WEEK 9 - Visual Cognition Pt 2.

• Even though the image on our retinas is 2D, how do we perceive a 3D world? Our minds use different kinds of cues consistently associated with depth to perceive depth
• Where do cues for depth come from? Learnt association between depth cues across our lives which become automatic through consistent, repeated exposure
• What are the 3 different kind of cues that our minds use to perceive depth?

  1. oculomotor cues – bits of information about depth that our minds get from muscles in an around our eyes

  2. monocular cues – bits of information that can be extracted from images by either eye independently

  3. binocular cues – bits of information that can only be extracted from images by both eyes working together
• Explain oculomotor cues: our body and mind able to sense position of our eyes and degree of muscle tension controlling our eye position
• What are 2 types of ocular motor cues?

  1. ocular convergence

  2. lens accommodation
• What is ocular convergence? An oculomotor cue based on ‘inward’ movement of our eyes when we focus on nearby objects – muscles around eyes tense – this tension is a depth cue
• What is lens accommodation? Change in shape of lens when focusing on near vs far objects
• When an object is close, lens is shorter and fatter – ciliary muscles relax

  
• When object is far, lens is thinner – ciliary muscle tense
• What are 2 types of monocular cues?

  1. pictorial cues

  2. non-pictorial cues
• What type are most monocular cues? Pictorial
• What are pictorial cues? Information that can be extracted from static 2D image

  What are non-pictorial cues? Information that cannot be extracted from a static 2D image
• What are monocular pictorial cues?

  1.occlusion

  2. relative height

  3. Perspective convergence

  4. Familiar size

  5. atmospheric perspective

  6. texture gradient

  7. shadow
• 1. What is occlusion? When one object partially covers another. The occluded object is typically deeper in the visual field than the occluding object. No sense of absolute distance but tells us relative distance.

  
• 2. What is relative height?

  BELOW the horizon, Objects higher in field of vision are more distant

  ABOVE the horizon, objects lower in field of vision are more distant

  Objects of EQUAL size, the closer one will take up more visual field
• What is perspective convergence? Parallel lines appear to approach one another at greater distances

  
• What is familiar size? Knowledge of sizes of familiar objects can help us infer which object is closer e.g. 5c coin closer than 20c coin as it appears larger to us in this image. Only works If you are familiar with coin sizes

  
• 5. what is atmospheric perspective? Distance objects are blurrier/ fuzzier, having sometimes a blue/grey tint depending on atmospheric conditions

  
• 6. What is texture gradient? Equally spaced elements more closely packed as distance increases e.g. cobblestone looks more packed further away it is

  
• 7. What is shadows? Position of objects shadows can be useful cue to their depth

  
• When are these monocular pictorial cues most effective? When multiple are given as they give consistent information
• What are nonpictorial cues? Motion parallax
• What is motion parallax? Near objects glide rapidly past but Objects in distance move slowly
• Changes in pictorial occlusion are non-pictorial cues e.g. deletion (object being covered) and accretion (when object is uncovered)
• Explain how depth perception is typically extracted from binocular cues: this typically involves comparing slightly different images presented to our left and right eyes. Retinal images are similar but not exactly the same thus extra information gained from comparing images
• What is binocular disparity? Some objects in world fall in same retinal location on both eyes and some fall in different retinal locations in different eyes
• What does same location binocular disparity mean? Images fall on corresponding points
• What is meant by corresponding point? Places on retina that would overlap if 2 retinas were overlaid on one another
• What is the fovea? Always a corresponding point, whatever you are focusing on in the world, is on the same place on both retinas

  
• What else in the world falls on corresponding points of retina? Objects on the horopter arc
• What is the horopter arc? Distances in the world that fall on corresponding points in the 2 eyes

   

  The larger the disparity of two retinal images (binocular disparity), the further away it is from the horopter arc

  
• What is crossed disparity? Object is inside the horopter and therefore appears closer

  
• What is uncrossed disparity? Object is outside the horopter and therefore appears further

  
• What is random dot stereograms? Use binocular disparity to create depth

  How does random dot stereogram work? Mind detects binocular disparity between same information

  
• How are size perception and depth perception deeply interrelated? Size information is a cue for depth, and depth information is a cue for size
• What is the measure of retinal image size? Visual angle

  What is visual angle? Amount of retina that is occupied by an image

  
•  When depth cues absent, object’s perceived size is determined by its retinal image size

  Explain how we have an eclipse of the sun? moon’s disk almost exactly covers the sun during an eclipse because the sun and moon have almost the same visual angle despite having very different distances.

  
• Explain the Holway & Boring 1941 experiment about depth cues:

  An observer is seated at intersection of 2 long hallways

  A screen in which luminous circles are presented are displayed at each hallway

  On the right hallway, there is a far luminous test circle (30-40m away)

  On the left hallway, a close comparison luminous circle was presented about 3m away

  Task: look at both luminous circles and adjust so comparison luminous circles looked the same size by changing diameters of circles-   Experimenters look at the comparison circle and observe how large the participant sees the test circle to be
• Results of Holway & Boring 1941 experiment about depth cues:

  y-axis, as you go up, participant thinks the test circle is larger

  no changes between monocular and binocular cues

   

  when depth cues removed, people made judgements less on actual physical size of test circle but more on visual angle that is taken up on the retina

  
• What do findings of Holway and Boring 1941 demonstrate? The concept of size constancy