Perceptual Systems and Multimedia

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Perceptual Systems and Multimedia

• CS4005

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Course outline

• Perception and measuring perception
• Physiology of perception central nervous system
  (brain, spinal cord) neurons, communication
• Visual system
• Auditory system (pitch, loudness, location)
• Touch system
• Object perception, depth and spatial perception,
  attention using the visual system as an example

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Required reading

• Sensation and Perception. E. Bruce Goldstein 7th
  edition bundled with a lab manual available
  in the book shop
• Lecture notes on web http//www1.csisdmz.ul.ie/cur
  students/modules/1stsem/cs4005/
• Many other sensation and perception text books in
  the library which cover similar topics
• Bring the book to class as I may refer you to
  explanations / diagrams within it

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Module assessment

• 1 tutorial per week
• 1 lab per week 20 of final mark
• mid term assessment 10 of final mark
• Final exam is 70
• Repeat exam is 70
• Headphones for lab work

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Introduction

• Aim of this module to understand how our senses
  work in order to perceive the world around us
• How much is known about this?
• Detectors eyes, ears, skin and the receptors in
  the nose and mouth.
• Where does the information from these detectors
  go and what happens next?

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Why study perception?

• Necessary to pass this course
• Career choice e.g. the development of
  technology that helps improve performance of a
  sense that may be impaired - the cochlear implant
  in hearing to achieve this we need to know and
  understand how our perceptual systems work
• Interesting, enhances our curiosity and
  ultimately results in a greater appreciation in
  the workings of our sensory systems allows us
  to take advantage of our perception of the world
  e.g. through art

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The perceptual process

• Perception is the end result of complex processes
  many of which we are unaware of
• The environmental stimulus all of the things in
  our environment that we can potentially perceive
• Look / listen for items of interest focus our
  attention on the attended stimulus
• Transduction transformation of one form of
  energy into another
• Stimulus item of interest something that can
  cause a response

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The perceptual process

• In addition to perception we also have
  recognition and action these are outcomes of
  the perceptual process
• recognition our ability to place an object in a
  category that gives it meaning
• Action includes motor activities such as moving
  the head or eyes and travelling through the
  environment
• Knowledge - information we bring to the
  situation learned - can influence our
  perception bottom up and top down processing

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Measuring perception

• Psychophysical analysis the relationship
  between the physical properties of a stimulus and
  our perceptual response to these (e.g. if we
  increase the light intensity entering our eye
  what in our perception changes?)
• Physiological analysis stimulus physiology
  physiology perception
• How does our knowledge, memories and expectations
  influence how we perceive the environment?

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Measuring perception

• Methods used to measure the stimulus- perception
  relationship
• Absolute threshold smallest amount of stimulus
  energy necessary to detect a stimulus e.g.
  measure the faintest light we can see, softest
  sound we can hear, lightest touch we can feel
• Difference threshold (difference limen) smallest
  difference between two stimuli that a person can
  detect e.g. how much louder does a sound have to
  be in order for it to be heard as louder than a
  previous sound?

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Measuring perception

• Method of limits stimuli presented in ascending
  or descending order e.g. a light of increasing or
  decreasing intensity observer can / cant see
  it yes / no response at each presentation
• Method of adjustment the light intensity is
  adjusted until it is just detectable this taken
  as the absolute threshold.
• Method of constant stimuli - lights of different
  intensities presented in a random order
  determine of times that the observer perceived
  each intensity. Absolute threshold is taken as
  the intensity that results in 50 detection

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Measuring perception

• We use Magnitude estimation to measure above
  threshold perceptions
• does doubling the intensity of a light cause it
  to look twice as bright?
• Procedure present a standard stimulus to the
  observer e.g. a light of moderate intensity.
  Assign a value to this standard (say 10). Lights
  of different intensities are then presented and
  it is the task of the observer to assign values
  to these based on the standard value.

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Signal Detection Theory

• In the previous measurement methods how can the
  experimenter be sure that the person is
  responding accurately - e.g. what if the person
  answers yes I can see the light - on every
  trial?
• how would you account for this (the bias of the
  observer) in the experimental procedure?
• Signal Detection Theory was designed to take
  account of the characteristics of the observer
  and of the sensory system
• factors other than a persons sensitivity to the
  signal can influence our results.

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Signal Detection Theory

• E.g. to measure the absolute threshold for a
  sound
• Use a single low intensity tone that is difficult
  to hear
• Present it only on some trials so in other
  trials no sound is presented mix these tone /
  no tone trials at random
• 4 types of response hit, miss, false alarm,
  correct rejection
• Hit a correct response the observer responds
  yes when a stimulus is present

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Signal Detection Theory

• Miss an incorrect response the observer says
  no when a stimulus is presented
• False alarm an incorrect response the observer
  responds yes when there is no stimulus
• Correct rejection a correct response the
  observer says no when no stimulus is present
• Signal and noise
• Plot hits v.s. false alarms to get receiver
  operating characteristic (ROC) curve (check do
  they do this in a lab)