Discussion
This study sought to examine the accuracy and efficiency of perception of facial features, more specifically eyelid asymmetry when viewed from different rotational perspectives. The first finding suggests that for both laypeople and surgeons, correctly identifying asymmetry appeared to depend on the type and severity of asymmetry. For both groups, the odds of correctly identifying asymmetry were highest for more severe asymmetry, and higher for ptosis when compared with peak. The second result relates to the rotation of the facial depiction. There was a significant inverse linear relationship between the proportion of correct responses and the degree of rotation away from upright. The greater the degree of rotation away from upright, the less accurate observations were for both peak and ptosis, in all groups, with the greatest error found in the inverted position, even for oculoplastic surgeons trained at perceiving eyelid asymmetry. Even expert oculoplastic surgeons with extensive practice experience suffer from significant inversion disability, in that they are less accurate at defining small differences in contour and eyelid position when viewed in an inverted position relative to upright.
Expectedly, surgeons were overall more accurate (and spent more time on each trial) than laypersons. However, oculoplastic surgeons are subject to similar perceptual challenges with the rotational task, leading to diminished perceptual accuracy as rotation is increased, with particular significant challenges performing symmetry assessments in the inverted position. Stated alternatively, surgeons were more accurate overall but the trend line to worsening accuracy in the inverted position was the same (the regression functions are roughly parallel). The impact of such observations on visual processing of facial asymmetries may be exemplified by further discussing eyelid surgeries such as ptosis surgery, which require iterative recognition of minute asymmetries at different orientations in space. The most common complications of ptosis surgery are uneven or asymmetric eyelid height, poor contour or eyelid peaking.12 Intraoperatively, correction of eyelid ptosis is frequently evaluated with the patient in an upright primary position.9 ,12 This process is time-consuming as well as logistically and ergonomically challenging, both for patients and surgeons. Beyond the time and logistics, there are also safety concerns in sitting patients up with significant variations in haemodynamic measures and oxygenation in sitting versus supine position.13 However, surgeons often perform this task, and the findings from this study suggest that upright positioning is crucial to achieving the highest perceptual accuracy. It is not clear what aspects of positioning the patient upright are critical. It is unlikely that gravity plays a role,14 and the eye projection is also likely stable.15 Purported stimulation of the wakeful state in the patient (or at least enhancing arousal) may contribute to the upright assessment, though this is unproven. The role of the surgeon’s perceptual challenges has been underappreciated in this process. This study suggests that, whether consciously or not, the surgeon can increase perceptual ease and improve accuracy of asymmetry assessment by viewing the patient upright. Whether this is by assessing the patient in the upright position or some other perceptual trick such as using a mirror or changing surgeon position to view from below.
This study further demonstrates that this process is roughly linear, in that accuracy decreases in a stepwise manner as the image is rotated away from upright. Oculoplastic surgeons may be seven times less likely to accurately perceiving a lateral peak and nine times less likely to perceive small amounts of ptosis in the inverted position. However, even for smaller degrees of rotation away from upright such as 45° (as might be created by head tilt in the office), surgeons are almost three times less likely to correctly perceive lateral peak. While a 45° rotation at bedside (achieved via surgeon positioning) may result in increased accuracy compared with the inverted position, it may not be practically useful in the operating room. Rather, the 45° rotation away from upright demonstrates a step in the linear process of perception disability from upright to inverted positioning. These findings are not surprising in the context of the broader visual perception literature, where now classic experiments in the 1970 were able to show similar findings in that accuracy in identifying pairs of rotated three-dimensional (3D) structures decreased linearly with degree of rotation.7 Overall, these findings suggest that although oculoplastic surgeons are experts at perceiving eyelid asymmetry, they are subject to similar perceptual challenges as the lay population. Recognising these limitations may improve aesthetic and functional surgical outcomes.
Interestingly, both laypersons and surgeons were best able to correctly appreciate severe ptosis, and among surgeons there was no significant difference in perception of severe ptosis in the upright versus inverted position. Thus, the degree of asymmetry affects accuracy of perception, with larger asymmetries being easier to recognise. This is not surprising given previous studies showing that ptosis asymmetry in laypeople can be recognised more than 90% of average observers can identify 2 mm of ptosis asymmetry accurately.16 This finding unfortunately has minimal application to the perceptual tasks involved in eyelid surgery. Although patients may present initially with severe ptosis, intraoperative adjustments in ptosis surgery involve appreciating more minute differences in peak and ptosis. These smaller adjustments would more closely correspond to the mild ptosis and medial or lateral peak in this study, which were significantly more accurately perceived in upright position. Thus, even in cases of severe baseline ptosis, the perceptual challenges presented with inverted viewing are involved in the critical intraoperative tasks.
This study also demonstrated difference in sensitivity to asymmetry type, peak or ptosis. Although marginal peak is a major determinant of upper eyelid contour and horizontal position, both medial and lateral peak position were perceived less accurately by both surgeons and laypeople in this study. This is in line with past studies that have discussed the challenge in achieving and quantifying symmetric contour.17 18 Digital image analysis techniques to quantify peak have only further emphasised the importance of considering perception of peak asymmetry by showing that a nasal shift of peak can be extreme in cases of involutional ptosis, and correction of ptosis is associated with a temporal shift in peak.17 Such cases may be prime candidates for upright viewing.
These results may have implications beyond ptosis surgery, for other facial plastic surgeries. The production and execution of symmetric incision lines in surgery such as blepharoplasty, pretrichial brow surgery and cheiloplasty is critical in the determination of functional and aesthetic outcomes.19 Studies have shown an effect of handedness on surgical outcomes,20–22 which may interact with perceptual rotational challenges. Other facial procedures that involve setting of facial landmarks such as the lateral canthus, the nasal ala or earlobe position may also be subject to inverted viewing challenges. These types of procedures could benefit from upright viewing, whether by patient or surgeon positioning, when assessing resultant symmetry.
Visual perception frameworks for perceiving materials with complex appearances emphasise the power of supervised learning, using labelled data to improve perception and predict outcomes.23 The randomisation in this experiment mitigated the effect of learning on the data as presented, however, it may be possible that specific inverted image recognition training may improve overall perception of asymmetry both during baseline evaluation and intraoperatively. Though even with routine training, it should be noted that inverted tasks remain more challenging. This is demonstrated by the fact that even experienced oculoplastic surgeons, with previous extensive intraoperative and clinical training in multiple positions and rotations of gaze, performed significantly worse with rotation away from upright in this study just as laypeople did. Interestingly, studies have shown that 3D faces are recognised more accurately and faster than two-dimensional (2D) faces in the upright position but have similar face inversion effects.24 Therefore, even 2D image-based training on images at different degree of rotation may have implications for perception in 3D space. Future studies may explore the impact of training surgeons on perception of peak and ptosis in 2D images at different degrees of rotation on perception in 3D space.
This study is not without limitations. While MTurk allows for access to a large population of laypeople, the population is still less diverse than the general US population (eg, more highly educated and younger compared with the US population).25 In addition, this study focused on a unilateral eyelid pathology although patients commonly present with different types and severities of asymmetry. Intraoperatively, this could pose a greater challenge. Future studies would benefit from exploring the difference in accuracy and RT of patients with bilateral asymmetry with differing type and severity for each eye at different degrees of rotation. In addition, this study assesses perception of asymmetry for 2D images, however, the relationships noted may differ in 3D when evaluating patients in person.24 It is also important to note that this study considers perception of static asymmetry, however, this is not a direct proxy for effective management of asymmetry intraoperatively or differences in overall treatment success. Finally, this study did not quantify any operating room or clinical outcome findings and thus cannot be directly applied to a specific physical environment or task. Further studies trialling a mirror reversal apparatus in the operating room are ongoing.
Overall, this study highlights the challenges involved in the recognition of particularly small eyelid asymmetries under different rotational viewing conditions, demonstrating the significant inversion disability faced by oculoplastic surgeons. This study quantifies the extent to which inversion disability decreases accuracy in perception of asymmetries, even among experts with extensive training and experience. Understanding the extent of such limitations and considering how to improve them is integral to achieving optimal aesthetic and functional results. Beyond the implications for our understanding of visual processing of facial structures at different orientation in space, these findings may have practical implications for the assessment of patients under different viewing conditions, particularly in the inverted position as is common in various facial plastic surgeries performed at the head of the bed.