The Nature of Impulsivity: Visual Exposure to Natural Environments Decreases Impulsive Decision-Making in a Delay Discounting Task Meredith S. Berry*¤a, Mary M. Sweeney, Justice Morath¤b, Amy L. Odum, Kerry E. Jordan Department of Psychology, Utah State University, Logan, Utah, United States of America Abstract The benefits of visual exposure to natural environments for human well-being in areas of stress reduction, mood improvement, and attention restoration are well documented, but the effects of natural environments on impulsive decision-making remain unknown. Impulsive decision-making in delay discounting offers generality, predictive validity, and insight into decision-making related to unhealthy behaviors. The present experiment evaluated differences in such decision- making in humans experiencing visual exposure to one of the following conditions: natural (e.g., mountains), built (e.g., buildings), or control (e.g., triangles) using a delay discounting task that required participants to choose between immediate and delayed hypothetical monetary outcomes. Participants viewed the images before and during the delay discounting task. Participants were less impulsive in the condition providing visual exposure to natural scenes compared to built and geometric scenes. Results suggest that exposure to natural environments results in decreased impulsive decision-making relative to built environments. Citation: Berry MS, Sweeney MM, Morath J, Odum AL, Jordan KE (2014) The Nature of Impulsivity: Visual Exposure to Natural Environments Decreases Impulsive Decision-Making in a Delay Discounting Task. PLoS ONE 9(5): e97915. doi:10.1371/journal.pone.0097915 Editor: Malte Friese, Saarland University, Germany Received October 29, 2013; Accepted April 25, 2014; Published May 19, 2014 Copyright: ? 2014 Berry et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The Cambridge Center for Behavioral Studies supported this research. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: meredith.berry@umontana.edu ¤a Current address: Department of Society and Conservation, University of Montana, Missoula, Montana, United States of America ¤b Current address: Department of Psychology, Salt Lake Community College, Salt Lake City, Utah, United States of America Introduction The natural world has long been the subject of human enjoyment and fascination [1,2], and is often depicted in philosophical writings as healing and rejuvenating [3]. Adults also prefer viewing scenes of the natural world, such as mountains or forests, over human-made environments [4]. Beyond mere preference, exposure to natural environments decreases stress [5,6], increases happiness [7], improves mood [8,9], and restores attention [10]. These benefits have led some researchers to characterize exposure to nature as ‘the therapy with no side effects’ [11]. Natural environments rich in biodiversity are also vital to our physical health for medicines, medical research, combating infectious diseases, and food production [12]. Despite the known health and cognitive benefits of interacting with or viewing scenes of nature, it remains unknown whether natural environments may also promote healthy human decision- making. Developing techniques that decrease impulsive, maladap- tive human decision-making could promote human and ecological health, as many grave societal and environmental (e.g., climate change) issues can be partially attributed to impulsive human decisions [12,13]. Impulsivity is a multi-faceted construct that encompasses a number of meanings and can be measured in different ways [14]. The complexity of ‘impulsivity’ is highlighted by the various uses of the term including the failure to wait, inability to withhold a response, and lack of sensitivity to negative or delayed consequences, all of which likely represent different underlying processes [15]. For example, within self-reports, impulsivity manifests across one or more behaviors and personality traits including sensation seeking, distractibility/urgency and behavioral (dys)control [16]. Each of these personality traits can contribute uniquely to ‘impulsivity’ [16]. The focus of the present experiment was to better understand how impulsive decision-making within delay discounting (described below) may be affected by exposure to natural scenes. For this reason, we use delay discounting as a framework to develop the concept of impulsivity. Impulsivity in many contexts refers to the inability to delay gratification [17,18], and is associated with the choice of a smaller immediate reward over a larger delayed reward (e.g., the choice to continue eating high fat foods now over a healthier body in the future). One way in which the choice of a smaller immediate reward versus a larger delayed reward can be measured is by delay discounting. Delay discounting refers to the decline of the value of a reward (e.g., money) with the increased time to its receipt [18]. Odum [13,19] proposed that degree of delay discounting may be a personality trait that is relatively stable across time and contexts. For example, test-retest reliability is good for degree of delay discounting across time and alternate and same versions of the discounting task [20–25]. Those who discount money steeply also tend to discount other commodities steeply [13], and degree of delay discounting is similar across real and hypothetical rewards PLOS ONE | www.plosone.org1May 2014 | Volume 9 | Issue 5 | e97915

[26]. In other words, an individual who is considered ‘impulsive’ will likely be impulsive across various situations (i.e., impulsivity is not limited to one context or situation). This does not mean, however, that degree of delay discounting is unalterable. Just as other personality characteristics change with time [27], so does degree of delay discounting as part of normal development processes (i.e., older adults are less impulsive in delay discounting tasks relative to younger adults) [28]. Beyond the change in degree of delay discounting observed as a result of natural development, several training techniques also reduce degree of delay discount- ing. Some techniques are explicitly designed to decrease impulsive decision-making through a fading procedure, which gradually reduces the delay to the smaller sooner reward, when delays to the smaller sooner and larger later reward were initially equal. These fading procedures have been successful in reducing impulsive decision-making across human and nonhuman populations, and when follow-up measures of delay discounting were assessed, reduced impulsive decision-making remained in the experimental group [29–39]. Other techniques have been shown to reduce delay discounting, despite no direct manipulation of the delay discounting task itself [40]. For example, Bickel and colleagues [41] showed working memory training reduced degree of delay discounting. Similarly, financial management training has been associated with decreased impulsive decision-making in a delay discounting task relative to the control group. Importantly, this same financial management training was associated with decreased impulsive decision-making in real world situations unrelated to money [42]. For these reasons, generating general techniques to reduce impulsive decision- making in a delay discounting task may be a useful avenue for future research, and is the focus of the present experiment. Delay discounting is influenced by both genetic [43] and environmental [44] factors. For example, the heritability of degree of delay discounting has been estimated at up to 50% [43], and the context in which the task is administered can influence degree of discounting [44]. Delay discounting also offers predictive validity and generality across domains and populations [13,19]. Mitigating impulsive decision-making in delay discounting, therefore, may have broad implications for promoting healthy human choices. Delay discounting refers to the decrease in subjective value of a delayed reward [18] and is assessed by calculating indifference points: the point at which the value of an immediate outcome is equal to the value of a delayed outcome (e.g., $80 now versus $100 in a year). The more steeply indifference points decline with delay, the more impulsive the decision-making. Delay discounting is well described by a simple hyperbola [18]: V~A=(1zkD) ð1Þ where V is the subjective value of the indifference point, A is the amount of the delayed reward, D is the delay to receipt of the reward, and k is the degree to which the value of the reward decreases with delay. The values of A and D are predetermined based on the values used within the research context. For example, if the delayed reward used is 100 dollars, the numerator would be 100. The value of D would be the delay at which the indifference point is generated. Once indifference points are generated at each delay, across a range of delays (e.g., one day to 25 years), then Equation 1 is fit to the indifference points using nonlinear regression, and resulting k parameter values (degree of discounting) are compared. The degree of discounting then serves as a comparison across groups or individuals, and offers a measure of impulsive decision-making. If exposure to natural environments reduces impulsive decision- making (reflected with smaller k values), this would empirically support the development of future research endeavors that investigate the potential benefits of nature-based interventions for impulsive decision-making. The present experiment was therefore designed to determine whether exposure to natural environments results in decreased impulsive decision-making relative to built environments. Using a titrating hypothetical monetary discounting task [45,46] in which indifference points were determined across a range of delays, we tested the effects of viewing photographs of natural or built environments or geometric shapes on impulsive decision-making. Geometric shapes were chosen as a control condition, allowing for assessment of potential differences in impulsive decision- making as a function of viewing effort [10]. Scenes that require minimal effort to view can restore attention following mentally fatiguing tasks [10]. Kaplan [47] has hypothesized that natural environments require very little effort to view, while human-made environments require more effort to view (and are thus ‘restor- ative’ versus ‘nonrestorative’, respectively). This is because natural environments do not require directed attention, while built environments do [47]. Differences in eye movements (e.g., saccades, fixations) occurring while viewing natural relative to built environments supports Kaplan’s depiction of less effortful viewing of natural scenes [48]. Geometric shapes, like natural scenes, also require minimal effort to view [10]. Geometric shapes, however, do not depict environments in which humans spend time and are thus considered non-natural. Because natural environ- ments and geometric shapes require little viewing effort [10], these comparisons allow for assessment of potential differences gener- ated across effortless natural and effortless non-natural stimuli. Previous studies have shown that exposure to scenes of the natural world influences mood, attention and time perception. Specifically, mood is enhanced, attention is restored, and the perception of time slows with viewing scenes of natural environ- ments [8–10,49]. Degree of delay discounting can also be influenced by mood [50], attention [51], and time perception [14]. Some positive mood induction techniques increase impulsive decision-making in delay discounting. Increased attention, as well as slowed time-perception, however, decrease impulsive decision- making in delay discounting. Considering these effects of viewing natural scenes on mood, attention, and time-perception–and the influence that these same cognitive processes exert on degree of delay discounting–we predicted that viewing scenes of nature would result in decreased impulsive decision-making in a delay discounting task relative to viewing scenes of built environments or geometric shapes. Method Participants Ethics Statement. recruited from an introductory psychology course. Participants provided their written informed consent and received course credit for participation. The Utah State University Institutional Review Board approved all experimental procedures. Undergraduate students (N=204) were Setting and Apparatus Participants were tested individually in a room equipped with a computer. Experimental manipulations and data recording were programmed using E-Prime 2.0. The Nature of Impulsivity PLOS ONE | www.plosone.org2May 2014 | Volume 9 | Issue 5 | e97915

Stimuli The picture sets used as stimuli have been used in previous experiments testing attention restoration across environments [10]. In the natural condition, participants viewed photographs of natural environments (e.g., forests). In the built condition, participants viewed photographs of human-made environments (e.g., buildings). In the geometric condition (control), participants viewed photographs of geometric shapes (e.g., triangles). There were 25 photographs in each picture set. See Figure 1 for examples of the stimuli used in each condition. Procedure The participant was first seated at the computer and given instructions to choose whichever option they preferred in the delay discounting task [46]. The same instructions were provided on the computer screen to lead the participant through the task. Participants used the mouse to progress through instructional screens and make their choices. Participants were randomly assigned to one of three conditions– natural (n=63), built (n=59), or geometric (n=63)–using block randomization. Following practice trials, participants viewed condition-specific photographs for 10 s each on the computer screen both prior to the experimental delay discounting task (25 photographs) and between each delay block (5 photographs, selected randomly from the original set of 25 photographs). Excluding photograph type, all aspects of the experiment were identical across conditions. Participants were tested in the delay discounting task using hypothetical monetary outcomes. All choice screens presented the wording ‘‘Would you rather have [amount] now or [amount] in [delay]?’’ Participants selected either the immediate or delayed outcome to progress; the side of immediate or delayed amount varied randomly across trials. Before experiencing the condition- specific stimuli and completing the titrating amount procedure described below [45,46], participants completed 10 practice trials. The practice trials were designed to introduce the participant to the interface and choice tradeoffs between amount and delay. Practice trials began with a choice of $10 now or $100 dollars in one week. The immediate option increased by $10 until the final option was a choice between $100 now or $100 in one week. Because practice trials did not titrate based on participant response, they are not included in the delay discounting analysis [46]. Following the 10 practice trials and stimuli exposure, all participants experienced the titrating amount discounting proce- dure. Delays tested were 1 day, 1 week, 1 month, 6 months, 1 year, 5 years, and 25 years, in that order. For each trial in the titrating amount delay discounting procedure, participants chose between immediate and delayed options. The first trial at each delay began with the choice of $50 now or $100 after a delay, and the immediate amount increased or decreased based on the participant’s response with each subse- quent trial. If the immediate outcome was selected, the amount of the next immediate outcome decreased; if the delayed outcome was selected, the amount of the next immediate outcome increased. The adjustment on the first trial was half of the difference between the immediate and delayed outcomes (i.e., $25); for each subsequent trial, the adjustment was half the previous adjustment. For example, if the participant selected the delayed option on the first two trials, the third trial would be $100 delayed option versus $87.50 immediate option. If the participant selected the immediate option on the first two trials, the third trial would be $100 delayed option versus $12.50 immediate option. There were 10 trials at each delay. The indifference point was the last value of the immediate outcome for each delay. See Figure 2 for a schematic diagram of the choice portion of the experiment. Data Analysis Of 204 individuals that participated, data for 19 were not considered due to nonsystematic discounting, which is similar to previous percentages eliminated based on these criteria [52]. Delay discounting data were considered systematic and used if a.) participants discounted more than $5 across any delay (which assumes delay decreases the value of a reward), and b.) indifference points did not increase across consecutive delays by more than 35% of the larger later reward. Substantial increases in the value of a reward across delays suggests that the value of a reward is enhanced with increased delay. These criteria are based on the expectation of a monotonically decreasing discounting function, and are similar to the algorithm used by Johnson and Bickel [52]. Figure 1. Examples of the stimuli used in natural, built, and geometric conditions. Examples of the stimuli used in the natural (left stimuli), built (center stimuli) and geometric (right stimuli) conditions. doi:10.1371/journal.pone.0097915.g001 The Nature of Impulsivity PLOS ONE | www.plosone.org 3May 2014 | Volume 9 | Issue 5 | e97915