Week 2: Physiology of Early Visual Processing

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Week 2 Physiology of Early Visual Processing
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Review Important Concepts from Week 1

• Monisim vs. Dualism A scientific approach to
  studying sensation and perception (SP) must make
  a strong monist-materialist assumption that there
  is a one-to-one correspondence between the state
  of the brain and what we experienceevery unique
  experience has a corresponding unique brain state
• Understanding SP requires research at
  complimentary levels of analysis computational,
  representation and algorithm, and implementation
• Psychophysical Methods

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Week 2 Outline

• Physiological Foundations Neurons and Neural
  Signaling
• Light, Light Reception and Image Formation
• The Retina and Transduction
• Dark Adaptation and the Absolute Luminance
  Threshold

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Physiological Foundations Neurons

• The structure of neurons
• Dendrites
• Cell Body
• Axon
• Myelin Sheath
• Axon Terminal Branches
• Axon Terminal Buttons

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Physiological Foundations Neural Signaling

• Information in neurons nerve impulses
• Measuring nerve impulses electrophysiology
• More information http//www.blackwellscience.com/
  matthews/channel.html

mV -70
Oscilloscope
Micro-electrodes
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Action Potentials

• The nerve impulse or action potential
• Axon lies in solution of positive and negative
  ions
• Axon membrane can become selectively permeable to
  sodium (Na) and potassium (K) ions
• Concentration of ions determines voltage

mV -70
-
Na
Na
-
-
K
K
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Neuron at Rest

• Initially the axon is at rest
• More negative ions inside axon than outside
• Resting potential -70 milli-volts (mV)
• Sodium (Na) ions concentrated in solution
  surrounding axon
• Potassium (K) ions concentrated in solution
  within axon

mV -70
-
Na
Na
-
-
K
K
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Axonal Polarization

• Membrane becomes selectively permeable to sodium
  (Na)
• Negative charge inside neuron attracts Na ions
• Axon develops local positive charge of 40 mV
  (polarization) for approximately 1/1000 s (1 ms)

mV -70
-
Na
Na
-
-
Na
K
Na
K
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Axon Returning to Resting Potential

• Membrane becomes selectively permeable to
  potassium ions (K)
• Negative charge outside neuron attracts K ions
• Neuron returns to -70 mV resting potential

mV -70
K
K
-
-
-
Na
K
Na
K
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Ion Exchange

• The sodium-potassium pump exchanges sodium and
  potassium ions so the process can begin again
• Requires energy supplied by glucose in blood
• Link between rate of neuron firing and blood flow
  (fMRI, PET imaging)

mV -70
-
K
Na
-
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Propagation

• Change in neuron membrane potential occurs
  locally, starting at base of cell body (the axon
  hillock), and traveling (propagating) down the
  length of the axon
• Myelination and axon diameter effect the speed of
  action potential propagation

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Physiological Foundations Neural Codes

• Sensory Coding in nerve impulses
• all or none response
• Changes in stimulus intensity change firing rate
• refractory period 1 ms
• Limits firing rate to 800-1000 firings per sec
  (Hz)
• spontaneous activity
• adaptation
• The Synapse
• excitatory vs. inhibitory synapses

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Neurons and Neural Signaling

• Sensory Coding in Neurons
• Doctrine of Specific Nerve Energies (Johannes
  Muller, 1826)
• Cortical Receiving Areas
• Across-fiber-pattern (population) coding
• Specificity Coding the grandmother cell
• Revisiting the mind-body problem

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Physical, Psychological, and Physiological

• In studying Sensation and Perception one must
  always keep in mind the distinctions between
• Physical stimuli the objective reality our
  senses are presumably measuring
• Psychological Phenomena what we perceive and
  experience psychophysics is the science that
  describes the relationships between physical
  stimuli and psychological phenomena, typically
  measured as overt behavior (judgments,
  discriminations, adjustments, eye movements)
• Physiological Effects (how the senses and brain
  respond) psychophysiology is the science that
  that explains the relationships between physical
  stimuli and physiological effects (heart rate,
  breathing rate, galvanic skin response, EEG,
  cerebral blood flow, neuronal responses)

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The Visual System

• Physical Stimulus Structured Light
• Spatial distribution of intensities of different
  wavelengths
• Psychological Phenomena
• Perception of color, space, objects, movement,
  etc.
• Physiological
• Response of light-sensitive neurons in the
  retina, optic pathways, and visual centers in the
  brain

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The Nature of Light

• Electromagnetic Radiation
• Lights dual physical nature Waves and Particles
• Wavelength and Electromagnetic and Visible
  Spectrums
• Psychological Phenomenon Perception of Color
• Intensity
• Physical Measures Radiance, Luminance,
  Illuminance
• Radiometric vs. Photometric Units
• Psychological Phenomenon Brightness we have a
  17 log-unit range!

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Light Reception

• Light sensing vs. Image Formation
• A comment on compound eyesHollywood has it
  wrong!
• Structure of The Vertebrate Eye
• Similar to a camera, the eye serves to focus
  light onto a photosensitive surface that
  records the spatial-temporal structure of the
  image for further analysis by the brain

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Image Formation

• Structures that focus light
• Cornea 80 of focusing power
• Lens fine-tunes focus depending on fixation
  distance, a process called accommodation
• Accommodation
• Flattening or bulging of lens to focus light rays
  reflecting (or emitting) from a target

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Accommodation

• Near Point
• Refractive Error
• Presbyobia, increase in near point distance with
  age (gt40 or so). Degraded accommodation due to
  hardening of the lens
• Myopia (near-sightedness)
• Hyperopia (far-sightedness)
• Emmetropia (normal sightedness)
• Chromatic aberration, spherical aberration, and
  astigmatism
• Corrections
• Eyeglasses and contact lenses
• Corrective Eye Surgery

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The Pupil

• Wyatts reflex pupil regulates illuminance of
  retina under different light intensities by
  dilating and constricting
• But, the pupil does more than just regulate light
  intensity!
• Constriction in bright light helps minimize
  spherical and chromatic aberrations, and improve
  focus
• Pupil size changes with level of interest
  (arousal, attention, workload)
• Depth of field and pupil size

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Depth of Field and Pupil Size
Deep Depth of Field Shallow Depth of Field
pupil aperture closed pupil aperture open
(non-attentive)
(attentive) background in focus background
out of focus plant doesnt stand out plant
stands out
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The Retina A Biological Image Sensor

• The retina is an extension of the brain into our
  eyes
• Mosaic of photoreceptors that transduce light
  energy into electrochemical energy in the brain
• Rods 120-130 million
• Cones 6-8 million

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The Retina Transduction

• The transduction process
• Light absorbed by the visual receptors (rods and
  cones) triggers a bleaching process of
  isomerization, a change in the chemical structure
  of rhodopsin (a light sensitive pigment)
• Rhodopsin light ? retinal opsin
• Vitamin A Retinal opsin ? rhodopsin
• Vitamin A deficiency leads to night blindness due
  to inability to reverse the bleaching process

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The Retina Post-receptor neurons

• Intermediate retinal neurons Bipolar,
  horizontal, and amacrine cells
• Final retinal neurons ganglion cells
• The fovea
• backward organization of retina
• The optic nerve
• The blind spot

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The Retina Dark Adaptation

• Dark adaptation a change in the absolute
  luminance threshold measured over time when an
  observer goes from a brightly lit environment to
  a dark environment
• Two parallel processes
• Cone adaptation (fast, 5 min, but limited
  sensitivity high steady-state threshold)
• Rod (slower, 20-40 min, but greater sensitivity,
  lower steady-state threshold)
• Dark adaptation curves and the break-point

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Absolute Luminance Threshold

• A constant luminance threshold (C) depends on
  factors other than light intensity, or luminance
  (I)
• Retinal Area (A) of Stimulus (Riccos Law)
• C AI
• Fovea if stimuli lt 10 min
• Parafovea (4-7 deg) if stimuli lt 30 min
• Peripheral retina if stimuli lt 2 deg
• Duration (T) of Stimulus (Blocks law)
• lt 100 ms exposure C IT
• gt 100 ms exposure C I
• Wavelength
• Retinal Location

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Luminance Threshold and Wavelength

• Photopic, Mesopic, and Scotopic Vision
• Photopic Vision
• Light levels above cone sensitivity, and so
  bright that rods are saturatedcone vision
• Mesopic Vision
• Light levels above cone sensitivity, but still
  low enough for rods to functionrod and cone
  vision
• Scotopic Vision
• light levels below cone sensitivityrod vision
  only
• Lowest threshold (highest sensitivity) at
  scotopic peak wavelength
• Red light and dark adaptation

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The Purkinje Shift

• Spectral sensitivity and the Purkinje Shift
• photopic spectral scotopic spectral
• sensitivity sensitivity

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End of Week 2

• Be sure to write a journal entry!
• For the next week read Chapters 3 and 4 of your
  text book
• Whats coming up in Week 3?
• Physiology of Higher-Order Visual Processing