Month: July 2021

Current Directions in Psychological Science 2014, Vol. 23(6) 460 –465 © The Author(s) 2014 Reprints and permissions: sagepub.com/journalsPermissions.nav DOI: 10.1177/0963721414550705 cdps.sagepub.com

People are remarkably varied in their ability to perform under stressful or challenging circumstances and in their tendencies to engage in health-related behaviors. To understand and encourage behavior change, the sources of these individual differences need to be uncovered. This review focuses on the personality dimensions outlined in the Five Factor theory of personality (McCrae & Costa, 2008). Investigations have explored associations between personality traits and performance-related outcomes (e.g., academic and occupational success) and between per- sonality traits and health-related outcomes (e.g., diet and stress). Amid the academic articles available, a consider- able number have centered on personality-trait associa- tions in the context of sport and physical activity. The development of this field originated when researchers started to notice particular traits among athletic adoles- cents (e.g., Fleming, 1934), and their articles tended to emphasize the contribution of athletic behavior to person- ality change. Over time, the focus shifted to consider per- sonality traits as precursors to two core outcomes: sport performance (athletic success) and physical activity (exer- cise participation). We review each of these areas in turn,

focusing on recent advances before considering new research questions and avenues of further study.

Personality in Sport Performance

Personality traits predict a number of performance markers in competitive contexts such as work and academia (see, e.g., Poropat, 2009). Organized sport represents another competitive context in which some personality traits have been found to coincide with greater levels of success. For example, athletes competing in national or international competitions report higher levels of conscientiousness and lower levels of neuroticism than do those competing in club or regional competitions (Allen, Greenlees, & Jones, 2011), and athletes selected for the Paralympic Games report higher levels of tough-mindedness (a component of agreeableness) and lower levels of anxiety (a component

550705CDPXXX10.1177/0963721414550705Allen, LabordeThe Role of Personality in Sport and Physical Activity research-article2014

Corresponding Author: Mark S. Allen, School of Psychology, University of Wollongong, Northfields Avenue, Wollongong NSW 2522, Australia E-mail: mark_allen@uow.edu.au

The Role of Personality in Sport and Physical Activity

Mark S. Allen1 and Sylvain Laborde2,3 1School of Psychology, University of Wollongong; 2EA 4260, University of Caen; and 3Department of Performance Psychology, German Sport University Cologne

Abstract There is now good evidence that athletic success and participation in physical activity can be predicted by personality traits. In this article, we review new studies that have contributed to our understanding of these relationships and outline potential avenues of inquiry to support the development of personality-trait research in exercise and sport. Our review identified a number of novel findings from contemporary studies. In the context of sport performance, new studies have demonstrated that personality traits relate to long-term athletic success, interpersonal relationships, and athletes’ psychological states before, during, and after competitions. In the context of health-related exercise, new studies have demonstrated that personality traits relate to leisure-time sitting time, strength and mobility in old age, and unhealthy (addictive) exercise behaviors. There is also evidence that physical activity contributes to personality change. Our recommendations include a more targeted focus on adolescence (as this is the age of greatest change in personality and sport participation) and a greater consideration of consultant personality traits in applied research and professional practice (given their role in intervention effectiveness).

Keywords exercise participation, Five Factor model, physical fitness, developmental changes, applied psychology

The Role of Personality in Sport and Physical Activity 461

of neuroticism) than do those not selected for the Paralympic Games (Martin, Malone, & Hilyer, 2011). Longitudinal studies have also found that adolescent ath- letes with particular personality traits are more likely to progress to professional sport (Aidman, 2007) and that adult athletes with high levels of conscientiousness or low levels of neuroticism have more successful performance statistics over the course of a competitive season (Piedmont, Hill, & Blanco, 1999). In particular, Aidman (2007) assessed the personality traits of 32 elite youth athletes and moni- tored their progress over 7 years. The study found that per- sonality traits, in combination with coaches’ ratings of players’ potential, could distinguish those players who had progressed to become professional athletes (n = 13) from those who had withdrawn from participation (n = 19) with 100% accuracy (84% of athletes were correctly classified on the basis of personality traits alone).

To better understand the processes connecting person- ality traits to athletic success, researchers have recently begun to explore associations between personality traits and discrete athletic behaviors. It has been found that sport participants with high levels of conscientiousness use better preparation strategies (Woodman, Zourbanos, Hardy, Beattie, & McQuillan, 2010), take fewer reckless risks (Merritt & Tharp, 2013), and use more effective cop- ing strategies (Kaiseler, Polman, & Nicholls, 2012) before and during athletic competitions. It has also been found that sport participants with high levels of extraversion or low levels of neuroticism respond to unsuccessful out- comes with more positive cognitive and emotional symp- toms (Allen, Greenlees, & Jones, 2014) and that sport participants with high levels of agreeableness or conscien- tiousness report more favorable relationships with their teammates ( Jackson, Dimmock, Gucciardi, & Grove, 2010) and coaches ( Jackson, Dimmock, Gucciardi, & Grove, 2011). Little is known about how personality traits contrib- ute to aggressive or unethical behavior or how they might relate to important choices or clinical symptoms between competitions (e.g., eating disorders, doping). Nevertheless, the studies available might go some way toward explain- ing why conscientious athletes and athletes with low neu- roticism tend to be more successful.

The relationship between personality and athletic suc- cess is often thought of as unidirectional—the general assumption being that personality traits cause some indi- viduals to perform better or worse in athletic situations. However, it is also possible that athletic success, and the life changes that accompany this success, contribute to personality change. There is evidence from occupational settings that career success permeates personality change and, in particular, contributes to increases in levels of extraversion (Le, Donnellan, & Conger, 2014). This is because success in occupational roles often accompanies changes in interpersonal activities (e.g., greater leadership

behavior) that might challenge personality stability in adulthood. In athletic settings, success can also generate greater interpersonal activities, but, more importantly, it can lead to considerable income and media attention (e.g., changes in privacy, adoration by fans), and it remains unclear how these factors might contribute to personality change. We can speculate that the additional life stress that coincides with these factors might, under certain cir- cumstances, contribute to undesirable changes (e.g., increases in neuroticism). Longitudinal studies are needed to explore bidirectional associations and potential mod- erator variables (e.g., social support) during important personal or career transitions (e.g., the transition from amateur to professional athlete).

Personality in Health-Related Exercise

Physical inactivity has been identified as the fourth lead- ing risk factor of global mortality, causing an estimated 5.3 million deaths each year (Lee et al., 2012). It is there- fore unsurprising that psychologists are targeting the identification of factors associated with participation in regular exercise. There is now good evidence that per- sonality traits relate to physical-activity levels. For exam- ple, new studies of adult sitting behavior have shown that low levels of conscientiousness, extraversion, and open- ness and high levels of neuroticism predict a greater occurrence of leisure-time sitting time (Ebstrup, Aadahl, Eplov, Pisinger, & Jørgensen, 2013). Comprehensive meta-analyses have also demonstrated that high levels of extraversion and conscientiousness and low levels of neuroticism relate to high levels of physical activity (Rhodes & Smith, 2006; Stephan, Boiché, Canada, & Terracciano, 2014). In particular, industriousness (a com- ponent of conscientiousness) and activity (a component of extraversion) are often identified as important physi- cal-activity correlates (Rhodes & Pfaeffli, 2012).

The majority of research on personality and physical activity has targeted young adults. More recently, there has been a shift toward understanding physical-activity levels across the life span. Studies have demonstrated that older adults with high levels of extraversion have greater muscular strength (Tolea, Terracciano, et al., 2012) and that individuals with high levels of extraver- sion, openness, or conscientiousness or low levels of neuroticism record greater energy expenditure at peak walking pace (Terracciano et al., 2013). Importantly, a 3-year longitudinal study of older adults (mean age = 75) found that high levels of conscientiousness related to a faster initial walking speed and a slower decline in walk- ing speed over the course of the study (Tolea, Costa, et al., 2012), and a 6-year follow-up found that high lev- els of openness related to a reduced risk of acquired walking limitations (Tolea, Ferrucci, et al., 2012). It has

462 Allen, Laborde

also been found that positive attitudes toward aging mediate a positive association between openness and levels of physical activity (Emile, Chalabaev, Stephan, Corrion, & d’Arripe-Longueville, 2014) and that high neu- roticism relates to low levels of physical activity, but only when education levels are particularly low ( Jaconelli, Stephan, Canada, & Chapman, 2013).

The identification of important mediating and moder- ating variables has been a central focus in studies of behavior change. In their systematic review, Rhodes and Pfaeffli (2012) identified 17 studies that had explored mediating relationships—most often components of the theory of planned behavior (Ajzen, 2012)—and found evidence for a partial mediation of attitudes toward exer- cise and perceived behavioral control in the relationship between extraversion and levels of physical activity. Studies have also explored demographic variables that might moderate the relationship between personality traits and exercise behavior and the personality traits that might moderate associations between psychological states and behavior change. Associations between per- sonality dimensions and physical-activity levels appear relatively consistent across age groups, culture, gender, and activity modes (Rhodes & Pfaeffli, 2012), but there is evidence that exercise intentions are more strongly related to exercise behavior when levels of conscien- tiousness are particularly high (Rhodes & Dickau, 2013).

It is customary for association studies to consider per- sonality traits as precursors to physical-activity levels despite the bidirectional associations theorized in earlier articles (e.g., Fleming, 1934). Until recently, the potential for exercise participation to contribute to changes in per- sonality remained unclear. However, a recent longitudi- nal study of personality change in adulthood found that physically active adults declined less on conscientious- ness, extraversion, openness, and agreeableness than did less physically active adults (Stephan, Sutin, & Terracciano, 2014). These initial findings suggest that an active life- style might help to prevent undesirable personality change and support original proposals that personality traits are shaped through exercise participation. The pro- cesses through which this occurs are unknown, but the authors speculated that physical activity might help to maintain desirable personality traits by lowering the inci- dence of cognitive decline or facilitating better coping responses to adverse life events.

The benefits of regular physical activity are well estab- lished. However, excessive exercise can have adverse effects, including depression, body-image concern, and disordered eating behavior (Berczik et al., 2012). New lines of research have begun to uncover the personality traits that relate to unhealthy (obsessive and compulsive) exercise behavior. Studies have found that high levels of neuroticism, extraversion, and conscientiousness and low levels of agreeableness relate to exercise addiction

(Andreassen et al., 2013; Lichtenstein, Christiansen, Elklit, Bilenberg, & Støving, 2014), with excitement seeking (a component of extraversion), achievement striving (a component of conscientiousness), and straightforward- ness and compliance (components of agreeableness) fea- turing the strongest associations (Lichtenstein et al., 2014). These findings indicate that people who are sus- ceptible to exercise addiction might be identifiable through personality-trait profiling, which has implica- tions for the early identification and treatment of this clinical disorder.

Moving Forward

The studies reviewed so far have provided some interest- ing perspectives on personality-trait associations in sport and physical activity. The greater focus on older adults has been an important development in the field—as have the greater use of longitudinal designs and variations in out- come measures (e.g., leisure-time sitting time, energy expenditure). In terms of research progression, the field might benefit from a more targeted focus on adolescent populations. In the context of sport performance, research- ers might explore behavior-change interventions that tar- get continued participation in sport among adolescents who have great athletic potential but personality traits associated with a high risk of drop-out. In the context of health-related exercise, researchers might explore changes in personality traits throughout adolescence and their rela- tionship to changes in exercise motives. Longitudinal designs that target bidirectional associations might help to explain the shift from environmental to genetic influences on sport participation (Aaltonen, Ortega-Alonso, Kujala, & Kaprio, 2013) and the decline in exercise participation throughout adolescence (Kimm et al., 2002).

Adolescence is a critical developmental stage during which we observe not only the greatest changes in levels of physical activity (Hallal et al., 2012) but also the great- est changes in personality (McAdams & Olson, 2010). There is reason to consider that these associations might not be mutually exclusive. Studies have found that sport participation contributes to the development of impor- tant life skills (Gould & Carson, 2008) and long-term health-related quality of life (Vella, Cliff, Magee, & Okely, 2014). Through sport participation, adolescents gain con- fidence, learn new physical capabilities, develop impor- tant social relationships, develop leadership skills, and gain satisfaction by working toward goals (Gould & Carson, 2008). It is reasonable, therefore, to consider that sport participation might contribute to personality change during adolescence. A critical question is whether differ- ent modes of physical activity have similar effects on per- sonality change. For instance, team sports combine physical activity with high levels of social interaction, whereas individual sports often combine high levels of

The Role of Personality in Sport and Physical Activity 463

physical activity with low levels of social interaction. It would be interesting to explore the potential differences between activity modes in their effects on personality sta- bility and change during adolescence.

The role of personality in sport teams and exercise groups has received little empirical attention. In other (non- athletic) settings, it has been found that greater variation among group members in some personality traits (e.g., extraversion) and greater similarity in others (e.g., agree- ableness) relate to more successful group performances (e.g., Kramer, Bhave, & Johnson, 2014). Similar associations might be observed in team sports, and this could have implications for applied practice in terms of selecting or deselecting players to fit the needs of a particular team or competition. In physical-activity settings, people often choose to take part in structured (group) exercise classes, and individual behavior in these contexts (e.g., adherence levels) might also depend on the personality traits of other class members. For example, adherence levels might be higher when the group as a collective (or the group leader) scores highly on desirable personality traits (e.g., extraver- sion) or when the target person has personality traits simi- lar (or dissimilar) to those of other group members.

Personality similarity is one potential contributing fac- tor to adherence levels in exercise programs, and this extends beyond other members of the exercise class to the personality traits of the consultant or service provider administering the physical-activity intervention. Consultants are often responsible for recruiting participants into behavior-change interventions and monitoring their prog- ress. It is reasonable to expect that clients’ engagement and commitment to behavior-change recommendations might be contingent on their interactions with their ser- vice provider (e.g., relatedness) and that these interac- tions are conditioned by personality traits (Cuperman & Ickes, 2009). Psychologist–client personality similarity is considered an important factor in turning intentions into behavior (Bare, 1967), and researchers would do well to consider the personality traits of consultants when design- ing and implementing physical-activity interventions and evaluating their success. As far as we are aware, the per- sonality traits of service providers have not featured in studies of behavior change that have targeted leisure-time physical activity. To critically evaluate intervention suc- cess, it is important to consider all potential moderator variables, and these include the personality traits of the people delivering the interventions.

Conclusions

The available evidence shows that sport performance and physical-activity levels can be predicted by personality traits. Conscientiousness and neuroticism appear to be most important for athletic success, and these associations

seem to mirror those observed in other performance contexts (e.g., academic and occupational success). Personality-trait dimensions also relate to interpersonal relationships in athletic dyads and can predict athletes’ psychological states before, during, and after competi- tions. Little is known about the contribution of athletic success to personality change or how personality-trait similarities among team members relate to group success. In the context of physical activity, extraversion and con- scientiousness (and, to a lesser extent, neuroticism) pre- dict physical-activity levels in nonclinical samples. These dimensions also relate to strength and mobility in older adults and, along with agreeableness, can predict unhealthy (addictive) exercise behavior. Importantly, new research has suggested that leisure-time physical activity can contribute to personality change during adulthood. We recommend additional longitudinal studies that explore bidirectional associations between personality traits and various modes of physical activity. This informa- tion would be particularly valuable to researchers pursu- ing the various environmental factors that contribute to personality-trait development, and might also be of value to health professionals targeting the promotion of physi- cal activity and healthy living in developed nations.

Recommended Reading

Allen, M. S., Greenlees, I., & Jones, M. V. (2013). Personality in sport: A comprehensive review. International Review of Sport and Exercise Psychology, 6, 184–208. A detailed narrative review of personality in sport performance that includes population-based differences and issues in applied practice.

Jackson, B., Dimmock, J. A., Gucciardi, D. F., & Grove, J. R. (2011). (See References). An investigation of personality- trait contributions to relationship commitment and relat- edness in coach-athlete dyads that explores actor effects, partner effects, and actor–partner similarities.

Rhodes, R. E., & Pfaeffli, L. A. (2012). (See References). A sys- tematic review of the relationship between personality traits and participation in physical activity that explores facet-level relationships and moderator variables including age, culture, and physical-activity mode.

Stephan, Y., Sutin, A. R., & Terracciano, A. (2014). (See References). A longitudinal study of the relationship between physical-activity levels and personality stability and change during adulthood and old age.

Tolea, M. I., Costa, P. T., Terracciano, A., Ferrucci, L., Faulkner, K., Coday, M. M. C., . . . Simonsick, E. M. (2012). (See References). A longitudinal investigation of personality and walking-speed decline in older adults using data from the Health, Aging and Body Composition study.

Declaration of Conflicting Interests

The authors declared that they had no conflicts of interest with respect to their authorship or the publication of this article.

464 Allen, Laborde

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Current Directions in Psychological Science 2015, Vol. 24(5) 374 –378 © The Author(s) 2015 Reprints and permissions: sagepub.com/journalsPermissions.nav DOI: 10.1177/0963721415593725 cdps.sagepub.com

The eyes’ pupils constrict (shrink) in brightness and dilate (expand) in darkness. This is the pupillary light response. The light response has been studied for more than a millennium (Loewenfeld, 1958) but was histori- cally considered a low-level reflex without any cognitive component. However, recent studies have shown that the light response is far more than a reflex, and reveals what you attend to, how you interpret what you see, and even what you think about. Here, we review these recent advances in our understanding of the pupillary light response. In addition, we discuss how changes in pupil size help to find the optimal balance between visual acu- ity (how sharp you can see) and sensitivity (how well you can detect faint stimuli) and are therefore a crucial aspect of how you perceive the world.

The Light Response Reflects Awareness, Interpretation, and Mental Imagery

Cognitive effects on the light response were first shown using binocular rivalry (e.g., Harms, 1937; Naber, Frassle, & Einhauser, 2011). In binocular rivalry, different stimuli are presented to each eye. When the stimuli are too different to be fused into a single percept, visual awareness flips back

and forth between the left and right eye. (You can experi- ence binocular rivalry by looking at your own nose. Although each eye sees your nose from a different angle, you consciously perceive your nose from only one angle.) With respect to the light response, something remarkable happens when images of different brightness are presented to each eye: The pupil constricts when the bright stimulus, relative to the dark stimulus, dominates awareness (see Fig. 1). The light response therefore reflects which stimulus you consciously perceive at a given moment.

Similarly, recent studies have shown that the pupil responds to the perceived brightness of pictures, which is not always the same as their actual brightness (Binda, Pereverzeva, & Murray, 2013b; Laeng & Endestad, 2012; Naber & Nakayama, 2013). For example, a picture of a sun is generally perceived as brighter, and elicits a stron- ger pupillary constriction, than a picture of an indoor scene—even when both pictures are really equally bright. Strikingly, your pupil even constricts when you imagine a bright stimulus, without any visual stimulation (Laeng &

593725CDPXXX10.1177/0963721415593725Mathôt, Van der StigchelPupil Size and Active Vision research-article2015

Corresponding Author: Sebastiaan Mathôt, Aix-Marseille University, CNRS, Laboratoire de Psychologie Cognitive, UMR 7290, 3 Place Victor Hugo, Centre St. Charles, Bâtiment 9, Case D, 13331 Marseille, France E-mail: s.mathot@cogsci.nl

New Light on the Mind’s Eye: The Pupillary Light Response as Active Vision

Sebastiaan Mathôt1 and Stefan Van der Stigchel2 1Laboratoire de Psychologie Cognitive, CNRS, Aix-Marseille University, Marseille, France, and 2Department of Experimental Psychology, Helmholtz Institute, Utrecht University

Abstract The eye’s pupils constrict (shrink) in brightness and dilate (expand) in darkness. The pupillary light response was historically considered a low-level reflex without any cognitive component. Here, we review recent studies that have dramatically changed this view: The light response depends not only on a stimulus’s brightness but also on whether you are aware of the stimulus, whether you are paying attention to it, and even whether you are thinking about it. We highlight the link between the pupillary light response and eye-movement preparation: When you intend to look at a bright stimulus, a pupillary constriction is prepared along with the eye movement before the eyes set in motion. This preparation allows the pupil to rapidly change its size as your eyes move from bright to dark objects and back again. We discuss the implications of these recent advances for our understanding of the subtle yet important role that pupillary responses play in vision.

Keywords pupil size, eye movements, visual perception, arousal

Pupil Size and Active Vision 375

Sulutvedt, 2014). Together, these studies show that the light response is not driven solely by the amount of light that enters the eye but is related to high-level vision and even mental imagery.

The Light Response Reflects Visual Attention

You always see multiple objects. While you work on your computer, you look at your screen but, from the corner of your eye, might also see your keyboard and cup of coffee. You do not fully process everything you see, but selectively attend to only a few objects at a time. If you attend to an object, you respond to it more quickly, and perceive it more clearly (Carrasco, 2011). A crucial ques- tion is whether attention affects vision even at the earliest possible stage: as light enters the eye through the pupil.

The effect of attention on the pupillary light response was recently tested in several studies (Binda, Pereverzeva, & Murray, 2013a; Mathôt, van der Linden, Grainger, & Vitu, 2013; Naber, Alvarez, & Nakayama, 2013). In one of

our experiments, participants looked at the center of a display that was divided into a bright and a dark half (Fig. 2a; Mathôt et  al., 2013). Participants identified a target stimulus that could appear on either side of the screen, on the bright or the dark background. We used a staircase procedure to ensure that there was no differ- ence in how easily the target could be identified on a dark or a bright background. Just before the target stim- ulus appeared, a cue (a voice saying “left” or “right”) indicated the probable location of the target. Participants used this information to anticipate the location of the target and shifted their attention to the cued side of the screen while keeping their eyes on the display center (i.e., covert attention).

Figure 2d shows the results of this experiment. First, overall pupil size increased over time, regardless of the brightness of the attended side. This was related to the effort that participants invested in the task, which affects pupil size in a way that is more or less independent of the light response (reviewed in Beatty, 1982; Goldwater, 1972; Laeng, Sirois, & Gredebäck, 2012). More impor- tantly, when participants attended to the bright side of the screen (orange line), their pupils constricted relative to when they attended to the dark side (blue line). This difference arose about 0.6 seconds after the cue was presented.

This experiment showed that your pupils adjust to an object if you attend to it, even if you do not look at it directly. This is important, because elements in a visual scene can differ strongly in brightness: Your keyboard might be dark, whereas your monitor might be bright. While looking at your monitor, you may covertly (i.e., without moving your eyes) attend to your keyboard to localize your fingers. As we will describe later in this review, perception benefits from an optimal pupil size, so if you attended to the keyboard with a pupil size that was tuned to the brightness of the monitor, this would result in suboptimal perception. Therefore, even though the benefit is presumably small, a link between the pupillary light response and visual attention is beneficial.

In the experiment described above, participants shifted their attention voluntarily. But attention can also be drawn involuntary toward a location. Such reflexive shifts of attention are typically studied by presenting a salient cue, such as a sudden movement, in your visual periph- ery (Fig. 2b). Even when this cue is irrelevant for the task, it still captures attention.

We recently showed that the pupillary light response is also affected by reflexive shifts of attention in the absence of eye movements (Mathôt, Dalmaijer, Grainger, & Van der Stigchel, 2014). As shown in Figure 2e, partici- pants’ pupils first constricted when the cue was pre- sented on a bright background (orange line), relative to a dark (blue line) background, but relatively dilated for longer intervals after cue presentation (i.e., the pupil- size difference, indicated in green, switched from

From Dark to Bright

From Bright to Dark

Time Since Reported Switch (s)

Pu pi

l S iz

e (z

s co

re )

2–2 0

0.0

0.2

–0.2

0.0

0.2

–0.2

Pupil-Size Difference

Fig. 1. The effect of visual awareness on the pupillary light response in a binocular rivalry experiment (Naber, Frassle, & Einhauser, 2011). When awareness switches from a dark stimulus (presented to one eye) to a bright stimulus (presented to the other eye), the pupil constricts (orange line). Conversely, when awareness switches from a bright to a dark stimulus, the pupil dilates (blue line). On the x axis, 0 seconds corresponds to the moment that the participant indicates that his or her awareness has switched. Error bands indicate standard errors.

376 Mathôt, Van der Stigchel

positive to negative). The early constriction reflects a rapid reflexive shift of attention to the cued location, whereas the later dilation reflects a phenomenon called inhibition of return. In a behavioral response-time task, inhibition of return refers to the finding that responses are slower when a target is presented at a cued location, relative to an uncued location, for long intervals between the cue and the target. This is likely a “been there, done that” mechanism that prevents attention from being drawn to the same location over and over again. Interestingly, participants who showed strong pupillary inhibition (i.e., the negative pupil-size difference in Fig.  2e) also showed strong inhibition of return (i.e., slowed responses to targets that appeared on the cued side of the screen).

Attention may be drawn not only to locations but also to features such as color and shape. For instance, while approaching a bookshelf looking for a specific red book,

you can attend to the red books only. Binda, Pereverzeva, and Murray (2014) showed that pupil size is also an index of feature-based attention. In their experiment, two sets of dots (one bright, one dark) were presented at the same location (Fig. 2c). Participants could therefore select the cued set of dots only on the basis of its brightness. Crucially, the pupil constricted when the bright dots, rela- tive to the dark dots, were attended. This shows that the pupil is not only an index of spatial attention but a proxy for various forms of selective attention: Whatever visual information is important (be it feature or location) will be echoed by the pupil.

The Light Response Reflects Eye- Movement Preparation

In the experiments described above, participants did not move their eyes. This is artificial, because in daily life you

Voluntary-Attention Results

Time Since Cue Onset (s)

Attend to DarkAttend to Bright

Pu pi

l S iz

e (n

or m

al iz

ed r

el at

iv e

to p

re -c

ue b

as el

in e)

Pupil-Size Difference

0.5

1.08

1.04

1.00

1 1.5 10 0 0.5 1.5 2

0.0 0.2

–0.2

Reflexive-Attention Results d e

Reflexive-Attention Experiment

Feature-Based Attention Experiment

“Attend White Dots”

“Attend Black Dots”

Voluntary-Attention Experiment

“Attend to Right”

“Attend to Left”

a b c

Fig. 2. The effect of covert visual attention on the pupillary light response. Panel (a) shows an example of a voluntary- attention experiment in which participants direct their attention to the left or right side of the screen based on an auditory cue (cf. Mathôt, van der Linden, Grainger, & Vitu, 2013). Panel (b) shows an example of a reflexive-attention experiment in which attention is drawn to the left or right by a sudden movement (cf. Mathôt, Dalmaijer, Grainger, & Van der Stigchel, 2014). Panel (c) shows an example of a feature-based-attention experiment in which participants attend to one of two intermingled sets of dots (cf. Binda et al., 2014b). The pupil is larger when attention is voluntarily directed at a dark (blue), relative to a bright (orange), surface (d; Mathôt et al., 2013). After a reflexive shift of attention, the pattern is initially similar (i.e., a larger pupil when attending to a dark surface) but inverses after about 1 second, corresponding to inhibition of return (e; Mathôt et al., 2014). Error bands correspond to 95% confidence intervals.

Pupil Size and Active Vision 377

usually look directly at what you attend to. Therefore, numerous researchers have proposed that attention and eye movements are linked (Rizzolatti, Riggio, Dascola, & Umiltá, 1987): Whenever you shift your attention, you also prepare an eye movement to the attended location. To come back to our daily-life example: When you look at your monitor but want to localize your fingers, you quickly make an eye movement to the keyboard. Before this eye movement is executed, attention has already shifted to the end point of the eye movement. However, the to-be-fixated object (the keyboard) might have a dif- ferent brightness than the currently fixated object (the screen). An important question is whether the pupil pre- pares for this change in brightness. We recently showed that this is indeed the case: When you prepare an eye movement toward a bright object, a pupillary constriction is prepared along with the eye movement itself, before the bright object has been fixated (Mathôt, van der Linden, Grainger, & Vitu, 2015). This is useful, because it reduces the effective latency of the light response, which is long (approximately 0.25 s). This way, preparation allows the pupil to track the rapid changes in visual input that occur as your eyes shift from dark to bright objects and back again.

A Balance Between Visual Acuity and Sensitivity

Although the light response is the primary determinant of pupil size, the pupil also dilates in response to arousal in a way that is independent of the light response. Here, we use “arousal” in its broadest sense, sometimes described as “the intensity dimension of thought” ( Just & Carpenter, 1993). In general, anything that increases arousal also elicits a pupillary dilation: sexy pictures, mental arithme- tic, keeping something in working memory, effortful lis- tening, etcetera (reviewed in Beatty, 1982; Goldwater, 1972; Laeng et al., 2012). Irene Loewenfeld, one of pupil- lometry’s pioneers, aptly summarized that “man may either blush or turn pale . . . but his pupils always dilate” (1958, p. 237).

But why does the pupil respond to arousal and light in this way? The same pupillary responses are found across many vertebrate species and have even evolved indepen- dently in squids and octopuses (Douglas, Williamson, & Wagner, 2005), strongly suggesting that they serve an important function. Although there is no definite answer, there are several credible hypotheses that each explain one aspect of the pupillary response. Below, we synthe- size these hypotheses to provide a comprehensive under- standing of the important role that pupillary responses play in vision.

One function of the light response is to find a balance between visual acuity (how sharp you can see) and sen- sitivity (how well you can detect faint stimuli). The eye’s

lens is imperfect and distorts light in ways that reduce acuity. The severity of these distortions depends on pupil size: the smaller the pupil, the sharper the image (Campbell & Gregory, 1960). Another benefit of a small pupil is that it sees sharply across a wide range of dis- tances (i.e., increased depth of field). However, a small pupil also has disadvantages: It does not capture much light, which leads to reduced sensitivity, and provides a slightly reduced field of view. Crucially, the optimal size of the pupil depends on how much light is available. In darkness, vision is limited by the scarcity of light, and the pupil therefore dilates to capture more light. In bright- ness, light is abundant, and the pupil therefore constricts to obtain the sharpest image. Plausibly, cognitive effects on the pupillary light response may serve to optimize pupil size specifically for objects that you attend to, or prepare an eye movement toward.

But why does arousal trigger a pupillary dilation, apparently perturbing the balance between visual acuity and sensitivity? This may be related to Aston-Jones and Cohen’s (2005) proposal that there are two modes of behavior, exploitation and exploration, that are linked to pupil size. During exploitation, arousal is low (compared to exploration), and you are focused on one task, such as reading a book, that requires fine visual discrimination. In this mode, visual acuity is more important than sensi- tivity, and the pupil therefore constricts. During explora- tion, arousal is high, and you are in a vigilant state, ready to detect mates, predators, and other things that require immediate action. In this mode, visual sensitivity is more important than acuity, and the pupil therefore dilates. Pupil dilation in the absence of light changes may thus reflect a shift from exploitation to exploration mode, and a concomitant shift in the optimal balance between visual acuity and sensitivity.

Taken Together . . .

. . . it is clear that the pupillary light response is far more than the low-level reflex that it was historically thought to be. The extent to which a bright stimulus triggers a pupil- lary constriction depends on many cognitive factors: visual awareness (are you consciously aware of the stim- ulus?), interpretation (how bright does the stimulus sub- jectively appear?), and visual attention (are you paying attention to the stimulus?). We have emphasized the link between pupillary responses and eye-movement prepa- ration: When you prepare to look at a bright stimulus, a pupillary constriction is prepared along with the eye movement itself. Preparation allows the pupil to rapidly adjust its size, as your eyes shift from dark to bright objects and back again.

We have highlighted the important role that pupillary responses play in vision. The pupillary response to light balances visual sensitivity, which is highest for large

378 Mathôt, Van der Stigchel

pupils, and acuity, which is highest for small pupils (Campbell & Gregory, 1960; see Woodhouse & Campbell, 1975, for other functions). The function of pupillary dila- tion in response to arousal is less clear but may be under- stood in the same way: Arousing situations are generally those that require enhanced visual sensitivity, and the pupil therefore dilates when aroused. In summary, the pupillary light response reflects mental state in exquisite detail. It is truly a mind’s eye.

Recommended Reading

Binda, P., Pereverzeva, M., & Murray, S. O. (2013a). (See References). Shows that the pupillary light response is affected by covert visual attention.

Laeng, B., Sirois, S., & Gredebäck, G. (2012). (See References). A review of recent advances in pupillometry that focuses mostly on pupillary dilation and arousal, and is therefore a useful complement to our review, which focuses mostly on the light response.

Loewenfeld, I. E. (1958). (See References). A classic but remark- ably current review of pupillary responses.

Mathôt, S., van der Linden, L., Grainger, J., & Vitu, F. (2015). (See References). Shows that the pupillary light response is linked to eye-movement preparation.

Naber, M., Alvarez, G. A., & Nakayama, K. (2011). (See References). A study using binocular rivalry to show that the pupillary light response is linked to visual awareness (see also Fig. 1 in this review).

Declaration of Conflicting Interests

The authors declared that they had no conflicts of interest with respect to their authorship or the publication of this article.

Funding

The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under Research Executive Agency Grant Agreement n° 622738 awarded to S. Mathôt. In addition, this research was funded by Vidi Grant 452-13-008 from the Netherlands Organization for Scientific Research to S. Van der Stigchel.

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