Psychology 263: Introduction to Cognitive Processes Visual illusions and what they tell us about perception

Visual illusions

• What can visual illusions tell us about perception?

• Note: some of the explanations here will be overgeneralizations

The Ternus illusion

• Three discs appear on a screen for 1000 ms • A blank screen is then presented for a variable ISI (0-100 ms) • A second screen then appears with the same three discs shifted to the right • Depending on the duration of the blank screen, participants see one of two types of motion: Element or group • Similar to this week’s CogLab demo What does the Ternus illusion tell us about perception? • In order to combat saccadic blindness (saccadic suppression), our visual systems do a lot of work to ensure pattern persistence (we continue to see scenes as we move our eyes) • At short ISIs, therefore, pattern persistence endures, and the simplest percept between the two frames is element motion • Note: Element motion is impossible to get rid of, no matter how you try

Kramer & Yantis

• Reduces, but does not eliminate, element motion Dodd, McAuley, & Pratt

• Here, element motion is unaffected, despite the fact that there is now only one object to perceive on the screen

• In the previous example, pattern persistence is so powerful that people report seeing a 3D visual illusion which facilitates element motion • Some percepts are so powerful they can not be eliminated, even when common sense/knowledge dictates that the perception is untrue…this is an unfortunate consequence of how our visual system is designed to combat things like saccadic blindness

CogLab demo for this week

• Apparent motion • Guess what, as of yesterday, you guys ruined it!

• What methods did we employ in this experiment? • On each trial of the experiment, a dot appeared and disappeared on both the left and right sides of the display. You changed the duration of a blank between the dots. You stopped the trial when the duration of the blank seemed to give the most convincing impression of motion for the dots. This is the Best ISI for that trial. Although the dot does not actually move across the display, with a properly chosen ISI, the apparent motion of the dot can be fairly convincing. The independent variable in this experiment was the spatial separation of the dots. On some trials the dots were close together and on other trials the dots were far apart. Apparent motion

• What do we predict participants will do? Why? • The graph below plots the average Best ISI (in milliseconds) as a function of dot distance from the center of the screen. You should find that the Best ISI increases with distance. This relationship was first noticed by Korte (1915), one of the earliest researchers on apparent motion. The relationship between spatial separation and ISI threshold is consistent with a variety of theories that hypothesize that the visual system builds a motion percept. For larger separations, the stimulus must "move" a farther distance, which presumably requires a greater length of time. This experiment allows you to see apparent motion in one of its simplest forms and to demonstrate Korte's law for yourself. Apparent motion is the basis of movement in all television, movies, and computer animation. The screen actually shows a rapid succession of still images; the perceived motion is entirely "apparent" and illusory.

What the results should look like

Data summary: Statistics are based on 2162 participants.

What did you guys do? Data summary: Statistics are based on 6 participants.

Averages across participants What does the presidential illusion tell us about perception • Context strongly influences perception • The visual system is not concerned with specific details of an image as much as the overall meaning of the image (the position of the people, hairline, clothes, etc. contribute to the illusion)

What can the Thatcher illusion tell us about perception? • Some people have argued that the Thatcher illusion is evidence that certain parts of the brain are specialized for rightside-up face perception (fusiform face area…but this may be expertise and not face specific) • A better account though revolves around knowledge/expertise…we are used to perceiving faces holistically, taking the configuration to be the most important information…when faces are upside down, however, our ability to process configuration is disrupted • Instead, we can only see features, which look normal, but the configural cues are what leads to the rightside-up difference

Hollow mask illusion (Chaplin)

• Experience with face perception also leads to the hollow mask illusion Hollow mask illusion (mask vs. egg carton) • Notice that changes in luminance affect face perception, the accompanying egg carton though (which we are accustomed to seeing) does not elicit the same effect (does for some though)

Similar effects without the face

• Ames Window

What can these illusions (Thatcher, hollow mask, Ames Window) tell us about perception? • Though we perceive the world in a reasonable manner most of the time, knowledge of how things are “supposed” to be perceived can strongly bias how things are perceived (even when this perception is false) Extra explanation for Ames window

• The window is actually a trapezoid but when viewed from the correct angle, it appears to be a square…even though it is continually rotating it looks like it stops every 180 degrees and turns back…that’s because our knowledge of how a square/cube should look from a certain angle conflicts with the appearance of the trapezoid from other angles

Similarly, these twins appear very different in size when in fact this is a trapezoidal room which looks like a cubic room only when viewed at a certain angle

Ames room More fun with context/experience

• The figure in the back appears much larger than that in the the front, even though they are the same size • Your visual system is using the relation to the horizon as a cue for judging size/distance/depth relationships

Importance of depth cues/experience • Nothing looks bizarre here…but what if the guy in the background is brought to the foreground

More fun with context

• Muller-lyer illusion Magnetic Hill

• Moncton, New Brunswick

• The layout of the land makes it appear that a slight slope downwards is actually a slope upwards, so people feel as though they are going backwards up a hill

What can the ball in a box shadow illusion tell us about perception • Without a shadow, motion is ambiguous…your visual system can not decide between the two simplest possibilities (moving diagonally across the floor or rising diagonally along the same depth plane) • Again, context strongly influences perception

Context

• These are all examples of the rare times when knowledge/context/experience, can actually cause you to misperceive something • Note: our visual system is designed to work properly “in most situations” but can lead to these failures Afterimages (and what they tell us about perception) • Black & White afterimages

Visual illusions (and what they tell us about perception) • Colour afterimages What do afterimages tell us about perception? • Perception is mediated by various cells in the retina which respond to light/colour, etc. • They are caused by fatigued cells in the retina responding to light. The most interesting color afterimages are negative afterimages. If you stare at the red color for 30 seconds or more, the cells in your retina that respond to red will fatigue and will fire less. When you switch over to a white surface, your eyes subtract the red and you see its complementary color green • Kind of like pulling a rope that someone else lets go of (opponent process)

Hermann Grid

Hermann Grid

• “Ghostlike” black dots that disappear when you look directly at them What can the Hermann Grid illusion tell us about perception • We’ve already seen the importance of semantic context in perception (e.g. president illusion), this provides an example of how other aspects of context (e.g. contrast between items in a scene) can influence perception • Receptive fields, lateral inhibition, etc. (beyond the scope of this course) • The manner in which the receptors in the retina perceive colour is directly influenced by the colour of other items in the visual field

In case you want the real reason:

• Wikipedia! • The effect of both optical illusions is commonly (and falsely) explained by a neural process called lateral inhibition. The intensity at a point in the visual system is not simply the result of a single receptor, but the result of a group of receptors called a receptive field. • A retinal ganglion cell pools the inputs of several photoreceptors over an area of retina, the area covered by the photoreceptors is the ganglion cells "receptive field". In the center of the receptive field the individual photoreceptors excite the ganglion cell when they detect increased luminance. The photoreceptors in the surrounding area inhibit the ganglion cell. Thus, since a point at an intersection is surrounded by more intensity than a point at the middle of a line, the intersection appears darker due to the increased inhibition.

Another contrast/colour illusion

• Stepping feet Same reason the two small grey squares appear to be different shades of grey even though they are the same (principle of simultaneous contrast)

Simultaneous contrast

• The manner in which two colors affect each other Magic!

• Many magicians use the principle of simultaneous contrast to conceal parts of their magical apparatus (e.g. to conceal supports of a floating body) • The surrounding parts are brightly illuminated, shiny metallic objectsm white cloth, etc…the parts to be revealed are black, in front of a black background and appear even darker to the viewer who is dazzled by the rest of the display • The eye can not make out the details in the darker parts

Twisted cord illusion

• The white lines here are all straight and parallel • Attributable to interaction between orientation- sensitive cells in the striate cortex • This illusion CAN’T be overcome

What can the breathing squares illusion tell us about perception? • The visual system is unable to resolve ambiguous information about the rotating square’s true direction of movement…the more information provided, the better motion can be distinguished • Even though the visual system normally does a good job of “filling in the blanks”, it can’t do so under all circumstances