L2 Sensation and Perception

• What determines the hue or colour or a light wave?

  The length of the wave.
• What determines the brightness of a light wave?

  The amplitude/ intensity.
• What makes up the saturation of a light wave?

  The number of wavelengths that make up the light - purity.
• How does our vision work?

  1. Light passes through a clear, smooth outer tissue called the cornea.
  2. The cornea bends the light wave and sends it through the pupil.
  3. The iris controls the size of the pupil and the amount of light that can enter the eye.
  4. The thickness and shape of the lens adjust to focus the light on the retina, where the image appears upside down and backwards.
  5. Light-sensitive receptor cells in the retinal surface, excited or inhibited by spots of light influence the specialized neurons that convey nerve impulses the brain.
• What are charcateristics of cones?

  • Detect colour
  • Operate under normal daylight conditions 
  • Allow us to focus on fine detail
  • Each retina contains about 6 million cones
  • Cones adapt to the dark within 8 minutes but aren't too sensitive 
• What are characteristics of rods?


  • Become active only under low light conditions
  • Much more sensitive than cones
  • Provide no information about colour and sense only shades of grey
  • About 120 million rods are distributed around each retina except in the very centre, the fovea, where there are no rods.
• What is sensation?

  Detection of simple properties
• What is perception?

  interpretation of sensory signals
• Single-cell recording (Hubel & Wiesel, 1962)

  • Electro-physiological response of a single neuron can be observed by inserting a microelectrode
  • Electrical responses are monitored in a single cell when bright lines in different orientations are projected onto a small area of the retina
  • Particular cells are selectively active in response to a particular stimulus in a particular orientation
• Early processing of visual information:

  • Interconnections mean that the retina does not function as a simple light detector
  • Retinal processing involves ‘cleaning up’ of image and beginnings of feature extraction
  • Input feeds into the visual cortex in which separate structures exist for extracting information about shape, colour position, motion etc
• Perception:

  • Requires computational processing of sensory data, including segmentation and object recognition, and construction of 3 dimensional representation
  • Many of these processes are automatic or innately determined
• What is involved in segmentation?


  • Visual features that belong to the same object are grouped together
  • Figure-ground perception

  
• What are the Gestalt principles of grouping?

  • Grouping of elements to make a figure is determined by a set of basic principles that are automatic and innate
  • We group by:
  • similarity
  • good figure
  • proximity
  • connectedness
• The back of the retina is flat, but your brain automatically perceives depth and constructs a 3D world
• The visual cliff:

  • Of 36 infants, 27 were willing to move onto the shallow side, but only 3 onto the deep side
  • Shows they can perceive depth early on.
  • A one day old goat, when placed on the deep side, jumps to the safety of the start board - depth perception is innate?
• What is perceptual constancy?


  Our brains implicitly assume that objects are stable and unchanging.