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Light enters the cornea, crosses the lens, hits the retina. Electric sinal travels from retina through the optic nerve, reaches the chiasma, crosses and makes its way to the visual cortex.

My question is: given a fixed, absolute scotoma in one of the eyes, what parts of the vision could be at fault?

Initially, I believe the cornea/lens couldn't be, otherwise the scotoma wouldn't be absolute and fixed. Moreover, I suspect that, after the chiasma, signals from the same visual position are joined, so, the problem couldn't be after it, otherwise the scotoma would be binocular. As such, am I right to conclude that a fixed monocular scotoma could only be caused by issues on either the retina, or on the optic nerve?

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Yes. You're right.

A scotoma only affecting one eye (i.e., one that is not perceived when one eye is closed) is diagnostic of a lesion upstream from the optic chiasm.

See Brust's Practice of Neural Science, Fig. 3-2. The figure demonstrates a full cut, with complete blindness in one eye, but the principle is the same for a scotoma.

enter image description here

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  • $\begingroup$ Great answer! But could I ask just a quick confirmation? Notice this image. By the way it is drawn, we could conclude a small lesion on the highlighted area would cause a monocular loss of the right field of the right eye. That was the source of my confusion. Can I just assume this image is misleading in that sense, and that any lesion that far would result in binocular vision loss? $\endgroup$
    – MaiaVictor
    Jul 6 '18 at 4:23
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    $\begingroup$ @MaiaVictor Yes, the image you link to in your comment is misleading. Any lesion at or downstream from the optic chiasm is going to present as a binocular loss. The optic radiations (from the LGN of the thalamus to the visual cortex) are not structured in such a way that a lesion would impact only one eye. This is one of the few absolutes: a monocular loss must be upstream from the chiasm. $\endgroup$
    – De Novo
    Jul 6 '18 at 4:33
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    $\begingroup$ Actually, I think this may be a counter-example to the answer, although seemingly extremely rare. $\endgroup$
    – MaiaVictor
    Jul 12 '18 at 2:38
  • $\begingroup$ By the way, do you happen to be a doctor? If so, is it reasonable to ask here for a recommendation? It has been very hard to find competent doctors. $\endgroup$
    – MaiaVictor
    Jul 12 '18 at 2:42
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    $\begingroup$ @MaiaVictor that is a fascinating article. Thank you for sharing. I appreciate the trust in asking me for a recommendation, but it wouldn't be appropriate on this site. Most doctors are quite good, but we all make mistakes every once in a while. $\endgroup$
    – De Novo
    Jul 12 '18 at 4:31

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