Sensory functions

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Picture formation

The eye constitutes a complicated light refracting optic system (cornea, aqueous humor, lens, and vitreous body) and fully adapts to the need to zoom the light beam on the retina. If a person notices any object, the light from it firstly comes to the cornea, where light refraction and light centralization first occur. Further on the light penetrates the aqueous humor and pupil to the lens and a second refraction occurs. The lens zooms the beams penetrating the vitreous body so that they merge delicately on the retina, where a small upside-down picture of the observed object is projected. It is recorded by photosensitive cells. With the help of chemical processes, light energy is converted into nerve impulses transmitted by the optic nerve to the appropriate brain region, where the real picture is formed.

Visual acuity

Visual acuity (lat. visus) is the eye’s ability to differentiate two points in space. The best visual sharpness is in the yellow spot, which decrease towards the periphery. It depends not only on the optic system’s abilities to zoom the light beam onto the retina but also on the intensity and color of the light, the contrast and transparency of the light refracting parts and the amount of photoreceptors in the appropriate retinal place. The feeling that a person sees sharply and accurately all the time is due to the fast and constant re-zooming from place to place without our noticing it. However, if a person focuses on a nearby object, he perceives the distant objects as blurred.

Accommodation

Accommodation means eye adjustment to near or distant objects, in other words changing the optic lens size. Thanks to this feature, sharp vision of objects at various distances is possible. Accommodation is done by lens flattening and arching with the help of the ciliary body muscles. In addition, lens elasticity is involved as well. As a lens ages, it loses its elasticity and consequently its ability to change shape while looking at near distances.

Visual field

The visual field is the part of space that the eye is able to capture during fixed gazing straight ahead in the direction from which the light beam comes. The numerical description of the visual field is the visual angle. It reaches, in the human eye, from side to side around 90° from the head axis, up to the nose 50° and up and down approximately 50°. The size of the visual angle depends mainly on light intensity and on the size and color of the picture that is being observed. The biggest angle is for white, then yellow, blue, red and finally green.

While looking with both eyes simultaneously, visual fields overlap in greater portion (binocular vision). There is only one blind spot inside the visual field, corresponding with the optic nerve entry.

Spatial perception

3D spatial perception enables a person to have binocular perception – each eye perceives and observes an object or scene from a different angle and by means of these different pictures spatial perception in the brain occurs. When looking at far distances using mainly just one eye (monocular perception), the spatial perception is influenced by other factors, such as contour overlap, light intensity, perspective etc. Eye movement contributes to spatial perception as well, e.g. when travelling on a train closer objects in the visual field move faster than distant ones.

Color sensitivity

Color sensitivity is the ability of the eye to differentiate colors or light of various wavelengths. The human eye can detect light beams within the electromagnetic spectrum that are of a wavelength of 400 – 760 nanometers. The ability to differentiate colors is possible thanks to light sensitive cells on the retina – cones containing substances sensitive to light. There are three types of cones in the human eye and each of them contains different pigment with maximal sensitivity to one of three colors – red, green, and blue. A person perceives colors the best at the yellow spot, while towards the periphery the ability to perceive colors decreases. However, in normal light a person is able to differentiate around 150 colors and more than 2000 color tones.

Eye adaptation

Adaptation is the ability of the eye to adapt automatically to various light intensities. We differentiate adaptation to light (switching from darkness to light) and adaptation to darkness (switching from light to darkness). Unlike accommodation, adaptation does not occur immediately but happens due to gradual changes in the pupil and by small processes in the retina. If the amount of eye adaptation is not relevant to the light intensity, dazzling occurs. Dazzling can be caused directly by excessive glare or it can be caused by reflection.

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