Perception Chapter 4
Visual Process beyond the Retina
The optic nerve: the communication cable carrying the output of retinal
stimulation to the visual information processing centers of the brain.
The optic nerve is comprised of the axons of the RGC from each eye.
Hence, there are over one million "wires" in each optic nerve
Primary Visual Pathways
Perception Outline #5
Visual Process beyond the Retina
a) optic chiasm: crossover point where fibers from two eyes meet
b) ipsilateral fibers: the half of the optic nerve from each eye emenating from
the temporal half of each retina, which do not cross at the optic chiasm
c) contra-lateral fibers: the other half of the optic nerve, emanating from the
nasal half of each retina, which do cross.
d) optic tracts: term referring to the post-chiasm optic nerve, this represents
combined fibers from both eyes.
Why do half the fibers cross?
Ans: Nasal half of one eye, and temporal half of other eye, monitor same
visual field, therefore, fibers emanating from same areas monitoring same
physical space are combined.
About 80% of these fibers will project to the Lateral Geniculate Nucleus of
the thalamus, the other 20% will project to a structure in the mid-brain
called the Superior Colliculus.
Visual info processing in the Superior Colliculus:
SC is the more phylogentically primitive projection site, in some creatures such as fish and frogs, SC is major site of visual info processing.
1) Cells in SC have ill-defined "On" "Off" regions, therefore will respond to most any visual stimulus regardless of shape, color, or
orientation.
2) SC cells seem most interested in location of visual stimulus, not identity of visual stimulus, therefore, SC has often been referred to as
the "where" system, rather than the "what" system.
3) SC appears to have important role in directing ballistic eye movements toward a visual stimulus in the periphery
4) Most of the fibers projecting to SC are from the M type of RGC, therefore, messages often get to SC more quickly than to LGN.
So what does all this mean?
Ans: it appears that SC is very important for regulating visually guided reflexive behaviours, for example, the SC would quickly signal the
presence of an object coming toward the head from the visual periphery. SC only cares that there is something there, LGN will figure
later what it was!
Visual info processing in LGN:
Structure:
1) There are 6 distinct layers in LGN,
first two layers have large bodied
cells, these are called the
magnocellular layers, the next four
layers have smaller bodied cells and
are called the parvocellular layers.
2) Contra-lateral fibers from the nasal
retina of one eye contact with layers
1, 4, and 6, while the ipsilateral
fibers from the temporal retina of
the other eye make contact with
layers 2, 3, and 5. The same
arrangement is true for each LGN.
3) Input from both eyes is "mapped"
onto LGN layers in such a way as to
preserve the retinal topography.
Retinotopic maps of LGN.
LGN: Retinotopic maps
Spatial layout on retina is preserved when transmitted to layers of LGN
Visual info processing in LGN:
Function:
1) LGN contains cells with similar receptive field characteristics as
RGC, except that "off" portions of cells exert even stronger
inhibitory affect. This serves to accentuate to an even greater
extent the "edges" and "borders" already identified by RGCs.
2) Parvocellular layers contain cells with receptive fields which respond
differentially to color. These are called color opponent cells, most
are red/green or blue/yellow color opponent. These cells receive
most of their input from the P type of RGC.
3) Magnocellular layers are color blind and receive input mostly from
M type of RGCs.
4) Both Magnocellular cells and Parvocellular cells can signal
movement, but Magno cells respond to fast movement, Parvo
respond to slow movement.
5) LGN may also receive "top-down" visual processing biases due to its
interconnections with visual cortex.
6) Interconnections with RAS may serve as “volume control” for visual
inputs.
Visual info processing in the Primary Visual Cortex:
• PVC: the area in the very back of the occipital lobe which first
receives info from connections to LGN, also known as "striate
cortex," "area 17,“ or V1
Visual info processing in the Primary Visual Cortex:
Like RGCs and cells of LGN, cells of the PVC have receptive fields which monitor a
restricted zone in the retina, however, the number of cells responding to input
from the foveal area of retina far exceeds the number responding to input from
more peripheral regions. This is known as cortical magnification.
• Cortical magnification: the disproportionally large amount of cortical tissue
dedicated to processing the foveal retina compared to the peripheral retina.
• Cortical magnification stands to reason if you consider the number of RGCs
connected to photoreceptors in fovea vs. RGCs connected to photoreceptors in
periphery. Remember degree of convergence?
Functional properties of cortical cells:
Orientation specificity: cells in PVC
differentially respond to stimuli
of varying orientations. This
was not true of LGN cells or RGC
cells, both of which contain
circular receptive fields
insensitive to orientation.
Receptive fields in PVC cells
tend to be more oblong shaped.
Simple cells: Well defined
excitatory region; location
important. Complex cells: less
well defined excitatory region;
location less important. Hypercomplex: length of contour
important
oblique effect
Functional properties of cortical cells:
• Direction specificity: cells in PVC also differentially respond to
movement in different directions. One cell may respond vigorously
if a visual stimulus moves from left to right across the receptive
field, but not respond as much if movement is from right to left,
other cells do just the opposite (also up/down). Simple cells: slower
movement. Complex cells: faster movement
Functional properties of cortical cells:
• Binocular cells: Two receptive fields, one for
each eye. Stronger response to one eye
(ocular dominance). Cells give a most vigorous
response when a stimulus of the same size,
shape, and orientation is located in a
particular position in 3-d space across the two
retinae. Cells are important for the the visual
system's use of the depth cue called retinal
disparity (more on that later).
6 layers of PVC
Visual cortex organization: Columns and hypercolumns
• Cell columns: in the PVC
cells (s,c,hc) are arranged in
layers by their preferred
orientation.
• Cell hypercolumns: the
layers of orientation specific
cells are arranged such that
an orderly incrementing of
orientation change exists
across layers until a
complete cycle has been
achieve. Include both eyes.
• Layer 4: input layer,
monocular cells
• Blobs: non-orientation
specific cells for color
processing
Cortical cells as feature detectors
• "Families" of cortical cells as feature detectors: the
responding of any single cell may be ambiguous as to the
true nature of the visual stimulus. For example, a vertical
preferring cell may start to increase its responding as a
stimulus approaches a more vertical orientation, but does
that mean the stimulus is approaching vertical from a
clockwise or counterclockwise position? Don’t know just
from the responding of that one cell (ambiguity problem),
however, the responding of another cell who prefers
certain types of oblique orientations may clear up the
ambiguity. This points to the importance of cells working
as aggraded sets in order to process visual info.
Two visual pathways beyond the PVC
Dorsal: “where” or “how” pathway
Ventral: “what” pathway
M vs. P pathways and Dorsal/Ventral pathways
Seeing vs. Acting
distinction.
Segregation but
not complete
separation.