Seeing Things 2 Visual Processing in the Brain
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Transcript of Seeing Things 2 Visual Processing in the Brain
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Seeing Things 2Visual Processing in the Brain
How Your Brain Works - Week 4
Dr. Jan [email protected]
HowYourBrainWorks.net
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Visually Guided Behaviour
• To catch a prey, your sensory system has to “represent” the target to be caught in a manner that can “instruct” the appropriate motor commands.
• In reptiles and amphibia, this representation most likely resides in the optic lobe, also called the optic tectum, or (in mammals) superior colliculus.
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Motor Maps in the Superior Colliculus
• Microstimulation studies have shown that the SC contains a “motor map”, which is in register with the retinotopic sensory map
Retinotopic MapRetinotopic Map Motor MapMotor Map
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There is more to vision than visual reflexes
• Often we have to balance the desire to catch one object with the need to dodge another, or choose which from a variety of objects is most worth pursuing.
• Which objects need catching, and which need dodging, may change over time. This creates a need for quite abstract representations of objects within a flexible, rapidly adaptable system. Is that what sensory cortex is for?
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Primary and “Extrastriate” Visual
Cortex
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Decoding Brain Activity
• Miawake et al. Neuron 2008
• Observed activity of ca 1500 voxels of (3mm)3.
• Reconstructed the image shown from recorded activity
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Seeing Lines
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Simple Cell Receptive Fields
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LGN Cortex
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Cortical Layers
• 1: “tufts” of apical dendrites receive cortico-cortical connections.
• 2/3: gets input from layer 4. Many simple cells. Outputs to other parts of cortex.
• 4: gets most input from LGN. Many LGN-like, non-oriented cells. Output to layers 2/3.
• 5/6: inputs from layers 2/3. Output to subcortical targets
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Cortical Columns as “Computational Modules”
• Surface
• Supra-• granular
• Granular
• Infra-• Granula
r
• White• matterFrom
ThalamusSubcorticalTargets
I
II/III
IV
V
VI
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Representing Shape and Position Within an “Orientation Map”
• Pseudocolour “orientation tuning” map of ferret primary visual cortex (revealed with intrinsic optical imaging).
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Binocular Vision
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Binocular Fusion
• Try “shooting a hole” into your hand by rolling up a piece of paper into a tube, holding it in front of one eye, and holding your free hand flat in front of the other eye, as shown here.
• Your brain will try, as best it can, to paint a single scene out of the disparate images seen by each eye.
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Stereopsis (Stereo vision for depth)
BAB
AB
A
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Ocular dominance
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Cytochrome Oxidase Blobs
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Cortical Hypercolumns
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Break
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Cortical Hypercolumns
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Stripe Rearing
Distribution of orientation tuning in V1 of kittensreared in a vertically...
... or horizontally striped environment.
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What would the world look like to a stripe reared kitten?
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Three-eyed Frogs
means that if you want to predict the PSTH ofmeans that if you want to predict the PSTH of
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Strabismus
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Amblyopia
• Inputs from each eye are thought to “compete” for cortical territory during early development.
• If one eye is “weaker” (e.g. due to an optical defect), it may fail to get properly connected to the visual cortex.
• This in principle essentially healthy eye can then become functionally blind.
• To prevent amblyopia, children at risk sometimes have their stronger eye temporarily deprived of input.
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Meltzoff & Moore 1977
means that if you want to predict the PSTH of
• Neonates are said to be able to mimic facial or hand gestures after 14 to 21 days.
• Wilderbeast run with the herd after just a few hours.
• Experience dependent maturation of the visual system may need to be rapid.
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Enriching Early
Experience
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Parallel Pathways
Retina
M
LGN V1 Extrastriatecortex
MagnocellularLayer IVCαβthen IVB
V5 (MT)
P ParvocellularLayer IVCβinterblob
V2
non-Mnon-P
Koniocellular blob V4
mot
ion
shap
eco
lour
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Higher order Visual Pathways
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Shape processing hierarchy
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Face Cells
means that if you want to predict the PSTH of• Infero-temporal cortex
contains neurons that appear to be selective for visual objects, such as faces or hands.
• Damage to these areas can lead to “visual agnosia”, and inability to recognize objects by sight even though there is no blindness.
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Motion Sensitivity
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Newsome’s Moving Random Dots
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Neurometric Curves
• Hatched Bars: responses to movement in preferred direction• Filled black bars: responses to movement in null direction• Open (white) circles: psychometric function (animal’s choices)• Filled (black) circles: neurometric function (neuron’s “choice”)• From Newsome, Britten, Movshon (1989) Nature 341:52
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Microstimulation Biases Perceptual Choice
• From Salzman, Britten, Newsome (1989) Nature 346:174
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The Motion Aftereffect Illusion
http://www.michaelbach.de/ot/mot_adapt/index.html
Go
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Hemineglect Syndrome
• Drawing of a clock by a patient with a lesion in the right posterior parietal lobe.
means that if you want to predict the PSTH of
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Form from Motion
means that if you want to predict the PSTH ofmeans that if you want to predict the PSTH of