Perception Chapter 1
Three aspects of Perception
1) The physical world or environment: we must study and understand the properties that are present in the
environment which are available to be perceived.
2) The functioning of the nervous system: we must study and understand the nervous system which
transduces environmental properties into brain events (neuronal impulses) which are the biological bases for
perception.
3) The conscious experience or the perception: we must study and understand the meaningfulness that the
organism attaches to the brain events evoked by the physical world.
Because of aspects 2 and 3 we must recognize that perception is governed largely by internal events and
therefore is unique to each organism and person (when dealing with humans).
Psychophysics: Study of the relation between changes in physical world and changes in perceptual
experience
The world we humans perceive is not the same world perceived by other creatures or by machines. The only
reason we perceive the world as we do is because of the design of our perceptual systems.
Ex: snakes "see" infra-red; bees "see" ultra-violet; bats "see" acoustic patterns of reflected sound; Arnold in
predator.
It’s all in your head!
• We expect “perception” to correspond to “reality,” but
that need not be so. The brain and nervous system are
designed to be functional, not truthful.
• Unperceivable reality and not real perception
• Bonnet’s syndrome (visual hallucinations due to
damaged visual system), phantom limbs, dreams, etc.
• Changes in perception over time – pain in childbirth.
Philosophical issues: Can perception be reduced entirely to
brain function?
• “No” Dualism: Mind and brain are separate substances; perception
involves brain function and non-material mental processes (Eccles).
Conforms with subjective experience, but violates laws of physics
• “Yes” Materialism: Perception arises from brain function. Hard
position (Churchlands): Subjective experience is “in” brain even if
we can’t find it. Softer position (Sperry): No all subjective
experience is reducible to brain function. Emergence: Mind is
emergence property of brain; depends on brain, but has own causal
power.
Overview of Human sensory systems:
Near senses:
1) Touch -- mechanoreceptors in skin sensitive to pressure, texture, temperature.
2) Taste -- Chemical receptors on the tongue sensitive to dissolving substances in
mouth
3) Smell (olfactory sense) -- olfactory receptor cells in ceiling of nasal cavity sensitive
to the chemical properties of vaporous molecules.
1 & 2 require direct contact with to-be-perceived object; 3 requires close contact.
Far distance senses:
1) Hearing -- receptor cells in cochlea of the inner ear sensitive to air pressure
changes.
2) Vision -- photoreceptors in the retina of eye sensitive to a narrow range of
electromagnetic energy
•Note how the to-be-perceived object may be quite a distance away form perceiver
and still detected.
•Note: importance of "active" nature of perception.
Philosophical approaches to perception
Plato: Fallibility of perception (chained in a cave)
John Locke: Primary vs. Secondary qualities
Primary: inherent to stimulus (bulk, number, motion
shape
Secondary: subjective (color, sound, taste, smell)
Humean skepticism: we can never be sure of reality
because we cannot distinguish between what sensations
arise from the object vs. what arises from the mind. -this may be a bit too strong as constructionists would
argue that the elaborations of the mind are do follow
logical rules and therefore is predictable.
Naïve Realism: the world really is as it appears. Reasons to
reject: individual variability in perception, ambiguous
signals.
Subjective Idealism or solipsism: Only perception is real.
Berkeley’s Idealism
George Berkeley
God in the Quad
There was a young man who said "God
Must find it exceedingly odd
To think that the tree
Should continue to be
When there's no one about in the quad."
Reply:
"Dear Sir: Your astonishment's odd;
I am always about in the quad.
And that's why the tree
Will continue to be
Since observed by, Yours faithfully, God."
(poem actually written by: Monsignor Ronald
Knox)
Levels of analysis in studying perception
Psychological: Behavioral measures of detection (verbal
response, button pressing, etc.)
Phenomenological or Naturalistic: recording subject
perceptual experiences in natural setting (colors of
sunset). Least formal, only good for human adults
(generally); are subjective reports trustworthy?
Anton’s syndrome: Blindness with denial.
Experimental approaches: Controlling and
manipulating stimulus presentation in order to
isolate those properties that affect perception. More
formal. Concerns: Ecological validity; cross-modal
interactions.
Levels of analysis in studying perception
Biological: Connection perceptual experience with biological (brain,
nervous system) activity.
Lesion studies: brain damage and loss of function
Transcranial magnetic stimulation: delivery of magnetic energy to
specific brain regions to disrupt function.
Event related potentials: use of electrodes to monitor electrical
activity of brain
Positron Emission Tomography: blood flow followed by radioactive
tracer (limited spatial resolution)
Function magnetic resonance imaging: blood flow followed by
changes in magnetic potential of oxygenated blood (good spatial,
not so good temporal resolution)
Magnetoencephalography (MEG): Measures external magnetic fields
(expensive, but great temporal resolution)
Brain imaging technologies
Magnetoencephalography (MEG)
Measures external magnetic fields
Transcranial magnetic stimulation
Produces ‘virtual’ lesions
Brain imaging technologies
• Positron Emission Tomography (PET)
• Makes use of radioactive tracer to measure cerebral
blood flow.
Brain imaging technologies
• Functional Magnetic Resonance Imaging
(fMRI)
• Uses electromagnets to measure oxygen levels
in brain.
Theoretical approaches
• Interactive levels of explanation (highest to lowest)
1. The perceptual problem to be solved (e.g.
recognizing a face or hitting a base-ball)
2. The “programming rules” used to address the
problem (figure/ground – face from background;
up/down vs. left/right – eyes from mouth, etc.)
3. Neural machinery used to implement rules
(fusiform face area in temporal lobe)