Lecture #22

Higher order visual processing4/18/13

With acknowledgement to Prof Dan Butts

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

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

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 +

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

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

Center surround demo

Ganglion cell simulationsDr Jon Krantz – Hanover College

But stimuli are not often dots

Larger scale objects are detected by the edge

What about color?

• What if white dot were replaced by red dot or green dot?

• What if dots were large

We don’t often look at tiny 2 um spots

• What about a large block?

What about other colors?

1

2

3

Visual image

What the photoreceptors see is made of dots

What the brain sees are objects

UmbrellaHatTreeFace

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

RGC respond to edges or annuli

Neuroscience Purves et al eds. Fig 11.2

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

Neuroscience Purves et al eds. Fig 11.2

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

α and β ganglion cell receptive fields: change across retina

LGN

• Relays ganglion cell outputSame center / surround wiring as in retina

RGC receptive LGN receptive field field

Lateral geniculate nucleus• geniculate = bent• Layered• Bottom two layers

have larger cellsMagnocellularMagno- = large

• Top four layers have smaller cellsParvocellularParvo- = small

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

LGN layers• Topographic mapping

Eyes and fields within the eyes

• 1,2 Magnocellular (motion)

• 3-6 Parvocellular (color)

Layers of LGN

Lateral geniculate nucleus is a relay station between retina and cortex

Webvision

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

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

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

Wiring to visual cortex

Cortical magnificationFovea gets more neurons than periphery

Index finger demo

Wolfe et al 2006 Sensation and perception

Primary visual cortex, V1 = striate cortex

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

David Hubel and Torsten Wiesel

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

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.”

LGN center surround converted to edge detectors

Same as ganglion cells

Orientation detection• Hubel and Wiesel

discovered cells that were sensitive to bars

• Some responded to orientation

Orientation sensitivity changes with penetration through cortex

Stimulus size match to cell response area (complex cells)

Get higher response if stimulus matches the cell’s receptive area

Striate receptive fields

Factors affecting cortical response

• Bar width• Bar orientation• Cell type• End stopping• Edge- and bar-detectors• Receptive field size• Motion• Color• Ocular dominance:

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

Found orientation sensitive columns

Alternate in which eye they are most sensitive to - ocular dominance columns

Feature detectors

Psychophysical tests

• By saturating a given class of neurons, we can show that they exist

• Color after images• Tilt aftereffects• Motion detection

Union jack inversion

Ocular dominance columns

Ocular dominance columns

Purves et al fig 11.3 Cells with different amounts of input from two eyes

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

Ocular dominance in layer IV

Effect of monocular deprivation

More finely define critical period

Projections from LGN are altered in response to deprivation

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

He .. Quinlan 2007

Rat eyes are more contralateral than ipsilateral

Eyes

Ipsilateral - same side

Contralateral - other side

Rat eyes are more contralateral than ipsilateral

LGNLGN

Eyes

Binocular visual cortex - ocular dominance columns

Use visual evoked potential - record from cortex when expose eyes to light

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

Normal rearing experiment

Normal rearing 12 hr light:12 dark

Monocular deprivation in adult

VEP recording

Deprived eye

Nondeprived eye

Cont Ips

Rearing with dark period experiment

Normal rearing 12 hr light:12 dark

Monocular deprivation VEP recording

Rat in the dark for 3-10 days

If put in dark, get cortical plasticity

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

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

Dan Butts lab