This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ Preprints) and either DOI or URL of the article must be cited.
The contralateral organization of the forebrain and the crossing of the optic nerves in the optic chiasm represent a long-standing conundrum. According to the Axial Twist Hypothesis (ATH) the rostral head and the rest of the body are twisted with respect to each other to form a left-handed half turn. This twist is the result, mainly, of asymmetric, twisted growth in the early embryo. Evolutionary selection tends to restore bilateral symmetry. Since selective pressure will decrease as the organism approaches symmetry, we expected a small control error in the form of a small, residual right-handed twist. We found that the mouth-eyes-nose (rostral head) region shows a left-offset with respect to the ears (posterior head) by up to 0.8° (P<0.01, Bonferroni-corrected). Moreover, this systematic aurofacial asymmetry was larger in young children (on average up to 3°) and reduced with age. Finally, we predicted and found a right-sided bias for hugging (78%) and a left-sided bias for kissing (69%). Thus, all predictions were confirmed by the data. These results are all in support of the ATH, whereas the pattern of results is not explained by existing alternative theories. As of the present results, the ATH is the first theory for the contralateral forebrain and the optic chiasm whose predictions have been tested empirically. We conclude that humans (and all other vertebrates) are fundamentally asymmetric, both in their anatomy and their behavior. This supports the thesis that the approximate bilateral symmetry of vertebrates is a secondary feature, despite their being bilaterians.
Histogram of systematic eccentricity of the endocanthion of the eyes
Left: for the 3D database (Troje & Bülthoff, 1996), and Right: for the 2D analysis of frontal pictures of the face in a British population of mostly male subjects. Red curves: fitted normal distribution
Correlations between the asymmetries of some of the landmarks from the analysis of the 3D Database of Troje & Bülthoff (1996)
Each panel shows the correlation of two of the landmarks. The lowest (Nasion-Menton) and the highest (Exocanthion-Endocanthion) are the lowest and highest correlations respectively of all pairs of landmarks.
Matlab script for analysing the data base of adult faces
Matlab script for computing the Aurofacial Asymmetry as predicted by the Axial Twist hypothesis. The draft of this script was written by Niko Troje. The author (MdL) wrote most of the analysis. The script needs the symmetric face and the individual faces from the Faces database, and the Facerenderer functions that come with it. Access can be obtained by contacting http://faces.kyb.tuebingen.mpg.de