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I want to address your query about when the brain begins to make constructions and adjustments to sensory visual information, and to question it further. Critical periods exist for the development of many cognitive and neural behaviors.

Landmark research examined newborn kittens, placed in environments which limited their experience to a few of many visual elements, and examined their sensitivity to visual elements that were not within their environments until a certain point in development (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1309349). It turns out that critical periods exist for many aspects of vision, which leads me to consider the constructions of the mind (and adjustments in what is physically recieved by sensory systems of the brain, prior to a signal's travel to BA V4 (the visual system in the occipital lobe).

It is clear from the kitten research that it is physically possible to recieve horizontal input and not produce any sort of "horizontal output" in the form of conscious awareness/perception of horizontal line. How, then, do these constructions, such as the perception of horizontal lines, manifest themselves in terms of critical periods for more holistic processing? What about squares? They include horizontal lines--are they percieved once the critical period has passed without recieving input? Does the brain fill them in despite never having experienced them (in which case the plastic areas of the brain which recieve input and produce output for horizontal lines may not yet have atrophied)?