Demonstration of Importance of Edges

1
Demonstration of Importance of Edges

• Following are five illustrations that show how
  edges are very important to the visual system.
• Mach Bands
• Craik-Cornsweet Effect
• Simultaneous Contrast
• Effect of Minimal Contours
• Hermann Grid

2
Mach Bands
Click to add successively lighter bars. Watch
the edge to the right side of the last bar.
3
Edges and Mach Bands

• What happened to the edges of the bars?
• What does that say about the role of edges in
  vision?

4
Craik-Cornsweet
5
Craik-Cornsweet Described
The figure above is an exaggerated map indicating
the light levels across the image on the previous
slide. Note how the center and edges have
identical luminance. That can be seen by sitting
far enough away from the screen
6
Edges in the Criak-Cornsweet

• What did the edges do here in the Craik-Cornsweet
  illusion?
• How is this different from the role of edges in
  the Mach Bands?
• Can we apply what we learned here to the Mach
  Band illusion?

7
Simultaneous Contrast
8
Edges and Simultaneous Contrast

• What happened in this demonstration?
• How it is similar to Craik and Cornsweet?
• Have we learned something new about the
  importance of edges to visual perception?

9
Minimal Contours
10
Minimal Contours Described
There are two circles below. Both circles have
the same luminance (intensity level) at the
center. Click on your mouse and This one changes
abruptly watch as the edges are blurred to
the level at the center. and the circle
disappears.
11
Edges in Minimal Contours

• So what does the lack of edges tell us here?
• Does this illusion add to what we have already
  learned? In what way?

12
Hermann Grid
13
Edges and the Hermann Grid

• What was the effect of edges on the left Hermann
  Grid?
• On the Right Hermann Grid?
• How is this the same and different from the other
  demonstrations?
• Does this tell us anything about what goes on in
  the blind spot?
• About tunnel vision?