Visual Cortex; the vanishing cow

Here's a nice overview of visual cortex, with a few cool experiments you can try on yourself:http://www.tutis.ca/Senses/L2VisualCortex/L2V1.pdf
When testing your blind spot, I suggest moving the test screen back and forth a bit while fixating the X target: you will see that at one particular distance the center of the blind spot becomes invisible - or, better, gets "filled in" with the rest of the pattern. It's at this distance that the central patch falls exactly on the blind spot (use 1 eye only, and don't cheat: you must fixate the X.) Here's another example: fixate the + and find the distance from the screen where the cow vanishes. Why and how does the cow vanish?
from http://www.psy.ritsumei.ac.jp/~akitaoka/moten01.jpg