PvP exam

• 4 interpretations of depth


  • Absolute distance
  • Relative distance
  • Local surface orientation
  • Global surface shape
• Absolute distance


  Distance from observer to object, size/distance relationship (visual angle), distance and depth inferred by relative size eg larger things perceived to be closer
• Relative distance


  Distance from object to object, gives info about order – what is in front of what and therefore closer? Using known sizes (people, buildings etc) to judge relative distances
• Local surface orientation


  Orientation of a line perpendicular to the surface of an object at any location on the surface. These perpendicular lines = surface normals – can be defined by slant or tilt
• Slant


  Movement along vertical axis (left/right)
• Tilt


  Movement along horizontal axis (up/down)
• Global surface shape


  Orientation of a region of the object's surface or the variation of it – more generalised than local surface orientation
• Monocular depth cues


  • Motion parallax
  • Accommodation
  • Pictorial
  • Angular declination
• Pictorial depth cues


  • Relative size
  • Familiar size
  • Linear perspective
  • Texture
  • Interposition
  • Clarity
  • Lighting & shadow
• Levels of binocular vision (low to high)

  • Simultaneous vision
  • Fusion
  • Stereopsis
• Benefits of binocular vision


  • Space capacity – loss of one eye doesn't mean complete vision loss, increased FOV, overlapping fields – each field covers defects in the other, increased sensitivity – summation – probability of detection of stimuli is higher, stereopsis = more precise perception of depth than what is possible with just 1 eye
• Consequences of binocular vision


  • Double vision in some parts of visual field, confusion and rivalry with stimuli that cant be fused, changed visual directions – locations in space different in RE, LE and binocular viewing, distortion in 3D perception
• Disadvantages of binocular vision


  • Imbalance of refractive error, difficulty in converging on the same point, conflict between accommodative and convergence systems. Can cause eye strain, headaches, double vision
• Each monocular field = 155 deg, Total visual field = 200 deg, Binocular visual field = 120 deg
• Dioptic


  Single stimulus seen by both eyes – like natural binocular viewing
• Dichoptic


  2 stimuli, 1 presented to each eye, each stimuli can be independently controlled – used for research/clinical assessment
• Methods of dichoptic viewing


  • Dissociation by filter – red/green, perpendicular axes of polarisation
  • Dissociation by location – monocular imgs presented at different locations in space – free fusion
• Wheatstone's stereoscope


  2 monocular images facing each other, 45deg mirrors, 2 images adjusted so they overlap (due to being reflected off the mirrors), reflected into eyes. This is the combining of 2 images that can be independently manipulated
• Brewster's stereoscope


  Similar to Wheatstone's. 2 imgs placed side by side, prisms used instead of mirrors, imgs perceived as coming from one point due to prism position therefore 2 images fused. Prisms make it easier to fuse images – base out therefore images displaced medially. Partition eliminates extra diplopic images
• Types of free fusion


  • By convergence – 2 stimulus imgs placed side by side, cross eyes to a point in FRONT of the imgs, keep accommodation constant. Images fuse at binocular plane of fixation.
  • By divergence – similar principle as by convergence but fix on a plane BEHIND the images
• Why use a septum/aperture to free fuse


  Free fusion can be difficult due to 3 images (binocular in centre, 1 monocular on either side) – septum or aperture can block out the 2 monocular images. Free fusion also difficult due to requirement of voluntary adjustment of accommodation and convergence. Lenses/prisms can be used to aid viewing distance and vergence eg Brewster stereoscope
• Things that help fusion


  • Contour, luminance, colour and contrast SIMILARITIES between 2 monoc imgs
• Things that rival fusion


  • Contour, luminance, contrast, colour and geometric DIFFERENCES between 2 monoc imgs
• Stages of the visual pathway


  • Retina
  • Optic nerve
  • Optic chiasm
  • LGN
  • Optic radiations
  • Visual cortex
• Left visual field data goes to the right side of the brain, right visual field data goes to the left side of the brain
• Binocular correlation


  Finding a match between similar features in 2 monocular images
• Disparity


  Matching elements appear in slightly different locations in the monocular imgs
• Correlation


  Matching elements in same locations
• Suppression of uncorrelated images


  Uncorrelated monocular imgs cause confusion, visual system resolves confusion by suppression, generally weaker img suppressed
• Types of suppression


  • Gross – entire img from one retina suppressed
  • Central – suppression of fovea in one eye by img in the other
• If the 2 monocular images are the same 'strength', suppression alternates between the 2
• Levels of binocular vision (from high to low)


  • Simultaneous
  • Fusion
  • Stereopsis
• Corresponding retinal points


  When both eyes fix on the same point, that point is imaged on both foveae – foveae are corresponding retinal points. At the same time, other points in space will be imaged on other corresponding points
• Horopter


  Locus of corresponding points ie points with zero disparity. Closer fixating distance = smaller horopter
• Panum's area


  an area that lies close to the horopter, both externally and internally. If a feature is within this area, disparity is small and images can still be fused. Outside this, disparity is large and physiological diplopia occurs (eg 2 images of 1 object)
• Disparity gradient


  Ratio of the separation of 2 points
• For crossed disparities, objects have to be nearer than the horopter. For uncrossed disparities, objects have to be further than the horopter.
• Positive offset


  Offset towards right
• Negative offset


  Offset towards left
• Disparity equation


  Disparity = L target offset – R target offset. Positive value = positive disparity = moving towards midline, Negative value = negative disparity = moving away from midline