the visual system. the retina light passes through the lens, through the inner layer of ganglion...
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The visual system
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The retina
• Light passes through the lens, through the inner layer of ganglion cells and bipolar cells to reach the rods and cones.
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The retina• 0.5 mm thick
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The retina• 0.5 mm thick• The photosensors (the rods and cones) lie outermost in the retina.
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The retina• 0.5 mm thick• The photosensors (the rods and cones) lie outermost in the retina.
• Interneurons
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The retina• 0.5 mm thick• The photosensors (the rods and cones) lie outermost in the retina.
• Interneurons • Ganglion cells (the output neurons of the retina) lie innermost in the retina closest to the lens and front of the eye.
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The retina• Receptive field:
– The location where a visual stimulus causes a change in the activity of the visual neuron
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The retina• Receptive field:
– The location where a visual stimulus causes a change in the activity of the visual neuron
– For a rod or a cone, you could think of it as a pixel.
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The retina• Receptive field:
– The location where a visual stimulus causes a change in the activity of the visual neuron
– For a rod or a cone, you could think of it as a pixel.
– For the rest of the visual system, receptive fields are more complicated and more interesting!
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Retinal interneurons
• Photoreceptors synapse onto many interneurons.
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Retinal interneurons
• Photoreceptors synapse onto many interneurons.
• The interneurons synapse onto one another and onto ganglion cells.
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Ganglion cells
• There are about a million ganglion cells.
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Ganglion cells
• There are about a million ganglion cells.
• There are at least 18 different morphological types of ganglion cell in the human retina.
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Ganglion cells
• Most ganglion cells have center-surround receptive fields.
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Ganglion cells
• Most ganglion cells have center-surround receptive fields.
• About 50% of those are OFF-center ON-surround.
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Ganglion cells
• Most ganglion cells have center-surround receptive fields.
• About 50% of those are OFF-center ON-surround.
• These are like the “bug detectors” in the Lettvin paper.
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OFF-center ON-surround ganglion cells
• ON-center OFF-surround ganglion cells
• Many cells respond best to a small spot of light on a dark background.
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Sustained vs. transient responses
• Some respond transiently
• Some give sustained responses.
transientcell
sustainedcell
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Other types of ganglion cells
• Some ganglion cells don't have center-surround receptive fields and are involved in detecting novel stimuli but not in form detection.
• Some ganglion cells have huge receptive fields and are involved in setting circadian rhythms
• Some respond best to particular color combinations (red/green or yellow/blue).
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Pathway from the eye to the cortex
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Striate cortexAlso called V1, primary visual cortex, or area 17
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Cortical circuitry
• The basic arrangement of circuitry is perpendicular to the surface of the brain: columnar.
• Axons from lateral geniculate terminate in layer 4.
pia
white matter
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Receptive fields in striate cortex
• Most cells in layer 4 have circular center-surround receptive fields.
• Most cells in the other layers respond only weakly to spots of light or dark. Instead, they respond best to lines or edges.
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An elongated bar in the correct orientation and the correct position is the best stimulus for a simple cell.
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Examples of other types of orientation-selective
receptive field preferences
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Model of how center-surround cells can be
building blocks for simple cells.
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Complex cells also prefer oriented lines but they don’t require a
particular location.
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Many complex cells are directionally-selective.
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Model of how simple cells could be building-blocks for
complex cells
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Orientation columns
• Cortex -- as a whole -- is organized into columns.
• In striate cortex, the orientation column is the basic unit.– All of the cells outside of layer 4 will respond best to stimuli of the same orientation.
– Some cells will be direction selective, others not.
1 mm
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Output of striate cortex: more than 40 other regions!
"where" (parietal)
"What"(temporal)
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The “where” stream
• Cells in some of these regions respond almost exclusively to moving objects.– Some respond to circular or spiraling movement.
– Some respond to “visual flow.”– Some respond best to approaching or receding objects.
– ….
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Area MT is a well-studied “motion area.” Most of the cells in MT respond best to moving stimuli, and most of those cells have well-defined direction preferences.
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Area MT, like many other cortical areas, shows columnar organization.
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Cells in area MST often respond best to more complex types of
motion.
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Lesions of the “where” stream
• loss of speed motion perception• visual neglect in peripersonal space
• loss of ability to follow moving objects
• loss of ability to use visual information to grasp objects
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The “what” stream
• Some areas have many cells that are color-selective.
• Some areas have cells that respond to complex shapes.
• Some areas are particularly important for face perception.
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Lesions of the ventral stream
• Achromatopsia: loss of color vision
• Prosopagnosia: loss of face recognition– Some regions have cells that respond best to objects such as particular faces.
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Example of a ventral occipito-temporal cell that responds best to faces
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Recommended reading
• Lettvin, J. Y., H. R. Maturana, et al. (1959). "What the frog's eye tells the frog's brain." Proc. Inst. Radio Engr. N.Y. 47: 1940-1951.
• Hubel, D. H. (1982). "Exploration of the primary visual cortex, 1955-78." Nature 299(5883): 515-524.
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Supplementary material starts here.
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Organization of orientation columns
• Adjacent columns usually have cells with slightly different orientation preferences.
• This orderly pattern is interrupted by occasional “color blobs” -- columns of color-selective cells with center-surround receptive fields.
1 mm
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In vivo imaging
• Present horizontal bars.
• Collect image intensity data.
• Repeat with vertical bars.
• Subtract the difference on a point-by-point basis.
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Orientation columns form a “pinwheels” with 360° of
orientations surrounding a zone of non-oriented cells.
Center: non-oriented, color ‘blobs’.
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Ocular dominance columns
• Left-eye and right-eye layers of the lateral geniculate project to adjacent zones in layer 4.
• Each zone is called an ocular dominance column and is about 1 mm wide.
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R
a b
L R L R L
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Hubel and Wiesel noticed that they tended to find groups of cells
dominated by one eye, then a group dominated by the other eye, etc.
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Ocular dominance columns as seen by in vivo imaging.
In humans, each stripe is about 1-2 mm wide.
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Ocular dominance
• Each ocular dominance column contains:– about 20 orientation-selective columns
– about 1 color blob
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Development of ocular dominance columns
• At birth, the geniculo-cortical projection to layer 4 is much less well segregated than in the adult.
immature matureR L R L R L R L R L R L
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Development of ocular dominance columns
• The connections from the geniculate sort themselves out by remodeling of axons.
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Development of ocular dominance columns
• The connections from the geniculate sort themselves out by remodeling of axons.– New branches are made.
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Development of ocular dominance columns
• The connections from the geniculate sort themselves out by remodeling of axons.– New branches are made.– Some old branches are retracted.
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Development of ocular dominance columns
• The connections from the geniculate sort themselves out by remodeling of axons.– New branches are made.– Some old branches are retracted.
• The basic segregation of geniculocortical projections is complete at 4-6 months postnatal.
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Development of ocular dominance columns
• Each eye will normally wind up controlling about 50% of layer 4.
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Development of ocular dominance columns
• Each eye will normally wind up controlling about 50% of layer 4.
• However, if one eye isn't functioning properly during development, it won't get its share of cortical territory.
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Result of uncorrected left eye cataract
• Instead of a 50:50 division of left-eye & right-eye inputs from the lateral geniculate to layer 4, the deprived eye occupies only ~20% of the territory.
immature matureR L R L R L R L R L R L
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Result of uncorrected left eye cataract
• Instead of a 50:50 division of left-eye & right-eye inputs from the lateral geniculate to layer 4, the deprived eye occupies only ~20% of the territory.
• The open eye occupies ~80%.
immature matureR L R L R L R L R L R L
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Result of uncorrected left eye cataract• The good eye takes over 100% of the cells in the other
layers.
immature matureR L R L R L R L R L R L
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Take-over by non-deprived eye
• Why do all of the cells in the layers above and below layer 4 respond only to the good eye?
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Take-over by non-deprived eye
• Why do all of the cells in the layers above and below layer 4 respond only to the good eye?– Anatomy: Most of the projections to the other
layers come from the layer 4 cells that are controlled by the good eye.
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Take-over by non-deprived eye
• Why do all of the cells in the layers above and below layer 4 respond only to the good eye?– Anatomy: Most of the projections to the other
layers come from the layer 4 cells that are controlled by the good eye.
– Physiology: Intrinsic inhibitory circuits exaggerate the imbalance between the two eyes.
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The deprived eye becomes functionally blind.
• The deprived eye completely loses its ability to activate most cells in striate cortex.
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The deprived eye becomes functionally blind.
• The deprived eye completely loses its ability to activate most cells in striate cortex.
• Since most of the visual projections to other parts of cortex come from these cells, those regions also become monocular.
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The deprived eye becomes functionally blind.
• The deprived eye completely loses its ability to activate most cells in striate cortex.
• Since most of the visual projections to other parts of cortex come from these cells, those regions also become monocular.
• The child has no binocular (stereoscopic) depth perception.
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Critical periods
• This dramatic effect happens only if there is a deficit during the critical period of development, the first few months of postnatal life.
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Critical periods
• This dramatic effect happens only if there is a deficit during the critical period of development, the first few months of postnatal life.
• A cataract that develops in an adult will produce no permanent deficits.
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Critical periods
• This dramatic effect happens only if there is a deficit during the critical period of development, the first few months of postnatal life.
• A cataract that develops in an adult will produce no permanent deficits.
• Even if the cataract is present for years, vision will be restored when the cataract is removed.
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Critical periods
• If a cataract in a baby is removed soon enough (within 1 or 2 months), there will be a partial or even complete
recovery from effects.
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Critical periods
• If a cataract in a baby is removed soon enough (within 1 or 2 months), there will be a partial or even complete recovery from effects.
• The worse the cataract and the longer the delay to removal, the less recovery will occur.
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Axons that fire together, wire together
• What mechanisms allow visual input to influence formation of ocular dominance columns?
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Axons that fire together, wire together
• What mechanisms allow visual input to influence formation of ocular dominance columns?– Left-eye and right-eye inputs compete for
synaptic territory on cells in layer 4.
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Axons that fire together, wire together
• What mechanisms allow visual input to influence formation of ocular dominance columns?– Left-eye and right-eye inputs compete for
synaptic territory on cells in layer 4.– Normally, vigorous activity from each eye
helps axons cooperate in stabilizing territory.
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Model of activity-mediated segregation
• Initially, inputs from both eyes via the lateral geniculate connect to each cell in layer 4.
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Model of activity-mediated segregation
• Initially, inputs from both eyes via the lateral geniculate connect to each cell in layer 4.
• By the time of birth, about half the cells have more left-eye input, while others have more right-eye input.
right eye
left eye
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Model of activity-mediated segregation• Consider a cell with mostly left-eye input.
right eye
left eye
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Model of activity-mediated segregation• Consider a cell with mostly left-eye input.
• If visual input is normal, the more numerous left-eye inputs reinforce each other. The less numerous right eye inputs are eliminated.
right eye
left eye
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Effect of monocular deprivation• Monocular deprivation tips the scales in favor of the good
eye.
right eye
left eye
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Effect of monocular deprivation• Monocular deprivation tips the scales in favor of the good
eye.
• The good eye takes over all the cells where it started out with a majority of inputs and can also take over many other cells since the other eye’s activity is very weak.
right eye
left eye
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Other aspects of visual function have their own critical periods.
• Uncorrected astigmatism can lead to permanent deficiencies in acuity for lines of particular orientations.
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Other aspects of visual function have their own critical periods.
• Uncorrected astigmatism can lead to permanent deficiencies in acuity for lines of particular orientations.
• This defect need not be corrected until early grade school age.
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Strabismus
• Strabismus (cross-eyedness or wall-eyedness) can lead to loss of stereopsis.
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Strabismus
• Strabismus (cross-eyedness or wall-eyedness) can lead to loss of stereopsis.
• Strabismus is often be treated by surgery followed by patching the eye with the stronger vision.
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Strabismus
• Strabismus (cross-eyedness or wall-eyedness) can lead to loss of stereopsis.
• Strabismus is often be treated by surgery followed by patching the eye with the stronger vision.
• A better understanding of critical periods has led to earlier attention to the problem in recent decades.
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ON-center OFF-surround ganglion cells
• Lighting up the center of the field excites the ganglion cell (turns it "ON"). photoreceptors
interneurons
ganglion cell+
* * * *
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ON-center OFF-surround ganglion cells
• Lighting up the center of the field excites the ganglion cell (turns it "ON"). photoreceptors
interneurons
ganglion cell+
* * * *
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ON-center OFF-surround ganglion cells
• Lighting up the surrounding part of the field inhibits the ganglion cell (turns it "OFF").
photoreceptors
interneurons
ganglion cell-
* * * *
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OFF-center ON-surround ganglion cells
• The idea is the same, but now light in the center leads to inhibition, and light in the surround leads to excitation.
photoreceptors
interneurons
ganglion cell-
* * * *
photoreceptors
interneurons
ganglion cell+
* * * *
+
-