How we see an object, Color vision, Extraocular muscles, Vestibulo-ocular reflex, Smooth pursuit movement

How we see an object

The steps of how we see an object:

• The light rays enter the eye through the cornea (transparent front portion of eye to focus the light rays)
• Then, light rays move through the pupil, which is surrounded by Iris to keep out extra light
• Then, light rays move through the crystalline lens (Clear lens to further focus the light rays )
• Then, light rays move through the vitreous humor (clear jelly like substance)
• Then, light rays fall on the retina, which processes and converts incident light to neuron signals using special pigments in rod and cone cells.
• These neuron signals are transmitted through the optic nerve,
• Then, the neuron signals move through the visual pathway: Optic nerve → Optic Chiasm → Optic Tract → Optic Radiations → Cortex
• Then, the neuron signals reach the occipital (visual) cortex and its radiations for the brain's processing.
• The visual cortex interprets the signals as images and along with other parts of the brain, interpret the images to extract form, meaning, memory and context of the images.

Color vision

What is seen as color is essentially different combinations of certain ranges of wavelengths in the electromagnetic spectrum. In humans at least, there are three different kinds of cones for three ranges of wavelengths, roughly red, green and blue light. Each color of cone picks up the intensity of light in its range of wavelengths, and the combination is translated by the brain to a perceived color. Of course, some people lack the ability to see some or all of the colour spectrum: they are referred to as being 'color blind'.

Extraocular muscles

Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn. This is an almost pure rotation, with only about one millimeter of translation (Carpenter, 1988). Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.

Rapid eye movement

Rapid eye movement typically refers to the stage during sleep during which the most vivid dreams occur. During this stage, the eyes move rapidly. It is not in itself a unique form of eye movement.

Saccades

Saccades are quick, simultaneous movements of both eyes in the same direction controlled by the frontal lobe of the brain.

Microsaccades

Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output. Microsaccades move the eye no more than a total of 0.2° in adult humans.

Vestibulo-ocular reflex
Three-neuron arc, during a head movement to the right. 8th vestibulocochlear nerve, from the peripheral vestibular sensors to vn, the vestibular nuclei in the brainstem. VI abducens nucleus. The medial lateral fascicle (mlf) projects from the abducens nucleus to III, the oculomotor nucleus. The left lateral rectus muscle lr and the right medial rectus muscle mr get contracted, turning the eyes to the left. The green objects are excited, the orange ones inhibited.

The vestibulo-ocular reflex (VOR) is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa. Since slight head movements are present all the time, the VOR is very important for stabilizing vision: patients whose VOR is impaired cannot read, because they cannot stabilize the eyes during small head tremors. The VOR reflex does not depend on visual input and works even in total darkness or when the eyes are closed.

The "gain" of the VOR is defined as the change in the eye angle divided by the change in the head angle during the head turn. If the gain of the VOR is wrong (different than 1)—for example, if eye muscles are weak, or if a person puts on a new pair of eyeglasses—then head movements result in image motion on the retina, resulting in blurred vision. Under such conditions, motor learning adjusts the gain of the VOR to produce more accurate eye motion. This is what is referred to as VOR adaptation.

The main neural circuit for the VOR is fairly simple. Vestibular nuclei in the brainstem receive signals related to head movement from the Scarpa's ganglion located on CN VIII, or the vestibular nerve From this Vestibular nuclei excitatory fibers cross to the contralateral CN VI nerve nucleus. There they synapse with 2 additional pathways. One projects directly to the lateral rectus of eye. Another nerve tract projects from the CN VI nucleus to the oculomotor nuclei, which contain motorneurons that drive eye muscle activity, specifically activating the medial rectus muscles of the eye.

The cerebellum is essential for motor learning to correct the VOR in order to ensure accurate eye movements. Motor learning in the VOR is in many ways analogous to classical eyeblink conditioning, since the circuits are homologous and the molecular mechanisms are similar.

Smooth pursuit movement

The eyes can also follow a moving object around. This is less accurate than the vestibulo-ocular reflex as it requires the brain to process incoming visual information and supply feedback. Following an object moving at constant speed is relatively easy, though the eyes will often make saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.

While still, the eye can measure relative speed with high accuracy, however under movement relative speed is highly distorted. Take for example, when watching a plane while standing -- the plane has normal visual speed. However, if an observer watches the plane while moving in the same direction as the plane's movement, the plane will appear as if were standing still or moving very slowly.

When an observer views an object in motion moving away or towards himself, there is no eye movement occurring as in the examples above, however the ability to discern speed and speed difference is still present; although not as severe. The intensity of light (e.g. night vs. day) plays a major role in determining speed and speed difference. For example, no human can with reasonable accuracy, visually determine the speed of an approaching train in the evening as they could during the day. Similarly, while moving, the ability is further diminished unless there is another point of reference for determining speed; however the inaccuracy of speed or speed difference will always be present.

Optokinetic reflex

The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window in a moving train, the eyes can focus on a 'moving' tree for a short moment (through smooth pursuit), until the tree moves out of the field of vision. At this point, the optokinetic reflex kicks in, and moves the eye back to the point where it first saw the tree (through a saccade).


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