Lecture Overview

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Lecture Overview

• Sensory coding issues
• Anatomy of the retina
• Receptive Fields
• Organization of striate cortex
• Feature analysis
• Spatial frequency analysis
• Visual association cortex

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Definitions

• Afferent vs. efferent
• Stimulus any energy capable of exciting a
  receptor
• Mechanical
• Chemical
• Thermal
• Photic
• Sensory energies are measurable (unlike ESP)

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Sensory Receptors

• Receptors specialized nerve cells that transduce
  energy
• Act as dendrites that eventually induce A.P.s
• Receptors are mode specific
• Only detect a small range of energy levels
• Eye 400-700 nM
• Ear 20-20,000 Hz
• Taste buds specific chemicals
• Law of Specific Nerve Energies

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• Anatomy of the Visual System
• The Eyes
• Orbits
• Bony pockets in the front of the skull.
• Sclera
• The white tissue of the eye.
• Conjunctiva
• Mucous membranes that line the eyelid and protect
  the eye.

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• Anatomy of the Visual System
• The eyes
• Cornea
• Transparent outer covering of the eye that admits
  light.
• Pupil
• Adjustable opening in the iris that regulates the
  amount of light that enters the eye.
• Iris
• Pigmented ring of muscles situated behind the
  cornea.

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• Anatomy of the Visual System
• The eyes
• Lens
• Consists of a series of transparent, onion-like
  layers. Its shape can be changed by contraction
  of ciliary muscles.
• Accommodation
• Changes in the thickness of the lens,
  accomplished by the ciliary muscles, that focus
  images of near or distant objects on the retina.

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• Anatomy of the Visual System
• The eyes
• Retina
• The neural tissue and photoreceptive cells
  located on the inner surface of the posterior
  portion of the eye.
• Rod
• Photoreceptor cells of the retina, sensitive to
  the
• light of low intensity.
• Cone
• Photoreceptor cells of the retina maximally
  sensitive to one of three different wavelengths
  of light and hence encodes color vision.

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• Anatomy of the Visual System
• The eyes
• Fovea
• Area of retina that mediates the most acute
  vision. Contains only color-sensitive cones.
• Optic Disk
• Location on the retina where fibers of ganglion
  cells exit the eye responsible for the blind
  spot.

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Receptive Fields

• Receptive Field (RF) Those attributes of a
  stimulus that will alter the firing rate of
  sensory cell
• Can measure RF at each level of sensory system
• There are as many RFs as there are cells in a
  sensory system

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Sensory System Issues

• How many synapses (order of system)
• Degree of decussation (crossover) ?
• Projects to what area of thalamus?
• Projects to what area of cortex?
• Does cortex show topical organization?
• Does cortex show columnar organization?
• Modification of sensory coding?
• Experience, hormones?

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Visual Systems

• Function of visual systems is to detect EMR
  emitted by objects
• Nature of visible light (400-700 nM)
• Functions of vision
• Locate figure vs. ground
• Detect movement (predator/prey?)
• Detect color (adaptive value of color?)

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Eye Details

• An eye consists of
• Aperture (pin hole, pit, or pupil)
• Lens or not?
• Photoreceptive elements (retina)

Source http//www.nei.nih.gov/nei/vision/vision2
.htm
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Cross-section through Retina

• Light passes through several layers of retina to
  reach the photoreceptors

Source http//insight.med.utah.edu/ WebVision/ima
geswv/husect.jpeg
Light
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Rods and Cones
Source http//insight.med.utah.edu/Webvision /ima
geswv/rodcoEM.jpeg

• Rods 120 million
• Light sensitive (not color)
• Found in periphery of retina
• Consist of stacked protein disks
• Low activation threshold
• Cones 6 million
• Are color sensitive
• Found mostly in fovea
• One continuous membrane

6.11
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Retinal Circuitry

• Photorec. 1st order
• Bipolar 2nd order
• Ganglion cell 3rd order
• Rods more diffusely connected to bipolar cells

Source Carlson, 5/e, 6.13
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Visual Transduction

• Photopigments
• Consist of opsin and retinal
• In the dark, NA channels are open -gt glutamate
  is released
• Light breaks opsin and retinal apart-gt
• Activates transducin (G protein)-gt
• Activates phosphodiesterase-gt
• Reduces cGMP -gt closes NA channels
• Net effect light hyperpolarizes the retinal
  receptor

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Receptive Fields Ganglion Cell

• Ganglion cells exhibit low baseline firing rates
• Receptive fields circular in shape with
  ring-shaped surround
• ON-Cell
• Light placed on center increases firing
• Light placed on surround decreases firing

OFF
ON
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Visual Pathways

• Within retina
• Photoreceptor -gt bipolar cell - gt ganglion cell
• Beyond retina
• Ganglion cell -gt through optic chiasm -gt lateral
  geniculate -gt primary visual cortex (striate)

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Retina-LGN Details

• Magnocellular system
• Cells from retina terminate in LGN layers 1,2
• Carry info on contrast and movement (color
  insensitive)
• Carry input from A retinal ganglion (Y type)
  cells
• Parvocellular system
• Cells from retina terminate in LGN layers 3-6
• carry info on fine detail, and color
• Carry input from B retinal ganglion cells (X
  type)

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LGN Receptive Fields

• LGN shows circular receptive fields
• Size larger for magnocellular than
  parvocellular
• Color sensitivity only for parvocellular
• Color receptive fields (circular on-off)
• e.g. red-on, green-off

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Cortical Receptive Fields

• Hubel and Wiesel were interested in the visual
  stimuli that would excite a nerve cell in area 17
• Anesthetized a cat, inserted microelectrode into
  area 17, recorded AP pattern during presentation
  on stimuli to retina
• Cells responded to features of visual field
• Shape
• Orientation
• Movement

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Complexities of Visual Cortex

• Columnar Organization (input from same part of
  retina)
• Ocular dominance cells respond to only one eye
• Columns for left and right alternate
• Orientation columns
• Cells respond to same orientation, adjacent cells
  are shifted by 10 degrees
• Are at right angle to ocular dominance column
• Do a 180 degrees in 1 mm
• Color blobs-stained for cytochrome oxidase
• -Show up every 0.5 mm (one blob for each eye)
• Removal of one eye - alternate rows of blobs
  disappear

6.1
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Spatial Frequency?

• Visual neurons respond to a sine wave grating
• Alternating patches of light and dark
• Low f large areas of light and dark
• High f fine details

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Visual Association Cortex

• From striate cortex (V1) see 2 streams
• Dorsal where an object is
• Projects to post. parietal association cortex
• Ventral what an object is (analysis of form)
• Projects to extrastriate cortex (V2, V3, V4, V5)
• and to inferior temporal cortex (TEO, TE, STS)
• Dorsal mostly magnocellular input
• Ventral equal mix of magnocellular and
  parvocellular input

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Summary of Visual Cortex

• V1 responds to color, orientation, eye dominance
  
• V4 responds to color constancy (and form
  perception)
• Lesions impair color constancy
• V5 responds to movement
• Inf. temporal cortex
• TEO coding of object features (2-d patterns,
  color)
• TE recognition of objects (a face or a hand)

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• Analysis of Visual Information The Striate
  Cortex
• Anatomy of the striate cortex
• David Hubel and Torsten Wiesel
• 1960s at Harvard University
• Discovered that neurons in the visual cortex did
  not simply respond to light they selectively
  responded to specific features of the visual
  world.

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• Analysis of Visual Information The Striate
  Cortex
• Orientation and movement
• Simple cell
• An orientation-sensitive neuron in the striate
  cortex whose receptive field is organized in an
  opponent fashion.

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• Analysis of Visual Information The Striate
  Cortex
• Orientation and movement
• Complex cell
• A neuron in the visual cortex that responds to
  the presence of a line segment with a particular
  orientation located within its receptive
  field,especially when the line moves
  perpendicular to its orientation.

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• Analysis of Visual Information The Striate
  Cortex
• Orientation and movement
• Hypercomplex cell
• A neuron in the visual cortex that responds to
  the presence of a line segment with a particular
  orientation that ends at a particular point
  within a cells receptive field.

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• Analysis of Visual Information The Striate
  Cortex
• Spatial frequency
• Sine-wave grating
• A series of straight parallel bands varying
  continuously in the brightness according to a
  sine-wave function, along a line perpendicular to
  their lengths.

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• Analysis of Visual Information The Striate
  Cortex
• Spatial Frequency
• Spatial frequency
• The relative width of the bands in a sine-wave
  grating, measured in cycles per degree of visual
  angle.

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• Analysis of Visual Information The Striate
  Cortex
• Retinal Disparity
• Retinal disparity
• The fact that points on objects located at
  different distances from the observer will fall
  on slightly different locations on the two
  retinas provides the basis for stereopsis or
  depth perception

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• Analysis of Visual Information The Striate
  Cortex
• Color
• Cytochrome oxidase (CO) blob
• The central region of a module of the primary
  visual cortex, revealed by a stain for cytochrome
  oxidase contains wavelength-sensitive neurons
  part of the parvocellular system.

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• Analysis of Visual Information The Striate
  Cortex
• Ocular dominance
• The extent to which a particular neuron receives
  more input from one eye than from the other.
• Cortical blindness
• Blindness caused by damage to the optic
  radiations or primary visual cortex.

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• Analysis of Visual Information The Visual
  Association Cortex
• Extrastriate cortex
• A region of the visual association cortex
  receives fibers from the striate cortex and from
  the superior colliculi and projects to the
  inferior temporal cortex.
• Regions respond to particular features of visual
  information such as orientation, movement,
  spatial frequency, retinal disparity, or color.

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• Analysis of Visual Information The Visual
  Association Cortex
• Dorsal stream
• A system of interconnected regions of the visual
  cortex involved in the perception of spatial
  location, beginning with the striate cortex and
  ending with the posterior parietal cortex.
• Ventral stream
• A system of interconnected regions of visual
  cortex involved in the perception of form,
  beginning with the striate cortex and ending with
  the inferior temporal cortex.

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• Analysis of Visual Information The Visual
  Association Cortex
• Color constancy
• The relative constant appearance of the colors of
  objects viewed under varying lighting conditions.

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• Analysis of Visual Information The Visual
  Association Cortex
• Studies with humans
• Achromatopsia
• Inability to discriminate among different hues
  caused by damage to the visual association
  cortex.
• Inferior temporal cortex
• In primates, the highest level of the ventral
  stream of the visual association cortex located
  on the inferior portion of the temporal lobe.

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• Analysis of Visual Information The Visual
  Association Cortex
• Studies with humans
• Agnosia
• Inability to perceive or identify a stimulus by
  means of a particular sensory modality.
• Visual agnosia
• Deficits in visual perception in the absence of
  blindness caused by brain damage.
• Apperceptive visual agnosia
• Failure to perceive objects even though visual
  acuity is relatively normal.

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• Analysis of Visual Information The Visual
  Association Cortex
• Analysis of form
• Prosopagnosia
• Failure to recognize particular people by the
  sight of their faces.
• Associative visual agnosia
• Inability to identify objects that are perceived
  visually, even though the form of the perceived
  object can be drawn or matched with similar
  objects.

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• Analysis of Visual Information The Visual
  Association Cortex
• Perception of movement
• Fusiform face area
• A region of the extrastriate cortex located at
  the base
• of the brain involved in perception of faces
  and other objects that require expertise to
  recognize.
• Akinetopsia
• Inability to perceive movement, caused by damage
  to area V5 of the visual association cortex.

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COLOR VISION

• REQUIRES
• At least 2 photoreceptor types
• A way to compare their responses
• Different wavelengths of light

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• Genes-gt Photopigments-gt Color Vision

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• THE MAGICIANS AND PSYCHOLGY
• Inattentional blindness
• if we dont attend to something we wont see it.
• Instead of a complete, detailed scene, we
• only see a small part which we are attending
• to!
• This is how magicians make things (dis)appear

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• Pick a card

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Your card disappeared.
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