An investigation of matching symmetry in the human pinnae with possible implications for 3D ear recognition and sound localization

Abstract

The human external ears, or pinnae, have an intriguing shape and, like most parts of the human external body, bilateral symmetry is observed between left and right. It is a well-known part of our auditory sensory system and mediates the spatial localization of incoming sounds in 3D from monaural cues due to its shape-specific filtering as well as binaural cues due to the paired bilateral locations of the left and right ears. Another less broadly appreciated aspect of the human pinna shape is its uniqueness from one individual to another, which is on the level of what is seen in fingerprints and facial features. This makes pinnae very useful in human identification, which is of great interest in biometrics and forensics. Anatomically, the type of symmetry observed is known as matching symmetry, with structures present as separate mirror copies on both sides of the body, and in this work we report the first such investigation of the human pinna in 3D. Within the framework of geometric morphometrics, we started by partitioning ear shape, represented in a spatially dense way, into patterns of symmetry and asymmetry, following a two-factor ANOVA design. Matching symmetry was measured in all substructures of the pinna anatomy. However, substructures that 'stick out' such as the helix, tragus, and lobule also contained a fair degree of asymmetry. In contrast, substructures such as the conchae, antitragus, and antihelix expressed relatively stronger degrees of symmetric variation in relation to their levels of asymmetry. Insights gained from this study were injected into an accompanying identification setup exploiting matching symmetry where improved performance is demonstrated. Finally, possible implications of the results in the context of ear recognition as well as sound localization are discussed.

Analyzing Fluctuating Asymmetry with Geometric Morphometrics: Concepts, Methods, and Applications

“…A few studies of shape in Drosophila found no significant directional asymmetry of wing shape [115][116][117][118][119] or mixed results [120][121][122][123][124][125][126], although a series of other studies did find it [15,16,54,56,62,77,85,123]. Similarly, one study on human skulls [127] found no directional asymmetry of shape, whereas several others reported directional asymmetry of the skull [47,68,84,86,90,94,109] and soft tissues of the face and ears [38,66,98,104,105,108]. Further non-significant results were reported from mites [128] and wings of Trichogramma egg parasitoids [129]-but both studies reported results only from relatively small subsamples (≤30 specimens per sample) and tiny organisms, raising questions about statistical power and possible artifacts from mounting very small specimens.…”

“…Because the coordinates of the landmarks serve as variables in the subsequent analyses, it is important that each variable is a coordinate of the same point for all the specimens in the sample. This correspondence of landmarks, or homology in the context of comparative evolutionary studies, is a central assumption of geometric morphometric methods [9], even for semilandmarks and similar methods that use points selected along a smooth outline or surface [104,[202][203][204][205]. Because fluctuating asymmetry is usually investigated within populations and the scale of variation is normally relatively small, correspondence of landmarks tends to be a less serious problem than in studies of evolutionary or ontogenetic variation.…”

“…Examples of methods for this include Fourier analyses [275][276][277], eigenshape analysis [278][279][280], semilandmarks [202,205,281,282], and a variety of methods of superimposing 3D surfaces to an overall best fit [104,203,204,283,284]. Note that, even though some authors have claimed that such methods are "homology-free" [285], all these approaches are based in one way or another on a one-to-one correspondence of points on the outline or surface, which is the basis for the computations of mutual fit or difference between shapes [286].…”

“…Asymmetry can be computed as left-right averages of the scores in an outline analysis, and variation among individuals can be characterized as the averages of both sides after the outline or surface on one side has been reflected. Studies of this kind have been done using elliptic Fourier analysis [291,292], eigenshape analysis [293], semilandmarks [87,169,171], and 3D-surface superimposition [104].…”

Fluctuating Asymmetry of Human Populations: A Review

“…Shape asymmetry, involving either matching symmetry or object symmetry [24][25][26] can be estimated from landmarks or semi-landmarks in either two or three dimensions [25][26][27][28][29][30][31][32], as well as continuous symmetry measures [12,[33][34][35]. Despite the apparent simplicity of fluctuating asymmetry, careless researchers can easily reach erroneous conclusions [8,12,22,23,36].…”