Psycophysics in Vision: Challenges in Studying Visual Perception

Question 1

Describe stimuli, physiological responses, perception?

Answer 1

• Physical characteristic of stimuli invokes physiological responses
• Physiological responses give rise to perception
• Studying relationship between physical stimuli and perception is psychophysics
• Studying relationship between physical stimuli and physiology and physiology and perception is visual neuroscience

Question 2

How do we see (2 theories)?

Answer 2

• Emission Theory:
  o Maintains that visual perception is accomplished by rays of light coming out of eyes & these rays are caught by visual objects
• Intromission Theory:
  o Vision is achieved by some copy or miniature version of object entering the eyes

Question 3

Describe the mind-body problem?

Answer 3

• How what we see, hear, feel, and think are related to the physical reality or processes in body
• Dualism: sees mind & body as two separate entities
• Monism:
  o Sees only mind or body as real
  o Depending upon which one is seen as real
  o Idealist: thinks only mind is real – for idealist, physical world does not exist & world around is mere creation of mind sent from God
   E.g. Bishop George Berkeley
  o Materialist: thinks only matter is real. Thinks that mind & conscious experience are just by products of material processes
   E.g. Ivan Pavlov – view held also by computer & neuroscientists today
• Dualism:
  o Both mind and body are real, separate entities & having different relations
   1st put forward by Rene Descartes
• No right or wrong answer & has not been proven

Question 4

What are the 3 challenges in studying perception?

Answer 4

1. Mind-body problem
2. Veridicality of perception
3. Vision as an inverse problem

Question 5

Describe the veridicality of perception?

Answer 5

• Veridicality: degree to which our perceptual experience accurately represents reality
• Some of our visual experience are consequences of active creation
• Perception as interference
  o Sensory info obtained through eyes
  o Also have prior knowledge of a situation
  o And have context of a situation
• Sensation vs Perception:
  o Sensation: process of transforming physical stimuli into electrical (neuronal) signal
   Data driven processing; bottom-up processing
  o Perception: process of interpreting these signals for conscious awareness or for action. Based on observer’s prior knowledge.
   Knowledge driven processing; top-down processing
  Illusion – when perception goes awry:
• Happens when perception of objects differs from their physical characteristics
• A window into how we perceive reality
• Experience something different than reality when:
  o Our own experience and knowledge conflict with sensory info
   E.g. ‘Hollow mask illusion’
  o Boundaries between objects are not clear – when there are multiple ambiguous sensory signals competing with each other for perceptual processing, brain will choose only one interpretation at a time
   E.g. ‘mask of love’
   BISTABLE PERCEPTION – viewer entertains 2 equally possible interchangeable perception
  o Things are upside down – details are lost
  o Our eyes get tired from looking at things for a long time
   Lilac Chaser: special illusion resulting from multiple visual phenomena.
   1st: motion illusion where there’s no real motion – dots are disappearing & reappearing in a very fast sequence. When they are fast enough, we experience the optical illusion of continuous motion.
   2nd: perception of the green dot is illusion from colour adaptation – green dot is never there. It is an after image emerging after the red-sensitive cells in retina get tired after the prolonged view of the red dots. The complementary green sensitive cells get relatively sensitive to overtake the grey gap as if a green stimulus had been presented.
   3rd: disappearance of pink dots is explained by Troxler’s fading, where static stimulus away from fixation fades away & disappear when we don’t move our eyes to refresh the scene

Question 6

Describe the veridicality of perception?

Answer 6

• Veridicality: degree to which our perceptual experience accurately represents reality
• Some of our visual experience are consequences of active creation
• Perception as interference
  o Sensory info obtained through eyes
  o Also have prior knowledge of a situation
  o And have context of a situation
• Sensation vs Perception:
  o Sensation: process of transforming physical stimuli into electrical (neuronal) signal
   Data driven processing; bottom-up processing
  o Perception: process of interpreting these signals for conscious awareness or for action. Based on observer’s prior knowledge.
   Knowledge driven processing; top-down processing
  Illusion – when perception goes awry:
• Happens when perception of objects differs from their physical characteristics
• A window into how we perceive reality
• Experience something different than reality when:
  o Our own experience and knowledge conflict with sensory info
   E.g. ‘Hollow mask illusion’
  o Boundaries between objects are not clear – when there are multiple ambiguous sensory signals competing with each other for perceptual processing, brain will choose only one interpretation at a time
   E.g. ‘mask of love’
   BISTABLE PERCEPTION – viewer entertains 2 equally possible interchangeable perception
  o Things are upside down – details are lost
  o Our eyes get tired from looking at things for a long time
   Lilac Chaser: special illusion resulting from multiple visual phenomena.
   1st: motion illusion where there’s no real motion – dots are disappearing & reappearing in a very fast sequence. When they are fast enough, we experience the optical illusion of continuous motion.
   2nd: perception of the green dot is illusion from colour adaptation – green dot is never there. It is an after image emerging after the red-sensitive cells in retina get tired after the prolonged view of the red dots. The complementary green sensitive cells get relatively sensitive to overtake the grey gap as if a green stimulus had been presented.
   3rd: disappearance of pink dots is explained by Troxler’s fading, where static stimulus away from fixation fades away & disappear when we don’t move our eyes to refresh the scene

Question 7

Describe vision as an inverse problem?

Answer 7

• Perception is fundamentally to solve an inverse problem
• Measure effect given the cause
• Visual system faces inverse problems every day, trying to reconstruct the world outside based on incomplete info from sensation  one of reasons why perception is act of active interference
• Visual system solves inverse problem without much trouble