WEEK 5: Visual Illusions ... and what they tell us about vision What is a visual illusion? -An optical illusion and visual illusion are two different things -A visual illusion is when perception conflicts with reality (what we perceive is a misperception (not real)) -The problem is that all of our perception is not a good representation of physical reality. Optical illusion: Mirage: the key thing is that the trick is in the light, it is not in the mind Visual illusion: -the trick is in the mind, not in the light Moon illusion: -optical illusion or visual illusion? Rainbow: -The wavelengths of light are changing as you go across the rainbow -Isaac Newton was the first person to understand wavelengths of light -A beam of light is separated into its wavelengths -A rainbow is a clear-cut example of perception conflicting with reality, but it never figures it out on virtual websites! Horizontal-Vertical Illusion -2 lines, both physically the same length in the shape of a T. but the stem of the T always looks longer than the tip of the T, but they are in fact the same length -We are normally very good at estimating relative length, but our perception of length is long here. Definition of a visual illusion (by Dejan Todorovic) -A misperception that stands out as an anomaly Visual Illusions are all about CONTEXT! -it is the context in which you put the stimulus that produces the illusion Why do visual illusions happen? -Theory -Illusions are perceptual hypotheses: when we are looking at stimuli we formulate hypotheses based on our knowledge of the physical world Alhazen -Arab philosopher who lived in the 12th century
Helmholtz -German physicist in the 19th century Gregory -British scientist of this century Perceptual Hypothesis -your visual system is making an assumption about light: light comes from the top/above -convex (inward) and concave (outward) Examples: 1) Motion movement from shadows -if the shadow is close: the object is moving in the same direction of the shadow 2) Depth from shadows: -the person looks like he is floating on top of the shadow but it is right next to him Well light looks like paint -We tend to see light as if it looks like paint -Ex: in New York, a person painted around the shadow, and when you do that it does not look like a shadow anymore. Why? Because our mind perceives it as a painted figure rather than a shadow. -We make a hypothesis based on the knowledge we are given in a specific picture Roshad Alakbarov: Creating shadow art from suspended objects -the face looks like it is painted with charcoal rather than there being shadows there. Photograph by Brian Mickethwait: -sunset illumination -when we see things that are brightly colored, we tend to see it as painted rather than light (we make a misperception) -we make the assumption that strongly colored things tend to be paint or stain rather than light Example: Shiny vs. Mat Demonstration -If