3D Space Perception

Physicalist Approach

  • Focuses on identifying physical image features associated with 3D perception.
  • Employs a cue-based approach, explaining perception based on the presence or absence of physical cues.

Da Vinci and Wheatstone

  • Da Vinci noted the absence of 'realness' in paintings.
  • Wheatstone demonstrated that stereoscopic pictures with binocular disparity could produce this 'realness'.
  • The cue-based physicalist approach explains 'realness' as a result of binocular disparity.

Problems with Physicalist Approach

  • It fails to explain why we perceive relatively undistorted 3D space when viewing pictures with both eyes.
  • It does not account for perceptual duality during binocular picture viewing.
  • It doesn't predict a shift in perceived depth magnitude between binocular and monocular viewing.
  • Unable to explain how stereopsis can be achieved without binocular disparity using monocular-aperture viewing.

Phenomenological Approach

  • Abstract Perceptual Model of Phenomenology
  • Analytic Description of Phenomenology
  • Perceptual Phenomenology
    1. Egocentric distance
    2. Relative (unscaled) depth structure
    3. Exocentric (scaled) distances

Depth Cues and Phenomenology

  • Sensory Input
  • Encoding
  • Phenomenology
  • The impression of stereopsis arises when the brain can derive visual scale (exocentric distances).
  • Deriving exocentric distance requires information on egocentric distance.
  • Exocentric distance is vital for visuo-motor planning and execution.
  • Stereopsis is associated with feelings of presence and tangibility.

Explaining Observations

  • Pictorial Duality

Pictorial Duality under Binocular Viewing

  • The picture surface is encoded by exocentric distance and appears "real."
  • The pictorial object is encoded by unscaled depth and does not appear "real."

Binocular Disparity and Stereopsis

  • Binocular disparities enable the brain to derive object distance and scale, causing stereopsis.
  • Disparity can be manipulated to position the object in space.

Monocular-Aperture Effect

  • Monocular aperture viewing causes the brain to assign a specific visual distance to the object, allowing for the scaling of pictorial depth cues.

Viewing Photographs

  • With both eyes, the brain doesn't assign a visual scale to the scene.
  • Monocular aperture viewing causes the brain to assign a specific distance and scale.

Cognitive Interpretation and Familiar Objects

  • Familiar objects (bridges, buildings) lead to a cognitive interpretation of large, distant objects.
  • Scale isn't visually fixed, but cognitive interpretation suggests far-away large objects.

Miniaturization Effect

  • Monocular aperture viewing should cause the scene to appear miniaturized, like a small model scene nearby.

Depth-of-Focus Blur and Tilt-Shift Miniaturization

  • Depth-of-focus blur is an optical cue to egocentric distance.
  • Applying depth-of-focus blur makes pictured objects appear closer.

Depth-of-Focus and Distance

  • Adding depth-of-focus blur forces the objects and scene to appear at a specific distance much closer to the observer, optically fixing the distance and scale.

Blur and Stereopsis

  • Adding blur should generate a phenomenology of stereopsis.

Tilt-Shift Phenomenology

  • Tilt-shift miniaturization shows an interaction between the distance perception effect of depth-of-focus blur and familiar-size information.
  • The brain interprets familiar objects as being close up and small by perceiving them as a "toy scene."
  • This creates cognitive dissonance: a 3D scene appears toy-like while being life-size.

Viewing the Real World

  • Little or no impression of stereopsis at far distance.
  • Moderate impression of stereopsis at mid distance.
  • Strong impression of stereopsis at near distance.

Scale and Distance Certainty

  • Reduction in certainty of scale and impression of stereopsis with distance in real scenes.

Neural Processing Predictions

  • Based on multiple space encodings and stereopsis phenomenology, predictions can be made about neural processing.

Two Visual Streams

  • Visuo-motor transformations for actions and perception of objects and space.
  • Stereopsis is linked to encodings of scaled depth needed for motor interaction.
  • The phenomenology of stereopsis is instantiated in the dorsal stream (posterior parietal cortex).

fMRI of Stereopsis

  • Monocular and binocular stereopsis conjunction analysis reveals an area in the posterior parietal cortex activated during stereopsis, regardless of the cue.
  • MNI: x y z = -24 -76 46

Model from Phenomenological Approach

  • The model explains depth in pictures and the specific changes in depth phenomenology (realness, distance perception).
  • It considers the phenomenology of scale as a central component of 3D space and objects.
  • It understands spatial phenomenology instead of focusing on how depth cues "mix".

Physical vs Perceptual Realm

  • Physical Realm: Physical description of stimulus.
  • Psychophysical or Neural Model: Neural/behavioral response.
  • Perceptual Realm: Analytic description of phenomenology.

Analytic Description of Phenomenology

  • 1. The phenomenology of egocentric distance
  • 2. The phenomenology of relative (unscaled) depth structure
  • 3. The phenomenology of exocentric (scaled) distances

Summary

  • A purely physicalist viewpoint doesn't provide a conceptual structure for a unified theory.
  • Phenomenology helps determine the structure of perception.
  • Both phenomenological and physicalist approaches are necessary.
  • To understand exocentric depth, understanding physical cues is essential.

Strabismus

  • Strabismus is caused by impairment in eye muscle function (usually congenital) and has a genetic element.
  • Depth perception was assumed to be severely impaired in strabismics.
  • "Seeing 3D" was often confused with "realness/stereopsis." Also, there were assumption that there is a single 3D spatial encoding
  • Traditionally, what aspects of 3D vision are impaired in strabismus has not been studied. Many figures were strabismic, such as da Vinci.

Depth and Distance Cues

  • Depth Cues:
    • Contour
    • Shading
    • Interposition
    • Relative size
    • Height in the field
    • Texture
    • Perspective
    • Blur
    • Binocular disparity
    • Motion Parallax
  • Distance Cues (near space):
    • Accommodation
    • Convergence
    • Depth of Focus Blur
  • Distance Cues (far space):
    • Declination from eye level
    • Ground-plane information

Distance Cues and Range

  • Lens accommodation (1m)
  • Ocular convergence angle (6m)
  • Declination from eye level (5m-20m)

Relative Depth Cues

  • Provide information about 3D shape and layout
  • The only cue compromised in strabismus is binocular disparity.
  • Traditionally, since binocular disparity has been considered the most important 3D cue, it was thought strabismics have very compromised depth perception and generally don't see in 3D

Strabismus and Distance Perception

  • Those with Strabismus can do blind walking as well as non-strabismics when target object is on the ground
  • However, they underestimate distance when the object is suspended in mid air

Blind Walking

  • Accurate (depends on declination from eye level)
  • Underestimated (task depends more on binocular disparity)

Relative Depth Judgement

  • Task: Adjust until they appear equal in 3D
  • Typical observer will set farther intervals to be larger (than they should be) in order to look equal
  • Someone with impaired 3D perception should have even greater underestimation

2D Judgement

  • Are the black bars equally spaced in on the picture plane (screen)?
  • Pictorial depth biases judgement and makes farther interval seem larger
  • Task: Adjust bars until they appear equidistant in the picture plane
  • Typical observer will set farther intervals to be smaller (than they should be) in order to look equal
  • Observer with impaired 3D perception should have less bias

Examining Relative Depth Perception in Strabismus

  • Compare performance of strabismic and non-strabismic observers
  • Tested both monocular and binocular viewing

Results

  • Strabismics have difficulty with precision motor tasks in near space (threading a needle, pegging a board)
  • Grasping in near space could be compromised
  • These tasks need perception of exocentric distance and egocentric distance in near space

Depth Perception in Strabismus

  • Perception of 3D shape and layout is the same as for non-strabismics (relative depth)
  • Egocentric distance estimation using blind walking shows that those with strabismus can accurately estimate distances to objects as long as there is ground contact
  • Compromised in motor tasks requiring exocentric (scaled) distance and egocentric distance in near space (especially arms reach) due to loss of binocular disparity and ocular convergence information
  • Strabismics can perceive depth and only compromised on specific aspects. Also, evidence that strabismics can perceive monocular stereopsis
  • This suggests that compromise is only on the binocular contribution to exocentric and egocentric distance perception
  • Provides support for view that 3D perception based on multiple encodings, not a single one