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Seeing Things 2Visual Processing in the Brain
How Your Brain Works - Week 4
Dr. Jan [email protected]
HowYourBrainWorks.net
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.
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
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?
Primary and “Extrastriate” Visual
Cortex
Decoding Brain Activity
• Miawake et al. Neuron 2008
• Observed activity of ca 1500 voxels of (3mm)3.
• Reconstructed the image shown from recorded activity
Seeing Lines
Simple Cell Receptive Fields
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LGN Cortex
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
Cortical Columns as “Computational Modules”
• Surface
• Supra-• granular
• Granular
• Infra-• Granula
r
• White• matterFrom
ThalamusSubcorticalTargets
I
II/III
IV
V
VI
Representing Shape and Position Within an “Orientation Map”
• Pseudocolour “orientation tuning” map of ferret primary visual cortex (revealed with intrinsic optical imaging).
Binocular Vision
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.
Stereopsis (Stereo vision for depth)
BAB
AB
A
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Ocular dominance
Cytochrome Oxidase Blobs
Cortical Hypercolumns
Break
Cortical Hypercolumns
Stripe Rearing
Distribution of orientation tuning in V1 of kittensreared in a vertically...
... or horizontally striped environment.
What would the world look like to a stripe reared kitten?
Three-eyed Frogs
means that if you want to predict the PSTH ofmeans that if you want to predict the PSTH of
Strabismus
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.
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.
Enriching Early
Experience
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
Higher order Visual Pathways
Shape processing hierarchy
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.
Motion Sensitivity
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Newsome’s Moving Random Dots
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
Microstimulation Biases Perceptual Choice
• From Salzman, Britten, Newsome (1989) Nature 346:174
The Motion Aftereffect Illusion
http://www.michaelbach.de/ot/mot_adapt/index.html
Go
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
Form from Motion
means that if you want to predict the PSTH ofmeans that if you want to predict the PSTH of
