Answer to Question 1
Answer: Bottom-up processing occurs as features are processed and combined to activate different letters. Additionally, the letters are combined to activate possible words. Top-down processing occurs as the possible words are used to fill in the missing pieces of the letters.
Answer to Question 2
Answer: If there are light and dark surfaces next to each other forming an edge separating the surfaces, center-surround receptive fields are excited by the light surface but inhibited by the darker surface. Additionally, the center-surround receptive fields on the border between the light and dark surfaces respond differently since these receptive fields startle both the light and dark sides. On the light side, the center of these receptive fields is excited by the light and a portion of the surround is excited by the dark, producing a heightened response. Conversely, on the dark side, the center is inhibited by the dark and a portion of the surround is inhibited by the light, producing a more negative response. The resulting perception is an edge but one with Mach bands.
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