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In Search of the Most Attractive Lip Proportions and Lip Vol… : Plastic and Reconstructive Surgery


Minimally invasive lip augmentation using soft-tissue fillers is one of the most desired aesthetic procedures in female patients.1 Whereas the increase in lip volume is mostly requested, aesthetic providers need to also consider lip contouring, facial proportions, and the ratio between the upper and the lower lips to obtain a harmonious and aesthetically appealing result. Despite current trends in social media to substantially increase the lip volume and alter lip proportions, the most accepted ratio for beautiful lips is 1:1.6, with the lower lip being larger than the upper lip.2,3 This ratio was previously termed the “golden ratio” and is considered to be the achievable standard in lip augmentation procedures.4

The increased lip volume, the undulated shape, and the redness of lips are thought to be evolutional developments in humans compared to primates and to play a key role in nonverbal communication between individuals.5 These nonverbal signals are capable of generating positive emotions that have been shown to alter the behavior of the observer.6 Altering these signals (i.e., changing the volume and the shape of lips) can have different effects on the observer.

Recent technologic advancements have enabled researchers to quantify and to analyze emotions by capturing eye movements and gaze patterns with eye-tracking devices.7 The time until first fixation and the duration of a stable eye fixation toward a visual stimulus have been shown to be reliable and predictable markers when it comes to decoding emotions8 and analyzing differences in eye movement patterns.9 Using eye tracking technology could be a useful tool to objectively quantify the response of altered lip volume and lip shapes and to reevaluate the most aesthetically pleasing lip volume and lip shape. The results could help to confirm or to challenge current lip augmentation strategies and could provide guidance for aesthetic practitioners.

Therefore, it is the objective of this study to objectively analyze differences in the eye movement patterns of independent observers when images of lips with different volume and different proportions are displayed. The results of this study will also be guided by the aesthetic rating of the same images by the same study population to allow for comparison between involuntary (eye movement pattern) and voluntary (aesthetic assessment) responses to the presented images of the lips.

PATIENTS AND METHODS

Study Sample

A total of 59 White study participants with a mean age of 32.73 ± 9.4 years (range, 22 to 58 years), of which 27 (45.8 percent) were men and 32 (54.2 percent) were women, were included into the study. The study participants were recruited from the Department of Hand, Plastic and Aesthetic Surgery, Ludwig-Maximilian University of Munich, Munich, Germany. Of those, three (5.1 percent) were board-certified dermatologists, 10 (16.9 percent) were board-certified plastic surgeons, and 46 (78.0 percent) were volunteers without medical (aesthetic) background.

Before enrolment into the study, participants were informed about the aim and the scope of this study and provided written informed consent for the use of their research- related and demographic data. The study was approved by the institutional review board of Ludwig-Maximilian University Munich (protocol number 20-1018).

Eye Movement Analysis

A Tobii Pro Nano binocular eye-tracker (Tobii Pro AB, Stockholm, Sweden) was used to capture eye movements of the 59 volunteers. The eye-tracking device operated at a frequency of 60 Hz and was positioned at the inferior aspect of a laptop monitor. The laptop monitor was a 15-inch commercially available laptop (Surface Laptop 3; Microsoft Corp., Redmond, Wash.) with a screen size of 340 × 244 mm. Eye movement was captured within a distance of 65 cm to the monitor and a lateral and cranial distance of 35 cm and 30 cm, respectively.

Visual Stimulus Presented

Lip Proportions

To identify differences in eye movement pattern between the 59 volunteers, four different images of lips (upper and lower lip pairs) were displayed for the duration of 6 seconds each on the laptop monitor with a resting interval of 2 seconds in between. The four images showed the lips of a female face in frontal view with different proportions between the upper and lower lips: 1:1, 1:1.6, 1:2, and 1.6:1, respectively. The lips of the female display image were edited with Adobe Photoshop Version 21 (Adobe Systems, Inc., San Jose, Calif.) based on the original upper and lower lip size of a previously commercially acquired stock image (Fig. 1).

Fig. 1.:

Image showing the four different assessed lip proportions: 1:1, 1:1.6, 1:2 and 1.6:1.

Lip Volume

In addition, the (upper and lower) lips of the female stock image were edited (see above) to obtain various volume ratios based on the lip proportion of 1:1.6 (upper versus lower lip). The following different lip volumes were generated: 70, 85, 100, 115, and 130 percent of the original size. The five different images were displayed for 6 seconds each with a resting interval of 2 seconds in between (Fig. 2).

F2
Fig. 2.:

Image showing the five different assessed lip volumes: 70, 85, 100, 115, and 130 percent.

Data Analysis

Eye Tracking.

Eye movement pattern was captured and processed by the internal software toolkit and analyzed the following variables:

  • Time until first fixation (interval between initial display of the image and a stable eye fixation of the lips = 0.08 second).
  • Time of fixation (duration of a stable eye fixation on the lips within the time of visual stimulus exposure = 6 seconds) (Figs. 3 and 4).
F3
Fig. 3.:

Image showing the heat map, based on the gaze of the participants, superimposed onto the four different assessed lip proportions: 1:1, 1:1.6, 1:2, and 1.6:1.

F4
Fig. 4.:

Image showing the heat map, based on the gaze of the participants, superimposed onto the five different assessed lip volumes: 70, 85, 100, 115, and 130 percent.

Aesthetic Rating.

In addition to the eye-tracking analysis, the 59 study participants were asked to rate the four different lip proportions and the five different lip volumes according to a five-point Likert scale: 1 = very unattractive, 2 = unattractive, 3 = neutral, 4 = attractive, and 5 = very attractive.

Statistical Analysis

Differences in eye movement pattern across the four different lip proportions and across the five different lip volumes revealed nonnormally distributed data (Kolmogorov-Smirnov test with p < 0.001); nonparametric testing was performed using the Kruskal-Wallis test. The same test was used for computing differences for ordinal variables between the aesthetic lip assessments based on the classification according to a five-point Likert scale. All calculations were performed using IBM SPSS Version 26 (IBM Corp., Armonk, N.Y.) and results were considered statistically significant at a probability level of p ≤ 0.05 to guide conclusions.

RESULTS

General Results

No differences between male or female observers were detected for any of the recorded variables obtained from the eye tracking or lip assessment (p > 0.05) even though the displayed image showed a female face. Stratification analyses revealed that there was no statistically significant difference in the recorded variables obtained from eye tracking and lip assessment when investigating the influence of the medical background on the results, with all values of p > 0.05.

Lip Proportions

The lip proportion (ratio in size between upper versus lower lip) with the ratio 1.6:1 (upper to lower lip) had a statistically significant shorter time until the first stable eye fixation occurred at 0.86 ± 0.9 second when compared pairwise to all other proportions with p ≤ 0.05. The time until first fixation for the other lip ratios was as follows: 1.20 ± 1.0 seconds for the ratio 1:1.6, 1.15 ± 0.8 seconds for the ratio 1:2, and 1.84 ± 0.9 seconds for the ratio 1:1, with p < 0.001 across groups (Fig. 5).

F5
Fig. 5.:

Bar graph showing the time when the first stable eye fixation occurred after initial image exposure (in seconds) for the four different lip proportions. Whiskers indicate ±1 SD. p, probability value.

The lip proportion that had the longest duration of a stable eye fixation during the stimulus exposure (6-second interval) was the image that displayed the lip ratio of 1.6:1, with a total fixation time of 3.34 ± 0.7 seconds (p ≤ 0.002). The lip ratio 1:2 had a duration of 2.76 ± 0.6 seconds, whereas the lip ratio 1:1 had a duration of 2.45 ± 0.6 seconds and the lip ratio 1:1.6 had a duration of 2.18 ± 0.7 seconds (p < 0.001 across groups) (Fig. 6).

F6
Fig. 6.:

Bar graph showing the duration of stable eye fixation (in seconds) during the 6-second image exposure interval for the four different lip proportions. Whiskers indicate ±1 SD. p, probability value.

The lip proportion that received the highest rating according to the aesthetic assessment of the 59 study participants was the lip ratio of 1:1.6 (upper to lower lip) with a rating of 4.21 ± 0.7. The lip ratio of 1:1 received a rating of 2.95 ± 0.7, whereas the lip ratio 1:2 had a rating of 2.16 ± 0.7 and the ratio 1.6:1 had a rating of 1.27 ± 0.4, with a value of p < 0.001 across groups (Table 1 and Fig. 3).


Table 1. -
Mean Time until First Fixation and the Mean Duration of Fixation during the 6-Second Interval, and the Mean Aesthetic Rating for the Respective Lip Ratios






Lip Proportion Time until First Fixation (sec) Duration of Fixation (sec) Aesthetic Rating
1:1 1.84 ± 0.9 2.46 ± 0.6 2.95 ± 0.7
1:1.6 1.20 ± 1.0 2.18 ± 0.7 4.21 ± 0.7
1:2 1.15 ± 0.8 2.76 ± 0.6 2.16 ± 0.7
1.6:1 0.86 ± 0.9 3.35 ± 0.7 1.27 ± 0.4

Lip Volume

The interval between image exposure and the first stable eye fixation on the five different lip volumes displayed statistically significant differences across groups (p < 0.001). The lip volume with the shortest time was the image that displayed lips at a volume of 70 percent of its original size with 0.68 ± 0.6 second. The lip volume of 130 percent had an interval of 0.70 ± 0.6 second, followed by the volume of 100 percent with 0.81 ± 0.8 second, the volume of 115 percent with 0.96 ± 1.0 second, and the volume of 85 percent with 1.10 ± 0.8 seconds (Fig. 7).

F7
Fig. 7.:

Bar graph showing the time when the first stable eye fixation occurred after initial image exposure (in seconds) for the five different lip volumes. Whiskers indicate ±1 SD. p, probability value.

The duration of a stable eye fixation during the stimulus exposure (6-second interval) displayed statistically significant differences across groups (p < 0.001). The lip volume with the longest fixation duration was the image that displayed the lip volume of 130 percent with a total fixation time of 3.35 ± 0.7 seconds. The lip volume of 70 percent had a duration of 3.03 ± 0.5 seconds, whereas the lip volume of 85 percent had a duration of 2.79 ± 0.7 seconds, followed by the lip volume of 115 percent with a duration of 2.46 ± 0.7 seconds and lip volume of 100 percent with a duration of 2.22 ± 0.8 seconds (Fig. 8).

F8
Fig. 8.:

Bar graph showing the duration of stable eye fixation (in seconds) during the 6-second image exposure interval for the five different lip volumes. Whiskers indicate ±1 SD. p, probability value.

The lip volume which received the highest rating according to the aesthetic assessment of the 59 study participants was the lip volume of 100 percent with a rating of 4.56 ± 0.5. The lip volume of 115 percent received a rating of 4.48 ± 0.5, the lip volume of 85 percent had a rating of 2.52 ± 0.6, the lip volume of 70 percent had a rating 1.70 ± 0.5, and the lip volume 130 percent had a rating of 1.56 ± 0.5 (p < 0.001 across groups) (Table 2 and Fig. 4)


Table 2. -
Mean Time until First Fixation and Mean Duration of Fixation during the 6-Second Interval, and the Mean Aesthetic Rating for the Respective Lip Ratios







Lip Proportion Time until First Fixation (sec) Duration of Fixation (sec) Aesthetic Rating
70% 0.68 ± 0.6 3.00 ± 0.5 1.70 ± 0.5
85% 1.10 ± 0.8 2.79 ± 0.7 2.52 ± 0.6
100% 0.81 ± 0.8 2.22 ± 0.8 4.56 ± 0.5
115% 0.95 ± 1.0 2.46 ± 0.7 4.48 ± 0.5
130% 0.69 ± 0.6 3.35 ± 0.7 1.56 ± 0.5

DISCUSSION

Even though the displayed image showed the face and lips of a female individual,10,11 there was no gender-related difference in the response of the observers, a group that was composed of 45.8 percent men and 54.2 percent women. Previous eye tracking studies have indicated an influence of the observer’s experience on the assigned task12; however, stratified analyses have revealed that there was no statistically significant difference in any of the investigated parameters when comparing aesthetically trained observers versus aesthetically inexperienced observers. This may be reflective of the task assigned, as viewing female lips and assigning an aesthetic rating score is a rather generic and common assignment and can be completed independent of the observer’s background and training.13

The results revealed that the lip proportion that was rated to be most attractive was the 1:1.6 ratio (upper-to-lower lip, 4.21 of a possible 5). This confirms previous investigations that have suggested this to be the golden ratio.2,4 This lip proportion, however, was not the first one to be viewed on initial image display (1.20 seconds) and had the shortest duration of stable eye fixation within the 6-second image display interval (2.18 of 6 seconds). A similarly interesting trend was observed for lip volume: The lip volume that received the highest aesthetic rating was the 100 percent volume (original lip size), with 4.56 of a possible 5. This lip volume, however, had the third longest interval between initial image exposure and the first stable eye fixation (0.81 second) and had the shortest duration of stable eye fixation during the 6 seconds of image display (2.22 of 6 seconds).

These results indicate an interesting trend that is in line with previous eye tracking investigations: objects that are viewed and perceived by the observer as pleasing or attractive are viewed shorter and need more time to catch the observer’s attention than less pleasing objects.12,14 This is intriguing, as one would expect the opposite: the more pleasing the image, the longer the need to observe. This finding could be explained by the fact that aesthetic perception of beauty follows a neural match versus nonmatch pattern in which the observed image is aligned to an internal representation of beauty. This internal representation or “internal standard” of beauty is influenced by a plethora of factors that include the sociocultural background, professional experience, or other environmental influences. If something is perceived as aesthetically pleasing and matches the internal standard, less time and less effort are needed to process the visual stimulus or to adapt to the observer’s internal standard of beauty. Conversely, it could be speculated that less aesthetically pleasing or less attractive objects require less time to catch the observer’s attention yet more time for visual processing. This could be attributable to a potential “mismatch” with the internal standard of beauty and requires more cognitive processing than aesthetically pleasing visual stimuli. Our results support this hypothesis.

The obtained results are interesting when trying to understand aesthetic trends and aesthetic behavior, especially in social media. The general belief that a person’s involuntary gaze and thus their attention is predominantly attracted by beauty or aesthetically pleasing objects should be reconsidered. Based on the results of this study, it seems that beautiful lips with harmonious proportions (1:1.6) and a natural lip volume (100 percent of original size), match the internal standards of beauty of the observers; therefore, less time is spent to observe and to view them compared to less attractive lips. This would fall in line with current trends in aesthetic medicine and surgery where the outcome of surgical or minimally invasive procedures should be more natural and less obvious without carrying the stigma that an intervention was performed. It can be now postulated that a “beautiful” and “aesthetically pleasing” outcome would fulfill such desired requirements where the patient is perceived by those around them with less cognitive effort; the perceived beauty matches the internal standard effortlessly.

Within the social media environment, our results could potentially indicate that the metrics of attention might not be associated with the “beauty” or “aesthetic appeal” of the content presented. Rather, it could be reflective of the cognitive processing of the observers: less attractive content captures the observer’s attention more quickly yet needs more time to be processed. Both are influenced by the sociocultural background, professional experience, and previous environmental exposure. On the same note, receiving substantial attention in real life after an aesthetic intervention might not be reflective of a “beautiful” outcome (i.e., matching to internal standards of the observer’s normative understanding of beauty). Physicians, and laypersons, should be mindful of such behavior to not misunderstand optic feedback. Visual attention, based on the results of this study, is not reflective of the beauty in the image assessed; rather, the opposite.

Practitioners should keep in mind that respecting lip proportions and lip volume (within the margins of the respective sociocultural environment) is key to aesthetically pleasing and beautiful outcomes. Alterations to these lip characteristics will influence the observer’s perception, and injectors should be mindful of those when performing lip volumization procedures, even if the patient’s desire differs; this should be discussed before each treatment.

CONCLUSIONS

The results revealed that the lip proportion and lip volume that were rated to be most attractive were the 1:1.6 ratio (upper-to-lower lip ratio) and the 100 percent, nonaugmented, natural volume; this confirms previous investigations and current trends in aesthetic medicine. The lips that were perceived as least attractive were those with a ratio of 1.6:1 and those having an exaggerated volume of 130 percent. Eye tracking–based analyses supported these findings: attractive lips captured the observer’s attention less quickly and were viewed for a shorter time.

REFERENCES

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9. Wan Y, Yang J, Ren X, Yu Z, Zhang R, Li X. Evaluation of eye movements and visual performance in patients with cataract. Sci Rep. 2020;10:9875.

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12. Cai LZ, Paro JAM, Lee GK, Nazerali RS. Where do we look? Assessing gaze patterns in breast reconstructive surgery with eye-tracking technology. Plast Reconstr Surg. 2018;141:331e–340e.

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