depth perception

• Depth perception


  Our ability to perceive the world in three dimensions and the relative distances of objects
• Our sensory systems have evolved over millions and millions of years
• Purpose of our sensory systems

  • Not to create a realistic view of the environment, but to ensure survival through processing of information (food, shelter, friends, procreation, avoiding danger)
• We only process information that we need to survive, we don't process information that we can't/don't have to use (⇒ energy overconsumption)
• Light


  The visual stimuli that allows us to perceive depth
• Evolution of organisms' ability to detect wavelengths and presence or absence of objects


  1. Single cell/small group of cells with protein that changed its chemical composition after being in contact with waves of certain frequencies (eye spots)

  2. Eventually effective mechanism to detect presence of potential predator or prey

  3. Thought to have triggered Cambrian explosion
• Evolution of the eye

  • Goal is to determine how much light is located where
  • Concavity allows the eye to detect directionality of light
  • Fluid filled cavity allows spherical shape and small aperture for better directionality detection
  • Tissue evolved (cornea) to direct light into specific spot on retina
  • Lens developed to bend light differently depending on where it's coming from, allowing for better directionality detection and image resolution
• Humans have worse perception than many animals because we don't need it (unnecessary energy consumption)
• Transduction


  The method by which we take a physical stimulus and transform it into a neural signal
• Purpose of vision


  To detect if something is coming or going, allowing organisms to avoid predators and detect prey
• Distal stimulus


  The constant properties of the physical stimulus out there in the environment, such as the size, distance, orientation, and shape
• Proximal stimulus


  The 2D upside-down image projected onto the back of the retina, which is ambiguous in terms of size, distance, orientation, and shape
• The perceptual system must solve the ambiguity in the proximal stimulus in order for the percept to resemble the distal stimulus
• Emmert's Law


  We need to know either the distance or the size of an object in order to determine the other
• Size Distance Invariance Hypothesis


  If an object projects the same size visual angle and it's farther away, it appears bigger. If an object projects the same size visual angle and it's closer, it appears smaller.
• Primary depth cues


  • Accommodation (lens bending light to focus on retina, providing a cue for close objects)
  • Retinal/binocular disparity (differences between the images in the two eyes, providing a cue for depth)

  
• Field of view/binocularity trade-off


  Overlap between the eyes allows for depth perception but decreases the field of view. Prey animals prioritize field of view, while predators prioritize depth perception.

  
• Convergence


  The amount of rotation required to put the fovea (focus) on an object, which can specify its relative distance (more rotation for closer objects)

  
• Pictorial depth cues


  • Derived from inferences made from what our perceptual system knows about perspective and Euclidean (geometrical) relationships
• Inverse projection problem: what we perceive is closer to the distal stimulus than the proximal stimulus