SFWR ENG 4D03: Design of Human Computer Interfaces

Software Engineering 4D03, Design of Human Computer Interfaces

Perceptional experiments and illusions
2002 November 26

In the lectures some visual effects were mentioned that you can easily observe yourselves. Try them out:

Draw a cross on a piece of paper and about 2 inches to its left, a dark circle. Close your right eye and look at the cross with your left eye. Slowly move the paper closer to your eye or farther away, always looking at the cross. At a certain distance, the circle will disappear because its image falls on your blind spot at the retinal end of your optic nerve. Try the same with colored paper; when the dot disappears, you will see the background color where the dot should be. These optical effects are illustrated on pages 38 and 39 of the book Sensation and Perception by Goldstein (fifth edition).
At night, adapt your eyes to darkness for some time (at least 12 minutes). Then look at a faint star. If you look straight at it (so that its image falls on your fovea, which is devoid of rods), you will not see it, but if you look a little to the side, it will appear.

The web page http://www.cami.jccbi.gov/AAM-400A/Brochures/Pilot_Vision2_2.htm has additional information on the night blind spot, the anatomical blind spot, the fovea (in particular, its narrow visual angle) and the horizontal and vertical angles of human vision.

In the lectures I used a slide showing the distribution of rods and cones in the retina. The same diagram is at web page http://people.bu.edu/takeo/takeo/fig(chapter%201).PDF#page=5 (and undoubtedly other locations as well).

Other examples of interesting optical effects of potential interest to designers of human-computer interfaces can be viewed at
http://www.ukl.uni-freiburg.de/aug/bach/ot
http://www.cs.ubc.ca/nest/imager/contributions/flinn/Illusions/Illusions.html
and many other web sites (search for "visual illusions").

Previously, the web site "illusionworks" had an interesting collection of visual illusions. The web site apparently no longer exists, but archived pages from it can be viewed beginning at
http://web.archive.org/web/20020124023351/http://illusionworks.com.

An interesting aural illusion is called Shepard's tones. Each tone is higher than the previous one, but overall, the sequence of tones does not rise at all. This aural illusion can be heard at
http://www.cs.ubc.ca/nest/imager/contributions/flinn/Illusions/ST/st.html. and other web sites (search for "Shepard's tones").