Filtered beauty in Oslo and Tokyo: A spatial frequency analysis of facial attractiveness. Morten Øvervoll et al. PLOS, January 14, 2020. https://doi.org/10.1371/journal.pone.0227513
Abstract: Images of European female and male faces were digitally processed to generate spatial frequency (SF) filtered images containing only a narrow band of visual information within the Fourier spectrum. The original unfiltered images and four SF filtered images (low, medium-low, medium-high and high) were then paired in trials that kept constant SF band and face gender and participants made a forced-choice decision about the more attractive among the two faces. In this way, we aimed at identifying those specific SF bands where forced-choice preferences corresponded best to forced-choice judgements made when viewing the natural, broadband, facial images. We found that aesthetic preferences dissociated across SFs and face gender, but similarly for participants from Asia (Japan) and Europe (Norway). Specifically, preferences when viewing SF filtered images were best related to the preference with the broadband face images when viewing the highest filtering band for the female faces (about 48–77 cycles per face). In contrast, for the male faces, the medium-low SF band (about 11–19 cpf) related best to choices made with the natural facial images. Eye tracking provided converging evidence for the above, gender-related, SF dissociations. We suggest greater aesthetic relevance of the mobile and communicative parts for the female face and, conversely, of the rigid, structural, parts for the male face for facial aesthetics.
Discussion
A
key question about what constitutes our sense of aesthetics is what
kind of visual information within the stimulus underlies our judgements.
Despite the spatial frequency structure of any visual stimulus is
processed very early by the visual brain and several studies have
addressed its role in the identification of facial identity and/or
expression [
81,
82],
very few studies have specifically investigated the role of visual
spatial frequency information in supporting our sense of facial
aesthetics. That different face relevant types of visual information can
be optimally channeled through different bands of spatial frequencies
is well known for emotional expressions, but the possibility that a
similar relationship occur for aesthetic cues has not been fully
explored yet. It is very likely that other facial information, seemingly
unrelated to visual spatial frequency, plays a relevant role in
judgments of attractiveness (e.g., skin’s tone [
83,
84]),
but spatial frequency may play a role beyond the coding of facial
shape. In particular, the optimal perception of several of surface and
texture cues may be confined within specific bands of spatial
information (e.g., the thin lines or creases revealing age or the colors
of small parts like the irises). The appreciation of the colors (or
discoloring) of small-width or thin facial parts (like the mouth lips)
may also depend on high frequency information that may be smeared and
significantly weakened in visibility at low spatial frequencies.
Hence,
for the present study, we gathered evidence that forced-choice
preferences when viewing specific SF bands of face images relate
positively to preferences when viewing the corresponding broadband
facial images. Our approach consisted in filtering spatial frequencies
out of the natural face’s photo image (
Fig 1).
We then presented the obtained face images, containing a narrow band of
SF information, in a “beauty contest” between same-sex face pairs (
Fig 2).
Although all of our photo images depicted faces of really existing
European individuals, the participants of the present study belonged to
different populations recruited in Europe and Asia (i.e., Norway and
Japan).
The
gender of the faces had a strong effect on which spatial frequencies
were closest related to the same individuals’ decisions when performing
the task with the unfiltered, natural looking, face pairs. That is,
female faces related 80% of the time to choices made with the broadband
faces, when viewing the highest of the four SF bands (
Fig 3).
In contrast, male faces related slightly above 80% of the time to
choices made with broadband faces when viewing the second lowest of the
four SF bands included in this study (
Fig 3).
We note that all of the SF bands related above 65% of time to choices
made with broadband faces, indicating that all SF bands contribute to
some extent with information relevant to aesthetics decision, although
apparently in different doses. Thus, it would appear that medium-low
spatial frequencies contains visual information that is most relevant
for aesthetic decisions made about male faces, but the high spatial
frequencies contain key information for decision about female faces.
This dissociation is to our knowledge a novel finding, which could lead
to identifying detailed gender-specific visual cues.
The
oculomotor behavior provided converging evidence for the relevance of
the medium-low spatial frequencies for male faces and high spatial
frequencies for female faces. The attractive face in a pair was not only
looked more in general than the unattractive, but gaze lingered the
most over the attractive female face when the face pairs were shown with
the highest (SF 4) filtering. Consistently, gaze dwelled the longest
over the attractive male face when seeing the face pairs with the
medium-low spatial (SF 2) filtering (
Fig 4).
In
addition, to get a sense of what information is contained and visible
in the stimuli and within each band of SF information, we provided
visualizations of how this content related to observers’ judgements.
There was high similarity for the female faces between the two highest
(SF 3 and 4) and the broadband faces within a small, bilateral, region
(overlapping the eye pupil and the lower eyelid and infraorbital
concavity but including the upper part of the zygomatic convexity;
Fig 8).
In addition, similarity between the filtered and unfiltered faces was
higher for male models than female models for the area around the brows,
nasal root, and eyes, and especially so for the (medium low) SF 2.
Instead, for the medium high and high spatial frequencies (SF 3 and 4),
the similarity was higher for female models than male models for the
area around the lower eyelid and infraorbital region. Remarkably, the
relative distributions of gaze when viewing these SF bands closely
matched these similarity profiles.
Statistical
analyses on the Lempel-Ziv complexity confirmed that the female faces
contained significantly more information than the male faces in the
above-described regions (
Fig 9).
However, information content was significantly high in the low SF bands
only for the male faces; in particular, for the central eyebrow region,
including the skull area immediately above (i.e., the supraorbital
process or brow ridge) in SF 1, and the glabella of the nose and lowest
nose region (including the nostrils) in SF 2.
Importantly,
there was a striking dissociation between SFs for male and female faces
in relation to the relevance of vertically oriented frequencies for
attractiveness (normative) ratings. As visible in
Fig 10,
different spatial frequencies related to the ratings, revealing that
for female faces, there was a significant positive correlation between
attractiveness and amplitude of high vertical frequencies for female
faces and at low vertical frequencies for male faces. Taking into
consideration also the eye-tracking data, participants had a strong
tendency to look at the faces along the whole axis of the nose (
Fig 7),
in particular in the European group, extending as low as the upper lip
(philtum) and the Cupid’s bow at the center of the mouth, more so with
increasing spatial frequency. This gaze behavior seems consistent with
the preponderant role of the central, vertically oriented, features for
attractiveness (normative) ratings.
Being
the nose at the center of both the vertical and horizontal axes of the
‘face’ (Nota Bene: below the hairline, not the head), it is presumable
that it constitutes an important element to focus gaze when evaluating
facial proportions, the configuration and global harmony or symmetry of
the face [
58]. When spatial frequency is high, the volumetric aspect of the nose, relatively more relevant for the male face (
Fig 11,
right panel), becomes less visible. The nose is the most sexually
dimorphic facial trait in its morphology, being on average
disproportionally larger in volume in male than female faces [
85–
87].
While the visibility of the nose’s volume decreases that of its shape
and symmetry increases with higher spatial frequencies and the latter
features appear more relevant for judging the attractiveness of the
female faces (
Fig 11, left panel). Since gaze scanning (
Fig 7)
revealed a strong tendency to focus gaze at the root of the nose, or
onto the central portion of the face that may correspond to the limiting
size for efficient summation of configurational properties of upright
(vertically oriented) face information in a single configurational face
template [
88]. The eyes, being paired features, horizontally centered together with the vertical prominence of the nose [
89],
may also convey essential information on a face’s proportions and
symmetry, and more clearly so in higher spatial frequencies conditions.
It
also seems of interest that the dispersion of gaze over the eyes, nose
and mouth region differs in our European and Asian groups (
Fig 7).
The typical T-shaped focus pattern appears mainly with the European
participants and increasingly so with higher spatial frequencies. In
fact, the pattern of fixations is consistent with previous reports that
Asians (i.e., Chinese) tend to look less at the eyes and distribute less
their gaze over the face [
90–
92].
Especially within Japanese culture, a prolonged eye contact may be
disrespectful and Japanese children are taught to look at others’ necks
instead of the eyes [
93,
94].
Perhaps
the most remarkable dissociation between female and male features
related to attractiveness, revealed by the present study’s Fourier
approach, is between the two faces in
Fig 11.
These show graphic representations of the spatial frequencies that
correlate positively with the stimuli’s normative attractiveness ratings
(collected independently of the present eye-tracking study and only
with Norwegian raters). A striking difference between the two genders’
images is that they show very different, little overlapping, SF
components. Moreover, these SF components impressively overlap with the
SF bands most relevant for forced choices, derived from the present
eye-tracking study (
Fig 3). For the male face (
Fig 11,
right panel), the attractiveness-correlated SF provide only a coarse
visual resolution of the face, which however clearly conveys the depth
or volumetric aspect of the head and face, with its overall size, extent
of the face contour (the jaw and chin), and skull’s bone structure.
These three-dimensional aspects of the male’s whole face or skull
structure may be important in judging overall proportions. In contrast,
the female face’s (in
Fig 11,
left panel) attractiveness-correlated SFs, not only show little overlap
with the male’s, but they suggest that female attractiveness may be
judged more on information carried by higher spatial frequencies. These
may reveal local information about the surface of the face and of
specific features at a level of detail that is optimal also for the task
of individual person recognition and the communication of emotional
signals.
In
particular, internal features of the female face like the brow ridge,
eyes, mouth, as well as the lower part of the face contour or chin, and
their immediately surrounding facial surface regions, are clearly
visible in the left panel image. We surmise that the high resolution of
the above traits allows a more precise evaluation of the arrangements,
spatial relations, or distance ratios between these features (e.g., the
inter-ocular distance). There are several suggestions in the literature
on facial beauty (also from anthropology, odontology, and aesthetic
medical surgery) that our sense of face attractiveness may seek a
“golden ratio” between facial traits like the eyes and mouth/teeth and
the general proportions of the face ([
95] but see [
96]).
We surmise that at HSF resolutions, information is optimal for spotting
the presence of skin blemishes and the smoothness surface skin (i.e.,
cues of age or poor health) as well as details of the eye region
affording the registering of subtle differences in eyelids’ and orbital
region shape. If smooth skin is crucial for attractiveness in female
faces and these properties of surface skin are best represented in high
spatial frequencies, then amplitudes of higher frequencies should
correlate with attractiveness ratings, since these frequencies make
visible these aspects. We also note that the irises’ colors as well as
the size of the pupils seem clearly delineated at such resolution.
Instead, the colors of the irises would be smeared at LSF and,
interestingly, previous research suggests that eye color may be more
relevant when judging female than male faces for attractiveness [
13].
Similarly, the highly mobile pupils may be particularly important for
signaling social agreeableness, interest and attraction [
8,
97]. We note that our behavioral and gaze results in the main experiment seem consistent with this ideas.
Moreover,
the lower portion of the nose (nostrils) and the fullness of the lips
(or vermilions) appear clearly visible within these
attractiveness-correlated spatial frequencies and shape imperfections
and coloring, luminance contrast between sides of the Cupid’s bow, may
be very salient at this high resolution. Thus, female faces’
attractiveness-related SFs may reveal subtle deformations over the face
surface, skin, and be related to the soft and malleable elements of the
face, instead of its rigid skull structure. These highly mobile parts of
the face like the mouth, eyes and eyebrows, all allow the display of
subtle affiliative emotions [
98], which may also play a key role when judging the attractiveness of an individual, even when just looking at static images [
72].
In the male image in
Fig 11
(right panel), a region around the ocular orbits, including the
eyebrows and the bony area immediately above (i.e., the supraorbital
process or ridge and glabella), as well as a region below the eyes and
cheeks’ zygomas, appears well delineated in volumetry. Interestingly,
the lower portions of the nose and of the mouth’s upper region play a
role for male attractiveness, despite at such a coarse level of
resolution the separations between the nostrils or lips are not
resolved. Instead, the three-dimensional or volumetric aspects of the
chin (in particular the protuberance of the mandible and its breadth)
appear to be very salient. A possibility is that the coarse LSF
prevalence in the image, by revealing the bony prominence of the brow
ridge and of the jaw and chin, conveys effectively the attribute of
masculinity inherent in the face [
62,
99,
100]. In addition, a large face size characterizes masculinity as opposed to femininity [
101].
However, several researchers have cautioned that masculinity may
predict attractiveness relatively weakly compared to other fluctuating
properties like skin color [
102–
104] or face and body symmetry [
105,
106], which signal immunocompetence. Said and Todorov [
18]
found a gender-specific dissociation in the effects of shape (e.g.,
face width) or reflectance (e.g., lightness and color of skin).
Increases towards masculinity in reflectance aspects of the male face
increased attractiveness, but doing the same in shape aspects decreased
it. We surmise that despite the coarse LSF male image (in
Fig 11)
both the reflectance of skin and of the brows are clearly visible.
Interestingly, the reflectance dimensions with the strongest effects on
female attractiveness involved the contrast around the eyes and the
redness of the lips, which may be both best visible at higher SF.
Indeed, the HSF prominence in the image of the female face’ in
Fig 11
yields a more detailed but somewhat less volumetric rendition (with
slightly “embossed” features to use an art metaphor). What is visible
appears related not only to highly mobile parts of the face that allow
the display of subtle affiliative emotions but also to several cues
associated with a sense of femininity [
107,
108]. Sexual dimorphism correspond to different directions in morphometric space [
108]
and the female direction is associated with horizontal reduction of the
chin, a forward movement of the gonion (jaw angle) and alveolar
prognathism. In
Fig 11, the male chin is clearly more visible than the female and appears larger in the morphed image.
The eye-tracking results confirmed that the beautiful faces are strong attractors of attention [
109], since participants spent about 10% more time dwelling onto the attractive face in a pair (
Fig 5)
than on the relatively less attractive one. It has been shown that the
attentional priority towards attractive faces can also occur
unconsciously [
110]
and that a decision about a face’s level of attractiveness can be
reached very rapidly (within 33 ms), and not very differently than when
having unlimited time [
111].
However, the present results are consistent with several previous
studies showing that we typically spend extra time looking at faces
considered attractive [
112–
114].
Finally, a previous study [
38]
used Fourier power spectrum analyses to describe the relation between
spatial frequency and power of the radially averaged (1d) Fourier
spectrum on a log-log scale. As the researchers point out, most natural
(complex) images show a linear relationship and the relative strength or
‘power’ of fine detail information or coarse structure in an image can
be, respectively, expressed linearly be the angle of the slopes in the
power plots. Importantly, enhanced HSF information leads to shallow
slopes, whereas enhanced LSF information leads to steep slopes. Given
that pleasing natural scenes and artworks share a shallow power slope of
-2 [
115],
the authors hypothesized that also faces approaching a Fourier power
slope of -2 (i.e., with enhanced HSF information) would be considered
more attractive than the same face, or others, differing from this value
(e.g., steeper slopes between -3 and -4). Remarkably, when participants
were given the opportunity to manually adjust the Fourier slope of the
images on screen, they did choose a mean value of -2.6, which is a bit
closer to that of pleasing natural scenes or artistic facial portraits.
The effect was significantly larger for female faces, which also seems
consistent with the present study’s findings of a bias for HSF
information for female faces. A limitation of the Fourier slope approach
is that it is informative about the relative distribution of frequency
power, but not specific frequency bands. We surmise that, by presenting
ranges of SF information separately, we are likely to reveal which
information contained in the natural stimulus directly related to the
aesthetic judgment about a face. In contrast, by strengthening or adding
one type
of visual information by distorting the natural
image, one can reveal directional biases and explore the limits within
which a face’s attractiveness can be enhanced [
17].