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Research on the processing of sexual stimuli has proved that such material has high priority in human cognition. Yet, although sex differences in response to sexual stimuli were extensively discussed in the literature, sexual orientation was given relatively little consideration, and material suitable for relevant research is difficult to come by. With this in mind, we present a collection of 200 erotic images, accompanied by their self-report ratings of emotional valence and arousal by homo- and heterosexual males and females (n = 80, divided into four equal-sized subsamples). The collection complements the Nencki Affective Picture System (NAPS) and is intended to be used as stimulus material in experimental research. The erotic images are divided into five categories, depending on their content: opposite-sex couple (50), male couple (50), female couple (50), male (25) and female (25). Additional 100 control images from the NAPS depicting people in a non-erotic context were also used in the study. We showed that recipient sex and sexual orientation strongly influenced the evaluation of erotic content. Thus, comparisons of valence and arousal ratings in different subject groups will help researchers select stimuli set for the purpose of various experimental designs. To facilitate the use of the dataset, we provide an on-line tool, which allows the user to browse the images interactively and select proper stimuli on the basis of several parameters. The NAPS ERO image collection together with the data are available to the scientific community for non-commercial use at http://naps.nencki.gov.pl.
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ORIGINAL RESEARCH
published: 10 September 2015
doi: 10.3389/fpsyg.2015.01336
Frontiers in Psychology | www.frontiersin.org 1 S
eptember 2015 | Volume 6 | Article 1336
Edited by:
Luiz Pessoa,
University of Maryland, USA
Reviewed by:
Jan Van Den Stock,
KU Leuven, Belgium
Desmond Jay Oathes,
Stanford University, USA
*Correspondence:
Małgorzata Wierzba and Artur
Marchewka,
Laboratory of Brain Imaging,
Neurobiology Centre, Nencki Institute
of Experimental Biology, 3 Pasteura
Street, Warsaw 02-093, Poland
m.wierzba@nencki.gov.pl;
a.marchewka@nencki.gov.pl
Specialty section:
This article was submitted to
Emotion Science,
a section of the journal
Frontiers in Psychology
Received: 27 March 2015
Accepted: 19 August 2015
Published: 10 September 2015
Citation:
Wierzba M, Riegel M, Pucz A,
Le
´
sniewska Z, Dragan WŁ, Gola M,
Jednoróg K and Marchewka A (2015)
Erotic subset for the Nencki Affective
Picture System (NAPS ERO):
cross-sexual comparison study.
Front. Psychol. 6:1336.
doi: 10.3389/fpsyg.2015.01336
Erotic subset for the Nencki Affective
Picture System (NAPS ERO):
cross-sexual comparison study
Małgorzata Wierzba
1
*
, Monika Riegel
1
, Anna Pucz
2
, Zuzanna Le
´
sniewska
2
,
Wojciech Ł. Dragan
2
, Mateusz Gola
3, 4
, Katarzyna Jednoróg
5
and Artur Marchewka
1
*
1
Laboratory of Brain Imaging, Neurobiology Centre, Nencki Institute of Experimental Biology, Warsaw, Poland,
2
Faculty of
Psychology, University of Warsaw, Warsaw, Poland,
3
Institute of Psychology, Polish Academy of Sciences, Warsaw, Poland,
4
Swartz Center for Computational Neuroscience, Institute for Neural Computations, University of California, San Diego,
San Diego, CA, USA,
5
Laboratory of Psychophysiology, Department of N europhysiology, Nencki Institute of Experimental
Biology, Warsaw, Poland
Research on the processing of sexual stimuli has proved that such material has high
priority in human cognition. Yet, although sex differences in response to sexual stimuli
were extensively discussed in the literature, sexual orientation was given relatively little
consideration, and material suitable for relevant research is difficult to come by. With this
in mind, we present a collection of 200 erotic images, accompanied by their self-report
ratings of emotional valence and arousal by homo- and heterosexual males and females
(n = 80, divided into four equal-sized subsamples). The collection complements the
Nencki Affective Picture System (NAPS) and is intended to be used as stimulus material
in experimental research. The erotic images are divided into five categories, depending
on their content: opposite-sex couple (50), male couple (50), female couple (50), male
(25) and female (25). Additional 100 control images from the NAPS depicting people in a
non-erotic context were also used in the study. We showed that recipient sex and sexual
orientation strongly influenced the evaluation of erotic content. Thus, comparisons of
valence and arousal ratings in different subject groups will help researchers select stimuli
set for the purpose of various experimental designs. To facilitate the use of the dataset,
we provide an on-line tool, which allows the user to browse the images interactively and
select proper stimuli on the basis of several parameters. The NAPS ERO image collection
together with the data are available to the scientific community for non-commercial use
at http://naps.nencki.gov.pl.
Keywords: emotion, erotic stimuli, homosexual, heterosexual, sexual orientation, Nencki Affective Picture System
Introduction
Sex is one of the most important driving forces in human life, yet very little has been uncovered
about its mysteries (Georgiadis and Kringelb ach, 2012). Undoubtedly, sexual behavior is crucial for
reproduction and thus may be viewed as the primary and fundamental mechanism of survival of
the species (Costumero et al., 2013). This, however, constitutes only a small part—perhaps even the
least interesting one—of what there is to explore about sex. With all its diverse manifestations,
human sexual behavior abounds in practices that do not serve reproduction (Georgiadis and
K
ringelbach, 2012) and there is hardly any sexual activity, no matter how elabora
te or bizarre, that
Wierzba et al. NAPS ERO: erotic pictures database
could not be considered a potential source of sexual pleasure
(Lewontin, 2000). The complexity of human sexuality is a
classic example of the interplay of nature and nurture, whereby
inherited, biologic ally determined mechanisms interact with
cultural and environmental factors (
LeVay, 1993; Georgiadis and
Kringelbach, 2012).
With this conceptual framework, many manifestations of
sexual arousal—defined as physical or psychological readiness
to engage in sexual behavior (Stoléru et al., 2012; Costumero
et al., 2013)—may be proposed: from vague and intangible sexual
desire or attraction to observable, physiological genital response.
Classically, this ambiguity was overcome in research on sexuality
by simultaneously measuring subjective sexua l experience (self-
report) and using penile or vaginal photoplethysmography
techniques (Freund, 1963; Rieger et al., 2015). Both methods
were criticized for serious limitations they impose on research.
Whereas self-report measures of sexual behavior were frequently
considered unreliable and prone to inaccuracy and deception
(
Lewontin, 2000), peripheral response measurements such as
photoplethysmography were regarded as relatively intrusive and
largely ineffective (Ponseti et al., 2009).
More recently, sexuality became the subject of neuroimaging
research (for a review see: Georgiadis and Kringelbach, 2012;
Stoléru et al., 2012). As psychophysiological response to
sexual stimulation begins in the brain, neuroimaging methods
could emerge as the most effective in exploring sexual
arousal. In fact, it has been shown that the human brain is
involved in all phases of the human sexual response cycle:
from evoking arousal through consummation and orgasm to
satiation (Georgiadis and Kringelb ach, 201 2). To date a number
of neural structures underlying different aspects of sexual
response have been identified, among which the following were
mentioned: amygdala, midbrain, hippocampus, orbitofrontal
cortex, nucleus accumbens, subcallosal cortex, ventral anterior
cingulate, mediodorsal thalamic nucleus, hypothalamus and
visual cortex (Safron et al., 2007; Stoléru et al., 2012; Sylva et al.,
2013). The implication of this vast network of structures is still
being explored and different functional roles were proposed to
explain their contribution to sexual behavior (Sylva et al., 2013).
Yet, it remains uncertain whether there is anything unique and
exceptional about functional neuroanatomy of sex t hat would
differentiate it from other pleasures (
Georgiadis and Kringelbach,
2012).
Most obviously, the idea of sexual pleasure that would be
the same to everyone irrespective of individual differences is
quite abstract (Georgiadis and Kringelbach, 2012). Indeed, in
various kinds of sexual responses, significant differences between
men and women were observed (Rupp and Wallen, 2008,
2009; Stoléru et al., 2012; Sylva et al., 2013). In particular,
as demonstrated by subjective self-report and genital response
measurements (Chivers et al., 2004; Bailey, 2009; Rupp and
Wallen, 2009), as well as in neuroimaging research (Safron et al.,
2007; Sylva et al., 2013), male sexual arousal patterns are category
specific, i.e., men experience sexual arousal when exposed to
preferred-sex stimuli and little or no arousal to other sexual
stimuli. By contrast, women’s sexual arousal patterns are less
differentiated, as t h ey tend to show similar response to sexual
stimuli irrespective of the preferred-sex or non-preferred-sex
category.
Research on sexual arousal largely focused on male
participants, which is especially evident in the neuroscientific
domain (
Stoléru et al., 2012; Sylva et al., 2013). Relatively few
attempts were made to compare female and male participants by
means of neuroimaging and t he findings were rather ambiguous
(Gizewski et al., 2006; Sylva et al., 2013). Although there is
evidence that t he patterns of neural activation in men and
women are similar, differences in the intensity of neural response
were shown to exist. In particular, women revealed relatively
less activation in some brain regions involved in sexual arousal,
including amygdala a nd hypothalamus (Karama et al., 2002;
Hamann et al., 2004).
Only recently neuroimaging studies have begun to examine
sexual response in the context of sexual orientation (Savic
et al., 2005; Berglund et al., 2006; Kranz and Ishai, 2006;
Ponseti et al., 2006, 2009; Safron et al., 2007; Hu et al., 2008,
2011; Paul et al., 2008; Savic and Lindström, 2008; Zhang
et al., 2 011; Sylva et al., 2013
). Most evidence collected so
far points to similarities between homosexual a nd heterosexual
response patterns to sexual stimulation. In particular, category
specificity in arousal is characteristic of men irrespective of
their sexual orientation. When sexual stimuli matched the
participant’s stated preference, increased activity across multiple
brain regions was observed that generally did not differ with
sexual orientation (Safron et a l., 2007; Sylva et al., 2013).
Although differences in several brain regions were reported,
including: amygdala (
Safron et al., 2007), nucleus accumbens,
extrastriate, hypothalamus, and thalamus (Sylva et al., 2013),
these differences were mostly marginally significant and did not
survive controlling for multiple comparisons. On the other hand,
Ponseti et al. (2006) reported brain activation driven by sexual
preference, independent of either subject’s sex or the stimulus.
Moreover, it was demonstrated that brain response patterns to
sexual stimuli contained sufficient information to predict sexual
preference with high accuracy (Ponseti et al., 2009).
Interpretation of differences due to sex and sexual orientation
remains a challenge, especially if the stimuli used in experimental
research are not properly controlled (Rupp and Wallen, 2009). It
is uncertain whether t h e observed differences represent different
ways of arousal processing or merely different levels of arousal
associated with the stimuli (
Safron et al., 2007). As previous
findings suggested, men and women’s interest in and response
to visual sexual stimuli may be dependent upon the activities
and situations depicted (Rupp and Wallen, 2009). Thus, in
order to dissociate the influence of sex from that of sexual
orientation it is necessary to perform counter-balanced studies
involving different t ypes of experiment al material that appeals to
different groups of subjects—male and female, homosexual, and
heterosexual—at a comparable level.
To date, different modalities of stimuli were used to trig ger
sexual response. The most common included visual cues, usually
either still images or films (for an extensive review see: Stoléru
et al., 2012
). Attempts were also made to use other types of
stimuli: verbal (e.g., Stevenson et al., 2011), auditory, i.e., erotic
audio content or erotic prosody (e.g.,
Ethofer et al., 2007),
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Wierzba et al. NAPS ERO: erotic pictures database
olfactory, i.e., (artificial) pheromones (e.g., Savic et al., 2005;
Berglund et al., 2006, 2008; Savic and Lindström, 2008), or
somatosensory, i.e., penile or clitoral stimulation (e.g., Holstege
et al., 2003; Georgiadis and Holstege, 2005; Georgiadis et al., 2006,
2010
).
Different strategies were applied to the selection of stimuli and
the description of thi s process was usually relatively imprecise.
The stimuli used in sexual research most often included the
depictions of opposite-sex intercourse or interactions, as well as
male and female nudes (Stoléru et al., 2012). Limited number
of studies used same-sex intercourse or interactions as sexual
stimuli (e.g., Safron et al., 2007; Paul et al., 2 008; Hu et al.,
2008, 2011; Zhang et al., 2011; Sylva et al., 2013). Usually the
stimuli were preselected to elicit comparable levels of perceived
sexual arousal or sexual attractiveness (Stoléru et al., 2012). Other
subjective measures controlled for included: emotional valence
and emotional arousal (e.g.,
Ponseti et al., 2006; Jacob et al., 2011),
emotional intensity (e.g., Walter et al., 2008), pleasantness (e.g.,
Savic et al., 2005; Berglund et al., 2006), or perceived erection
(e.g., Arnow et al., 2002; Moulier et al., 2006; Mouras et al.,
2008). However, the validity of these subjective measures was—
with some notable exceptions—mostly based on the author’s a
priori evaluation or the opinions of a very small number of
independent judges. In fact, e ven in the most recent research
the selection of sexual stimuli was constrained by the lack of
adequately validated experimental material (e.g., Sescousse et al.,
2010, 2013; Demos et al., 2012; Kühn and Gallinat, 2014; Voon
et al., 2014). Finally, efforts to provide arousing sexual stimuli for
use by other researchers were rather limited (
Lang et al., 2008;
Rupp and Wallen, 2009; Jacob et al., 2011).
One of the most commonly recognized sources of
standardized visual affective stimuli is the I nternational Affective
Picture System (IAPS; Lang et al., 2008). This database provides
pictorial material from a wide range of content categories,
including a limited number of sexual stimuli. However, since the
original IAPS pictures were collected several decades ago, the
sexual stimuli in this dataset are now considered obsolete and
inadequate for experimental research (Jacob et al., 2011).
Recently, Jacob et al. (2011) introduced a set of 100 erotic
pictures to complement the IAPS database. In this study ratings
of 20 erotic stimuli from IAPS and 100 new erotic stimuli were
collected and made available to rese a rchers. Since the set was
intended for research with female subjects, two different picture
categories were defined: heterosexual couples in intimate or
erotic interaction (not sexually explicit); attractive single males
(not sexually explicit, i.e., no genitals were depicted). Similarly
to the IAPS, the stimuli colle ct ed by
Jacob et al. (2011) were
rated with regard to valence, arousal and dominance using Self-
Assessment Manikin (SAM, Bradley and Lang, 1994) scales.
The authors, however, focused on non-explicit sexual scenes,
considered suitable for research on intimacy and attachment.
Moreover, the set was only validated on heterosexual females,
which significantly limits its usefulness for research on other
populations.
Another set of stimuli was introduced by
Rupp and Wallen
(2009). In this study, a set of 216 sexually explicit photographs
of heterosexual couples was viewed and rated with regard to
sexual attractiveness by heterosexual subjects (men, women
not using hormonal contraception, women using hormonal
contraception). The set was divided into several categories
depicting different sexual activities (oral sex to male, oral sex
to female, female dominant intercourse facing male partner,
female dominant intercourse facing away from male partner,
male dominant intercourse from front of female partner, male
dominant intercourse from behind female partner). Although it
was not the primary aim of
Rupp and Wallen (2009) to create a
standardized database of stimuli, the pictures used in their study,
together with attractiveness ratings, are available on request. This
material, however, is suitable only for research on heterosexuals.
As outlined above, sexual stimuli currently used in research
vary substantially and attempts to provide suitable standardized
material were unsatisfactory. Moreover, since the vast majority
of research on the processing of sexual stimuli investigated sex
differences in heterosexual subjects, no stimuli appropriate for
studying sexual responses of non-heterosexuals (homosexuals,
bisexuals, etc.) have been provided and validated. Finally, images
used as sexual stimuli are often not fully specified and their
technical parameters are not controlled in experimental setting.
Such technical features are known to influence the processing
of images and hence should be considered in the process of
stimuli selection (
Knebel et al., 2008; Willenbockel et al., 2010).
Controlling for low-level features of an image was pointed out
as an important issue in a variety of me asuring methods, such
as eye-tracking, functional magnetic resonance imaging (fMRI),
magnetoencephalography (MEG), and electroencephalography
(EEG) (Willenbockel et al., 2010).
The erotic subset of the Nencki Affective Picture System
(NAPS ERO) is designed to address some of these limitations.
This collection of images was selected to represent a wide
range of content applicable in research on subjects of different
sex and sexual orientation. In the process of selecting erotic
images, we were primarily interested in images derived from non-
professional collections, depicting sexual content in a natural
manner. The subset contains images depicting same-sex and
opposite-sex couples engaged in different kinds of sexual contact,
as well as individuals sexually appealing to recipients of different
sexual orientati on.
The NAPS ERO dataset consists of standardized sexual
images, as well as commonly used normative ratings of valence
and arous a l. Additionally, several image characteristics were
computed to provide meta-data (width, height, luminance,
contrast, complexity, entropy, and color composition) allowing
for the sele ct ion of physically matching stimuli. The subjects
invited to rate the images (males and females) were selected to
represent extreme orientation toward either homosexuality or
heterosexuality, to best distinguish the stimuli that specifically
appeal to t he respective groups.
The following issues will be explored: (1) Overv iew of the
NAPS ERO set: A broad characteristics of the distribution of
the NAPS ERO ratings is provided, together with meta-data
(physical properties of the images); (2) Influence of sex and
sexual orientation on the preferred category of sexual stimuli:
Normative ratings corresponding to respective content categories
were compared and preferred categories for each group of
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Wierzba et al. NAPS ERO: erotic pictures database
subjects: homosexual males (HoM), homosexual females (HoF ),
heterosexual males (HeM) and heterosexual females (HeF) were
identified; (3) Influence of sex and sexual orientation on individual
image evaluations: Stimuli that best differentiate between the
HoM, HoF, HeM, and HeF groups are identified by means of
statistical tests performed on ratings of individual images; this
provides additional guidance for the use of the NAPS ERO
stimuli.
We hypot hesized that NAPS ERO ratings would allow to
differentiate stimuli perceived as preferable (i.e., positive in
valence and highly arousing) to each of the compared groups. In
particular, we expected that HoM would find images depicting
males and male couples as most appealing. HoF were expected
to prefer pictures of females and female couples. We predicted
that images of females, female couples and opposite-sex couples
would be sexually appealing to HeM. As for HeF, we assumed
that most preferable images would be t h ose depicting males and
opposite-sex couples.
The NAPS ERO dataset is freely available for research
community, so as to ensure a certain level of standardization
across studies using sexual visual stimuli and facilitate
experimental research.
Materials and Methods
Stimuli
The NAPS ERO subset consists of 200 erotic images initially
divided into the following content categories: Opposite-sex
Couple (50 pictures), Male Couple (50), Female Couple
(50), Male (25), and Female (25). The Opposite-sex Couple,
Male Couple and Female Couple categories contain images
of opposite-sex, male same-sex and female same-sex couples
respectively, explicitly engaged in sexual intercourse or sexual
interaction. The Male category contains photographs depicting
male individuals, and the Female category—female individuals
in an erotic, sexual setting. Examples of erotic pictures from each
category are presented in Figure 1.
The erotic pictures were obtained from Flickr (https://www.
flickr.com), an image hosting Internet service. From the initial
pool of nearly 1000 images, 200 were selected to represent a
diversity of sexual content. Six independent judges (3 females, 3
males) participated in the selection process by assigning images to
above-mentioned content categories. The judges were provided
with a brief summary of the purpose of the selection process,
as well as t h e description of the categories. Only pictures for
which full agreement of the judges was reached were included in
the set. Selected pictures were either under a Creative Commons
license (https://www.flickr.com/creativecommons) or used with
a written permission of the authors.
The selected images were colorful photographs, resized to
match the resolution of 1600 × 1200 (landscape) or 1200 × 1600
(portrait) pixels. All th e images were inspected with regard to
technical parameters (e.g., resolution, color, contrast, brightness)
and adjusted if necessary. Images containing visible commercial
logotypes or inscriptions were edited.
For each image we computed its technical parameters using
the Python Image Library (PIL, version 1.1.7; Python version
2.7.3). Similarly to other NAPS images, the following parameters
are provided: width, heig h t , luminance, contrast, complexity
(JPEG compression rate), entropy, and color composition (CIE
L
a
b
).
Additionally, 100 control images depicting people in non-
erotic context were selected from the NAPS People (50 pictures)
and Faces (50 pictures) categories. The control stimuli were
chosen to evenly represent the valence-arousal affective space
based on the normative ratings from the NAPS (
Marchewka et al.,
2014).
Sexual Orientation Questionnaires
The sexual orientation of t h e subjects was assessed with the
Kinsey Scale (
Kinsey et al., 1948/1998) and the Sell Assessment
of Sexual Orientation (Sell, 2007) questionnaire. Although the
Kinsey Scale is the most widely used instrument, it only
allows for a very general des cription of sexual identity. Several
shortcomings related to the use of the Kinsey Scale have
been pointed out (Sell, 2007). The Sell Assessment of Sexual
Orientation, on the other hand, provides much more precise
information on sexual preferences and behaviors, enabling the
determination of several aspects of sexual orientation (Sell,
2007). Since the questionnaires were not available in Polish,
the Kinsey Scale, the Sell Assessment of Sexual Orientation and
the Kinsey-type measures of sexual attractions, sexual contact
and sexual orientation identity were translated into Polish (3
FIGURE 1 | A sample image from each category. All images were obtained from Flickr and were published under a Creative Commons license. Credits
(from left to right): Charles Roffey, CC BY-N C- SA 2.0; David Shankbone, CC BY 2.0; Georgie Pauwels, CC BY 2.0; Charles Roffey, CC BY-NC-SA 2.0; Lies
Thru a Lens, CC BY 2.0. For license terms see: CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/); C C BY-NC-SA 2.0 (https://creativecommons.org/
licenses/by-nc-sa/2.0/).
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Wierzba et al. NAPS ERO: erotic pictures database
independent translators) and back-translated (1 translator) to
confirm the accuracy of the translation. The Polish versions of
the questionnaires can be found in the Supplementary Materials.
Participants
A total of 80 subjects aged 18–35: 40 homosexual (20 F, 2 0
M; age: M = 23.7, SD = 4.0) and 40 heterosexual (20 F,
20 M; age: M = 22.3, SD = 2.3) were invited to rate the
images. Most of the participants were college students or young
graduates from various faculties and departments (including
management, social sciences, philology, biology, medicine, as
well as technical faculties) of several universities and schools in
Warsaw. Several recruitment channels were used, including mass
mailings arranged by student unions, non-profit organizations
supporting non-heterosexual minorities, as well as social media
and personal communication.
Before they participated in the study, subjects completed
a short on-line screening questionnaire to assess their sexual
orientation wit h the Kinsey-type measures (
Sell, 2007). Only
individuals who manifested extreme orientation toward either
homosexuality (i.e., exclusively homosexual or predominately
homosexual, only incidentally heterosexual) or heterosexuality
(i.e., exclusively heterosexual or predominate ly heterosexual, only
incidentally homosexual) with respect to sexual attractions, sexual
contact and sexual orientation identity were invited to participate
in the study. After the rating procedure, the initial assignment to
particular experimental groups was further confirmed with the
Kinsey Scale (Kinsey et al., 1948/1998) and the Sell Assessment
of Sexual Orientation (
Sell, 2007) questionnaires. The above
inclusion criteria were imposed by the primary aim of the present
study, i.e., to provide a standardized set of sexual stimuli for use
in experimental research and to facilitate the selection of stimuli
with the desired effect on subjects of a given sex and sexual
orientation.
Subjects received a financial gratification for their
participation in the amount of PLN 30 (approximately EUR 7).
The local research ethics committee of the University of Warsaw
approved the experimental protocol of the study. A written
consent was obtained from each participant and the possibility
to quit the experiment at any point without stating reasons was
ensured.
Procedure
During the assessment procedure each subje c t worked on a
separate computer station equipped wit h a standard mouse. A
web application running on a local server was used to collect the
normative ratings. No time constraints to complete the task were
introduced, but an obligatory short break was scheduled. Each
session lasted approximately 60 min.
Each participant was presented with each of the 300 pictures.
The stimuli were divided and randomized within equal-sized
(50 images each) categories: Opposite-sex Couple, Male Couple,
Female Couple, Male + Female, Faces, and People. Then the
stimuli were grouped into blocks consisting of exactly one picture
from each of the six categories. The order of the pictures within
the block was randomized, while ensuring that no consecutive
stimuli belonged to the same content category.
The assessment task was preceded by brief instructions.
Subjects were able to return to the instruction screen or ask
for assistance when in doubt. The full text of the instructions
in Polish and its English translation are enclosed in the
Supplementary Materials.
The images were assessed one at a time. A single picture in
full-screen mode was displayed for 3 s, and then presented in a
smaller size in the left part of the screen along with the valence
and arousal 9-point SAM (
Bradley and Lang, 1994) rating scales.
Additionally, semantic labels describing the scales were used. The
valence scale ranged from 1—this picture elicits very negative
emotions in me, to 9—this picture elicits very positive emotions
in me. The arousal scale ranged from 1—I feel weak emotions, I
am not emotionally aroused, to 9—I feel strong emotions, I am
emotionally aroused. Subjects were encouraged to indicate their
immediate, spontaneous reaction to images. As soon as an image
was rat ed on both scales, the next screen with the subsequent
image was displayed.
Results
Overview of the NAPS ERO Set
Each picture in the NAPS ERO set was rated by 80
subjects, subdivided into equal-sized samples: homosexual males
(HoM), homosexual females (HoF), heterosexual males (HeM),
heterosexual females (HeF). Mean (M) and standard deviation
(SD) of valence and arousal ratings were calculated for each image
and each sample separately. The complete listing of subjective
ratings and technical parameters (width, height, luminance,
contrast, complexity, entropy, and color composition) of
all NAPS ERO stimuli can be found in Supplementary
Materials.
The distribution of the mean ratings of the NAPS ERO images
in the valence-arousal affective space is presented in Figure 2.
For each group of subjects, a similar curviline a r pattern of
distribution emerged, i.e., stimuli rated as extreme in valence
were a t the same time rated as highly arousing, whereas neutral
stimuli were rated as unarousing.
Erotic content was mostly rated as positive by all the groups
of subjects, except HeM. The latter identified many of the
erotic pictures as negative, especially those depicting males
and male couples. For the members of each group of subjects
preferable (i.e., positive in valence and highly arousing) erotic
stimuli were identified. Most of the pictures preferred by HoM
turned out to represent males or male couples. HoF, on the
other hand, preferred photographs depic ting females and female
couples. HeM also gave high ratings to many photographs
depicting females and female couples, as well as to opposite-
sex couples. HeF singled out pictures of opposite-sex couples
as particularly positive and arousing. However, HeF preference
for any specific type of erotic content appeared to be least
pronounced.
Sex and Sexual Orientation Differences: Image
Categories
As outlined in the previous section, members of each group
found some type of erotic content to be preferable in terms of
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Wierzba et al. NAPS ERO: erotic pictures database
FIGURE 2 | Distribution of the mean ratings obtained for 200 erotic images from NAPS ERO and 100 control non-erotic images from NAPS in the
affective space of valence and arousal. Each plot represents mean ratings obtained from a different group of participants.
valence and arousa l. Figure 2 shows that for each group such
preferred sexual stimuli belonged mostly to one or two content
categories. To test the statistical significance of this category
preference, we examined whether the initially defined subsets of
the NAPS ERO collection differed significantly in terms of the
way the pictures were rated by the members of different groups:
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Wierzba et al. NAPS ERO: erotic pictures database
TABLE 1 | Descriptive statistics (M—mean, SD—standard deviation) in the NAPS ERO calculated for valence (Val) and arousal (Aro) in homosexual
females (HoF), homosexual males (HoM), heterosexual females (HeF), and heterosexual males (HeM) for each content category.
Category HoF HoM HeF HeM
Val Aro Val Aro Val Aro Val Aro
Non-erotic (n = 100) M 4.13 3.79 4.48 3.97 4.49 4.61 4.63 4.13
SD 1.85 1.10 1.73 1.31 1.98 1.18 1.47 1.02
Opposite-sex couple (n = 50) M 5.26 3.17 6.21 3.57 6.55 4.86 5.96 4.40
SD 0.75 0.57 0.53 0.66 0.66 0.61 0.60 0.78
Male couple (n = 50) M 5.13 3.27 6.88 4.84 4.77 4.20 3.47 4.10
SD 0.84 0.52 0.69 0.91 0.64 0.62 0.64 0.84
Female couple (n = 50) M 6.73 4.64 5.75 3.17 5.09 3.97 5.56 4.26
SD 0.79 0.75 0.70 0.51 0.68 0.60 0.80 0.96
Male (n = 25) M 4.39 2.72 6.88 4.96 5.64 4.56 3.91 3.58
SD 0.66 0.53 0.64 0.92 0.71 0.55 0.58 0.68
Female (n = 25) M 6.80 4.96 5.27 2.98 5.46 4.13 6.96 5.48
SD 0.90 0.67 0.54 0.67 0.55 0.54 0.34 0.51
HoM, HoF, HeM, and HeF. The mean ra tings for each picture
category are summarized in Table 1.
First, for each subject we obtained me an valence and
arousal ratings of pictures representing each content category.
Using those mean category ratings, we performed ANOVA
for valence and arousal ratings separately, with group as a
between-subject factor (four levels: HoM, HoF, HeM, HeF) and
content category as a within-subject factor (six levels: Non-
erotic, Opposite-sex Couple, Male Couple, Female Couple, Male,
Female). Since we analyzed the mean category ratings, the
normality assumption was satisfied by the laws of the central limit
theorem.
With regard to valence ratings, we observed the main effect
of group F
(3, 76)
= 4.22, p < 0.008, η
2
= 0.14, the main
effect of content category F
(3.84, 292.15)
= 69.70, p < 0.001,
η
2
= 0.48, as well as the interaction between these two factors
F
(11.53, 292.15)
= 38.19, p < 0.000, η
2
= 0.60. The analysis of
post hoc Bonferroni corrected pairwise comparisons revealed a
significant difference (p = 0.04) in the valence ratings of non-
erotic pictures between HeM (M = 4.63, SD = 1.47) and HoF
(M = 4.13, SD = 1.85). No significant differences in the valence
ratings of non-erotic pictures were observed between other
compared groups. In contrast, valence ratings of erotic pictures
depended strongly on group. The detailed results can be found in
Figure 3.
The main effect of group on arousal ratings was not significant
F
(3, 76)
= 1.24, p = 0.30, η
2
= 0.05. Still, arousal ratings
were shown to depend on picture content, with th e main
effect of picture category F
(3.53, 268.57)
= 4.53, p < 0.002,
η
2
= 0.06. The interaction effe ct of group and cate gory was
also significant F
(10.60, 268.57)
= 29.46, p < 0.001, η
2
=
0.54. The post hoc tests revealed no significant differences in
arousal ratings for non-erotic pictures. Arousal ratings of erotic
pictures depended significa ntly on whether the subject belonged
to HoM, HoF, HeM, or HeF, but these differences appeared
to be less pronounced than those for valence, as indicated in
Figure 3.
Sex and Sexual Orientation Differences:
Individual Images
In the previous section we identified differences in the category
mean ratings suggesting a strong preference for a particular
category of sexual stimuli as dependent on group. However,
the strength of this preference depends essentially on many
characteristics of the given erotic image. In particular, images
rated signific antly different by different groups are potentially
valuable as stimuli in experimental designs focused on sex and
sexual orientati on differences.
To facilitate the use of the mean ratings provided with the
NAPS ERO dataset, we investigated sex and sexual orientation
differences in erotic content evaluations of e ach individual
image. We identified stimuli marked by significant differences
(as shown by ANOVAs and t-tests) and provided the results
of this analysis along with the effect sizes as another set of
indexing parameters, in addition to mean ratings (i.e., Ms and
SDs). It should be noted that the normality assumption was
not met for the tests of individual ratings. Since violations of
the normality assumption generally have rather small effect on
the summary score distribution (e.g.,
Loftus and Loftus, 1988;
Howell, 2013), we present the results obtained from the statistical
tests as approximate parameters to guide the selection process of
the sti muli.
To this end, Two-Way (sex × sexual orientation) ANOVAs
were performed for valence and arousal for each picture
separately to identify those with significant main effect s and
interaction effects on affective evaluation. For both affective
Frontiers in Psychology | www.frontiersin.org 7 September 2015 | Volume 6 | Article 1336
Wierzba et al. NAPS ERO: erotic pictures database
FIGURE 3 | Pairwise comparisons of image ratings averaged over all subjects in a given group and over all images in a given category. Each comparison
is done separately for valence and arousal. Significant differences (p < 0.05) are marked with an asterisk. Notation: HoM, homosexual males; HoF, homosexual
females; HeM, heterosexual males; HeF, heterosexual females; N, Non-erotic; OC, Opposite-sex Couple; MC, Male Couple; FC, Female Couple; M, Male; F, Female.
dimensions the η
2
and p-value were provided for each effect:
sex, sexual orientation and sex × sexual orientation interaction.
Moreover, a series of t-tests were performed to distinguish
images that best differentiate between each of the possible
pairs of groups: HoM—HeM, HoM—HoF, HoM—HeF, HeM—
HoF, HeM—HeF, HoF—HeF. The difference of means, Cohen’s
d and p-value were obtained for each variable (valence and
arousal) and each picture in each comparison. Differences in
the means between the respective groups are presented in
Figure 4.
The complete ratings along with the ANOVAs a nd t-tests c an
be accessed online at http://naps.nencki.gov.pl as an interactive
data browser.
Discussion
In the present work we introduce the NAPS ERO—a database of
200 erotic images of broad applicability to experimental research
in a wide range of disciplines. The collection includes images
suitable for studies on subjects differing in terms of sex or sexual
orientation. To date, few attempts have been made to standardize
or validate sexual stimuli, the results of which were rather
unsatisfactory for reasons such as obsolescence and paucity of
stimuli (
Lang et al., 2008) or validity confirmed only for specific
populations (
Lang et al., 200 8; Rupp and Wallen, 2009; Jacob
et al., 2011). As a rule, stimuli used to trigger sexual responses
were selected on a priori basis and their properties were rarely
controlled or a dequately described. This variety of experimental
material may have accounted for some of the inconsistencies
between the findings in previous studies, as they might have used
stimuli of unequal interest to the compared groups. Substantial
discrepancies between effect sizes in comparisons of groups
differing in sex or sexual orientation reported in research may be
due to the use of uncontrolled, dissimilar experimental material
(Rupp and Wallen, 2009).
The NAPS ERO dataset provides erotic images together with
their normative ratings of valence and arousal, as well as technical
parameters of the images. Erotic images included in the database
differ in the explicitness of sexual content, providing both
nuanced and highly explicit scenes. Thus, valence and arousal
ratings can be treated as parameters guiding the selection of
stimuli according to the requirements of a given experiment a l
design. Additionally, the NAPS ERO images are divided into
several categories, as such classification has proven useful in
Frontiers in Psychology | www.frontiersin.org 8 September 2015 | Volume 6 | Article 1336
Wierzba et al. NAPS ERO: erotic pictures database
FIGURE 4 | Differences in the mean ratings between the respective samples (HoM—HeM, HoM—HoF, HoM—HeF, HeM—HoF, HeM—HeF, HoF—HeF)
as obtained for each of the NAPS ERO images (200 erotic, 100 non-erotic), for valence and arousal separately. Each bin represents a difference in the
mean ratings of a given image. Significant differences (p < 0.05) are marked by a darkening of a bin. Bins are sorted according to the size of a difference in mean
ratings. Images rated higher by one group in a given comparison are aggregated on both sides of the plot, whereas images for which no difference was obtained can
be found around the middle of the plot.
Frontiers in Psychology | www.frontiersin.org 9 September 2015 | Volume 6 | Article 1336
Wierzba et al. NAPS ERO: erotic pictures database
previous research. Since categorizations of sexual stimuli usually
differentiated opposite-sex and same-sex sc enes, as well as male
and female nudes (Stoléru et al., 2012), we decided to adopt
similar labeling and divide the NAPS ERO collection into
Opposite-sex Couple, Male Couple, Female Couple, Male, and
Female content categories.
Another advantage of the NAPS ERO is the possibility of
choosing stimuli according to both sex and sexual orientation
of the subjects. Since previous studies focused mainly on
heterosexual male participants, valid a ted stimuli suitable for
the exploration of sexual mechanisms in different groups are
hard t o come by. Thus, in the NAPS ERO we provide ratings
obtained from four distinct samples strictly controlled for sexual
orientation (
Kinsey et al., 1948/1998; Sell, 2007): h omosexual
males, homosexual females, heterosexual males and heterosexual
females. Our decision to collect ratings only from subjects
manifesting extreme orientation toward either homosexuality or
heterosexuality was imposed by the primary aim of the present
study, i.e., to provide a standardized set of sexual stimuli for
use in experimental research and to most reliably detect those
features of the stimuli t hat differentiate populations with respect
to sexual response. In this way, stimuli could well be used in
an empirical study on subjects of ambiguous sexual identity or
sexual orientation (e.g., transgender, bisexual) by interpolation
of their preferences for erotic stimuli from the initial measures
we provide. Since considerable evidence was provided for the
influence of sexual orientation on the perception and processing
of sexual stimuli (e.g.,
Ponseti et al., 2006, 2009), it appears to be
an important direct ion of future research.
The comparison of ratings obt ained from different groups of
subjects revealed that the evaluation of erotic content depended
substantially on sex and sexual orientation of the subjects,
showing that it is necessary to control for these two factors when
choosing experimental material. In line with previous research
on category specificity in sexual arousal patterns (
Chivers et al.,
2004; Safron et al., 2007; Bailey, 2009; Rupp and Wallen, 2009;
Sylva et al., 2013), our results provide evidence for the claim
that both h omosexuals and heterosexuals prefer speci fic erotic
content. Congruent with previous findings (e.g., Ba iley, 2009),
the sexual arous al pattern for heterosexual females appeared to
be less differentiated. Substantial variability of ratings within
the predefined content categories, however, indicates that no
strict categorization can be recognized as appropriate. Since the
strength of the preference for a particular type of sexual stimuli
depends essentially on many characteristics of an image, stimuli
intended to be especially appealing to a given group should
be chosen on the basis of mean ratings (e.g., valence, arousal).
This highlights the need for a more data-driven approach,
rather than the reliance on oversimplified a priori categories of
preferred vs. non-preferred stimuli. The interpretation of the
group differences was previously raised as an important issue.
As the stimuli used in research are not controlled for their
emotional impact, it is unresolved whether the observed group
differences result from different ways of arousal processing or
different level of arousal associated with the stimuli (
Safron et al.,
2007; Rupp and Wallen, 2009). The NAPS ERO provides such an
opportunity, allowing researchers to choose stimuli according to
the mean ratings of valence and arousal, as well as to control for
their technical parameters. Apart from mean ratings, we provide
pairwise comparisons between the mean ratings of different
groups. The statistical tests performed for individual stimuli,
along with t he effect sizes are approximate measures intended
to provide additional guidance in t h e selection process of the
experimental material. Such differential measures may be of
interest to researchers concerned with sex and sexual orientation
differences. The NAPS ERO images, as well as the normative
data are freely available from: http://naps.nencki.gov.pl, upon the
completion of the registration form. Moreover, the normative
data (ratings and statistical parameters) can be accessed in the
form of an interactive data browser (registration required).
Limitations and Future Directions
As pointed out in the previous sections, sexual stimuli were
referred to with t h e use of variety of subjective measures,
among which perceived sexual arousal was most frequently
mentioned (
Stoléru et al., 2012). Although simultaneous control
for emotional valence and emotional arousal possibly allows
distinguishing sti muli recognized as sexually arousing, it is
important not to confuse these two approaches. Additionally,
apart from the subjective ratings aimed to approximate the
level of sexual arousal experienced by the subjects differing in
terms of sex and sexual orientation, experimental material
provided with NAPS ERO should be further validated
with the use of more objective methods of measurement,
such as photoplethysmography, EDA, eye-tracking
or fMRI.
Another important issue concerns the methods to control
for t h e sexuality of the subjects. Since NAPS ERO ratings have
been collected from individuals manifesting extreme orientation
toward either homosexuality or heterosexuality, furthe r research
should validate the present material on other populations.
Moreover, although most of the previous studies roughly
identified their subjects’ sexuality with self-report methods,
such as Kinsey scale (Kinsey et al., 1948/1998), they certainly
are perceived as imprecise and prone to inaccuracy. Recently,
evidence was provided for the necessity to control for hormone
level in the research on sexual arousal, since the motivation for
and response to erotic content may be strongly influenced by
both the prenatal hormone level, as well as hormonal fluctuations
due to menstrual cycle (e.g.,
Gizewski et al., 2006; Rupp and
Wallen, 2009). Thus, group differences in the NAPS ERO
ratings should be further verified with the control of hormone
level.
Acknowledgments
We appreciate the help of Paweł Turnau in preparing the web
application for the purpose of ratings collection, as well as in
developing the interactive data browser (http://naps.nencki.gov.
pl). We are also grateful to Maciej Skorko and the VR Lab
(http://vrlab.edu.pl/) for the possibility of using GEx platform
to collect the questionnaire data. Finally, we would like to
thank Maksymilian Bielecki for his helpful recommendations
regarding statistical analysis. This s tudy was supported by: the
Frontiers in Psychology | www.frontiersin.org 10 September 2015 | Volume 6 | Article 1336
Wierzba et al. NAPS ERO: erotic pictures database
Polish Ministry of Science and Higher Education, Iuventus Plus
Grant No. IP 2012 042072 (A. Marchewka); the National Science
Centre, PRELUDIUM grant, number 2013/11/N/HS6/0178 6 (M.
Wierzba); and by the BST grant from the University of Warsaw
(W. Dragan).
Supplementary Material
The Supplementary Material for this article can be found
online at: http://journal.frontiersin.org/article/10.3389/fpsyg.
2015.01336
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05.021
Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2015 Wierzba, Riegel, Pucz, Le
´
sniewska, Dragan, Gola, Jednoróg and
Marchewka. This is an open-access article distributed under the terms of the Creative
Commons Attribution License (CC BY). The use, di stri bution or reproduction in
other forums is permitted, provided the original author(s) or licensor are credited
and that the original publication in this journal is cited, in accordance with accepted
academic practice. No use, distribution or reproduction is permitted which does not
comply with these terms.
Frontiers in Psychology | www.frontiersin.org 12 September 2015 | Volume 6 | Article 1336
Wierzba et al. NAPS ERO: erotic pictures database
Description of the database
ID Name identifying each of the images; the name contains
the information on the content c at e gory name, the ordinal
number and the subscript referring to the image orientation
(h horizontal, v vertical)
Category Name of the erotic content category, i.e. Opposite-
sex Couple, Male Couple, Female Couple, Male, Female
Nr Ordinal number of the image, as counted separately for
each of the content category
V/H Denotes image orientation: h horizont al, v - vertical
val_M_HoF - Mean (M) of valence ratings in the homosexual
female group, ranging from 1 to 9, where: 1 very negative
emotions, to 9 - very positive emotions
val_SD_HoF - Standard de via tion (SD) of valence ratings in the
homosexual female group
val_M_HoM - Mean (M) of valence ratings in the homosexual
male group, ranging from 1 to 9, where: 1 very negative
emotions, to 9 - very positive emotions
val_SD_HoM - Standard deviation (SD) of valence ratings in the
homosexual male group
val_M_HeF - Mean (M) of valence ratings in the heterosexual
female group, ranging from 1 to 9, where: 1 very negative
emotions, to 9 - very positive emotions
val_SD_HeF - Standard devia tion (SD) of valence ratings in the
heterosexual female group
val_M_HeM - Mean (M) of valence ratings in t h e heterosexual
male group, ranging from 1 to 9, where: 1 very negative
emotions, to 9 - very positive emotions
val_SD_HeM - Sta nd ard deviation (SD) of valence ratings in the
heterosexual male group
aro_M_HoF - Mean (M) of arousal ratings in the homosexual
female group, ranging from 1 to 9, where: 1 weak emotions,
being emotionally unaroused, to 9 - strong emotions, being
emotionally aroused
aro_SD_HoF - Stand a rd deviation (SD) of arousal ratings in the
homosexual female group
aro_M_HoM - Mean (M) of arousal ratings in the homosexual
male group, ranging from 1 to 9, where: 1 weak emotions, being
emotionally unaroused, to 9 - strong emotions, being emotionally
aroused
aro_SD_HoM - Standard deviation (SD) of arousal ratings in
the h omosexual male group
aro_M_HeF - Mean (M) of arousal ratings in the heterosexual
female group, ranging from 1 to 9, where: 1 weak emotions,
being emotionally unaroused, to 9 - strong emotions, being
emotionally aroused
aro_SD_HeF - Standard deviation (SD) of arousal ratings in the
heterosexual female group
aro_M_HeM - Mean (M) of arousal ratings in the heterosexual
male group, ranging from 1 to 9, where: 1 weak emotions,
being emotionally unaroused, to 9 - strong emotions, being
emotionally aroused
aro_SD_HeM - Standard deviation (SD) of arousal ratings in the
heterosexual male group
Width Image width in pixels
Height Image height in pixels
Luminance - Average pixel values of grayscaled image (PI L
library, ver. 1.1.7)
Contrast - Standard deviation of pixel values of grayscaled image
(PIL library, ver. 1.1.7)
JPEG_size80 JPEG size as an index of overall complexity of
an image. JPEG size was determined for color image with a
compression quality setting of 80 on a scale from 1 to 100 (PIL
library, ver. 1.1.7). The bigger the JPEG size, the more complex
an image
LABL Average luminance (L
) in the CIE L
a
b
color space
(L
= 0 for black and L
= 100 for white pixels) (PIL library,
ver. 1.1.7)
LABA - Average a
value in the CIE L
a
b
color space
(a
= 100 for green and a
= 100 for red pixels) (PIL library,
ver. 1.1.7)
LABB - Average b
value in the CIE L
a
b
color space
(b
= 100 for blue and b
= 100 for yellow pixels) (PIL library,
ver. 1.1.7)
Entropy - Entropy (H) of gray scaled image, computed from the
histogram of 8-bit gray-level intensity values x: H = 6p(x)log
p(x), where p represents t he frequency of intensity x (PIL
library, ver. 1.1.7). The higher the entropy, the more random
the image
Frontiers in Psychology | www.frontiersin.org 13 September 2015 | Volume 6 | Article 1336
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