The Brain Vision

The conjunctiva is a thin, transparent membrane that covers the front of the eye and the inside of the eyelid; it lubricates and protects the eye.

The cornea is the transparent, slightly convex covering of the front of the eye; it protects the eye and refracts light.

The cornea directly covers the iris, pupil, and aqueous humour.

The aqueous humor is a gelatinous, transparent fluid that fills the area between the lens and the cornea; it provides nutrients, maintains optimal eye pressure, and refracts light.

The pupil is a small hole located in the center of the iris; it allows light to enter the eye.

The pupil appears black because entering light rays are absorbed by tissues in the eye.

The iris is a thin, circular muscle that surrounds the pupil; it controls the diameter of the pupil and thus, how much light enters the eye.

The iris is the structure that gives the eye its characteristic color.

In the dark.

In the dark, pupil diameter will increase to let in more light.

The lens is a transparent structure inside the eye; it changes shape to effectively refract light as it enters the eye, properly focusing the light on the center of the retina.

The vitreous humor is the gelatinous, transparent mass filling the area between the lens and the retina; it effectively keeps the retina in place.

The vitreous humor is not naturally replenished; therefore, foreign material must be surgically removed.

The retina is a thin, light-sensitive tissue that lines the inside of the eye; it converts light energy to nerve impulses that are sent to the brain to be processed.

The sclera is the white, fibrous covering of the eye that is continuous with the cornea; it forms the hard outer protective wall of the eye.

The choroid is a highly vascularized layer of the eye that sits between the sclera and the retina; it carries nutrients to the iris and retina. With the help of melanin, it also helps prevent reflection of light that could damage the eye.

Refraction is the process by which the rays of light entering the eye bend in order to strike the proper place on the retina.

The following characteristics of the eye promote the refraction of light as it enters the eye:

1. The consistency of the aqeous humor and vitreous humor

2. The curvature of the cornea

3. The shape of the lens

The ciliary muscles are a ring of smooth muscle around the inside of the eye; they control the shape of the lens within the eye.

The suspensory ligaments attach the lens of the eye to the ciliary muscles; they force the lens to change shape as the ciliary muscles move.

When the ciliary muscles are relaxed, there is less refraction of light.

As the ciliary muscles relax, the tension on the suspensory ligaments increases, causing the lens to flatten out and decrease the degree of refraction.

When the ciliary muscles are relaxed, there is greater refraction of light.

As the ciliary muscles contract, the tension on the suspensory ligaments decreases, causing the lens to form a more natural convex shape, which increases the degree of refraction.

If light rays converge at a point past the retina, the person will experience far-sightedness, or hypermetropia, meaning they have difficulty seeing objects close up.

Far-sightedness occurs when the eye is too short from front to back, and can be corrected with eyeglasses that have convex lenses.

If light rays converge at a point in front of the retina, the person will experience near-sightedness, or myopia, meaning they have difficulty seeing objects at a distance.

Near-sightedness occurs when the eye is too long from front to back, and can be corrected with eyeglasses that have concave lenses.

Bifocal lenses are prescribed to people with presbyopia, or the hardening of the lens.

Presbyopia causes an inability of the lens to form the shapes needed for adequate vision at all distances.

Bifocal lenses are lenses in which the top half is concave, for distance vision, and the bottom half is convex, for close-up vision.

The visual field of the eye is the spatial area imprinted on the retina when the eye is focused on a distant point.

Centrally, the two eyes have overlapping visual fields, while the right eye’s visual field extends farther to the right, and the left eye to the left.

The optic nerve transmits visual information from each eye to the various brain structures that analyze these visual signals.

The optic chiasm is the point at which the optic nerves from both eyes intersect and allow some axons to cross over to the other side of the brain.

The optic chiasm is located beneath the hypothalamus, in close proximity to the pituitary gland.

Five relay structures between the retina and the primary visual cortex are:

1. The LGN of the thalamus

2. The suprachiasmatic nucleus of the hypothalamus

3. The pretectum of the midbrain

4. The superior colliculus of the midbrain

5. The pulvinar nucleus of the thalamus

The majority of the visual nerve fibers project to the lateral geniculate nucleus (LGN) of the thalamus.

the primary visual cortex of the brain

the optic radiation

Visual signals sent to the suprachiasmatic nucleus of the hypothalamus deal with the body's internal clock. The images it receives keep the body informed about the darkness or lightness of the environment.

This allows the body to coordinate the body's sleep-wake cycle based on daylight and night.

The visual signals sent to the pretectum of the midbrain deal with certain eye movements and the opening of the pupil.

The superior colliculus of the midbrain deals with visual signals involved in moving the eye; it allows a stimulus that initially appears in the peripheral vision to be clearly viewed.

Visual signals sent to the pulvinar nucleus of the thalamus are involved in the interpretation of images, visual attention and motion perception.

The primary visual cortex is located in the most posterior portion of the occipital lobe of the brain.

It functions as the brain's first interpreter of basic visual signal information, such as location, color, and shape.

The secondary visual cortex is located on the posterior part of the occipital lobe; it surrounds the primary visual cortex.

The secondary visual cortex functions to interpret more complex visual images and make associations with other sensory modalities to create a better picture of the world.

1. The ventral pathway

2. The dorsal pathway

The ventral visual pathway allows for the perception, recognition, and identification of an object based on the object's intrinsic characteristics, such as color and shape.

The dorsal visual pathway allows for the integration of visual-motor coordination by interpreting the extrinsic characteristics of an object, such as the object's size, position, and orientation in space.