Theory: The human visual system

The human visual system

In this section you will see how many processes are involved when we open our eyes and see something.

The whole process starts when we open our eyes:

How the eye an the brain work together
1. Light rays [reflecting from a twig shaped like a snake] enter the eyes by passing through the cornea, the aqueous, the pupil, the lens, the vitreous, and then striking the light sensitive nerve cells (rods and cones) in the retina.


2. Visual processing begins in the retina. Light energy produces chemical changes in the retina's light sensitive cells. These cells, in turn, produce electrical activity.

3. Nerve fibers from these cells join at the back of the eye to form the optic nerve.

4. The optic nerve of each eye meets the other at the optic chiasm. Medial nerves of each optic nerve cross, but lateral nerves stay on the same side. The overlap of nerve fibers allows for depth perception.

5. Electrical impulses are communicated to the visual cortex of the brain by way of the optic nerve.

6. The visual cortex makes sense of the electrical impulses, and either files the information for future reference or sends a message to a motor area for action. The visual cortex is divided into different sections as labeled below.

Visual Areals
Visual Areals

7. The neurons in the occipital region of the visual cortex start to recruit other neurons, starting the 'making meaning' process.

8. The process of recruitment moves from the rear of the brain towards the front (there is nowhere else for it to go) and down to the midbrain.

limbic system

9. In the limbic system (mid-brain) the neuronal activity takes two paths: It goes into the amygdala, which produces a response on an emotional (that is fear or not fear) level, and it goes into the hippocampus (which manages memory interpretation, storage and retrieval from the cerebral cortex).
When the neuronal activity reaches the amygdala, the image itself is only half interpreted, but that 'broad' interpretation (could be a twig / could be a snake) is enough for the amygdala to work on and react if it matches one of its (probably also broad) patterns that prompt the 'fear' reaction.

10. The amygdala sends two signals. One goes from the limbic system to the muscles, prompting the bodily reactions that we mentioned earlier, and the other goes back to the occipital region , prompting it to pay more attention to this potentially dangerous stimulus.

11. Recruitment of neurons continues towards the frontal lobes (the area of the brain where rational thought takes place). The signals direct from the optic nerve and those triggered by the amygdala combine, and the frontal lobes complete the process of identification (conceptualization and categorization) and interpretation.

12. The frontal lobe sends signals back to the amygdala which prompt it either to signal the body's alarm systems to relax, or to continue the fear response until the danger is past.

We can call this last step 'post-rationalization' in its truest sense: we rationalize our emotional reaction, and this in turn modifies the emotional reaction.

Emotions are emotionally 'Tagged'

One big danger in offering this kind of tidy step-by-step analysis is that you might assume that there are really two separate memories that operate within the brain: one memory that operates on the amygdala and another that operates on the hippocampus. This is not the case. The emotional reaction and the rational interpretation, the 'contextual memory', are not only formed at the same time, they are inextracably part of the same system. And the hippocampus does not only draw on the rational information the neuronal Gestalt provides, it also draws on the emotional information that is part of the Gestalt.

Aqueous - a clear watery fluid that fills the space between the cornea and the vitreous. It is responsible for nourishing the cornea, iris, lens, and maintaining the intraocular pressure.

Amygdala, (from Greek 'almond') is a almond-shaped groups of neurons located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.
Cones - light sensitive receptor cells mostly found in the central part of the retina. It is responsible for sharp visual acuity and the discrimination of color.
Conjunctiva - a transparent mucous membrane that covers the outer surface of the eyeball except for the cornea. It also lines the inner surface of the eyelid.
Cornea - the clear cone over the front part of the eye. It is responsible for most of the optical power of the eye.
Choroid - a layer of the eye found between the retina and the sclera. It contains major blood vessels and provides nourishment to the outer layers of the retina.
Fovea - is located in the center of the macula. It provides the sharpest vision.
Hippocampus - is a part of the brain located inside the temporal lobe (humans and other mammals have two hippocampi, one in each side of the brain). It forms a part of the limbic system and plays a part in memory and spatial navigation. The name derives from its curved shape in coronal sections of the brain, which resembles a seahorse (Greek: hippos = horse, kampi = curve).
Iris - a colored circular muscle that gives us the color of our eyes. It is responsible for controlling the amount of light that gets into the eye.
Lens - is responsible for bringing rays of light into focus on the retina. It is a clear oval structure suspended behind the iris which can contract and expand as needed to provide additional focusing power.
Macula - a small central area of the retina responsible for fine central visual acuity.
Optic Nerve - carries the light impulses for sight from the retina to the brain.
Retina - the most active area of the eye where the rods and cones are found. These receptors pick up the bits and pieces of the visual signals and transport them to the optic nerve for transmission to the brain.
Rods - light sensitive receptor cells mostly found in the peripheral part of the retina. They are responsible for night vision.
Sclera - the protective outer layer of the eye.
Vitreous - a clear jello-like substance between the lens and the retina that provides the structural support to the eye.

Last modified: Thursday, 1 February 2007, 11:44 PM