Visual Processing

ganglion cells

the only cells in the retina that fire action potentials

optic nerve

ganglion cells axons bundle together to form the…

optic chiasm

the optic nerves extend to the base of the brain and join the…

LGN

from optic chiasm, ganglion cells from the inner half of the eye cross over to the other side of the brain and synapse within the LGN

contralateral visual field

each brain hemisphere receives input from the…

single cells in the LGN

ganglion cells from the outer half of the eye stays on the same side to synapse with…

contralateral

part of the brain that is opposite to where the stimulus occurred

ipsilateral

part of the brain that is same side to where the stimulus occurred

lateral geniculate nucleus (LGN)

• 6 layers of neurons

• ganglion cells extending from the periphery are mostly magnocellular (M-type)

• they synapse with the inner (ventral) layers of the LGN (magnocellular layers)

• ganglion cells extending from center (fovea) of retina are mostly parvocellular (P-type)

• they synapse with the outer (dorsal) layers of the LGN

• ganglion cells that syanpse with layers of the LGN that are between magnocellular and parvocellular are called koniocellular (nonM-nonP-type)

M-type ganglion cells

• about 5% of ganglion cells

• physically larger and with larger receptive fields

• bursts rapid and fast action potentials

• not sensitive to color bc receive input from bipolar cells synapsed to rods and/or more than 1 type of cone

• better for motion perception

P-type ganglion cells

• consist of about 90% of ganglion cells

• physically smaller in size and with smaller receptive fields

• sustained, slower action potentials

• sensitive to color bc in the retina they receive information from specific bipolar cells synapsed to specific cones

nonM-nonP type ganglion cells

• about 5% of ganglion cells

• less known about these layers, but seem to help with color perception as they are primarily yellow-blue ganglion cells and tend to synapse with neurons that project along the ventral visual path

striate cortex

• from LGN to the primary visual cortex (VI)

• connections extend through a ventral stream into the temporal lobe and a dorsal stream into the parietal and prefrontal cortex

• like striate, has 6 layers arranged in bands

• likely organized in modules or functional units of clusters of neurons that specialize in processing a certain kind of info

• one module processes low-level information and passes it onto the next module, which will build on that info to build up a more complete understanding of a stimulus at each layer

dorsal visual pathway

• determines “where is it”

• inputs from mostly M-type pathways

ventral visual pathways

• determines “what is it”

• inputs from the P-type (and some M-type) pathways

Gozli et al., 2012

• placing hands near an object activates the M-pathway and prioritizes acting on those objects; better temporal processing and motion detection, but worse spatial perception

• placing your hands away from an object activates the P-pathway and prioritizes perception; better spatial perception, but low temporal processing

• ppts completed two diff tasks → one demanded high-temporal precision and one that demanded high-spatial precision

• in temporal task → ppts performed better when hands near computer screen

• in spatial task → ppts performed worse when hands near computer screen

extrastriate cortex

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