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Effects of Natural Environmental Stimulation Duration on Psychophysiological Recovery Benefits

HORTSCIENCE 56(11):1387–1394. 2021. https://doi.org/10.21273/HORTSCI16101-21

Effects of Natural Environmental

Stimulation Duration on

Psychophysiological Recovery Beneﬁts

Yen-Cheng Chiang

Department of Landscape Architecture, National Chiayi University, Chiayi

City 600355, Taiwan

Pei-Yi Weng

Department of Plant Industry, National Pingtung University of Science and

Technology, Pingtung 912301, Taiwan

Additional index words. attention restoration, stress reduction, viewing nature

Abstract. The rapid pace of modern life, as well as chronic work and academic over-

loading, contribute to our society’s progressively increasing stress levels. Chronic

stress can affect both physical and mental well-being. Numerous studies have con-

ﬁrmed that viewing a nature-based video presentation can reduce stress and result in

attention recovery. However, the discourse has been primarily focused upon stimula-

tion applied over a single duration, rather than over various durations. Therefore,

the present study explored the effect of various viewing durations with regard to for-

est-related videos on stress reduction and attention recovery. Before the videos’ pre-

sentation, the participants’ stress and distraction levels were increased through

stimulation. Data on stress, attention, and relaxation were collected through the

implementation of the State–Trait Anxiety Inventory and an electroencephalogram

instrument. Equal numbers from the 90 participants (i.e., 30) were assigned to watch

a short, medium-length, or long video (5, 10, and 20 minutes in duration, respec-

tively). The viewing of 20-minute forest-related videos signiﬁcantly promoted stress

reduction and physiological relaxation. The present ﬁndings advance the understand-

ing of the relationship between viewing nature scenes duration and psychophysiologi-

cal states; thus, they serve as a reference for future research on the natural-dose

concept, as well as the development of relevant activities and courses.

Under rapid urbanization, individuals in

modern society are exposed to various stimuli

and stressors and, thus, suffer from fatigue.

Several studies have indicated that the negative

physical and mental impacts of chronic stress

extend to work efﬁciency and the ability to

cope with the stresses of daily life (Brewer and

McMahan, 2003; Kamaldeep et al., 2016).

Rambo (1984) identiﬁed anxiety as an emotion

triggered in reaction to stress. Spielberger

(1966) asserted that the state anxiety response

is a behavior generated after an individual con-

sciously evaluates the situational stress they

perceive. State anxiety is characterized by dis-

tress, which may take the form of apprehen-

sion or worry. Anxiety and stress exert

negative impacts on mental health and, in the

long term, this can easily lead to the develop-

ment of speciﬁc chronic diseases (Schachter

and Singer, 1962). Physical disease can be pre-

vented and physical health can be promoted

through the relief of anxiety or depression,

and the generation of positive emotions; for

Received for publication 2 July 2021. Accepted

for publication 6 Aug. 2021

Published online 5 October 2021.

P.W. is the corresponding author. E-mail:

peweng@mail.npust.edu.tw.

This is an open access article distributed under the

CC BY-NC-ND license (https://creativecommons.

org/licenses/by-nc-nd/4.0/).

example, through contact with nature or en-

gagement in leisure activities (Farrow and

Washburn, 2019; Godbey, 2003; Jason, 2011;

Kaplan, 2001; Kim et al., 2019; Lee and Son,

2018; Roe and Aspinall, 2011; Weng and

Chiang, 2014; Wijndaele et al., 2007).

Existing studies have found people prefer

to carry out their lives in a natural environ-

ment as it can alleviate stress or restore atten-

tion fatigue (Hartig et al., 1991, 2003; Hartig

and Staats, 2006; Ulrich and Addoms, 1981;

Ulrich and Parsons, 1992). Several empirical

studies have reported that participants who

viewed natural landscapes after being sub-

jected to stress stimulation exhibited negative

and positive emotions to lesser and greater

extents, respectively, compared with their

counterparts presented with urban street

scenes (Herzog et al., 2003; Honeyman,

1992; Ulrich et al., 1991). Coming into con-

tact with natural environments, especially for-

ests, can promote mental health, bring a

sense of comfort, and soothe one’s mind (Ikei

et al., 2014; Kotera et al., 2020).

The bulk of studies on attention restora-

tion are based on attention restoration theory

(ART) (Kaplan and Kaplan, 1989) and the

stress reduction theory (SRT) (Ulrich, 1984;

Ulrich et al., 1991). ART, which focuses on

the cognitive level, argues that a direct atten-

tion restoration can be achieved by participat-

ing in activities in the natural environment

(Kaplan, 1995; Kaplan and Berman, 2010).

Kaplan and Kaplan (1989) noted that direct

attention requires mental focus. When some-

thing deserving of notice is unattractive, one

must pay more attention to prevent distrac-

tion. Furthermore, maintaining direct atten-

tion for a prolonged or excessive period of

time leads to attention fatigue. For people

whose attentional capacity is exhausted,

being in a fascinating natural environment

brings a deep sense of calm and comfort;

thus, direct attention can be maintained or

restored. Herzog et al. (1997) observed that

ordinary natural settings achieved more satis-

factory attentional recovery effects than

everyday urban or sports/entertainment set-

tings. Multiple studies have demonstrated

that recovery from stress and attention fatigue

is facilitated by exposure to a natural environ-

ment (Cimprich, 1993; Hartig et al., 2003;

Hartig et al., 1991; Staats et al., 2003; Taylor

et al., 2002; Tennessen and Cimprich, 1995;

Ulrich, 1984; Wells, 2000). According to the

ART, the natural environment features ele-

ments that trigger fascination, particularly

soft fascination. For example, nonthreatening

natural environments (forests, parks, and

waterfalls) are considered soft and alluring

because they draw the attention effortlessly

while providing pleasant emotional experien-

ces (Kaplan, 1995; Kaplan and Kaplan,

1989). The natural environment is not only

conducive to attention restoration but also to

factors such as cognitive performance, work-

ing memory, attentional control, visual atten-

tion, vigilance, and cognitive ﬂexibility

(Stevenson et al., 2018). Through a critical

review and meta-analysis of 31 papers, a

study reported that exposure to natural envi-

ronments had signiﬁcantly positive effects,

thus supporting the ART (Ohly et al., 2016).

SRT explains emotional and physiological

responses to natural spaces that facilitate

stress reduction (Ulrich et al., 1991). When

people are in contact with nature, their atten-

tion is redirected to the landscape such that

their negative emotions are replaced by posi-

tive ones; thus, the balance of their physiolog-

ical system is regained (Hartig and Staats,

2006; Hansmann et al., 2007). As these

emotions become positive, stress-induced

impairments in cognitive operations and

performance may also be mitigated (Parsons

et al., 1998). SRT highlights the beneﬁts of

positive emotions, as well as the reduction

and enhancement of negative and positive

emotions, respectively (Ulrich, 1979; Ulrich

and Addoms, 1981; Ulrich et al., 1991). Hon-

eyman (1992) compared the effects of vary-

ing amounts of vegetation in countryside and

urban scenes on the degree of perceived

stress: participants were exposed to scenes of

rural villages, cities with vegetation, and cit-

ies without vegetation, with the results deter-

mining that urban greening signiﬁcantly

reduced stress. In a similar vein, other studies

have indicated that viewing natural environ-

ments can reduce stress, promote more posi-

tive emotions, trigger positive physiological

functions, beneﬁt cognitive functioning, and

facilitate recovery from discomfort or illness

HORTSCIENCE VOL. 56(11) NOVEMBER 2021

1387

(Berto, 2005; Knopf, 1987; Leather et al.,

1998; Ulrich et al., 1991). In summary, the lit-

erature conﬁrms the positive effects of a natu-

ral environment on both physical and mental

health (Berto, 2014; Chun et al., 2017; Her-

zog et al., 2003).

Viewing nature scenes can generate posi-

tive beneﬁts, in particular, mysterious natural

settings. A study indicated that as presenta-

tion durations were extended, images depict-

ing scenes perceived as high in mystery

achieved greater improvements in recognition

performance than images depicting scenes

perceived as low in mystery (Szolosi et al.,

2014). Therefore, presentation duration affe-

cts not only the psychophysiological beneﬁts

obtained therefrom but also memory recogni-

tion performance. Furthermore, in-depth res-

earch on the presentation duration of nature

scenes remains lacking. Speciﬁcally, the pre-

sentation duration required to maximize the

associated psychophysiological beneﬁts re-

mains unclear. To ﬁll this gap in the litera-

ture, this study explored the impacts of the

presentation duration of nature scenes on the

psychophysiological beneﬁts derived.

Relationship between natural

environmental stimulation of varying

durations and stress reduction

Stress refers to the physiological response

that threatens the autonomic nervous and

endocrine systems. The body tends to choose

either an active response or passive avoidance.

When the stress endured exceeds a tolerable

level, homeostasis is disrupted (Taylor, 2006).

Anxiety, which can be classiﬁed as trait and

state anxiety, is a complex process contained

within stress; if a stressful situation is per-

ceived as a threat or danger, a temporary state

of anxiety will be induced (Spielberger et al.,

1972). Trait anxiety—a chronic emotional

state—refers to stable anxiety tendency. By

contrast, state anxiety is a transitory emotional

state characterized by apprehension, perturba-

tion, worry, and unease. State anxiety is

induced when an individual believes or per-

ceives an external stressor as dangerous or

threatening (Spielberger, 1966). Therefore, the

restoration of psychophysiological functions

after a period of rest or stimulation by natural

environments following the development of

such pressure is termed stress reduction.

Ulrich (1979) demonstrated that students

who viewed slides of nature-based scenes for

18 min after class experienced reductions in

stress, sadness, and anger/aggression and

increases in positive perceptions; the response

of the students who viewed slides or urban

scenes was the opposite. In a later study by

Ulrich and Simons (1986), the participants

watched a stress-inducing video before view-

ing slides of various scenes, and the psycho-

physiological effects were then determined.

Blood pressure levels, muscle tone, and the

electrodermal activity of participants who

viewed slides of water-based nature videos

were lower; only 4–6 min elapsed before fear

and anxiety decreased, and positive emotions

were provoked. In a study by Ulrich et al.

(1991), 120 participants viewed a 10-min,

black-and-white, stress-inducing video in

which negligence or disregard for safety pro-

cedures caused several wood factory employ-

ees to sustain serious injuries. Subsequently,

the participants viewed six natural and urban

scenes with color and sound for 10 min. Phys-

iological and verbal assessments revealed the

participants recovered from stress faster when

exposed to the natural environments. Kotera

et al. (2020) performed a meta-analysis of 20

studies on the mental health impacts of shin-

rin-yoku (i.e., forest bathing) practices (e.g.,

breathing exercises, walking, and yoga). The

interventions were effective in mitigating

depression, anxiety, stress, and anger—espe-

cially anxiety—in both clinical and nonclinical

samples. The authors concluded that negative

mental health symptoms could be reduced

through shinrin-yoku of durations ranging

from 15 min to 9 d.

Liszio et al. (2018) placed participants

under stress before exposing them to a natural

virtual environment, presented using virtual

reality (VR) technology or a computer desk-

top screen. After stress was induced through

a Trier Social Stress Test, a total of 62 partic-

ipants were randomly assigned to one of two

relaxation conditions (i.e., VR environment

or desktop screen), and systematic changes in

physiological and psychological indicators

were examined. The VR group experienced

signiﬁcantly less stress and higher positive

effects, indicating that VR can relieve emo-

tional strain under acute stress. Thus, such

technology can be used to design feasible sol-

utions for individuals for whom conventional

nature therapy is not an option.

Jiang et al. (2014) established a dose–res-

ponse curve reﬂecting stress reduction through

exposure to the natural environment. Psycho-

social stress was ﬁrst induced through the Trier

Social Stress Test, before 160 participants

watched a 6-min three-dimensional video ran-

domly selected from a selection of 10. The

density of trees in the video ranged from 1.7%

to 62%. Individual responses and stress recov-

ery were evaluated through the measurement

of various physiological indicators. The dose–

response curve of tree density indicated

nonsigniﬁcant results for women, but for men,

the dose–response curve was an inverted-U

shape. Wang et al. (2016) conducted a stress

recovery study in which 140 Chinese university

students (aged 18–24 years) were randomly

assigned to watch one of seven videos (six

depicting an urban park, one presenting a city

road) during the stress recovery stage, after

undergoing a stress-inducing oral exam. After

data on psychophysiological measures, includ-

ing stress and attention, were collected, stress

recovery was found to correlate to exposure to

natural landscape elements, and outdoor park

scenes without people exerted greater restor-

ative beneﬁts than scenes in which people were

present. These results are consistent with the

existing body of research (Chiang et al., 2017;

Gidlow et al., 2016; Peschardt and Stigsdotter,

2013).

Chun et al. (2017) randomly divided

patients who had suffered chronic strokes into

a forest group (residing in a recreational forest

site) and an urban group (residing in an urban

hotel), with both groups required to perform

identical activities for the same duration. The

patients were evaluated using the Beck

Depression Inventory, Hamilton Depression

Rating Scale, and State–Trait Anxiety Inven-

tory at pretest and posttest. The researchers

concluded that forest therapy was effective in

alleviating symptoms of depression and anxi-

ety in chronic stroke patients, particularly

those for whom standard drug therapy is inef-

fective. Whether for patients with speciﬁc dis-

eases or the general population, direct or

indirect contact with the natural environment

has been found to strongly activate the para-

sympathetic nervous system, the beneﬁts of

which include physical and mental relaxation

and stress reduction, as well as reduced cardio-

vascular risks, enhanced cognitive function,

improved sleep quality, and the prevention of

dementia and attention deﬁcit hyperactivity

disorder (Farrow and Washburn, 2019; Ikei

et al., 2014; James et al., 2015; Kim et al.,

2019; Kuo and Taylor, 2004; Lee and Son,

2018; Lee et al., 2014; Ochiai et al., 2015).

Relationship among varying durations of

natural environmental stimulation,

attention, and relaxation

Attention refers to selective concentration.

Individuals consciously respond to various

stimuli in a situation and select those to

which they wish to respond; perception is a

key component of this process (Kaplan and

Berman, 2010). Pretty (2004) proposed three

beneﬁcial levels of nature engagement for

humans: remotely viewing nature, being in

the presence of nearby nature, and active par-

ticipation in and involvement with nature. In

essence, nature participation at any level can

positively affect stress and attention (Mackay

and Neill, 2010). In a study on the restoration

of attentional capacity through exposure to

restorative environments (Berto, 2005), par-

ticipants were asked to choose photographs

depicting either restorative or nonrestorative

environments, or geometrical patterns; the

viewing duration for each photograph was

less than 15 s. Only participants who viewed

the restorative scenes improved upon their

score in the ﬁnal sustained attention test. In a

study by Parsons et al. (1998), participants

watched a 10-min simulated driving video in

either a natural environment or urban land-

scape; their physiological responses and

ability to concentrate were assessed. When

comparing scores for calculation processing,

concentration levels, and attention spans, par-

ticipants who viewed the natural landscape

video outperformed those who viewed the

urban landscape. Hartig et al. (2003) tested

changes in blood pressure, mood, and atten-

tion of adults before and after a 20-min walk

in either a natural or urban ﬁeld setting. The

participants assigned to the natural environ-

ment outperformed those assigned to the

urban environment, and the urban group

exhibited a downward trend in attention.

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Existing studies have demonstrated that

walking in nature facilitates attention restora-

tion more so than walking in urban areas (Har-

tig et al., 1991, 2003; Martens et al., 2011). In

a study on green exercise, Pretty et al. (2005)

asked participants to run on an indoor tread-

mill while viewing pleasant rural, unpleasant

rural, pleasant urban, and unpleasant urban

landscapes; greater beneﬁts to blood pressure,

self-esteem, and mood were observed in the

pleasant rural condition. In a later study, Pretty

et al. (2007) noted that participants engaging

in green exercise exhibited lower levels of

anxiety. A study by Hartig et al. (1991)

divided participants into two groups: one

walked through a wilderness setting, while the

second walked through a city; both partici-

pated in passive relaxation activities (listening

to music and reading magazines) for 40 min.

The researchers found that restorative effects

were most substantial in the wilderness group.

Hansmann et al. (2007) examined restoration

and stress relief in Zurich residents before and

after visits to green spaces (i.e., an urban forest

and a city park). The overall reduction in stress

and headaches following the visits were 87%

and 52%, respectively, with the changes

accounting for 40% of the possible mental bal-

ance improvement. Gidlow et al. (2016) ran-

domly assigned participants to a self-paced,

30-min walk in one of three environments: res-

idential (urban), natural (green), and natural

with water (blue). Psychophysiological res-

ponses (e.g., emotion, cognitive function, and

various physiological indicators) were mea-

sured at baseline (T1), at the end of the walk

(T2), and 30 min after departure from the

walking environment (T3). Indicator improve-

ments were noted at T2 and T3 in all environ-

ments, and associations between walking in

the natural (green and blue) environments and

better restorative experiences, improved cogni-

tive function, and reduced stress were

observed—effects that persisted to T3. A

meta-analysis study provided some evidence

of the positive beneﬁts of a walk or run in a

natural environment in comparison with in a

synthetic environment. With regard to the psy-

chological indicators, the ﬁndings were more

highly consistent among studies, whereas

those on physiological indicators (e.g., blood

pressure and cortisol concentrations) exhibit

greater discrepancies (Bowler et al., 2010).

In the present study, the parameter of atten-

tion—we measured participants’ brainwave

through the electroencephalogram (EEG)

instrument (MindBand, NeuroSky)—refers to

the participants’ attention level at that time.

Relaxation (measured on MindBand) refers to

the participants’ relaxation level at the time,

and reﬂects their mental rather than physical

state. Relaxation levels cannot be enhanced

over a short period of time, such as through an

entire-body muscle relaxation routine. None-

theless, for most people, physical relaxation

facilitates mental relaxation under normal cir-

cumstances. An increase in the relaxation lev-

els is clearly associated with a reduction in

brain activity. Long-term observations have

shown that closing the eyes reduces brain

activity because it eliminates the need for the

brain to process visual stimuli. Therefore, this

simple action is typically effective in raising

relaxation levels.

Research purpose and hypotheses

Multiple studies have conﬁrmed that expo-

sure to natural environments beneﬁts human

health (e.g., Farrow and Washburn, 2019; Her-

zog et al., 2002; Laumann et al., 2003; Li and

Sullivan, 2016). However, the duration of

exposure under which notable psychophysio-

logical effects are produced remains unknown.

Therefore, the present study sought to investi-

gate the effect exposure duration has using the

following psychophysiological indicators:

stress, attention, and relaxation. Furthermore,

state anxiety was used as a proxy for stress on

the basis of the premise advanced by Spiel-

berger (1966) that the transitory emotional state

is caused by anxiety or tension-induced stress.

Relevant studies on the relationship bet-

ween people and the natural environment

primarily use photographs as stimuli in simu-

lated situations, with the psychophysiological

responses assessed following a set period of

time. Therefore, determining exactly how long

it takes for natural landscape exposure to yield

restorative beneﬁts is challenging. Moreover,

whether the psychophysiological beneﬁts in

this situation increase over time has yet to be

elucidated.

Thus, the present study explored the effect

of varying durations of natural environmental

(forest) stimulation on psychophysiological

responses, namely stress, attention, and relaxa-

tion. The following hypotheses were proposed:

H1: Varying viewing durations lead to

signiﬁcant differences in the psychological

response of the individual with regard to

stress reduction.

H2: Varying viewing durations lead to

signiﬁcant differences in the physiological

response (EEG brainwave) of the individual

with regard to attention and relaxation.

H3: The pretest–posttest results of the psy-

chophysiological measurements of the individ-

ual differ signiﬁcantly by viewing duration.

Materials and Methods

Participants

Undergraduate and postgraduate students

were recruited to participate in our study.

Fig. 1. Screenshots of the forest video.

Volunteers were recruited through announce-

ments posted around campus. We employed a

convenience sample of 90 participants. For all

participants, questionnaire completion and

physiological data collection (i.e., brainwaves)

were conducted in the same indoor space. The

participants were divided into three groups of

30 individuals that were randomly assigned to

watch one of three nature videos of varying

durations. A total of 90 valid participants of

whom 47 (52.2%) indicated their gender as

female and 43 (47.8%) indicated as male.

Their age between 19 and 26 years (M = 22.1

years; SD = 1.14 years). All participants gave

their informed consent for inclusion before

they participated in the study.

Stimuli

“Traveling around the National Forest Rec-

reation Areas”—a video released by the Tai-

wan Forestry Bureau in 2007—was edited into

three time lengths: 5, 10, and 20 min. To

prevent the inﬂuence of different forest envi-

ronments on participants, the three videos, fea-

turing identical content, were played from the

same starting point for the 10-min and 20-min

videos, the 5-min video was looped twice and

four times, respectively. Regarding the content,

the videos showcase the botanical beauty and

diversity of Taiwan’s high-elevation coniferous

forests, 1000-year-old trees in cloud forests,

sacred trees, medium- and low-elevation prime-

val broad-leaved forests, and monsoon forests

(Fig. 1). The videos contained the sounds of

natural environments such as insects singing,

birds chirping, and leaves rustling.

Assessment tools

State–Trait Anxiety Inventory. State anxi-

ety, which refers to the varying degrees of

anxiety responses produced immediately

upon exposure to a situational stressor, and

the perception that the stressor presents threat

or danger (Spielberger et al., 1972), was

assessed using a Chinese adaptation of the

State–Trait Anxiety Inventory (Chung and

Long., 1984). This self-reporting question-

naire comprises an equal number of items

measuring state anxiety and trait anxiety.

Since state anxiety is an immediate emotional

response, the inventory only measured state

anxiety as a stress measurement tool, with

possible scores between 20 and 80 points, and

higher scores indicating greater anxiety and

HORTSCIENCE VOL. 56(11) NOVEMBER 2021

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stress. A reliability analysis for state anxiety

was conducted. The Cronbach’s a obtained

was 0.84, indicating high internal consis-

tency. The content validity of the scale has

been reported to be favorable (Bados et al.,

2010; Weng and Chiang, 2014).

Attention and relaxation. Data were

recorded using MindBand EEG headsets

(NeuroSky ThinkGear, San Jose, CA): nonin-

vasive biofeedback instruments that measure

responses through electrodes attached to the

scalp. Recordings were made at 256 Hz from

the forehead at the FP1 site. Raw EEG data

were sent to a personal computer via Blue-

tooth technology, before being saved to disk.

Studies investigating the validity of Think-

Gear dry-sensor technology have indicated its

superior validity and reliability as research-

grade equipment (Bhatti et al., 2016; John-

stone et al., 2012; Rogers et al., 2016).

The EEG device is used to measure the

four different brainwave frequency signals a,

b, u, and d, through the eSense brain chip

algorithms are used to calculate the context

of the attention and meditation (relaxation)

parameters (Neurosky Inc., 2009; Perhakaran

et al., 2015). Previous studies have docu-

mented well that focused on obtaining atten-

tion and meditation (relaxation) states of

mind from the characteristics of EEG (Jacobs

and Friedman, 2004; Kaur and Singh, 2015;

Liu et al., 2013). The attention value reﬂects

the intensity of a participant’s level of mental

“focus” or “attention” during increased men-

tal effort. Meditation (relaxation) is a mea-

sure of a person’s mental levels, not physical

levels, so simply relaxing all the muscles of

the body may not immediately result in a

heightened meditation level, while the medi-

tation value point to participant’s mental

“calmness” or “relaxation.” Each metric pro-

vides a relative measure of state from 1 to

100, these are not absolute values, where a

value from 1 to 20 indicates “strongly low-

ered” levels of the state, and a value from 80

to 100 points heightened levels of that state

(Maskeliunas et al., 2016).

Study procedure

Before being ﬁtted with the equipment,

participants were informed of the experimen-

tal procedure and, subsequently, completed

the questionnaire. The questionnaire’s ﬁrst

section was with concerned basic informa-

tion, while the second comprised the atten-

tion-distraction test: Trail Making A Test

(TMAT) (Charoenkitkarn et al., 2009). The

TMAT caused the participants to experience

physiological and physiological fatigue, such

as brain and body fatigue (Gotts et al., 2015).

Participants were presented with 49 sequen-

tial numbers, randomly scattered in a 7 Â 7

grid, and were instructed to complete the con-

nection, as well as identify repetitions and

missing values. High levels of concentration

while completing the task are expected to

achieve attentional distraction following the

test (Chiang et al., 2017). Next, participants’

brainwaves were recorded as they completed

the State–Trait Anxiety Inventory, and while

Fig. 2. Study procedure.

they viewed the video assigned to their condi-

tion. Lastly, as they completed the inventory

again, the participants’ brainwave data were

collected concurrently (Fig. 2).

Statistical analysis

All analyses used IBM SPSS Statistics

22.0. A one-way analysis of variance

(ANOVA) was performed to test for variations

in the levels of stress reduction, attention, and

relaxation according to the stimulation dura-

tion, as well as to validate H1 and H2. A

dependent samples t test was conducted to test

the pretest–posttest differences under H3. The

effect sizes were further analyzed. Cohen

(1988) stated that “there is a certain risk inher-

ent in offering conventional operational deﬁni-

tions for those terms for use in power analysis

in as diverse a ﬁeld of inquiry as behavioral

science” (p. 25). Cohen’s d can be interpreted

in terms of the nonoverlap percentage of the

pretest scores with the posttest scores. The pre-

sent effect sizes are based on unadjusted coef-

ﬁcients of correlation (i.e., in a within-group

design). Therefore, the effect sizes were com-

puted on the basis of the formula adapted for

correlated designs developed by Dunlap et al.

(1996) (p. 171, Eq. [3]): d = tc(2 [1Àr]/n)1/2.

According to the benchmarks developed by

Cohen, effect sizes can be classiﬁed as small

(d = 0–0.2), medium (d = 0.3–0.5), and large

(d = 0.6–0.8). In addition, analysis of covari-

ance (ANCOVA) was used to compare the

posttest results for stress, attention, and relaxa-

tion among the groups exposed to videos of 5-,

10-, or 20-min duration, with pretest results as

covariates to avoid the inﬂuence of different

baselines between the treatment conditions.

Results

Duration-dependent differences in

psychophysiological responses

To determine the effect of watching vid-

eos of varying durations on the participants’

stress, attention, and relaxation levels, the

posttest scores were subjected to a one-way

ANOVA. Viewing videos of varying length

were found to signiﬁcantly affected stress,

F(2, 87) = 3.065, P < 0.05, as well as relaxa-

tion, F(2, 87) = 6.542, P < 0.01 (Fig. 3).

However, attention was not signiﬁcantly

affected, F(2, 87) = 0.578, P = 0.56. Regard-

ing the Scheffe post-hoc pairwise comparison

results, a comparison of the 10- (Mposttest =

29.65) and 20-min (Mposttest = 34.43 for both)

durations revealed that greater stress reduc-

tion effects were achieved in the 10-min con-

dition. Watching the 20-min video (Mposttest =

64.56) had signiﬁcantly better effects on

relaxation compared with watching the 5- or

Fig. 3. Results from the analysis of variance of

psychophysiological responses by stimulation

duration.

10-min videos (Mposttest = 56.58 vs. 55.60).

This ﬁnding indicates that viewing a longer-

length nature video had excellent psycho-

physiological beneﬁts, including relaxation.

Pretest–posttest differences in

psychophysiological benefits under

varying stimulation durations

The stress, attention, and relaxation data

of the participants in the 5-min condition

before and after the viewing were examined

through a dependent samples t test. The pre-

test–posttest differences were signiﬁcant for

stress, t(29) = À6.89, P < 0.001, as were

for relaxation, t(29) = 2.19, P < 0.05, but not

for attention, t(29) = 0.18, P > 0.05 (Table 1).

These results demonstrate that stress was

reduced and relaxation was increased after

the 5-min video was presented. Moreover,

regarding effect sizes, the results on stress

and relaxation under all three durations exhi-

bited a satisfactory effect size (Table 1).

Data of the participants in the 10-min con-

dition were also examined through a depen-

dent samples t test. The pretest–posttest

differences were signiﬁcant for stress, t(29) =

À5.65, P < 0.001, as were for relaxation,

t(29) = 2.78, P < 0.01, but not for attention,

t(29) = 1.92, P > 0.05 (Table 1). In other

words, signiﬁcant reductions in stress and

increases in relaxation were observed, but no

differences in attention were detected.

Similarly, performance of the dependent

samples t test on the data from the 20-min con-

dition revealed signiﬁcant pretest–posttest dif-

ferences for stress and relaxation, t(29) = À3.90

and 2.98, P < 0.01, respectively), but not for

attention, t(29) = 0.44, P > 0.05 (Table 1). In

short, stress decreased and relaxation increased

following the video’s presentation.

Effect of duration on stress, attention,

and relaxation

To determine whether the effects of dura-

tion on stress, attention, and relaxation varied

between pretest and posttests, pretest scores

were set as the control variable and ANCOVA

was conducted. The stress analysis results indi-

cated that by excluding the covariate variable

(i.e., pretest) and dependent variable (i.e., post-

test), stimulation time exhibited pronounced

effects on stress posttest scores. To further un-

derstand the effects of the varying simulation

time on stress, the Bonferroni post-hoc analy-

sis was performed, which revealed that the

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HORTSCIENCE VOL. 56(11) NOVEMBER 2021

Table 1. Pretest–posttest differences in psychophysiological effects under varying stimulation

durations.

Pretest

Posttest

5 min (n = 30)

Stress

Attention

Relaxation

10 min (n = 30)

Stress

Attention

Relaxation

20 min (n = 30)

Stress

Attention

Relaxation

40.93

49.24

52.33

41.50

46.54

48.30

41.80

47.41

51.64

8.92

10.38

5.53

10.18

10.62

6.83

8.68

8.63

4.24

30.15

49.61

56.68

29.65

50.82

55.60

34.43

48.20

64.56

5.80

10.68

8.67

5.93

8.11

7.80

7.04

6.62

8.99

Lower stress scores represent a lower level of psychological stress.

*P < 0.05; **P < 0.01; ***P < 0.001.

À6.89***

0.18

2.19*

À5.65***

1.92

2.78**

À3.90**

0.44

2.98**

À0.91

0.04

0.56

À0.89

0.45

0.78

À0.93

0.10

0.81

effect of the 10-min video (M = 29.65) on

stress was lower than that of the 20-min video

(M = 34.43). In other words, the 10-min video

had a more favorable performance in terms of

stress reduction than the 20-min video did

(Table 2).

The attention analysis results indicated

that when excluding the covariate variable

(i.e., pretest) and dependent variable (i.e.,

posttest), stimulation time did not exhibit pro-

nounced effects on attention posttest scores;

signiﬁcant differences were observed only

between the pretest and posttest. The relaxa-

tion analysis results indicated that stimulation

time exhibited pronounced effects on relaxa-

tion posttest scores. Signiﬁcant differences

were also observed between the pretest and

posttest. Bonferroni post-hoc analyses reve-

aled that the effects of the 20-min video (M =

64.56) on relaxation were greater than those

of the 5-min video (M = 56.68) and 10-min

video (M = 55.60). In other words, the 20-

min video yielded greater relaxation effect

than the 5- and 10-min video did (Table 2).

Conclusions and Recommendations

Numerous studies have noted that a natu-

ral environment can beneﬁt human health.

The presentation of nature scenes is found to

exert positive mental effects, with studies

using videos that run anywhere between 4

and 20 min (Berto, 2005, 2014; Herzog et al.,

2003; Liszio et al., 2018; Ulrich and Simons,

1986; Wang et al., 2016). In the current

study, varying stimulation durations were

used to assess the effects of a natural environ-

ment on stress, attention, and relaxation. Vie-

wing the nature videos—regardless of their

duration—signiﬁcantly reduced stress levels,

and increased relaxation levels.

Regarding the pretest–posttest differences

in stress, participants experienced substantial

reductions in stress after watching the nature

videos, regardless of duration. Taking at least

20 min out of your day to stroll or sit in a place

that makes you feel in contact with nature will

signiﬁcantly lower your stress levels (Hunter

et al., 2019). Because the rapid pace of modern

life makes it difﬁcult to engage in activities in

nature, the present experiments were not con-

ducted outdoors. Nevertheless, stress reduction

can be achieved even through the brief viewing

of nature-related videos. The present results

are consistent with those of existing studies in

that stress reduction was achieved through

exposure to nature (Kim et al., 2019; Ulrich,

1979, 1986; Ulrich et al., 1991), regardless of

the duration. Regarding the exposure duration,

brief viewing (5–10 min) contributed to stress

reduction, whereas the beneﬁts of extended

viewing (>20 min) were limited, likely be-

cause the sensory stimulation remained incom-

parable to real experiences in nature.

Regarding the pretest–posttest differences

in attention, no signiﬁcant effect was observed

Table 2. Covariance analysis of the effect of duration on the participants’ stress, attention, and relaxa-

tion during the pre- and posttests (dependent variable: posttests).

Source

Sum of squares

Stress

Pretest

312.671

Duration

233.125

Error

3119.529

R2 = 0.151 (Adj R2 = 0.121)

Attention

Pretest

651.409

Duration

142.010

Error

7905.345

R2 = 0.088 (Adj R2 = 0.056)

Relaxation

Pretest

315.726

Duration

826.610

Error

4977.851

R2 = 0.203 (Adj R2 = 0.176)

df Mean square F Sig. Partial h2

1 312.671 8.620 0.004 0.091

2 116.563 3.213 0.015 0.070

36.274

Post-hoc

10 < 20

1 651.409 7.086 0.009 0.083

71.005 0.772 0.465 0.018

91.923

1 315.726 5.455 0.022 0.060

2 413.305 7.140 0.001 0.142 5 < 20, 10 < 20

57.882

under any duration, and an overall declining

trend was noted. This is likely because the

attention–distraction test administered before

the video presentation required complete con-

centration, thereby reducing the participants’

attentional capacity following the test. How-

ever, this test was modiﬁed on the basis of rel-

evant literature. In other words, the modiﬁed

test may have insufﬁciently distracted the par-

ticipants; their calmness and concentration

during the video viewing may have caused a

slight drop in attention. Another possible rea-

son is the discrepancies between the measure-

ment methods used in the present study and in

numerous other studies. Previous studies used

self-reported psychological assessments of

attention, such as the Perceived Restorative-

ness Scale (PRS) (Hartig et al., 1997) and

the Restorative Components Scale (RCS)

(Laumann et al., 2001). By contrast, the pre-

sent study obtained physiological brainwave

data. According to Bowler et al. (2010), the

consistency of the ﬁndings on psychological

indicators among various studies is high,

whereas those on physiological indicators are

inconsistent. In short, ﬁndings on physiologi-

cal indicators might be affected by factors

such as study design, sample size, type of natu-

ral environment, type of exposure to and

engagement with nature, and duration of

exposure.

For relaxation, all video durations (i.e., 5,

10, and 20 min) increased relaxation scores

(from pretests to posttests). The increases

were most pronounced with a video duration

of 20 min. This may be because the partici-

pants in the lengthier video condition were

mentally refreshed as a consequence of the

extended viewing duration and comfortable

environment, which may have contributed to

greater physiological relaxation. Our results

are in respects similar to those in previous

study examining the relaxing effects of forest

bathing on humans (Tsunetsugu et al., 2010).

According to the operational deﬁnition of

relaxation in this study, relaxation can induce

calmness, and the longer the duration, the

greater the beneﬁts.

Once the pretest scores were controlled,

the effects of stimulation time on stress and

relaxation varied considerably by video dura-

tion, where the 20-min forest video caused

more stress but induced more relaxation than

did the other videos. This may be because the

20-min video made the participants more irri-

tated mentally, but more relaxed physically.

There are a number of limitations associ-

ated with this study. First, the current experi-

mental group comprised only undergraduate

and postgraduate students. Various studies

have also used university students as the exp-

erimental group (Chiang et al., 2014; Hartig

and Staats, 2006; Wang et al., 2016; Weng

and Chiang, 2014), with signiﬁcant results.

Nonetheless, because of the relatively limited

demographics, extrapolation of the present

ﬁndings to other populations warrants cau-

tion. Second, although research conﬁrms that

responses to static color photos or dynamic

videos generalize well to on-site response

(Herzog and Kutzli, 2002; Stamps, 1990),

HORTSCIENCE VOL. 56(11) NOVEMBER 2021

1391

some researchers raise legitimate concerns

about the generality of such results (Heft and

Nasar, 2000; Scott and Canter, 1997). Thus,

research would do well to test the results on-

site or in response to dynamic displays.

Finally, no control group (e.g., assigned to

watch videos depicting urban or neutral

scenes) was enrolled for comparison. Thus,

only differences among viewing durations

could be analyzed.

In future research, ﬁrst, researchers are

recommended to alter the stimulation dura-

tions and incorporate videos depicting other

types of landscapes for cross-comparison pur-

poses. In addition, researchers should observe

the participants’ behavior while they watch

the videos and inquire upon their feelings

afterward; this would form a basis for any

auxiliary analysis of physiological indicators,

as well as add other psychological measure-

ment tools to explore the relationship

between physiological and psychological

health. Second, this study only measured the

duration of a single stimulation, but contact

with nature is not limited to one-time experi-

ences. Thus, assessing the frequency and

quality of nature stimulation in future studies

is recommended for gaining deeper insights

into the beneﬁts of nature on human health.

In practical recommendations, considering

the rapid pace of modern life, ofﬁce workers

and students can obtain psychological bene-

ﬁts through the brief viewing of nature

scenes. Although the short durations of rest at

work or school do not allow for breaks in

nature, psychophysical relaxation can be

achieved by viewing nature-related videos,

and work efﬁciency can be improved sub-

stantially. In addition, the present ﬁndings

can be used to develop a reference test with

regard to the time dose to understand how

varying durations of exposure to the natural

environment affect physical and mental

health. Moreover, the current ﬁndings can

provide a clinical reference with respect to

the dose-of-nature concept and can contribute

toward the development of treatment that

involves exposure to natural landscapes for

both the general public and patients. In addi-

tion to facilitating scientiﬁc and practical

applications of the natural-dose concept,

these ﬁndings can also promote and encour-

age people to engage in outdoor activities, as

well as create awareness of the importance of

leisure, all with a view to relieving the stress

of urban residents and improve mental health

on a public scale.

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