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A diagram of the visual pathway is shown here. Light from the left visual field reaches the left nasal retina and the right temporal retina. On the other hand light from the right visual field reaches the right nasal and the left temporal retina.

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The optic fibers from the nasal parts of the retina cross, whereas the fibers from the temporal hemispheres don't. This configuration leads to the projection of the left visual field to the right occipital cortex and vice versa.

My question is, why can't light from the left visual field reach the temporal left retina and vice versa? I have illustrated this in the picture below with blue arrows.

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Obviously, if it would, it would interfere with the normal pattern of projection. Now my question is what prevents light from the left field of view reach the left temporal retina? Or is there some overlap between the visual fields?

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    $\begingroup$ Very interesting and well researched question +1d. I hope you get a few more upvotes. $\endgroup$ – AliceD Sep 27 '16 at 20:35
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    $\begingroup$ @Christiaan Just wanted the answer! Not the upvotes. Thanks :) $\endgroup$ – Polisetty Sep 27 '16 at 20:48
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There is overlap between the left and right visual fields, as shown in Fig. 1. This overlapping part is referred to as the binocular field of view. The monocular field of view (FOV) is 160o (w) x 175o (h), and the binocular FOV is 200o (w) x 135o (h). The region of binocular overlap is 120o (w) x 135o (h) (Kolb, 2016). The visual disparity in the binocular FOV of the both retinal projections forms one of the most important depth cues that allows us to estimate the distance of objects.

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Fig. 1. The left and right visual fields have an overlapping area, the binocular visual field. source: Harvey Mudd College

Fig. 2 below shows why only a certain part of the visual fields overlap. Firstly, the retina has a finite size and secondly, the anatomical location of the pupils restricts the angles of the visual field. The most eccentric parts of the left visual field simply cannot reach the temporal left retina (and vice versa), because the angle of the pupil with respect to the retina simply doesn't allow for it. The same holds for the most central part of the left FOV, which cannot reach the nasal part of the right retina; it can only project to the the temporal right retina because of the restrictions due to the localization of the pupil, the retinal size limitations, as well as the nose blocking a certain part of the FOV.

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Fig. 2. The left and right visual fields have an overlapping area, the binocular visual field. source: Ohio State University

Reference
- Kolb, Facts and Figures concerning the human retina. In: Kolb et al (eds). Webvision, The Organization of the Retina and Visual System. Utah University (2016)

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    $\begingroup$ But one doubt remains. If the fields of view are overlapping, a lesion in the visual pathway like, lesion 2 (bitemporal hemianopia) in the picture in the question - shouldn't be so well defined/ straight line. Is the picture wrong in showing the visual defect to be etched out so clearly? $\endgroup$ – Polisetty Sep 27 '16 at 18:40
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    $\begingroup$ @Polisetty - the picture is valid and the symptoms are indeed pretty much as depicted, as the retinal hemispheres are divided in half. $\endgroup$ – AliceD Sep 27 '16 at 19:48

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