– English Lyrics

Lyrics & Translation

[English]

Vision is the perception of objects based  on the light that they emit or reflect.  

Human eyes can only detect visible light -  a narrow range of electromagnetic radiation,  

roughly from 400 to 750 nm in wavelengths. 

The eye consists of optical components, which  work like a camera, capturing and focusing images;  

and neural components that convert these images  into nerve impulses and send them to the brain. 

The main optical components are the cornea,  the lens, and the iris. The cornea and the  

lens refract light and focus the image on  the retina. The iris acts as an aperture,  

it controls the amount of light that enters  the eye by adjusting the size of the pupil. 

The neural components are the retina - a  light-sensitive tissue lining the inner  

surface of the eye, and the optic nerve. Light  is absorbed by photoreceptor cells in the retina.  

The optical information is then passed through  several cell layers, where it is converted into  

action potentials and sent, via the optic  nerve, to the visual cortex of the brain. 

The fovea is the central part of the retina  where the sharpest central vision is achievable. 

The optic disk, where the optic nerve  leaves the eye, has no photoreceptor cells.  

It corresponds to the blind spot in the  visual field. If an object falls on that spot,  

it would generate no visual information. However,  instead of leaving a black hole in the vision,  

the brain fills it in with visual  information from around the object. 

The major photoreceptor cells of the retina  are rods and cones. Rod cells are responsible  

for night vision. They can detect dim light,  but provide low-resolution images and cannot  

differentiate colors. Cones function in bright  day light. They detect colors, and provide high  

resolution details. There are 3 kinds of cones  named after the color that they absorb best:  

red, green and blue. A color is perceived based  on proportions of signals coming from these cones.  

Color blindness occurs when a person  lacks a certain kind of cones. 

The ability of photoreceptor cells to detect light  

is due to their light-receptor  molecules, called visual pigments.  

It’s rhodopsin in rods, and iodopsins in  cones. These molecules consist of 2 components:  

a protein called opsin, and a vitamin A-derivative  called retinal. The retinal component is  

identical for all visual pigments, but the opsin  is different for rods and each type of cones.  

Different opsins absorb different wavelengths,  allowing detection of different colors. 

In the dark, there is a so-called  dark current in photoreceptor cells.  

This is due to the presence of cGMP, which  permits a constant influx of sodium. The cells are  

depolarized, they release the neurotransmitter  glutamate at the synapse with bipolar cells. 

The retinal exists in 2 conformations:  cis and trans. In the dark,  

the cis-form is bound to opsin, keeping  it inactive. As the retinal absorbs light,  

it changes to trans-form and dissociates from the  opsin, which now becomes an active enzyme. The  

enzyme degrades cGMP, sodium channel closes, dark  current stops and so does glutamate secretion.  

The drop in glutamate tells the bipolar  cells that light has been absorbed.  

The information is then transmitted  to ganglion cells - the only cells  

within the retina that generate action  potentials and send them to the brain. 

On average, each ganglion cell receives  signals from over a hundred of rods. This  

degree of convergence is at the basis of the high  sensitivity of rod cells. A dim light produces  

only a weak signal in a rod, but together,  hundreds of these signals converge and become  

one strong signal acting on a single ganglion  cell. However, as the signal comes from a large  

area of the retina, the image resolution is poor. The cones have a much lower degree of convergence.  

The fovea in particular has only cones and  no rods, and each cone conveys signal to one  

ganglion cell. Because one ganglion cell receives  input from a very small area of the retina, this  

setup produces high resolution images. But high  resolution comes with low sensitivity, because  

each cone must be stimulated with a signal  strong enough to generate action potentials  

in the ganglion cell. This also explains  why there is no color vision in dim light. 

There exist other cell types that form  connections between photoreceptor cells,  

or bipolar cells. They detect changes in light  intensity within an image and provide additional  

information about contrast and edges of objects. Some of the ganglion cells also absorb light  

directly, but not for the purpose of forming  images. They transmit information about light  

intensity to the brainstem, as part of the reflex  that controls pupil size; and to the hypothalamus,  

as input for the sleep-wake cycle. The bipolar cells are first-order neurons,  

and ganglion cells are second-order neurons. The  axons of ganglion cells form the optic nerve.  

The 2 optic nerves from the 2  eyes converge at the optic chiasm.  

Here, the medial half of nerve fibers from  each eye cross to the other side of the brain.  

Most of the fibers then continue to the thalamus  and synapse with third-order neurons, whose axons  

project to the primary visual cortex. Some fibers  take a different route: they terminate in the  

midbrain and are responsible for pupillary light  reflex and accommodation reflex, among others. 

Note that objects in the left visual field  are perceived by the right side of the brain,  

which also controls motor responses of the  body’s left side – the same side as the objects.

Key Vocabulary

Vocabulary	Meanings
vision /ˈvɪʒən/ B1	noun - the ability to see; sight
light /laɪt/ A1	noun - the natural agent that stimulates sight and makes things visible noun - low-powered verb - to illuminate
eye /aɪ/ A1	noun - the organ of sight
detect /dɪˈtɛkt/ B1	verb - to discover or identify the presence or existence of
visible /ˈvɪzəbl̩/ B1	adjective - able to be seen
radiation /ˌreɪ.diˈeɪ.ʃən/ B2	noun - energy emitted in the form of waves or particles
wavelength /ˈweɪv.lɛŋkθ/ C1	noun - the distance between successive crests of a wave
cornea /ˈkɔːr.ni.ə/ C2	noun - the transparent layer forming the front of the eye
lens /lɛnz/ B2	noun - a transparent optical component that focuses light
iris /ˈaɪrɪs/ C1	noun - part of the eye controlling pupil size noun - a flower
pupil /ˈpjuːpl/ B1	noun - the opening in the eye allowing light entry noun - a student
retina /ˈrɛtənə/ C2	noun - the inner layer of the eye containing photoreceptor cells
optic /ˈɒptɪk/ C1	adjective - relating to sight or the eye
nerve /nɜːrv/ B1	noun - a bundle of fibers that transmits impulses
cell /sɛl/ A2	noun - the smallest structural and functional unit of an organism
rod /rɒd/ C1	noun - a type of photoreceptor cell for low-light vision
cone /koʊn/ C1	noun - a type of photoreceptor cell for color vision
brain /breɪn/ A2	noun - the organ of the central nervous system