I am not sure if this question should be asked in the Philosophy or Neuroscience forum as both domains are relevant to this inquiry. Will have a go anyway.
One of the defining characteristics of CBS are the hallucinations that appear as a result of some impairment to vision. They range from simple to complex. Examples are given from geometrical shapes to people and landscapes and many have studied and provided commentary on this and similar phenomenon for many years. When thinking about them (and imaginal phenomena in general) I recall the ancients and find just how relevant and valuable their inquiries are today. Let us consider the following as an example:
On "The Enneads of Plotinus, Volume 1: A Commentary" By Paul Kalligas page 557 III 6. 'On impassibility' he states:
"Matter is, at heart, a place where images are produced and thus itself may be characterized as an image [eidolon] that gives rise to fleeting and deceptive illusions but that can never be pinpointed or apprehended on its own. At the risk of simplification we could say that it is the indeterminate and imperceptible constituent of corporeal entities considered as images. For while the configuring elements of an image, which in themselves are subject to our powers of perception, may all be referred, in one way or another, to the intelligible models they figure, the nature of the image as such cannot be explained solely by the specification of its relation to these models. Something further is required: a declaration of its otherness with respect to these, in order that the dependent character of its nature as an image may be rendered perspicuous. The 'non-intelligible' element, which makes images be images and their subsistence be purely phenomenal --- an "apparition" [phantasma] or "ghostly image of bulk" [phantasma onkou]--- is matter. On this see also the highly pertinent, if somewhat cryptic, observations of Gerson 1994, 112. It is, in other words, a necessary condition for the possibility of images being produced: for it does not itself, literally, produce them. It merely has a predisposition to produce them and a "propensity" or "aspiration" to give them subsistence so that in this sense it "longs for that which is destructive [sc., of itself]" (Oregetai tou phthartikou: cf. Arist. Metaph. N 4, 1092a2 and my discussion of this passage in my introduction to I 8) notwithstanding that in reality it cannot incur anything and consequently cannot suffer destruction. It may be perceived indirectly because there appear fleetingly upon it all sorts of conflicting and mutually contradictory qualities such as those that Plato invoked in his unwritten doctrines in order to designate his own material ontological principle. On this see Arist. PH I 4, 187aI6-18, Metaph. A 6, 987b2o; and Hermod. Fr. 7 (apud Dercyllides Porph. 146F apud Simpl. in Ph 247.34-248.8): "Plato, assuming matter to be ranged with the unlimited and indefinite, clarifies its nature on the basis of these as belonging to those things which admit the more-and-less of which the great-and-small are one."<
In "A Vocabulary of the Philosophical Sciences
(Including the Vocabulary of Philosophy, Mental, Moral and Metaphysical, by William Fleming, from the 2d Ed., 1860: and the 3d, 1876, Ed. by Henry Calderwood, LL. D.)" By Charles Porterfield Krauth, William Fleming, Henry Calderwood · 1877 it mentions Hobbes suggesting that:
"Space is the mental image (phantasma) of a thing existent as existent, that is, no other accident of that thing being considered, except that it appears exterior to the person having the image."<
What is known is that a transformation (or conversion if you will) is occurring via transduction, in the case of CBS the impairment thereof with the brain accommodating for same, but what is brought into question is the topography (locus of manifestation) of visual experiences [e.g. "where is it that I go when I dream?"].
The horopter seems to be the topos for experiences like optical illusions, hypnogogic imagery or eidetikos and the like. As though they were projected from the fovea and into the vieth-muller circle, against Panum's fusion area and unto the zones of disparity; like any other image that is projected onto a screen. They surely cannot be plastered, so to speak, on the lens or cornea of the eye.
This is an utterly doubtful situation either way...but why does it appear as though they are "exterior" as Hobbes describes? Is the "apparition" or "phantasma" matter? If so then of what kind? If not then why?