NROC64H3 Study Guide - Motion Perception, Parietal Lobe, Peripheral Vision

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13 Apr 2012
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Sensory & Motor Systems 4/13/2012
Chapter 10- The Central Visual System
Anatomy of the striate Cortex
The Primary visual field (also known as striate cortex or V1):
- Is Brodmann’s area 17
- Is located in the occipital lobe of the primate brain
- Much of area 17 lies on the medial surface of the hemisphere, surrounding the calcarine fissure
Retinotopy: is an organization whereby neighboring cells in the retina feed information to neighboring
places in their target structures- in this case, the LGN and striate cortex.
- In this way the two-dimensional surface of the retina is mapped onto the two-dimensional surface
of the subsequent structures.
- The mapping of the visual field onto a retinotopically organized structure is often distorted,
because visual space is not sampled uniformly by the cells in the retina.
*** Recall: there are many more ganglion cells with receptive fields in or near the fovea than in the
periphery. Thus, the representation of the visual field is distorted in striate cortex: the central few degrees
of the visual field are overrepresented or magnified, in the retinotopic map.
- A discrete point of light can activate many cells in the retina, and often many more cells in the
target structure, due to the overlap of receptive fields. The image of a point of light on the retina
actually activates on a large population of cortical neurons; every neuron that contains that point
in its receptive fields is potentially activated.
oThus, when the retina is stimulated by a point of light, the activity in striate cortex is a
broad distribution with a peak at he corresponding retinotopic location.
Lamination of the Striate Cortex
The cells of different Layers:
- Spiny Stellate cells: are small neurons with spine-covered dendrites that radiate out from the cell
body
oThey are seen primarily in the two tiers of layer IVC
oAxons make local connections only within the cortex
- Pyramidal cells: are also covered with spines and are characterized by a single thick apical
dendrite that branches as it ascends toward the pia mater and by multiple basal dendrites that
extend horizontally
oThese are found in the outer layer IVC
oOnly pyramidal cells send axons out of the striate cortex to form connections with other
parts of the brain.
- Inhibitory neurons: lack spines and are sprinkled in all cortical layers
oThese form only local connections.
Inputs and Outputs of the Striate Cotex
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Sensory & Motor Systems 4/13/2012
Chapter 10- The Central Visual System
In the LGN, every layer receives retinal afferents and sends effecters to the visual cortex. In the visual
cortex, only a subset of the layers receives input from the LGN or sends output to a different cortical or
subcortical area.
- Axons from the LGN terminate in several different cortical layers, with the largest number going
to layer IVC.
- Magnocellular LGN neurons project to layer IVCx and parvocellular LGN neurons project to
layer IVCb
oTherefore the layer IVC contains two overlapping retinotopic maps, one from the
magnocellular LGN and the other from the parvocellular LGN
- Koniocellular LGN axons follow a different path, bypassing layer IV to make synapse in layers II
and III.
Innnervation of other Cortical Layers from Layer IVC:
Most intracortical connections extend perpendicular to the cortical surface
along radial lines that run across the layers from white matter to layer I.
- This pattern of radial connections maintains the retinotopic organization
established in layer IV.
oTherefore, a cell in layer VI, for example, receives information
from the same part of the retina as does a cell above it in layer
IV.
- However, the axons of some layer III pyramidal cells extend collateral
branches that make horizontal connections within layer III.
- Layer IVC Stellate cells project axons radially up mainly to layers IVB
and III where, for the first time, information from the left eye and right
eye begins to mix.
Whereas all layer IVC neurons receive only monocular input, most neurons in
layers II and III receive binocular input coming from both eyes.
- Layer IVCx which receives magnocellular LGN input, project mainly to
layer IVB.
- Layer IVCb which receives parvocellular LGN input projects mainly to layer III.
- In layers III and IVB, an
axon may form synapses
with the dendrites of
pyramidal cells of all layers.
Striate Cortex Outputs
- Layer II, III and IVB
pyramidal cells send their
axons to other cortical areas
- Layer V pyramidal cells
send axons all the way
down to the superior
colliculus and pons
- Layer VI pyramidal cells
give rise to the massive
axonal projection back to the LGN
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