Lecture # 22
description
Transcript of Lecture # 22
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Lecture #22
Higher order visual processing4/18/13
With acknowledgement to Prof Dan Butts
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Wiki assignments• Introductory page is due at midnight• Get the first “page” of your project up on the
wikiCan be a linked subpage or can be a section on the main page500-1000 wordsFigures or links to make it interestingREFERENCES
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The return of the center-surround
Cones are wired up so one cone is compared with one or more surrounding cones
The center and the surround cones send inputs to a ganglion cell
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Ganglion cells provide outputs to the combined cone inputs
Ganglion cell compares the inputs and provides an output
Same cones can be wired to different ganglion cells:center + / surround - and center - / surround +
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Center - surround color opponency
Webvision
Two cones provide input to 4 channels
Red ON green OFFRed OFF green ON
Green ON red OFFGreen OFF red ON
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Center surround types – receptive field
These show an area of the retina and its relative sensitivity to stimulation by a point of light
Kuffler’s Nobel prize winning work
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Center surround demo
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Ganglion cell simulationsDr Jon Krantz – Hanover College
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But stimuli are not often dots
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Larger scale objects are detected by the edge
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What about color?
• What if white dot were replaced by red dot or green dot?
• What if dots were large
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We don’t often look at tiny 2 um spots
• What about a large block?
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What about other colors?
1
2
3
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Visual image
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What the photoreceptors see is made of dots
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What the brain sees are objects
UmbrellaHatTreeFace
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Information flow from retina to cortex
Visual cortex 200M cells
1/3 human cortex1/2 monkey cortex
Retina6.5 M
cones120 M
rods1.2 M
ganglion cells
Lateral geniculate nucleus (LGN)2 M cells
Information bottleneck
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RGC respond to edges or annuli
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Neuroscience Purves et al eds. Fig 11.2
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Wiring to visual cortexTopographic map is retained
Optic chiasmRight side from both eyes to right visual cortex
Left side from both eyes to left visual cortex
Wolfe et al 2006 Sensation and perception
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Neuroscience Purves et al eds. Fig 11.2
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Two key ganglion cell types
• β (midget cells)High acuity – one to one wiring of cone to bipolar to ganglion cellColor vision
• α (parasol cells)Lower acuity – many to one wiring of cones to ganglion cellsBest for motion detection
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α and β ganglion cell receptive fields: change across retina
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LGN
• Relays ganglion cell outputSame center / surround wiring as in retina
RGC receptive LGN receptive field field
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Lateral geniculate nucleus• geniculate = bent• Layered• Bottom two layers
have larger cellsMagnocellularMagno- = large
• Top four layers have smaller cellsParvocellularParvo- = small
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Lateral geniculate nucleus• Parvocellular layer
receives input from β ganglion cells (midget cells)High acuity
• Magnocellular layers receive input from α ganglion cells (parasol cells)Motion detection
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LGN layers• Topographic mapping
Eyes and fields within the eyes
• 1,2 Magnocellular (motion)
• 3-6 Parvocellular (color)
Layers of LGN
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Lateral geniculate nucleus is a relay station between retina and cortex
Webvision
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Wiring to visual cortexCortex feeds back onto LGN
More input from cortex than from retina!!
•Inputs from other parts of brain
When sleep, LGN and rest of thalamus shuts down so no signals go to cortex
Wolfe et al 2006 Sensation and perception
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Wiring to visual cortexCortex feeds back onto LGN
More input from cortex than from retina!!
•Hard to studyUnder anesthesia, link to cortex is shut off
Wolfe et al 2006 Sensation and perception
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Wiring to visual cortex
Topographic map set up in retina, relayed through LGN and passed to cortex
Cortex also has 6 layers
LGN projects to layer 4 (V4)Magnocellular 4cα
Parvocellular 4cβ
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Wiring to visual cortex
Cortical magnificationFovea gets more neurons than periphery
Index finger demo
Wolfe et al 2006 Sensation and perception
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Primary visual cortex, V1 = striate cortex
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Arranged in layersAs move from top to bottom, cells are arranged in columns with different sensitivities
Orientation
Spatial frequency
Ocular dominance - prevalence of one eye
Color
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David Hubel and Torsten Wiesel
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Huber and Wiesel
• Record from visual cortex while illuminating cat retinaIlluminated retina with spots as their mentor Kuffler had done in his work to record from retinal ganglion cellsGot nothing
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Hubel, 1981 Nobel prize lecture“Our first real discovery came about as a surprise. We had been doing
experiments for about a month.. and were not getting very far: the cells simply would not respond to our spots and annuli. One day we made an especially stable recording… For 3 or 4 hours we got absolutely nowhere. Then gradually we began to elicit some vague and inconsistent responses by stimulating some where in the midperiphery of the retina. We were inserting the glass slide with its black spot into the slot of the ophthalmoscope when suddenlty over the audiomonitor the cell went off like a machine gun. After some fussing and fiddling we found out what was happening The response had nothing to do with the black dot. As the glass slide was inserted, its edge was casting onto the retina a faint but sharp shadow, a straight dark line on a light background. That was what the cell wanted, and it wanted it, moreover, in just one narrow range of orientations.”
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LGN center surround converted to edge detectors
Same as ganglion cells
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Orientation detection• Hubel and Wiesel
discovered cells that were sensitive to bars
• Some responded to orientation
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Orientation sensitivity changes with penetration through cortex
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Stimulus size match to cell response area (complex cells)
Get higher response if stimulus matches the cell’s receptive area
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Striate receptive fields
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Factors affecting cortical response
• Bar width• Bar orientation• Cell type• End stopping• Edge- and bar-detectors• Receptive field size• Motion• Color• Ocular dominance:
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Eyes start to come together in cortex
• Alternating columns are associated with each eye
• As if locating inputs more closely so they can be compared
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Found orientation sensitive columns
Alternate in which eye they are most sensitive to - ocular dominance columns
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Feature detectors
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Psychophysical tests
• By saturating a given class of neurons, we can show that they exist
• Color after images• Tilt aftereffects• Motion detection
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Union jack inversion
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Ocular dominance columns
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Ocular dominance columns
Purves et al fig 11.3 Cells with different amounts of input from two eyes
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Visual experience
• Connections to visual cortex require visual experience during early critical periodFirst 3-4 months in cats, primatesFirst 3-8 yrs in humans
• If no input from one eye then connections do not get made and can “never” be madePermanent loss of visual input from that eye
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Ocular dominance in layer IV
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Effect of monocular deprivation
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More finely define critical period
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Projections from LGN are altered in response to deprivation
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Monocular deprivation
• Some children have cataracts, crossed eyes, or very lazy eye early in life
• Brain turns off input from one eye• Only get input from other eye• So NO alterating occular dominance columns
No comparison of eyesMonocular vision
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He .. Quinlan 2007
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Rat eyes are more contralateral than ipsilateral
Eyes
Ipsilateral - same side
Contralateral - other side
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Rat eyes are more contralateral than ipsilateral
LGNLGN
Eyes
Binocular visual cortex - ocular dominance columns
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Use visual evoked potential - record from cortex when expose eyes to light
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Rat eyes are more contralateral than ipsilateral
Binocular visual cortex
LGNLGN
Recording from this region will give more signal for light in front of contralateral eye than ipsilateral
ContraIpsi
Vcontra : Vipsi = 2.4 : 1
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Normal rearing experiment
Normal rearing 12 hr light:12 dark
Monocular deprivation in adult
VEP recording
Deprived eye
Nondeprived eye
Cont Ips
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Rearing with dark period experiment
Normal rearing 12 hr light:12 dark
Monocular deprivation VEP recording
Rat in the dark for 3-10 days
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If put in dark, get cortical plasticity
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Rearing with monocular deprivation
12 hr light:12 dark
Monocular deprivation- MD VEP recording
Rat in the dark for 10 days
DE
Binocular vision- BVOr reverse occlusion- RO
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Cortical plasticity
• This implies putting subject in dark makes the cortical wiring plastic again as it was during early critical period
• Perhaps putting people in dark rooms for 3-10 days will enable cortex to wire up again correctly!
• Cure for amblyopia
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Dan Butts lab