Auditory Perceptual Load: a Review

Hearing Research xxx (2017) 1e9

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Hearing Research

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Review article Auditory perceptual load: A review

* Sandra Murphy a, , Charles Spence b, Polly Dalton a a Department of Psychology, Royal Holloway, University of London, United Kingdom b Department of Experimental Psychology, University of Oxford, United Kingdom article info abstract

Article history: Selective attention is a crucial mechanism in everyday life, allowing us to focus on a portion of incoming Received 23 September 2016 sensory information at the expense of other less relevant stimuli. The circumstances under which Received in revised form irrelevant stimuli are successfully ignored have been a topic of scientiﬁc interest for several decades now. 21 December 2016 Over the last 20 years, the perceptual load theory (e.g. Lavie, 1995) has provided one robust framework Accepted 5 February 2017 for understanding these effects within the visual modality. The suggestion is that successful selection Available online xxx depends on the perceptual demands imposed by the task-relevant information. However, less research has addressed the question of whether the same principles hold in audition and, to date, the existing Keywords: Auditory selective attention literature provides a mixed picture. Here, we review the evidence for and against the applicability of Perceptual load perceptual load theory in hearing, concluding that this question still awaits resolution. Distractor processing © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Contents

1. Introduction ...... 00 2. Principles of perceptual load from the visual domain ...... 00 2.1. Defining perceptual load ...... 00 2.1.1. The number of items in the display ...... 00 2.1.2. The level of similarity between targets and non-targets ...... 00 2.1.3. The number of perceptual operations required by the task ...... 00 2.2. Measuring irrelevant distractor processing ...... 00 2.2.1. Response competition ...... 00 2.2.2. Awareness report ...... 00 2.2.3. Neuroimaging ...... 00 2.2.4. EEG ...... 00 3. Auditory selective attention and perceptual load ...... 00 3.1. Dichotic listening ...... 00 3.2. Studies manipulating auditory perceptual load ...... 00 3.2.1. The number of items in the display ...... 00 3.2.2. The level of similarity between targets and non-targets ...... 00 3.2.3. The number of perceptual operations required by the task ...... 00 3.2.4. Alternative auditory load manipulations ...... 00 4. Summary and conclusions ...... 00 Acknowledgements ...... 00 References...... 00

1. Introduction * Corresponding author. Royal Holloway, University of London, Egham Hill, Egham, TW20 0EX, United Kingdom. In a world that is rich with sensory information, it is impossible E-mail address: [email protected] (S. Murphy). http://dx.doi.org/10.1016/j.heares.2017.02.005 0378-5955/© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Please cite this article in press as: Murphy, S., et al., Auditory perceptual load: A review, Hearing Research (2017), http://dx.doi.org/10.1016/ j.heares.2017.02.005 2 S. Murphy et al. / Hearing Research xxx (2017) 1e9 to perceive everything around us at any given time. Selective spatial senses. Despite some recent interest in this question within attention is thus a crucial mechanism because it allows us to focus the tactile domain (Adler et al., 2009; see Gallace and Spence, 2014; on relevant information and give less priority to irrelevant, for a review) and in crossmodal contexts (e.g. Macdonald and Lavie, potentially distracting information (see, for example, Awh et al., 2011; Murphy and Dalton, 2016; Otten et al., 2000; Raveh and Lavie, 2012; Chun et al., 2011; Dalton and Hughes, 2014; for recent re- 2015; Rees et al., 2001; Tellinghuisen et al., 2016; Tellinghuisen and views). A widely-researched topic within the area of selective Nowak, 2003), the current review will focus on audition. Although attention concerns the question of what determines whether or not Lavie and Tsal (1994) listed some examples of potential effects of distracting information can be successfully ignored. Why is it auditory perceptual load in their seminal consideration of the way sometimes near-impossible to ignore the radio playing in the in which earlier findings could be explained within the framework background, for instance, whereas in other situations we miss that of perceptual load, this question has subsequently received far less potentially important announcement at the train station? One attention than within the visual domain. Nevertheless, given that prominent approach over the last 20 years has come from hearing is often considered to act as an ‘early warning system’ (e.g. perceptual load theory (Lavie and Tsal, 1994; Lavie, 1995). The Dalton and Lavie, 2004; Spence and Driver, 1994; Spence and theory promises to explain this paradox by proposing that the main Santangelo, 2009) e continually scanning the environment in all determinant of whether or not irrelevant information can be suc- directions while the other senses focus on more restricted areas of cessfully ignored depends on the perceptual load (i.e. demands) of space e one might predict that mechanisms of distractor process- the relevant task. One of the critical aspects of the theory is that it ing would operate differently in audition than in vision. Indeed, proposes that our perceptual system has limited processing ca- from this evolutionary perspective, the processing of seemingly- pacity, and that it is beyond our volitional control as to how much irrelevant sounds is likely to have been beneficial for our survival of that capacity will be engaged at any given time. Instead, all of the (e.g. in allowing us to detect the sounds of predators approaching available information is automatically processed until an in- from behind). Furthermore, visual perceptual load has also been dividual's perceptual capacity is exhausted. The role of volitional suggested to alter the spatial focus of attention (Caparos and control is to influence what information gets prioritised for further Linnell, 2010), demonstrating a more narrow focus of resources as processing, thus allowing us to focus on relevant stimuli at the perceptual demands increase. In light of the early warning system expense of those that may be currently less relevant to us. account, it seems perhaps unlikely that an equivalent role of Taken together, this implies that the perceptual demand of the perceptual load would exist in hearing. Indeed, in recent years, relevant task that we are engaged with determines our success in mixed findings have been reported concerning the applicability of ignoring irrelevant information. If the relevant task is perceptually perceptual load theory to the auditory domain, creating the need simple (low perceptual load), only a small portion of our perceptual for a more thorough examination of auditory selective attention capacity will be allocated to it, with the remainder automatically and its underlying neural mechanisms. used to process surrounding irrelevant information. On the other hand, if the relevant task is perceptually demanding (high 2. Principles of perceptual load from the visual domain perceptual load), all, or most, of our processing capacity will be exhausted by the task and hence little or no capacity will remain We will begin by outlining the principles of perceptual load available to process any irrelevant information. The consequences theory as defined within the visual modality. This outline will then are thus that irrelevant distractor processing occurs to a much help structure the evidence from the auditory domain to help greater degree when the current goals involve relevant tasks of low assess how these findings fit within the existing framework of vi- (vs. high) perceptual load, simply because the perceptual capacity sual perceptual load. The literature is organised according to the will be spread between the processing of both relevant and irrel- particular measures and manipulations that have been used. evant stimuli in these contexts. However, it should be noted that we do not intend by this to sug- Although this review will focus solely on claims concerning gest that specific manipulations of perceptual load are computa- perceptual load, it is worth mentioning that load theory also ad- tionally equivalent for vision and audition when it comes to the dresses the influence of working memory (WM) load (e.g. de perceptual demands that they might impose. Indeed, the funda- Fockert et al., 2001; Lavie et al., 2004). The proposal is that suc- mental differences between the two modalities in terms of how cessful selection is not only determined by perceptual load, but also information is organised and processed make it near-impossible to by the availability of WM resources to maintain task focus. In other attempt direct comparisons of this type (see Allport, 1992). For words, if WM resources are exhausted through a concurrent (yet example, adding items in a visual display is not necessarily as unrelated) task, there are reduced top-down control resources demanding as adding the same number of concurrent sounds in an available to prioritise processing of task-relevant over irrelevant auditory task.1 Similarly, tasks that involve spatial selection may information. Load theory thus predicts that an increase in WM load impose different levels of demand on the auditory and visual sys- in an unrelated task will lead to greater distractor processing in a tems even though they appear superficially similar. Indeed, it may concurrent selective attention task. Indeed, this pattern of results be the case that more meaningful comparisons can emerge from has also been reported in the auditory domain (e.g. Berti and comparing tasks that emphasise different stimulus dimensions Schroger,€ 2003; Dalton et al., 2009; Muller-Gass and Schroger,€ according to the properties of the sensory modality in question. For 2007) although there is also some conflicting evidence (e.g. example, it has been argued that the closest equivalent to visual SanMiguel et al., 2008). A full consideration of this aspect of load spatial selection in audition is, in fact, frequency-based selection theory is certainly beyond the scope of the present review. How- over time (e.g. Kubovy, 1981, 1988). ever, it is important to consider the distinction between perceptual and WM load when reviewing different load manipulations, 2.1. Defining perceptual load because load theory predicts differing impacts of different types of load. One important question given the theory's suggestion that Evidence in support of perceptual load theory has been plentiful perceptual load determines successful attentional selection is what within the visual domain (see Murphy et al., 2016; for a recent review). This has prompted a growing body of research into whether the same principles might also hold within the other 1 We thank an anonymous reviewer for this example.

Please cite this article in press as: Murphy, S., et al., Auditory perceptual load: A review, Hearing Research (2017), http://dx.doi.org/10.1016/ j.heares.2017.02.005 S. Murphy et al. / Hearing Research xxx (2017) 1e9 3 defines perceptual load. The perceptual load or demand of a task distractor interference could relate to differences in post- has typically been operationalised in one or more of three different perceptual, response-related processing rather than an early in- ways within the visual domain. fluence determining whether irrelevant distractors receive any processing at all. Recently, however, perceptual load has been 2.1.1. The number of items in the display manipulated in paradigms that provide a more direct measure of The first definition concerns the number of relevant items in the the awareness of irrelevant stimuli, in the absence of response display requiring processing (e.g. Lavie, 1995). For example, in a competition (e.g. Bahrami et al., 2008; Bahrami et al., 2007; traditional flanker task (Eriksen and Eriksen, 1974), the task is to Cartwright-Finch and Lavie, 2007; Calvillo and Jackson, 2014; identify a target letter (e.g. X or N). Under conditions of low Macdonald and Lavie, 2008). For example, Cartwright-Finch and perceptual load, this target letter is presented on its own which Lavie adapted a task devised by Mack and Rock (1998) in order to requires little perceptual capacity to search for the target, whereas introduce a load manipulation. Participants were presented with a under high perceptual load it is presented amongst other similar cross comprising a horizontal line and a vertical line. In the low load looking non-targets. However, it has been argued that this type of condition they judged which of the two arms was blue (vs. green) manipulation is confounded with low-level effects of dilution (e.g., whereas in the high load condition they made a more subtle Benoni and Tsal, 2012; Tsal and Benoni, 2010). The suggestion is perceptual discrimination, deciding which of the arms was longer. that as set size increases, relevant as well as irrelevant information On the final (sixth) trial, a small square was presented unexpectedly is still processed to the same degree, but with the additional stimuli in the periphery of the task display and the participants were the neuronal representation of each stimulus is degraded and immediately asked afterwards whether they had noticed anything hence the interference of the irrelevant distractor is also reduced. else other than the target display. A significant decrease in awareness reports was demonstrated under high compared with 2.1.2. The level of similarity between targets and non-targets low load, suggesting that the level of perceptual load can determine A second approach is to vary the perceptual similarity between a awareness of irrelevant, unexpected stimuli. target and competing non-targets while the number of items However, although the inattention paradigms can provide an remain the same (e.g. Beck and Lavie, 2005; Forster and Lavie, index of perception of irrelevant stimuli, there is the potential 2008). A low load setting can, for example, comprise a target let- confound that any decrease in reported awareness with an increase ter X or N presented amongst small o's, whereas under high load in load could reflect memory failures rather than a genuine lack of the non-target letters will be perceptually similar to the target (e.g., perception (Wolfe, 1999). In an attempt to address this concern, H, K, M, V, W), requiring greater processing resources to identify the some studies have measured detection sensitivity to an expected target. and frequently-occurring stimulus whose presence or absence is reported on a trial-by-trial basis (e.g. Macdonald and Lavie. 2008). 2.1.3. The number of perceptual operations required by the task Typically, evidence of reduced detection sensitivity has been re- The third approach defines the level of load by the number of ported for high (vs. low) load, further suggesting that the level of perceptual operations that are required to discriminate the target visual perceptual load in the relevant task can determine the (e.g. Cartwright-Finch and Lavie, 2007). Typically, the stimulus perception of irrelevant information. display is identical between load conditions but under low load only one feature (e.g. colour) defines the target whereas under high 2.2.3. Neuroimaging load, a conjunction of two features are task relevant (e.g. blue The behavioural demonstrations of perceptual load have also square versus green triangle). However, it is worth noting that been supported with evidence from neuroimaging studies (e.g. manipulating perceptual load by varying processing demand levels Bahrami et al., 2007; Bishop et al., 2007; O'Connor et al., 2002; Rees may also lead to changes in other cognitive processes (e.g. working et al., 1997). For example, modulation by perceptual load of activity memory; Lavie and de Fockert, 2003; Tsal and Benoni, 2010). associated with the processing of irrelevant information has been demonstrated in those areas of visual cortex that are associated 2.2. Measuring irrelevant distractor processing with even the very early stages of perceptual processing, such as area V1 (e.g. Schwartz et al., 2005). In fact, O'Connor et al. The extent to which irrelevant information is processed has also demonstrated reduced activity in response to an irrelevant flick- been assessed using a range of different approaches involving both ering checkerboard pattern under high (vs. low) load as early as in behavioural and neural measures. the lateral geniculate nucleus (LGN), thus suggesting that an attentional modulation can occur as early as in subcortical regions 2.2.1. Response competition of the visual system. However, because of the poor temporal res- The most traditional measure is that of response competition: a olution of fMRI, it remains a possibility that the difference in ac- distractor with either the same or opposite identity to the target is tivity in the early visual areas as a function of load reflect later presented in an irrelevant location and reaction times (RTs) and feedback from higher-level areas. error rates are measured in response to the target as a function of the distractor being congruent (same identity as target) or incon- 2.2.4. EEG gruent (opposite identity to the target). Evidence in favor of the EEG studies which have better temporal resolution have also theory e demonstrating reduced distractor interference under high provided evidence for early attentional modulations (Fu et al., 2010; versus low load e has been plentiful (e.g. Beck and Lavie, 2005; Parks et al., 2013; Rauss et al., 2009, 2012; Wang et al., 2012). For Caparos and Linnell, 2010; Forster and Lavie, 2007, 2008, 2009; example, a number of experiments have demonstrated attentional Murphy et al., 2012). modulation as a function of load as early as around 80 ms post stimulus-onset (e.g. Fu et al., 2010; Rauss et al., 2009, 2012), which 2.2.2. Awareness report is in line with the predictions of the theory. Although the measure of distractor processing through response competition effects provides an index of irrelevant information 3. Auditory selective attention and perceptual load processing, it cannot offer much insight into the extent to which a stimulus was fully perceived or not. More specifically, a reduction in Perceptual load theory has undoubtedly received much support

Please cite this article in press as: Murphy, S., et al., Auditory perceptual load: A review, Hearing Research (2017), http://dx.doi.org/10.1016/ j.heares.2017.02.005 4 S. Murphy et al. / Hearing Research xxx (2017) 1e9 in the visual domain, as outlined above. However, the question of whether they had noticed anything else apart from the two con- whether it also holds within the auditory domain remains a matter versations. These findings suggest that auditory selective attention of considerable debate. We now turn to address this question, can operate at an early stage of information processing even in beginning with a review of the early dichotic listening work before more life-like and dynamic settings than the dichotic listening set- turning to a number of studies that relate to perceptual load more ups. Similar findings have also been reported for music, whereby a directly. It is important to note that many of the studies reviewed large proportion of participants counting the number of drum beats were not designed to test perceptual load theory specifically, but in a famous musical piece failed to notice an incongruous guitar instead were concerned with the more general question of how solo (Koreimann et al., 2014). auditory processing is affected when attention is focused on a However, while the research described so far clearly demon- concurrent auditory (or visual) task of varying complexity. They are strates that people are able to successfully ignore irrelevant sounds covered here nevertheless in an attempt to provide an exhaustive to the extent that they are not aware of the semantic content, there review of work both directly and indirectly relevant to the question have also been reports of conflicting findings demonstrating se- of whether perceptual load theory holds in the auditory modality. mantic processing of the information presented in the unattended ear. For example, when participants’ own names were presented to 3.1. Dichotic listening this stream, a higher rate of recognition was reported (Moray, 1959). This phenomenon has been replicated many times since An early body of work using the dichotic listening paradigm (e.g. Conway et al., 2001; Cowan and Wood, 1997; Rivenez et al., largely demonstrated that unattended auditory information in one 2006; Wood and Cowan, 1995a, b; and see Bronkhorst, 2015; for ear receives little processing when attention is focused on the other a recent review). There are also other demonstrations that the ear (e.g. Cherry, 1953, 1954; Moray, 1959). However, there were also processing of unattended sounds can occur to a semantic level findings demonstrating some semantic processing of unattended despite listeners being unaware of such effects. For instance, information, indicating a mixed pattern of results concerning the Corteen and Dunn (1974; see also Corteen and Wood, 1972) effects of auditory attention even at this early stage. Although conditioned participants with a small electric shock whenever they perceptual load was not typically manipulated directly in these heard a city name. City names were subsequently presented in the studies, they are likely to have induced a high perceptual load, as unattended channel in a dichotic listening task and participants described in more detail in the next paragraph. These findings can were instructed to make a response whenever they heard a city therefore be informative when it comes to assessing whether or not name. Although the participants hardly responded to any of the city perceptual load theory also holds in hearing. names (<1%), around a third of them elicited a galvanic skin In a classic dichotic listening task, participants are presented response (GSR) which is likely to reflect a fear response in associ- with two simultaneous auditory messages, one delivered to either ation with the expected electric shock previously paired with the ear (e.g. Cherry, 1953, 1954). The instructions are typically to attend city names. These findings, along with many others (see e.g. Driver, to one ear and shadow (i.e. repeat out loud) the speech sounds 2001; for a review) suggested that there may be many circum- while ignoring the concurrent speech in the unattended ear. The stances under which apparently-unattended information receives common finding from dichotic listening experiments is that when some level of semantic processing. asked about the content played to the unattended ear, participants Taken together, the overall pattern of findings from the dichotic are unable to give any details beyond its most basic physical char- listening studies suggests that although unattended information in acteristics. For example, although in many cases they are likely to some circumstances receives little processing, this is not consis- notice a change in the gender of the speaker (Cherry, 1953; tently the case. It is hard to draw any direct conclusions from these although see Zaske€ et al., 2013, for contradictory results and Fenn results concerning the applicability of load theory to the auditory et al., 2011, for a failure to notice a change in speaker identity), domain because of the lack of direct perceptual load manipulations they are often prone to miss important events, such as changes in in these experiments. However, they can still be informative the language being spoken (Cherry, 1953) or the same word being because the high load imposed by the standard dichotic listening repeated several times (Moray, 1959). task should, according to the predictions of perceptual load theory, Although the results of the dichotic listening studies published result in little or no processing of unattended information. In light to date suggest that unattended sounds are likely to receive very of this assumption, these mixed findings do suggest that auditory little processing, they are often performed in very unnatural set- information can, in some instances, be fully processed even during tings. However, more recent studies have also observed little pro- a high perceptual load task, which is not in line with the predictions cessing of unattended information in more lifelike and dynamic of perceptual load theory. settings. The inattention paradigm (e.g. Mack and Rock, 1998; Simons and Chabris, 1999) is often thought of as the most strik- 3.2. Studies manipulating auditory perceptual load ing example of the important interplay between attention and awareness, demonstrating how salient information (such as a In this section, we review those experiments that have directly person dressed in a gorilla outfit) can be missed when attention is manipulated perceptual demands in an auditory task, starting with engaged elsewhere (for example, on particular players in a those that have more closely followed the visual definitions of basketball game). Dalton and Fraenkel (2012) created an auditory perceptual load, followed by those that have used alternative analogue of this phenomenon, using a realistic three-dimensional auditory load manipulations. auditory scene with two concurrent conversations, one between two female voices and one between male voices. The participants 3.2.1. The number of items in the display had to attend to either the male or the female conversation in Thus far, only one experiment has directly manipulated auditory anticipation of subsequent content-specific questions about the perceptual load by varying the number of items in the relevant conversation. Unexpectedly, an additional male voice appeared and display. In particular, Fairnie et al. (2016) presented participants repeated the words “I am a gorilla” as he walked across the scene. with concurrent sounds emanating from different locations in Despite being clearly audible and repeated over 19 s, the majority of space. A target sound, either a lion's roar or a dog's bark, was pre- participants asked to focus on the female voice failed to notice the sented with a number of non-targets consisting of other animal unexpected male voice when asked immediately afterwards sounds, and load was manipulated through the number of non-

Please cite this article in press as: Murphy, S., et al., Auditory perceptual load: A review, Hearing Research (2017), http://dx.doi.org/10.1016/ j.heares.2017.02.005 S. Murphy et al. / Hearing Research xxx (2017) 1e9 5 target sounds. On half of the trials, an additional sound (the sound chosen because they sound more similar to the targets (P and T) of a car) was presented simultaneously to the target and non-target than the low load non-target (X). For example, the high load se- sounds, in a position perceived to be further away in space than the quences always contained at least one non-target letter (C or G) animal sounds. The task was first to determine the identity of the that rhymed with the target (P or T). Although the first experiment target and second to report the presence or absence of the car demonstrated clear effects of perceptual load such that perfor- sound. In line with the predictions of perceptual load theory, mance was significantly reduced under high (vs. low) load, the sensitivity in reporting the additional car sound was significantly magnitude of the congruency effect was no different between load reduced as the number of additional non-targets increased. Thus, it conditions. To address the possibility that the high load condition in seems that increasing perceptual load in the relevant task resulted this experiment had simply not been sufficiently demanding, a in fewer resources being left to perceive the more distant, task- second experiment shortened the duration of the letter sounds to irrelevant car sound. ensure a more rapid presentation of the stimuli with the aim of Although these findings seem to be in line with the predictions increasing the strength of the perceptual load manipulation. The of perceptual load theory, it is important to note that there is a intensity of the distractor was also decreased to reduce its relative potential alternative explanation for the reduction in sensitivity in salience. Despite the successful attempt to increase the strength of perceiving the car sound under high (vs. low) load. Although the perceptual load manipulation, no difference in the magnitude detection of this additional sound was specified as a secondary task, of the congruency effect was reported as a function of perceptual it is possible that the additional sound was treated as a second load, which is in direct contrast to the predictions of load theory. target to search for, following identification of the main target sound. With the increase of items under high load, the serial search 3.2.3. The number of perceptual operations required by the task task to identify the main target would have automatically taken Most of the experiments that have investigated auditory longer than under low load, thus leaving less time to then search for perceptual load have used paradigms with identical stimuli over the car sound. both load conditions and varied the task demands through the level Increasing the set size of auditory displays is also likely to reduce of perceptual processing needed to resolve the task. For instance, the baseline detectability of the sounds being presented, through Alain and Izenberg (2003) presented participants with two streams both energetic and informational masking. While energetic mask- (one presented to each ear) of both tuned and mistuned sounds in ing occurs because of acoustic overlap between competing sources, each, and instructed them which ear to attend to. Under low load, informational masking refers to any reduction in the ability to they were instructed to detect infrequent targets in the attended follow a given stream among competing streams due to higher- ear defined by being shorter in duration than the other sounds, level factors such as perceptual grouping and attention (e.g. whereas under high load they also discriminated the tuning (tuned Brungart et al., 2001; Kidd et al., 2008; Shinn-Cunningham, 2008). vs. mistuned) of these targets. The mismatch negativity (MMN), Typically, the ability to correctly report attended stimuli is reduced which is typically elicited by an ‘oddball’ sound deviating from a as the number of concurrent streams increases. Along similar lines, uniform sequence of sounds, was measured as a marker of pro- change deafness research has also found a reduced ability to detect cessing of unattended stimuli. As would be predicted by perceptual attended changes with an increase in the number of concurrent load theory, the MMN component elicited in response to short streams (e.g. Cervantes Constantino et al., 2012; Eramudugolla duration deviant stimuli appearing in the unattended ear was et al., 2005; Gregg and Samuel, 2008; Petsas et al., 2016; Sohoglu reduced during the high load task as compared with the low load and Chait, 2016). Thus, set size manipulations within the auditory task. modality are likely to be confounded by differences in both ener- However, although the unattended stream used stimuli that getic and informational masking, which will likely render all were identical to those of the attended stream, the conjunction task stimuli less perceivable (whether attended or irrelevant). Thus it is required participants to focus on two dimensions whereas the possible that any reduction in distractor interference seen in tasks feature task only required a focus on stimulus duration. Thus, the with higher set size reflects reduced baseline detectability of the attentional set is likely to have been more clearly focused on distractor, rather than reduced distractor processing per se. This duration in the low load task as compared with the high load task claim chimes with the previously-mentioned dilution account that where an attentional set would have been implemented for both has been suggested within the visual domain with regards to set duration and tuning. The difference in MMN amplitude in response size manipulations of perceptual load (e.g. Benoni and Tsal, 2012; to the duration-defined deviant could therefore be attributable to Tsal and Benoni, 2010). However, similarly to the visual domain, reduced priority of duration under high load compared with low other studies have used different load manipulations not open to load rather than to differences in available processing capacity. In this potential confound which we will now review. fact, there is substantial evidence to suggest that the attentional set required for the task plays a role in the extent to which task- 3.2.2. The level of similarity between targets and non-targets irrelevant stimuli capture attention, in vision (Bacon and Egeth, Manipulations of auditory load in which the same number of 1994; Folk et al., 1992) and in hearing (Dalton and Lavie, 2007). items are used for both high and low load displays are not open to Indeed, a similar study by Müller-Gass and Schroger€ (2007) which the confounds described above. Murphy, Fraenkel, and Dalton is not likely to have induced any differences in the attentional set (2013) manipulated perceptual load by varying the similarity be- between load conditions did not find any load modulation of the tween targets and non-targets using an auditory flanker task con- MMN amplitude in response to deviant sounds. Participants were sisting of a rapid sequence of six centrally located letter names, presented with tones and discriminated the duration of each tone. spoken in a female voice. The task was to identify the target (P or T) Task demands were manipulated by varying the difference in and to ignore the potential occurrence of a distractor letter (also P duration between the long and the short tones, which means that or T) spoken in a male voice and presented to the left or right on the attentional set in the two load conditions would not have been two thirds of the trials. Under low load, the five non-targets con- any different. Under these conditions, the amplitude of the MMN sisted of X's, making the target easily discriminable, whereas under elicited by occasional low frequency deviants did not vary as a high load the non-targets consisted of different letters rendering function of task load, which further suggests that Alain and Izen- the target identification process more perceptually demanding. berg's findings could be explained by differences in attentional set More specifically, the high load non-targets (A, C, H, G, J, K) were rather than perceptual load.

Please cite this article in press as: Murphy, S., et al., Auditory perceptual load: A review, Hearing Research (2017), http://dx.doi.org/10.1016/ j.heares.2017.02.005 6 S. Murphy et al. / Hearing Research xxx (2017) 1e9

Issues related to changes in attentional set between high and irrelevant speech. Although these findings provide support for the low perceptual load only concern task set-ups in which stimuli in role of perceptual load in audition, the load by congruency inter- the attended and unattended streams are highly similar. By action typically evident when a role of perceptual load on distractor contrast, a magneto-encephalography (MEG) study (Chait et al., processing is demonstrated was not significant (F < 1 in both re- 2012) presented two concurrent sequences of sounds: a mixture ported experiments). This therefore puts into question the of different auditory stimuli (pure tones, frequency-modulated robustness of these findings. tones, glides and white noise) to one ear and a sequence of brief Indeed, recent research that used the feature versus conjunction tones to the other ear as well as a sequence of visual geometrical manipulation of perceptual load failed to find the predicted load objects on the screen. The participants were instructed to attend effects. Alongside their aforementioned work, Murphy et al. (2013) either to the auditory or visual objects while ignoring the stream of reported two further experiments in which they used an inatten- brief tones and to detect a predefined target in the attended mo- tional deafness paradigm and measured the frequency of noticing dality. Task load was manipulated by varying the target object on an unexpected, task-irrelevant sound as a function of perceptual every trial under high load while keeping it constant under low load in the concurrent task. A tone was played to one ear while load. An increase in auditory load resulted in less cortical activity in white noise was played to the unattended ear. Half of the partici- response to a change in the pattern of the unattended stream, but pants made duration judgments to the tone (low load) while the only when the pattern changed from irregular to regular whereas a other half responded to a conjunction of duration and tone fre- change from regular to irregular was processed to the same extent quency (high load). On the final trial, the spoken word ‘cat’ was regardless of the load. However, perceptual load theory was not the unexpectedly presented in the unattended ear concurrently to the focus of this study, and indeed the task load manipulation is likely tone in the attended ear. Immediately after the target response, the to involve increases in memory demands rather than perceptual participants were asked if they had noticed anything other than the load under high (vs. low) load. target tone and white noise on the previous trial. Although there Similar issues of a memory rather than perceptual load was clear evidence of a perceptual load manipulation, the propor- manipulation also apply to a more recent study by Masutomi et al. tion of participants reported ‘deaf’ to the unexpected word was no (2015) that used a dual-task design. The participants received a different between high and low load. This was further confirmed in sequence of sounds in one ear containing a target sound presented a second experiment using a different critical word, replicating no concurrently with a distractor which changed on every repetition differences in level of inattentional deafness as a function of of the target or a sequence of identical target sounds. In the other perceptual load. ear, a sequence of pips were presented concurrently. Participants Taken together, the experiments using the feature versus were instructed to attend to the pips and either to count the conjunction manipulation provide mixed results with some number of pips (high load) or else to detect a salient deviant demonstrating evidence in favor of auditory perceptual load (present on 50% of trials; low load). At the end of each trial, par- whereas others have found no load modulation of distractor pro- ticipants were presented with a probe sound and had to judge cessing. However, as previously highlighted, this kind of load whether the sound had been present in the unattended sequence. manipulation which compares attending to a single feature versus a There was a decrease in performance on the probe task under high conjunction of two features has been questioned because there is a (vs. low) load, which could be suggestive of an influence of possibility that it could involve differences in higher level cognitive perceptual load. However, similarly to Chait et al. (2012), perceptual processes rather than perceptual load per se (see Tsal and Benoni, load theory was not the focus of this work either, and the load 2010). manipulation was therefore not restricted to the perceptual level. In fact, the load manipulation is likely to have influenced memory 3.2.4. Alternative auditory load manipulations rather than perceptual processes, because the low load task Given the structural differences between the auditory and visual involved simple detection whereas the high load task involved systems, it is perhaps not surprising that some studies have used maintenance and active updating of a number. Thus, this compli- alternative manipulations of auditory perceptual load than the cates the conclusions that can legitimately be drawn from these existing three definitions of perceptual load within vision. For two studies in relation to auditory perceptual load. example, Santangelo et al. (2007) used a dual-task paradigm in That being said, similar findings have been reported from a order to investigate whether responses to an exogenous cue would previous study, which used the traditional feature versus change as a function of auditory load. The participants in their conjunction task. Francis (2010) used an adapted auditory version studies were presented with a rapid central stream of spoken let- of the visual flanker task (Chan et al., 2005), presenting participants ters. On two thirds of the trials, a spoken target digit was presented with two streams of the words “bead” or “bad” along with a tone in the stream. An auditory spatial cue (either to the left or the right) which varied in pitch and amplitude modulation. The relevant appeared on the remaining third of trials, followed by a target stream of words was either determined by location (i.e. attend to sound which either occurred on the same or the opposite side as the central stream and ignore the stream appearing from the pe- the cue. The participants either had to respond to, or to simply focus riphery) or by gender (in which case both streams were centrally on, the central stream as well as to respond to the cueing task (high presented). Participants were instructed to respond to the target load) or to only perform the cueing task on its own (in the absence when the concurrent tone was of a specific identity whilst ignoring of the stream; low load). The cueing effect was reduced when the word in the irrelevant stream. Under low load, responses were participants were performing either of the two high load conditions made depending on whether or not the tone was modified in compared to responding to the cued target sound on its own. amplitude or not whereas under high load, participants attended to Santangelo et al. thus concluded that the perceptually demanding a conjunction of amplitude modulation and pitch. Regardless of central stream exhausted the processing resources, with little or no whether relevancy was determined by location or gender, a capacity left to perceive the auditory cues, in line with the pre- reduction in distractor interference was evident through smaller dictions of perceptual load theory. However, the two high load congruency effects under high than under low perceptual load. conditions both involved the presence of the auditory stream in Francis thus concluded that perceptual load seems to play a central addition to the cueing sounds, whereas the low load condition role in speech perception, determining the extent to which presented the cueing sounds on their own. The addition of the response competition occurs due to the inability to ignore stream under high load is likely to have changed perceptual factors

Please cite this article in press as: Murphy, S., et al., Auditory perceptual load: A review, Hearing Research (2017), http://dx.doi.org/10.1016/ j.heares.2017.02.005 S. Murphy et al. / Hearing Research xxx (2017) 1e9 7 other than perceptual load, such as the focus of spatial attention load. Similar findings were also reported in Pillay et al. (2016) using and perceptual grouping, which makes it hard to conclude that the the same task set up, again supporting a similar role for perceptual reduction in peripheral cueing seen under high load was due only load in hearing. to perceptual load differences and not to any of these other One potential criticism with this load manipulation is that the changes. SNR modulation is likely to have degraded the target sound. Lavie However, a number of experiments have instead manipulated et al. (2003) previously demonstrated that increasing task diffi- auditory perceptual load by varying the presentation rate of the culty through sensory degradation of visual targets (e.g. reducing attended stimuli with the idea that perceptual load should be contrast) does not result in the same effects as a perceptual load increased with a smaller inter-stimulus interval (ISI) compared manipulation typically does. The difference in distractor processing with a larger ISI. For instance, Parasuraman (1980) measured the N1 for perceptual versus sensory load was argued to relate to pre- (an early marker of attention) that was elicited by attended as well dictions of data limitations versus resource limitations (Norman as unattended stimuli (separated by ear). In the fast presentation and Bobrow, 1975). More specifically, because performance in a condition, the N1 in response to stimuli in the attended stream was task that suffers from data limitations cannot be improved by larger than the N1 elicited by stimuli in the unattended stream allocating more perceptual resources, manipulations of sensory whereas, in the slow presentation condition, the difference in load (which influence data limitations) should not affect the amplitude between the N1 for the attended and the unattended availability of perceptual processing resources and thus should not streams was smaller. These findings suggest that the unattended affect distractor processing. From this perspective, Sabri et al.’s stream received less processing in the presumably high compared results are not in line with perceptual load theory (which would with the low perceptual demand condition. In line with these predict no effects of SNR manipulations on distractor processing). findings, Woldorff et al. (1991) measured the MMN to an unat- However, the distinction between perceptual load and sensory load tended deviant and found a decreased response when the pre- has recently been challenged (Benoni and Tsal, 2012; Yeshurun and sentation rate for the attended stream was increased. Neelon, Marciano, 2013), and this remains a matter of ongoing debate. Williams, and Garell (2011) also manipulated the ISI and measured responses in event-related potentials (ERPs) to the 4. Summary and conclusions attended versus unattended streams using intracranial recordings. For the shorter ISI, only ERPs elicited by sounds in the attended ear Overall, the literature on auditory perceptual load reveals a were enhanced, whereas for the longer ISI, ERPs in response to both complex pattern of mixed results. Whereas some findings the attended and the unattended stream showed enhancement. demonstrate effects in line with the theory of perceptual load (e.g. Taken together, these findings from experiments that have Francis, 2010; Pillay et al., 2016; Sabri et al., 2013) others question manipulated the ISI suggest that with a faster rate of presentation whether the same principles do, in fact, hold in audition (e.g. of the attended stimuli, unattended stimuli receive less processing. Gomes et al., 2008; Murphy et al., 2013). We will attempt to This chimes well with the predictions of perceptual load theory and reconcile these conflicting findings. thus the findings do provide some support for the applicability of As previously reviewed, perceptual load in the visual domain the theory to the auditory modality. However, not all studies has typically been defined in one or more of three different ways: manipulating ISI have reported similar findings. For instance, either by varying the number of items in the relevant display, Gomes et al. (2008) presented participants with two auditory changing the perceptual similarity between targets and non- channels. Participants attended to one based on the frequency targets, or manipulating the complexity of perceptual operations whilst ignoring the other. Responses were made to tones of lower of the same stimuli. The majority of results demonstrating effects of intensity than standard, appearing in the attended channel. The Nd perceptual load within audition have manipulated task demands by component (the difference between ERP waveforms of stimuli varying the complexity of perceptual operations needed to perform when they are attended versus unattended) was used to measure the task. For example, Francis (2010) used a feature versus irrelevant distractor processing, and although it was predicted that conjunction task to modulate perceptual load and found reduced the Nd would be larger in amplitude in the fast ISI condition there congruency effects to an irrelevant distractor under high as was no difference in the Nd as a function of ISI. Thus, as yet, no compared with low auditory load. Similarly, Chait et al. (2012) and consensus has been reached regarding the impact of ISI manipu- Masutomi et al. (2015) also kept the stimuli constant across load lations on auditory distractor processing. It is also important to note conditions and manipulated load through task instructions. They that earlier work on auditory scene analysis (e.g. Bregman, 1990) also found reduced irrelevant sound processing under high (vs. has suggested that a smaller temporal separation between auditory low) load. Altogether, these types of manipulations all rely on stimuli can strengthen the processes of perceptual segregation in potentially more higher-level cognitive or memory load rather than the auditory scene. For this reason, manipulations of ISI potentially perceptual load per se. It may thus be that in hearing, load effects also introduce concurrent changes in the strength of perceptual are more likely to be seen under a higher-level ‘cognitive’ load than segregation, which can make it hard to draw any firm conclusions under a purely perceptual load. However, not all experiments based on this type of load manipulation. involving higher-level manipulations have demonstrated auditory Another alternative manipulation of auditory perceptual load perceptual load effects (Murphy et al., 2013). which does not suffer from the potential confound of the ISI vari- More mixed findings are evident among those experiments that ations is to vary the signal-to-noise ratio (SNR) between the target have used manipulations focused at a more strictly perceptual and non-target sounds. Sabri et al. (2013) presented participants level. On the one hand, Murphy et al. (2013) consistently reported with noise to one ear and, on a proportion of trials, a tone was no differences in the congruency effects elicited by an auditory presented concurrently with the noise. The participants’ task distractor as a function of load, using a load manipulation that involved reporting the presence or absence of the tone, and the SNR varied the similarity between non-targets and targets. On the other between the target and the noise was varied between load condi- hand, the experiments manipulating SNR (Pillay et al., 2016; Sabri tions, thus making the target harder or easier to detect from the et al., 2013) reported clear reductions in neural activity related to noise. A spoken syllable was presented to the unattended ear on distractor processing under high versus low load, thus providing some trials. Both fMRI and EEG activity in response to the irrelevant promising results in favor of perceptual load theory. Similarly, the syllables demonstrated a reduction under high (vs.) low perceptual majority of the experiments manipulating ISI demonstrated

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Gorillas we have missed: sustained inattentional deafness for dynamic events. Cognition 124, 367e372. Dalton, P., Hughes, R.W., 2014. Auditory attentional capture: implicit and explicit This work was supported by an Economic and Social Research approaches. Psychol. Res. 78, 313e320. Council research grant (ES/L007983/1) to Polly Dalton. This article Dalton, P., Lavie, N., 2004. Auditory attentional capture: effects of singleton dis- has been published under the terms of the Creative Commons tractor sounds. J. Exp. Psychol. Hum. Percept. Perform. 30, 180e193. Dalton, P., Lavie, N., 2007. Overriding auditory attentional capture. Percept. Psy- Attribution License (http://creativecommons.org/licenses/by/3.0/), chophys. 69, 162e171. which permits unrestricted use, distribution, and reproduction in Dalton, P., Santangelo, V., Spence, C., 2009. The role of working memory in auditory any medium, provided the original author and source are credited. selective attention. Q. J. Exp. 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Please cite this article in press as: Murphy, S., et al., Auditory perceptual load: A review, Hearing Research (2017), http://dx.doi.org/10.1016/ j.heares.2017.02.005