>A classic answer to this question is what I will call the Lines-As-Edges hypothesis. It says that drawings simulate natural images because line features activate edge receptors in the human visual system.

Why does the explanation need to go to that direction?

Line drawings have a liking to the thing being depicted.

They're a crude representation of it (compared to a photograph or a photo-realistic oil painting) but are nonetheless a represenation.

That's why they work, in the sense of people understanding what they show: they share similar patterns with the things being show. And we are pattern matches.

In this case the patterns are edge patterns, but it could just as well be non-edge patterns. Imagine a color drawing of a human face, where the ink drawn edges have been removed, and it's just blocks of color for the head, the eyes, the pupils, the mouth, the nostrils, ears, etc. We could still tell it's a face, even if we applied some gaussian blur to those blocks.

>The most basic statement of the problem with Lines-As-Edges is that the human visual system isn’t just an edge detector. You can see colors, you can see absolute intensities. You can tell the difference between a thin black line and the silhouette of a dark object against a light background; we have both kinds of receptors in the primary visual cortex, as well as others. Yet Lines-As-Edge supposes that the vision system discards all of this other information present in an image, for just this one special case. Why?

Isn't this taking things backwards?

It's not the vision system which "discards all of this other information present in an image" in our regular operation.

Rather, it's the line drawing with does away with (discards) all of this other information and only focuses on a thing's edges.

In other words, our visual system has capabilities A, B, C (say edge detection, color, 3d placement, etc). And, a line drawing gives it only A - which is still enough.

When our visual system also gets B and C, it can perceive objects even better. But for merely identifying something, A is apparently enough.

What I described here is totally compatible with Lines-As-Edges hypothesis, and makes sense too, so I don't see where the author's issue is, and why he thinks the fact that "the human visual system isn’t just an edge detector" invalidates the lines-as-edges hypothesis.

>Now you get a sense of the color of the object, and not just its outlines. How would one generalize Lines-As-Edges to account for these different types of depiction? The visual system is no longer ignoring everything aside some gradients; it’s now paying attention to some colors (and not others).

Yeah, so? It just means that the visual system can work with less or with more (and multi-type) information.

Lines-as-edges is a hypothesis for why line drawings "work" (are recognizable as the thing). The hypothesis doesn't say however that edges are the only thing the visual system can understand.

So, there's no need to "generalize Lines-As-Edges to account for these different types of depiction".

Lines-as-edges explains how line drawings are understood, period.

Line drawings with shading and color, add additional information, aside from the edges.

That's fine: no proponent of Lines-As-Edge ever said that the visual system only works with edges. Just that it works with edges when interpreting a line drawing which only offers edges.