Nasal Phosphenes and the Extent of the Visual Field
Of course you know this: If you press one corner of your eye, a bit of a spot will appear at the opposite end of your visual field, due to mechanical stimulation of the retina. (Or didn't you know that?) I find the effect clearest when I close one eye and press near the midline on the outside edge of the other eye. A phosphene -- seemingly a dark spot encircled by a bright ring -- appears right near the nasal edge of the visual field.
Now here's a challenge: See how far into the periphery you can get that nasal-side phosphene. Keep the other eye closed, half close the lid of the eye you're pressing, and rotate the pressure a bit toward the center of the eye. Can you see the phosphene go deeper in? At some point I find the phosphene disappears, but only well into the dark field -- not in the shade created by my nose but in what would, if the phosphene were located in space, be the interior region of my face. Who'd've thought we'd have retinal receptors that could refer stimuli there?!
Now if I open the other eye, I see that the phosphene is actually (barely) within the visual field created by both my eyes together. Well, that makes sense! Otherwise, there'd be some explaining to do about how the experienced visual field for phosphenes can extend beyond that for outward objects!
My question is this. When I close one eye, normally, does my visual field contract? My first impulse is to say yes. But now I wonder whether instead, my visual field might actually contain some of that dark region I've recently been exploring with the phosphene, the region inside my face -- a region I usually totally ignore, of course! If I turn my one open eye as far inward as it can go, I see my nose, and blackness farther in. But blackness, of course, isn't the end of the visual field. It's part of the visual field -- a part experienced as black. To see the contrast, rotate your eye as far outward as it can go. There's no blackness at the edge (or maybe just a little in the bottom half?) -- the visual field just stops.
So, when I stare straight ahead with one eye closed, does my visual field stop somewhere near my nose, like it stops at the outer edge, or does it include inward blackness, stretching over the entirety of the visual space that would normally be filled in by peripheral input from the other eye?