Introduction to perception Day 6

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Introduction to perceptionDay 6

• Music Cognition
• MUSC 495.02, NSCI 466, NSCI 710.03
• Harry Howard
• Barbara Jazwinski
• Tulane University

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Course administration

• Spend provost's money

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Introduction to perception

• Early perception and efficient encoding

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A question about perception

• What does perception do?
• Or, following up on our previous discussion, what
  question should a theory of perception answer?
• Take a few minutes to discuss it with someone.

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Your answer here!

• The class says that perception is
• 1. Attributing meaning to outside stimuli
• 2. Both conscious and subconscious
• 3. The way the brain reacts to and organizes
  sensory information
• 4. Linking the electrical signals caused by
  external stimuli to memories and emotions that
  are used to interpret these stimuli
• 5. Differentiation of stimulus type.

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Your answer here!

• I think there might be at least the following two
  schools of thought
• perception creates a faithful or truthful copy of
  reality (veridicality)
• perception creates a useful copy of reality

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What does 'perception' mean?

• In psychology and the cognitive sciences,
  perception is the process of attaining awareness
  or knowledge of sensory information.
• The word 'perception' comes from the Latin word
  percepio, meaning "receiving, collecting, action
  of taking possession, apprehension with the mind
  or senses".

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My favorite exampleWhat do you see?
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The evolution of color vision

• It has been suggested that trichromacy in
  primates and the reflectance functions of certain
  tropical fruits are aspects of a coevolved
  seed-dispersal system
• primate color vision has been shaped by the need
  to find colored fruits amongst foliage,
• and the fruits themselves have evolved to be
  salient to primates and so secure dissemination
  of their seeds.
• We show that the spectral positioning of the cone
  pigments found in trichromatic South American
  primates is well matched to the task of detecting
  fruits against a background of leaves.

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Normalized absorption spectra of human cone
(S,M,L) and rod (R) cells
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Increasing chromacy
top left Monochromatic view (no blues, greens,
or reds) top right Dichromatic view (only blues
and greens) bottom left Trichromatic
view (blues, greens, and reds)
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Revision

• Shall we revise our theory of perception in light
  of the example of the evolution of trichromacy?

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Taking natural selection into account

• In the theory of natural selection,
• favorable heritable traits become more common in
  successive generations of a population of
  reproducing organisms,
• and unfavorable heritable traits become less
  common.
• Over time, this process may result in adaptations
  that specialize organisms for particular
  ecological niches, i.e. evolution.
• With respect to perception, the process of
  natural selection guarantees a strong connection
  between the design of an organisms perceptual
  systems and the properties of the physical
  environment in which the organism lives.
• In humans, this connection is implemented through
  a mixture of
• fixed (hardwired) adaptations that are present at
  birth and
• facultative (plastic) adaptations that alter or
  adjust the perceptual systems during the lifespan.

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Segue to environmental statistics

• There is an enormous amount of information in the
  environment that a perceptual system could become
  attuned to

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A natural imageLet us say that the image is a
veridical representation of the world
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Is veridicality maintained at higher levels of
processing?

• That is, does visual processing propagate
  something like a photograph all the way up?

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Lets take a closer look
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Zoom in 300
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Zoom in 300 again
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Some redundancies
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Correlation
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The efficient coding hypothesis

• Motivated by developments in information theory,
  Attneave (1954) suggested that the goal of visual
  perception is to produce an efficient
  representation of the incoming signal.
• In a neurobiological context, Barlow (1961)
  hypothesized that the role of early sensory
  neurons is to remove statistical redundancy in
  the sensory input.
• Thus at least the early levels of a sensory
  system should be sensitive to the statistical
  properties of the environment.

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The environmental statistics of sound

• Ill talk about it next time

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Is that all?

• After all redundancies are removed, is there
  anything left?
• Hopefully, the objects that the organism is
  interested in (i.e. drive natural selection)
• For example, the purpose of trichromacy is not
  for monkeys to see red, but rather for monkeys to
  see ripe fruit

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Introduction to perception

• Late perception and object recognition

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What is an object?

• According to the Oxford English Dictionary,
  object means
• something placed before the eyes, or presented to
  the sight or other sense an individual thing
  seen or perceived, or that may be seen or
  perceived a material thing
• Its etymology explains its visuocentric
  connotation
• object derives from the Latin ob-, 'before' or
  'toward', and iacere, 'to throw' and used to
  mean
• something 'thrown' or put in the way, so as to
  interrupt or obstruct the course of a person or
  thing an obstacle, a hindrance
• Indeed, most visible things are obstacles or a
  hindrance to sight they prevent you from seeing
  something that lies behind them because they are
  opaque

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A cross-modal theory of objecthood

• Perceptual objects have the following properties
• they can be figures for figure-ground segregation
• they have edges (or contours or boundaries)
• they can be grouped

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What do you see?
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Most people see

• a white vase against a black background,
• two black heads facing each other in profile
  against a white background,
• and the image can switch rapidly back and forth
  between the two interpretations.
• It is known as the Rubin vase/profile illusion

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Figure and ground

• Notice that I couldn't describe the illusion
  without postulating an object against a
  background
• It is famous for showing the difference between
  figure and ground
• the figure is that which draws our attention,
• and stands out against an undifferentiated
  (back)ground
• The process of parcelling sensory input into
  figure and ground is called figure-ground
  segregation

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Auditory figure-ground segregation

• The same thing happens in audition, though it is
  difficult for me to put an example on a slide
• Can you think of one?

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What do you see?

• This left image has been processed by a computer
  program that attempts to retain the edges in the
  right image and throw away everything else
• The fact that the left is still informative shows
  the importance of edges to visual object
  recognition
• Edge an abrupt change in contrast (light dark)

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Edges in audition ba vs. pa
Edge an abrupt change in frequency
b a
p a
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What do you see?

• Most people see the image divided into rows, not
  columns
• This is grouping by similarity

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What do you hear?

• All around the mulberry bush, The monkey chased
  the weasel. The monkey thought 'twas all in fun.
  Pop! goes the weasel.
• A penny for a spool of thread, A penny for a
  needle. That's the way the money goes. Pop! goes
  the weasel.
• Up and down the City Road, In and out of the
  Eagle, That's the way the money goes. Pop! goes
  the weasel.
• Half a pound of tuppenney rice, Half a pound of
  treacle, Mix it up and make it nice, Pop! goes
  the weasel.

• The first 3 verses of each stanza have
  alternating weak/strong syllables
• This is a kind of grouping by similarity, which
  presumably accounts for the 'musicality' of poetry

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Vision vs. audition
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Summary outstanding questions

• What objects are in the (current) visual scene
  that enhance my survival?
• What objects are in the (current) auditory scene
  that enhance my survival?
• What will the next visual scene probably be?
• What will the next auditory scene probably be?
• What is the current note?
• What will the next note probably be?
• What did music evolve for, anyway?

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Back to our regularly scheduled program
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Ingredients of music cognition mostly receptive,
mostly from Levitin
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Next Monday

• Ok I will really talk about sound music
  perception
• 1-2 of Levitin