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°).
Highlights
The visual field, including the additional visual field gained through eye motion, may be tested in humans[6,7] but not in non-human apes
The ω max. plots according to the various eye abduction values (θ from 0 to 45°) for different φ values are displayed in Fig. 1: The human orbit gives far more visual field expansion with eye abduction than modified non-human ape-like orbits
In 1961, Hedblom reported that humans could extend their visual field through eye motion, mostly in the temporal sector where the facial relief does not interfere with vision[12]
Summary
The visual field, including the additional visual field gained through eye motion, may be tested in humans[6,7] but not in non-human apes. Visual field testing is not, an appropriate solution for appraising how the anatomical differences in the orbital margin between humans and non-human apes translate into visual field differences. To address this issue, we developed a mathematical model. We aimed to quantify the influence of the orbital morphology difference between humans and non-human apes on temporal visual field extent, including the additional visual field gained through eyeball abduction
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