As we navigate through our surroundings, a continuous stream of light images impinges our eyes,
in the back of each eye is a light-sensitive tissue: the Retina. It
converts patterns of light energy into electrical discharges known as
action potentials. These signals are conveyed along the axons of
retinal ganglion cells to connect for 80% to the LGN (Lateral
Geniculate Nucleus) a relay nucleus in the Thalamus and for 20% to the
SC (Superior Colliculus). Most of the output of the LGN is relayed directly to the Primary Visual Cortex (V1), and then to surrounding visual association areas."
The Primary Visual Cortex (V1) is the part of the Cerebral
Cortex (Grey Matter) that is responsible for processing visual stimuli,
because of its stripey appearance this area is also known as Striate
Cortex. It is the simplest and earliest cortical visual area, highly
specialized for processing information about static and moving objects
and it is excellent in pattern recognition. V1 has a very well-defined
map of the spatial information in vision, there is a very precise
correspondence between a given location in V1 and the visual field.
A large portion of V1 is mapped to the Fovea, a spot located in the center of the retina responsible for sharp vision, this is known as cortical-magnification. V1 neurons have
strong tuning to a small set of stimuli, the neuronal responses can
discriminate small changes in visual orientations, spatial frequencies
and colors, furthermore, these neurons have Ocular Dominance, tuning to one of the two eyes, and they tend to cluster together as cortical columns.
The LGN neurons mainly project to the Primary Visual Cortex, it
consists of 6 layers and several sub layers that are arranged in bands
parallel to the surface of the cortex. The axons from the LGN
terminate on cortical neurons in layer 4 of V1, the largest and most
important Visual Cortical Area, other areas are known as Extrastriate
Visual Cortex: V2, V3, V4 and MT (V5).
A retinotopic map gives us information of how the input from the
Retina is spread, a special technique is used to flatten the brain. The
Fovea is the part of the Retina that produces the sharpest images,
transfered to V1 it is situated all the way into the back, spreading
out to the front.
Picture's A -B of a Macaque Monkey's V1 shows us how the Ocular Dominance Collums are displayed, the
light columns correspond to the Ocular Dominance Columns of the other
eye. Thanks to the marks of the vains on the Retina it is possible to
position exactly which part of the view is represented in V1, the Blind
Spot (BS), where the Optic Nerve is attached to the Retina is also
clearly represented. (A is a redrawing of B).
MC is the Monocular Crescent-region, here is no input from the other eye so no pattern can
be develloped, an other way to look at this is area is from the
Alpha-Space point of view, this MC-region is Alpha-regulated: the more
you look sideways, the bigger MC gets and the more the Alpha-Space
disrupts our view, so this part of our view has to be "switched" off
and back on (black -> white) when we look the other way. This
happens 5-50 times a second (see: saccades
) so the brain is unable to construct a pattern. Also when we compare
the image input we get from the "Alpha-view" that is only half an inch
away (sometimes with serious light reflections) to "Monocular view"
that is further away (sometimes miles), the influence of Alpha must be
defining for this region. I believe this Alpha-regulated-area (MC) acts
more as a handhold to support and regulate our eye-movements.
Within V1, visual information is processed by cells
arranged within an elaborate system comprised of overlapping vertical
columns and horizontal layers, these stripes represents myelinated axons from the LGN terminating in layer 4 of V1.
The Parvo and Magno pathways are segregated into 3 separate and independent units:
1. Blobs: Inputs from both the Magno and Parvo system, process colour.
2. Interblobs: Inputs from only the Parvo system, process fine patterns in the stimulus.
3. Layer 4b: Input solely from the Magno system, respond to motion and very low contrast.
V1 is divided into 6 layers. Layer 4 which receives most visual
input from LGN, is further divided into 4 layers, labelled 4A, 4B, 4Cα,
and 4Cβ. Sublamina 4Cα receives most Magnocellular input from the LGN,
layer 4Cβ receives input from Parvocellular input.
Parvocellular-layers -> V1
Contrast and Movement (Colour Insensitive)
P-layers (1-2) neurons project their axons to neurons in the sub-layer 4Cβ
Magnocellular-layers -> V1
Colour: Red, Green and Fine detail
M-layer neurons (3-4-5-6) project to neurons in sub-layer 4Cα → 4B → V2 → V5.
Cells in layer 4B
selective for the direction of movement
some of these neurons are binocular and sensitive to retinal disparity
Koniocellular-layers -> V1
Colour: Blue
Inbetween-layers project to V1's "CO Blobs" (=special cells grouped together that integrate colour information).
V1 -> LGN:
Layer 6
There are three types of cells or neurons in the Primary Visual Cortex (V1):
Simple Cells:
Respond to bars of light, excited to a specific line of a particular
orientation placed in the center of its receptive field, and stops
firing if moved away from its center (see B).
Complex Cells: Respond
to line orientation in or out of its excitatory/ inhibitory zones, and
particularly so to movement (see C) Movement detector.
Hyper Complex Cells: Respond to moving corners or angles. Line end (edge) detectors.
Like
a bar broken in half and moving. These 3 respond to specific features
(i.e., bars, moving bars, corners), and are often called feature
detectors. Some of these properties are similar to those we have seen
in the LGN and the ganglion cells (e.g., size selectivity). However,
other properties are seen for the first time at the level of V1 (e.g.,
orientation selectivity and binocular integration)
In the next topic we take a closer look at a theory of the origin of these patterns.
5. Primary Visual Cortex 
The
purpose of this site is to present questions and new ideas about
the above subjects.
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