This is an area of some expertise for me. The macula is the part of the retina that is responsible for central vision. It is very much the "business area" of the retina with more tightly packed receptors enabling us to see fine detail, and colour. Look at a photo of a retina and you'll notice that the macula is much darker in colour due to the increased blood flow to that area. This area is very metabolically active; in other words, "working" hard.
Unfortunately, there is a cost to that increased metabolism. Without getting into too much detail, after about 70-80 years (give or take, there is a bell curve distribution) the macula starts to wear out, and receptors are lost: Macula Degeneration
Now I know that it is fashionable on here to underestimate the brain (

) but it is very good at making sense of the information it receives, even when there are bits of information missing. (As an aside in a condition like glaucoma the retina can lose up to about 40% of its ganglion cells before a measurable visual field defect can be found - again the brain is able to "fill in the gaps". This is remarkable.)
It appears that in some, with advanced macula changes, the blobs of missing receptors, can be made into recognizable shapes, often faces and people or animals, in a form of pareidolia.
The interesting part of this, IMO, is how under reported this phenomenom is. Elderly people often live alone, and are afraid of admitting to these hallucinations (remember, these folk are invariably in an age group who fear being thought of as "senile" or "losing it")
Following a remark at a conference a few years ago I decided to talk to patients with advanced macula disruption about hallucinations. The stories I continue to get back are amazing. Letting people know that it is a known side effect of their condition allows them to (with some relief!) discuss this symptom. Just last week I had an (extended!) chat to an elderly lady about the "man" who she lived with and talked to. I turned around to see her daughter (who was in the room) eyes wide with shock and her chin on the floor. This woman had told nobody about her hallucinations.