(I think, by the way, this is probably terminologically a little unfortunate, because this additional content doesn't have to be visual. It could be in any other sense modality as well. In fact, much of what makes the character of a particular perception visual, auditory, tactile and so on would be with this side of the content, rather than on the the propositional side.)
Now there seem to be some similiar findings in recent cognitive science which I stumbled upon, quoted in Colin McGinn's Mindsight:
"Suppose you ask subjects whether the navel is above the waistline or whether frogs have lips. Many people report forming an image and reading the answer off that image. [...] Clearly, they conjure up a memory image, derived from past perceptions, and extract the information from this. [...] Information is encoded in the image in some sort of form (often described as pictorial), and then it is transformed into another sort of informational encoding — the kind that corresponds to articulate beliefs."
(This doesn't have anything particular to do with unreality, but I found it interesting to find Socrates' mostly metaphorical way of putting it paralleled in relatively recent, modern research.)
 Colin McGinn, Mindsight. Image, Dream, Meaning. Cambridge/Mass: Harvard UP 2004, 19–20. McGinn quotes Steven Kosslyn, Image and Brain, which I haven't followed up myself yet.