Static Threshold

There is a static asymmetry threshold of perception for each facial subunit [33].

These thresholds are summarized in Table 2.8. The table represents the static asymmetry thresholds for different facial areas. The leftmost column gives the facial area, the column in the middle the threshold, and the rightmost column the reference.

As shown in the uppermost row, Hohman et al. [34] found the perceptive static threshold for eyelid closure asymmetry to be 2 mm. At that threshold, only 10 % of the observers perceived the eye closure to be symmetrical. In contrast, at the previous interval at 1 mm asymmetry, 85 % of the same informed observers experienced the eye closure to be symmetrical. Hohman et al. used 145 observers consisting of both laypeople and clinicians. The model used was a digitally modified female photograph. [34]

The second row in Table 2.8 lays out the perceptive static threshold for eyebrow asymmetry; at asymmetry of 3.5 mm 73 % (or 22 people out of 30) of the naive observers perceived the eyebrows to be asymmetrical [36]. When the asymmetry was digitally modified to be an interval less, 3 mm, then only 10 % of the observers stated the eyebrows to be asymmetrical. [36] Chu et al. used a digitally modified

Table 2.8 The facial subunits’ static asymmetry perception thresholds. Table is

photograph of a male as a model in the progressive asymmetry study [36]. Results gained by Hohman et al. [34] support this perception threshold of 3.5 mm. Hohman et al. used an interval of 1 mm and concluded that eyebrow lift with the asymmetry of 3 mm was perceived as symmetrical whereas 4 mm or more as asymmetrical.

In addition to the eyebrow asymmetry threshold, Chu et al. have reported the static threshold for oral commissure asymmetry in the same study [36]. The detection threshold is given in Table 2.8 on the third row and was found to be 3 mm by Chu et al. [36]. At that degree of asymmetry 73 % (or 22 people out of 30) of the naive observers identified the oral commissures to be asymmetrical. Furthermore, the asymmetry of 5 mm was found to be unacceptable, or in other words, to be the intervention threshold.

The static threshold for nasal asymmetry has been studied at least by Meyer-Marcotty et al. [35], Silva et al. [38], and Kwak et al. [39]. Meyer-Meyer-Marcotty et al. used a male 3D facial model, and 60 informed clinicians and 30 laypeople as observers. They concluded that the observation threshold of nose asymmetry was 4 mm; at the previous asymmetry interval of 2 mm the nose was still perceived to be symmetrical. The result is supported by Silva et al. who used a 2D digitally modified photograph of a female, and 100 laypeople as observers. Their interval of progressive asymmetry was smaller, 1 mm instead of 2 mm, and the study results the threshold to be the 4 mm. The study conducted by Kwak et al. measured the nasal asymmetry in degrees. Otherwise Kwak et al. used a similar model as Silva et al.; a digitally modified 2D photograph of a female, and such as Meyer-Marcotty et al. beacuse both clinicians (120) and informed laypeople (40) were observing. Kwak

et al. concluded 2.92 degrees to be the perceptive threshold, and the intervention threshold of nasal asymmetry to be 4.28 degrees.

The perceptive static threshold for chin asymmetry has been set by Meyer-Marcotty et al. in [35] to be 6 mm. In more detail, Meyer-Meyer-Marcotty et al. reported that the 6 mm progressive asymmetry was perceived as asymmetrical by the informed observers: clinicians and laypeople. At the previous interval of 4 mm the chin was recognized as symmetrical. This conclusion has been reinforced by Naini et al. [40]

that deviation of 5 mm was largely imperceptible. Naini et al. used female and male 2D models, and laypeople, clinicians, and patients as observers. On the other hand, their progressive interval was relatively big when compared to other studies;

the interval was set to be 5 mm. However, another study conducted by Silva et al.

[38] used smaller incerements of 1 mm and reported that the observers detected no asymmetry in deviations less than 6 mm.

Thus, the location of the asymmetry affects the static threshold [33] in the following way; the eye lid had the smallest threshold of 2 mm [34], and the eyebrow and oral commissure have the second most smallest thresholds of 3.5 mm and 3 mm, respectively [36]. Finally, the nose tip and chin have larger thresholds of 4 mm [35, 38] or 2.92^◦ [39], and 6 mm [35], respectively. However, for example Wang et al. [33]

suggest that further studies are needed to establish the static perception and static intervention thresholds.

In addition to the location of the asymmetry, the evaluation time affects the static perceptive threshold, too. To be more precise, the evaluation time influences the asymmetry perception in two ways: the detection threshold, and the number of detected asymmetries. This has been studied by Chu et al. in [36]. They investigated the detection of progressive asymmetry with two different observation times: 2 s that corresponds to a casual glance of a passer-by, and 10 s which resembles prolonged observation by an interlocutor.

Firstly, in case of a single defect of oral commissure or eyebrow, the static perception threshold decreased for both asymmetries as the observation time got longer. In other words, longer observation period causes smaller asymmetry perception thresholds.

[36] Secondly, Chu et al. [36] studied the relationship between observation time and multiple simultaneous defects. In more detail, the digitally modified 2D photograph had simultaneously an oral commissure and an eyebrow asymmetry. If the naive laypeople had 2 s to observe the photograph, 90% of the viewers detected a single defect. With the 10 s observation time, 77 % (or 23 people out of 30) of the observers perceived both defects. This result was reported to be statistically significant.

Thus, the observation time affects the number of perceived asymmetries. Chu et al. also suggested that with 2 s observation time and both studied defects present, the eyebrow asymmetry to be detected before the oral commissure anomaly. The

eyebrow asymmetry was detected first in 60 % (or 18 people out of 30) of the instances. However, this was not statistically proven. [36]