow us to conclude that environments that are more cluttered and require a greater amount of processing are likely to reduce change detection. Future studies should examine these effects at many levels of perceptual load, assessing whether perceptual load affects change detection as a continuum, with detection worse from low to medium to high load, or whether there is a qualitative “tipping point” at which perceptual capacity is consumed and change detection is affected. This would have interesting implications for both change blindness research and perceptual load model. The results of the current study also suggest that scene complexity ought to be controlled for in future change blindness studies. ORCID Gillian Murphy http://orcid.org/0000-0001-8898-139X RE FE RE NC ES Beck, D., & Lavie, N. (2005). 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Applied Cognitive Psychology, 31(2), 258–263. acp.3441 load affects change blindness in a real‐world interaction. Appl Cognit Psychol. 2018;32:655–660. https://doi.org/10.1002/ Copyright of Applied Cognitive Psychology is the property of John Wiley & Sons, Inc. and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.