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2.5.5 Perception and Sight

2.5.5.1 Overview

A detailed outline of Perception is explained here.

2.5.5.2 Details

2.5.5.2.1 Visual Perception in the Cerebral Cortex

Sensory perception would be a fundamental function to actualize high intelligence for the human brain. Most sensory perceptions such as sight and hearing process signal information in the cerebral cortex.
Since sight is the most important and intricate perception, mechanism of sight would be a sufficient example to understand the fundamental mechanism of sensory perceptions.

2.5.5.2.2 Basic Mechanism of Sight

2.5.5.2.2.1 Eyes

A diagram of a human eye and a top view of the visual system are shown below.
Light from objects enters the eyes. Since the eyes have crystalline lenses, images made of light are formed on the retinas turned 180 degrees. Light sensitive retinal neurons (photoreceptor cells) are arranged on the retinas. When part of light of an image hits a light sensitive retinal neuron (photoreceptor cell), the retinal neuron causes electric signals, while black part of images including no light wouldn't cause retinal neurons' electric signals. Since each light sensitive retinal neuron (photoreceptor cell) is independent, the retinal images are digital images represented by dots.


Diagram of Human Eye
*Attribution: https://en.wikibooks.org/wiki/File:Schematic_diagram_of_the_human_eye_en.svg


*Attribution: https://en.wikipedia.org/wiki/File:Neural_pathway_diagram.svg

* "Photoreceptor Cell in Wikipedia" https://en.wikipedia.org/wiki/Photoreceptor_cell
* "Visual System in Wikibooks" https://en.wikibooks.org/wiki/Sensory_Systems/Visual_System


2.5.5.2.2.2 Neurons Responding to Orientaion in V1 Area

Signals from both eyes are transmitted through LGN (Lateral Geniculate Nucleus) first to part of cerebral cortex called visual area 1 (V1) on the rear end surface of the Cerebrum. Since the Cerebrum consists of left and right hemispheres, visual area 1 (V1) is both on the left rear end surface and the right rear end surface. Objects placed on the left form images on the right part of the retinas (viewed from behind) of both eyes. In contrast, objects placed on the right form images on the left part of the retinas (viewed from behind) of both eyes.
Retinal neurons on the leftward part (viewed from behind) of both eyes are linked to V1 on the surface of the left cerebral hemisphere, signals caused in leftward retinal neurons of both eyes are transmitted to left V1 reflecting the digital image that the retinal neurons received. In contrast, retinal neurons on the rightward part (viewed from behind) of both eyes are linked to V1 on the surface of the right cerebral hemisphere, signals caused in rightward retinal neurons of both eyes are transmitted to right V1 reflecting the digital image that the retinal neurons received.


*Attribution: https://en.wikibooks.org/wiki/File:Wiki_brain_areas.png

Sites of both left and right V1 are similarly classified into 2 types. One is slender areas responsible for processing signals from the left eye. The other is slender areas responsible for processing signals from the right eye. Examplifying both slender areas of both V1 using yellow for the left eye and blue for the right eye, the back view would be schematically like below. Each slender area is called Ocular Dominance Column. The hight of the column (pillar) corresponds to the thickness of Cerebral Cortex.

On the other hand, most of neurons in V1 have an ability to respond to specific orientation of signals that formed a dotted line on a retina.
( i ) shows a back view of light-sensitive neurons on a retina. In ( ii ), light from an object comes, part of the light hits "b", "d", and "f". In (iii), "b", "d", and "f" caused signals. ( iv ) shows a postsynaptic neuron that exclusively respond to the association of "b", "d", and "f". Since the synapses from a, c, e, and g are Inhibitory Synapses, the postsynaptic neuron wouldn't cause a signal when either a, c, e, or g received part of light. Then the postsynaptic neuron responds only when "b", "d", and "f" exclusively received part of light, which formed a dotted line that inclined at an angle of 60°. In conjunction with that, the postsynaptic neuron's signal means a dotted line inclined at an angle of 60°.


Neurons would be classified according to the orientation to which they respond. In the figure below, neurons are tentatively color-coded. "-60°" is tentatively colored in orange.

As far as the distribution, similar neurons form stripes on V1 as follows.

Each area of color is called Orientation Column. The hight of the column (pillar) corresponds to the thickness of cerebral cortex.

2.5.5.2.2.3 After V1 Area to TE Area

The processes subsequent to the orientation process in V1 are shown below as Deep Hierarchies.

As shown below, after V1, signals go through V2 and V4, reach TEO area, where neurons respond to simple shapes.
After TEO, neurons respond to the whole shape in TE area. Then the whole shape is recognized by neurons in TE. This pathway is called Ventral Pathway.
On the other hand, some signals go from V2 to Dorsal Pathway through V3 area. This pathway recognizes additional pieces of information, to be precise.

*Attribution: https://en.wikibooks.org/wiki/File:Wiki_hierarchies.png









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