Hearing Research xxx (2017) 1e9 Contents lists available at ScienceDirect Hearing Research journal homepage: www.elsevier.com/locate/heares 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