Seeing Things 2 Visual Processing in the Brain

Seeing Things 2 Visual Processing in the Brain How Your Brain Works - Week 4 Dr. Jan Schnupp jan. schnupp@dpag. ox. ac. uk How. Your. Brain. Works. 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 Retinotopic Map Motor Map • Microstimulation studies have shown that the SC contains a “motor map”, which is in register with the retinotopic sensory 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 (3 mm)3. • Reconstructed the image shown from recorded activity

Seeing Lines

Simple Cell Receptive Fields

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 LGNlike, 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 I • • II/III Supragranula r IV • Granul ar • • Infra. Granul ar • • White matter V VI From Thalamus Subcortical Targets

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)

Ocular dominance

Cytochrome Oxidase Blobs

Cortical Hypercolumns

Break

Cortical Hypercolumns

Stripe Rearing

What would the world look like to a stripe reared kitten?

Three-eyed Frogs means 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 • Neonates are said to be able to mimic facial or hand gestures after 14 to 21 days. means • Wilderbeast runthat withif you want to predict the PSTH of the herd after just a few hours. • Experience dependent maturation of the visual system may need to be rapid.

Enriching Early Experience

Parallel Pathways colour shape motion Retina LGN V 1 Extrastriate cortex M Magnocellular Layer IVCαβ then IVB P Parvocellular Layer IVCβ interblob V 2 non-M non-P Koniocellular blob V 4 V 5 (MT)

Higher order Visual Pathways

Shape processing hierarchy

Face Cells • Infero-temporal cortex containsmeans neurons that if you want to predict the PSTH of 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

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 means that if you want to predict the PSTH of patient with a lesion in the right posterior parietal lobe.

Form from Motion means that if you want to predict the PSTH of