The visual system

The retina

• Light passes through the lens, through the inner layer of ganglion cells and bipolar cells to reach the rods and cones.

The retina• 0.5 mm thick

The retina• 0.5 mm thick• The photosensors (the rods and cones) lie outermost in the retina.

The retina• 0.5 mm thick• The photosensors (the rods and cones) lie outermost in the retina.

• Interneurons

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.

The retina• Receptive field:

– The location where a visual stimulus causes a change in the activity of the visual neuron

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.

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!

Retinal interneurons

• Photoreceptors synapse onto many interneurons.

Retinal interneurons

• Photoreceptors synapse onto many interneurons.

• The interneurons synapse onto one another and onto ganglion cells.

Ganglion cells

• There are about a million ganglion cells.

Ganglion cells

• There are about a million ganglion cells.

• There are at least 18 different morphological types of ganglion cell in the human retina.

Ganglion cells

• Most ganglion cells have center-surround receptive fields.

Ganglion cells

• Most ganglion cells have center-surround receptive fields.

• About 50% of those are OFF-center ON-surround.

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.

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.

Sustained vs. transient responses

• Some respond transiently

• Some give sustained responses.

transientcell

sustainedcell

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).

Pathway from the eye to the cortex

Striate cortexAlso called V1, primary visual cortex, or area 17

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

123456

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.

An elongated bar in the correct orientation and the correct position is the best stimulus for a simple cell.

Examples of other types of orientation-selective

receptive field preferences

Model of how center-surround cells can be

building blocks for simple cells.

Complex cells also prefer oriented lines but they don’t require a

particular location.

Many complex cells are directionally-selective.

Model of how simple cells could be building-blocks for

complex cells

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

Output of striate cortex: more than 40 other regions!

"where" (parietal)

"What"(temporal)

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.

– ….

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.

Area MT, like many other cortical areas, shows columnar organization.

Cells in area MST often respond best to more complex types of

motion.

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

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.

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.

Example of a ventral occipito-temporal cell that responds best to faces

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.

Supplementary material starts here.

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

In vivo imaging

• Present horizontal bars.

• Collect image intensity data.

• Repeat with vertical bars.

• Subtract the difference on a point-by-point basis.

Orientation columns form a “pinwheels” with 360° of

orientations surrounding a zone of non-oriented cells.

Center: non-oriented, color ‘blobs’.

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.

R

a b

L R L R L

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.

Ocular dominance columns as seen by in vivo imaging.

In humans, each stripe is about 1-2 mm wide.

Ocular dominance

• Each ocular dominance column contains:– about 20 orientation-selective columns

– about 1 color blob

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

Development of ocular dominance columns

• The connections from the geniculate sort themselves out by remodeling of axons.

Development of ocular dominance columns

• The connections from the geniculate sort themselves out by remodeling of axons.– New branches are made.

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.

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.

Development of ocular dominance columns

• Each eye will normally wind up controlling about 50% of layer 4.

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.

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

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

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

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?

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.

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.

The deprived eye becomes functionally blind.

• The deprived eye completely loses its ability to activate most cells in striate cortex.

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 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.

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.

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.

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.

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.

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.

Axons that fire together, wire together

• What mechanisms allow visual input to influence formation of ocular dominance columns?

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.

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.

Model of activity-mediated segregation

• Initially, inputs from both eyes via the lateral geniculate connect to each cell in layer 4.

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

Model of activity-mediated segregation• Consider a cell with mostly left-eye input.

right eye

left eye

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

Effect of monocular deprivation• Monocular deprivation tips the scales in favor of the good

eye.

right eye

left eye

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

Other aspects of visual function have their own critical periods.

• Uncorrected astigmatism can lead to permanent deficiencies in acuity for lines of particular orientations.

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.

Strabismus

• Strabismus (cross-eyedness or wall-eyedness) can lead to loss of stereopsis.

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.

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.

ON-center OFF-surround ganglion cells

• Lighting up the center of the field excites the ganglion cell (turns it "ON"). photoreceptors

interneurons

ganglion cell+

* * * *

ON-center OFF-surround ganglion cells

• Lighting up the center of the field excites the ganglion cell (turns it "ON"). photoreceptors

interneurons

ganglion cell+

* * * *

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-

* * * *

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+

* * * *

+

-