Download Check out figures to understand this tricky wiring pattern… After

Check out figures to understand
this tricky wiring pattern…
After leaving the retina, the
outputs of each eye are split
• The nasal (toward the nose) half of each
eye's visual field crosses from one side to
the other at the optic chiasm
• The temporal half (towards the temple)
remains on the same side as its eye-oforigin
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– This splitting and crossing re-organizes the retinal
outputs so that the left hemisphere processes
information from the right visual field, and the right
hemisphere processes information from the left visual
field
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Visual processing beyond the retina
Visual Pathway…..
• Major Pathways
– optic chiasm
• medial fibers
cross over
• left visual field
processed in right
hemisphere
• right visual field
processed in left
hemisphere
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• Retinal ganglion cells actually
come in (at least) 2 flavors:
– M (magnocellular)
– P (parvocellular)
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Thalamus contains…
Thalamus…..
• dorsal lateral geniculate nucleus (LGN)
– Left and right LGN
– 6 layers in each
• lower two –magnocellular
– M-cells - movement
• 4 upper layers – parvocellular
– P-cells – color, fine texture, depth
• Layers 1, 4, 6 from contralateral eye
• Layers 2,3,5 from ipsilateral
– Forms a “retinotopic map”
• Locations in LGN correspond to locations on the retina
– output of LGN forms the optic radiations
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LGN
Occipital Lobe
primary visual cortex (“striate cortex” due to
“striped” appearance)
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Primary Visual Cortex: V1
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Primary visual cortex….
• V1 has a topographic/retinotopic map of the
visual world
• This means that there is a "neural image"
retaining the spatial layout of the pattern of
light that falls on the retina
• This map has several interesting
characteristics
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Characteristics of Retinotopic Map
Striate Cortex Anatomy/Function
• Remember that there are 2 V1s in each
person (left and right hemispheres)
– Each V1 has a representation of the
opposite half of the visual field (e.g., left
V1 has a map of the right visual field, and
vice versa)
– Each V1 does not simply receive input
from the opposite eye; the outputs of each
retina are split (left half/right half) and
then run through the LGN to the
appropriate V1
• six major “layers”
• mapped to contralateral half of visual field
„Cortical magnification devotes 25% to foveal
vision
• processes the “features” of visual stimuli
– higher-order (that is, does not merely respond to
“spots” of light
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Characteristics of Retinotopic Map
• Just as the image of the world is inverted
when projected onto the retina, the
retinotopic V1 map is upside down (and the
right hemisphere's V1 has a topographic
map of the left visual field, and vice versa)
• Cortical magnification
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3 main types of cells in primary
visual cortex
• Simple
• Complex
• End-stopped (formerly Hypercomplex)
– more cortical space is dedicated to the fovea
than the periphery (remember the higher
density of photoreceptors in the fovea, hence
clearer vision)
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Simple
Complex
• Receptive fields often have a long, narrow
bar of light (ON) and flanking (OFF) parts
• Other types are the opposite (responding to
dark bars) or simply respond to a light/dark
edge
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• Bars of light must be oriented correctly, but can appear anywhere in
the receptive field
• Moving the bar through the field produces a sustained response
• Complex cells often show direction-selectivity:
– they fire more when the bar moves in one direction, and are
suppressed by motion in the opposite direction
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End-stopped (formerly
Hypercomplex)
Characteristics of Retinotopic Map
• Many simple and complex cells exhibit length
summation
– if an appropriate bar is placed in the visual field, they
fire action potentials; if the bar is made longer, they fire
more, up to the extent of the full receptive field
• However, end-stopped cells increase their
responses with increases in bar length up to a limit
that is smaller than the receptive field
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• Remember that there are 2 V1s in each person (left and right
hemispheres)
– Each V1 has a representation of the opposite half of the visual field
(e.g., left V1 has a map of the right visual field, and vice versa)
– Each V1 does not simply receive input from the opposite eye; the
outputs of each retina are split (left half/right half) and then run
through the LGN to the appropriate V1
• Just as the image of the world is inverted when projected onto the
retina, the retinotopic V1 map is upside down (and the right
hemisphere's V1 has a topographic map of the left visual field, and
vice versa)
• Cortical magnification
– more cortical space is dedicated to the fovea than the periphery
(remember the higher density of photoreceptors in the fovea, hence
clearer vision)
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Ocular Dominance Columns
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Architecture of V1
• Orientation columns:
– as you move perpendicular to the surface, the
preferred orientation of the cells changes
gradually from horizontal to vertical and back
again
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Orientation Columns combine
with Ocular Dominance Columns
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Defining and Separating Different
Brain Areas
Secondary Visual Areas
• There are approximately 30 visual areas
after V1
• Brain areas can be differentiated according to 4 main
criteria:
– Function: physiology
• Neurons in different parts of the brain are responsive to different
aspects of the stimulus (= do different things).
– Architecture: microanatomy can differ widely across brain areas
• For example, V1 is also referred to as "striate cortex" because it has a
series of stripes that run parallel to the surface; these stripes end
abruptly at the end of V1.
– Connections:
– The functional specialization hypothesis drives
much of the research about these areas
– Some areas seem specialized for processing a
certain aspect of visual information (e.g., MT motion, V4 - color (?))
• different areas feed forward and also receive backward-reaching
connections from distinct areas.
– Topography: e.g., retinotopy
• Each distinct visual area has its own retinotopic map.
Remember 'FACT' as a mnemonic
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Secondary Visual Areas
• Cortical areas dedicated to vision are
densely interconnected, and can seem quite
confusing at first glance
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Secondary Visual Areas
• However, a more general organization is evident in a pair
of parallel pathways
– What pathway
• Temporal lobe; recognition of objects
– Where pathway
• Parietal lobe; motion, spatial orientation,
localization
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