Visual Illusions Notes - lecture 6

Visual Illusions

Introduction

• Visual illusions demonstrate that perception is not always an accurate representation of reality.

• What we see can be quite different from what is physically present.

• Illusions reflect normal visual processing and distortions arise from comparisons.

• Comparisons can be made across time (motion) or space (brightness, color).

• The processing causing illusions increases sensitivity to change.

• Illusions occur when what you see doesn't correspond to what's physically present.

• Illusions tell us about raw visual processing.

Contrast Across Time

• Afterimage example: Look at the center of a light bulb filament for 20 seconds, then look at a blank screen.

  • You may see a light area where the filament was.

  • Areas where you viewed dark become light, and vice versa.

• Purple Smudges example: Focus on the central cross, and the purple smudges disappear.

Color Aftereffects

• Look intently at the central cross for 10-20 seconds.

  • Above the cross is a green area, below is a pink area.

• Switch to a blank white screen.

  • You may see the colored version.

  • If you don't see it, adapt for a longer time.

• The green area becomes pink, and the blue square becomes yellow, and vice versa.

Explanations for color aftereffects:

1. Pigment bleaching in the peripheral cones.

2. Neuronal fatigue: Overactive neurons become tired and require less frequent firing.

3. Opponent processing: The brain compares one color to another.

• Color Vision:

  • Different color receptors (short, medium, and long wavelengths, often referred to as blue, green, and red cones).

• Ganglion cells: Receive inputs from color receptors.

  • Example: A ganglion cell receives positive input from green cones and negative input from red cones.

• Opponent systems:

  • Red vs. green (green-on/red-off and red-on/green-off).

  • Blue vs. yellow.

  • Light vs. dark (and dark vs. light).

• Pigment Bleaching and Neural Fatigue: When viewing a stimulus light, green and red cones respond equally, resulting in white perception.

  • Looking at intense green for some time leads to maximal green cone firing and minimal red cone firing, resulting in green perception. After adaptation:

  • Green cones are pigment bleached and fatigued and when looking at a pale, light-colored area, the green cone system fires less than the red system.

  • Due to the opponent processing red cones fire more.

  • The perception is desaturated red or slightly pink when looking at white.

Motion Aftereffects

• Waterfall Illusion: After watching a waterfall for some time, trees appear to float upwards.

• Spiral Aftereffect: After watching a spiral for 40 seconds, static clouds appear to move in various directions.

Explanation:

• Detectors for movement exist for all different directions all over the retina.

• Adaptation occurs in local areas causing the effect.

• Vertical bars Example

  • Without adaptation: Detectors for left and right movement respond similarly and randomly, so perception is static.

  • Adapting to leftward movement: Left movement detectors respond more than right movement detectors, resulting in a perception of movement to the left.

• Mechanism

  • Looking at a static image after adaptation, the left movement detector is fatigued.

  • Right movement detector has more activity, so movement to the right is perceived.

• Comparator cells are needed to process movement motion detector comparator has positive input from leftward movement and negative input from rightward movement.

• The waterfall and spiral aftereffects are caused by nerve cell fatigue and opponent processing interaction.

• Similar to color vision, where black and white, red and green, and blue and yellow are compared.

Tilt Aftereffects

• Vertical bars adapting field: Look at vertical bars and cast eyes back and forth for 10-20 seconds, after adaptation vertical bars may appear tilted counterclockwise or bent.

• The explanation is more complicated then opponent processing.

• Orientation is processed by the pattern of activity in a population of cells.

• Cells in the visual cortex are tuned to orientation (vertical, horizontal, etc.).

• Looking at a static image with vertical gratings:

  • The largest response is with the cells tuned to the vertical axis, and there is some activity in the horizontal detectors, but it's much less than the vertical.

• Adapting to a counterclockwise tilted grating:

  • The activity profile shifts in favor of cells responding to counterclockwise rotation, more so than vertical orientation.

• After adapting for one minute:

  • The peak of activity is till at the orientation of bars tilted slightly counterclockwise.

• When switching back to vertical, the population activity is distorted relative to pre-adaptation:

  • Cells that have been firing at a high rate are fatigued.

  • The profile of activity across the population will be maximal at the cells tilted slightly clockwise.

• Illusions occur due to neural fatigue and population coding of orientation.

General Principles of Illusions Across Time

• Brightness, color, movement, and orientation aftereffects all occur as things change across time.

• Cells adapt to the steady state, so the perceptual system is more sensitive to change.

• The perceptual system overemphasizes the change. In aftereffects you're overemphasizing the change that happened.

Brightness Contrast

• Illusions that rely on comparisons across space.

• The brightness of an object is coded relative to the brightness of its surround.

• A uniform gray bar looks lighter on the dark side of a gradient and darker on the light side, even though it's the same lightness across the bar.

• The intensity of one region is coded relative to the surrounding region.

• If the surround is very bright:

  • There is lots of light, causing inhibition, which takes down response of the cell coding brightness.

• If the surround is dark:

  • There is less inhibition, so the cell fires more.

• Brightness of any object depends on the brightness of the surround.

Color contrast

• The color of this region is coded relative to the surrounding.

• One cross looks orange and the other green but they are the same color.

  • Since the surround is very red, it looks green.

  • When the surround is red/green looks red or orange.

Motion Contrast

• Moon and Clouds Example: On a clear night with clouds blowing in one direction, the moon may appear to move in the opposite direction.

• The moon is static, but the motion contrast illusion makes it appear to move.

Orientation Contrast

• Elongated lines don't appear parallel but appear divergent at one end and converging at the other.

• Coding the orientation of one object relative to the orientation of the surround causes the illusion.

General Principles of Illusions Across Space

• Brightness contrast, color contrast, motion contrast, and orientation contrast occur when regions over space are coded relative to the surround.

Geometrical Illusions

• Illusions of size and distance.

• Object size is inferred from distance (how far away it looks) and image size.

• If two objects have the same image size, the object that appears farther away is seen as larger.

• Explanation is working out the distance to the object and what retinal size is.

Ponzo Illusion

• Parallel lines converge, suggesting the point of convergence is further away.

• The top horizontal line appears larger because it seems further away.

Muller-Lyer Illusion

• The separation between fins may appear uneven, even if it's not.

• Perspective cues set up a false impression of depth and distance.

• This looks likes the corner of a building.

• Edges are receding away from you.

• So this height is larger because appears further away has to be larger.

Ames Room

• People on the left and right sides of the room appear to be different sizes.

• The room is constructed so that the back wall is not equidistant from the viewer.

• Monocular perspective is used (looking through a peephole) to remove stereo cues.

• The left and right corners aren't equally far away, so objects at different distances appear to be different sizes.

• Objects moving around will change in apparent size within a distance.

• Gregorys theory then, makes hypotheses about depth and distance objects that are interpreted in size based on the image evidence that it's actually true.

• Cross-cultural studies: People from different cultures are generally susceptible to the same geometric illusions.

Ambiguous Figures

• Figure-ground reversal example: Seeing two faces or a vase.

• Necker Cube: The interpretation of which face is in front changes spontaneously.

• Any two-dimensional image on the retina has multiple three-dimensional possibilities.

• Each interpretation can only be perceived one at a time.

Impossible Figures

• Seemingly three-dimensional structures that are contradictory when examined in detail.

• Examples include Escher's prints (e.g., a waterfall that seems to perpetually flow downwards to its own source) and stairs that perpetually ascend.

• The brain can't compute three dimensions consistently from all parts of the figure where this local area makes sense, but it contradicts the Q's here.

Penrose Triangle

• Appears to be a physically possible object from a certain vantage point.

• From a different viewpoint, the illusion is revealed because The edges don't line up.