Lecture #4 Sensation and Perception

• Sensation


  Nervous system detects or encodes information from the environment
• Perception


  How we interpret nerve sensations or give meaning to them
• We do not perceive everything in the physical world
• We miss events that happen around us, and even in front of us
• We perceive things that aren't there
• Perceptions are not true representations of the physical world
• Three themes of perception

  • Perception depends on context
  • Perception is about objects
  • Perception is multimodal
• Visible Light


  • Electromagnetic waves with wavelengths between 400 and 700 nanometers
  • Can be a wave, and also consists of particles known as photons

  
• Light reflection


  Light bounces off of objects
• Light refraction


  Light passes through objects and bends

  
• Lens


  Bends (refracts) light so that it focuses on retina

  
• Ciliary muscle


  Changes shape of the lens
• Corrective measures for vision

  • Nearsightedness (myopia): objects focus in front of retina, fixed by concave lens
  • Farsightedness (hyperopia): objects focus behind retina, fixed by convex lens

  
• Macula


  Center of retina
• Rods


  Detect dark vs light and motion, located in visual periphery

  
• Cones


  Detect colour and detail, three kinds, located in visual center
• Trichromatic theory of colour vision


  Colour vision is influenced by the output of all 3 cone types, not a single cone type

  
• Colour Contrast


  Colours are more vivid when they contrast with their background (context!)
• Colour Constancy


  Objects appear the same colour despite changes in the lighting conditions
• The lens of the eye focuses light waves on the retina; ciliary muscles control accommodation

  
• Rods and cones of the retina transduce light energy into neural signals

  
• The trichromatic theory states that colour vision is influenced by the output of all 3 cone types

  
• Perception of colour depends on context
• Gestalt Laws of Organization


  • Proximity: Perceive objects near each other as grouped together
  • Similarity: Perceive similar objects in groups
  • Continuity: Perceive parts as single, uninterrupted objects
  • Closure: Perceive stimuli as a whole entity, including illusory contours and negative space
• Linear perspective


  Parallel lines converge in the distance, diverge closer to us
• Monocular depth cues


  • Relative size: nearby objects appear larger than objects at greater distances
  • Relative height: faraway objects are higher in our view than closer objects
  • Familiar size: prior knowledge about the size of the object and how it should appear relative to other objects
  • Aerial perspective: distant objects appear more hazy because light is scattered by atmosphere
  • Motion parallax: distance estimated when viewing objects from two lines of sight
• Stereopsis


  We see depth because of binocular disparity, or the differences in images between left and right eyes

  
• Gestalt Laws of Organization let us group and combine visual features into coherent objects based on objects and past experiences
• Linear perspective, relative height and size, familiar size, aerial perspective, and motion parallax are monocular cues that let us see depth with one eye

  
• Binocular disparities, or the differences in images between left and right eyes, let us experience stereopsis, or using two eyes to see in 3 dimensions

  
• Perception is multimodal


  We perceive objects through more than one "mode" or sense
• Sound


  Mechanical vibrations we detect with our ears
• Sound amplitude


  Related to loudness
• Sound frequency


  Cycles per second, related to pitch
• Decibels (dB)


  • Ratio scales, can be used to describe any ratio of any value
  • dB sound pressure level (SPL): ratio of measured sound pressure amplitude to weakest sound pressure humans can hear at 1000 Hz
  • dB SPL (physical energy) is not the same thing as loudness (mental experience)
• Frequencies and sound levels we can hear

  • Frequency: 20 to 20,000 Hz
  • Level: -10 to ~140 dB SPL
• Cochlea


  Separates high and low pitches
• Sound transduction


  • Basilar membrane motion
  • Hair cells
• Speech perception is multimodal: hearing and vision combine
• Visual speech constrains the possible options for words that we hear