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Human rather than ape-like orbital morphology allows much greater lateral visual field expansion with eye abduction

Abstract : While convergent, the human orbit differs from that of non-human apes in that its lateral orbital margin is significantly more rearward. This rearward position does not obstruct the additional visual field gained through eye motion. This additional visual field is therefore considered to be wider in humans than in non-human apes. A mathematical model was designed to quantify this difference. The mathematical model is based on published computed tomography data in the human neuro-ocular plane (NOP) and on additional anatomical data from 100 human skulls and 120 non-human ape skulls (30 gibbons; 30 chimpanzees / bonobos; 30 orangutans; 30 gorillas). It is used to calculate temporal visual field eccentricity values in the NOP first in the primary position of gaze then for any eyeball rotation value in abduction up to 45° and any lateral orbital margin position between 85° and 115° relative to the sagittal plane. By varying the lateral orbital margin position, the human orbit can be made "non-human ape-like". In the Pan-like orbit, the orbital margin position (98.7°) was closest to the human orbit (107.1°). This modest 8.4° difference resulted in a large 21.1° difference in maximum lateral visual field eccentricity with eyeball abduction (Pan-like: 115°; human: 136.1°). The Hominoidea superfamily 1 ("hominoids") is comprised of modern humans (Homo sapiens) and non-human apes. Non-human apes, humans' closest relatives 2,3 , include gibbons (family Hylobatidae), orangutans (family Hominidae, genus Pongo), chimpanzees and bonobos (family Hominidae, genus Pan) and gorillas (family Hominidae, genus Gorilla) 1. Modern humans' orbital morphology is unique among the Hominoidea superfamily in that the human orbital width/height ratio is highest and, whilst convergent (front-facing), the human orbit has the rearmost temporal orbital margin 4,5. This orbital margin configuration increases the human median temporal visual field surface area by 46% with eye-abduction, which promotes effective visual and visual field exploration through eye motion rather than head motion 4,6,7. The neuro-ocular plane (NOP) is defined as the plane which, in the primary position of gaze (look-ing straight ahead in the distance), contains the centre of both crystalline lenses, optic discs, and optic foramina 8-10. This plane can be used to obtain head orientations in space to facilitate precise comparisons between human and non-human apes 4,10. In this plane, the human temporal orbital margin is 107.1°4 ,5 from the sagittal plane compared to 98.7°4 ,5 in humans' closest relatives 3,11 chimpanzees and bonobos.
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Eric Denion, Martin Hitier, Eric Levieil, Frédéric Mouriaux. Human rather than ape-like orbital morphology allows much greater lateral visual field expansion with eye abduction. Scientific Reports, Nature Publishing Group, 2015, 5 (1), pp.1229-1234. ⟨10.1038/srep12437⟩. ⟨hal-02191022⟩

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