how we see 2nd

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INTRODUCTIONThe ancient proverb the eyes are the window of the soul. Eye is the organ that

provides the sense of sight. According to Pedrotti (1999), eye forms continuum images of

objects from foot to infinity, scans scene such as the overhead sky, or focuses on detail asminute as the head of a pin. Without a well functioning eye, we will not be able to saw how

exactly the world looks like.

The process of how we see involve the component of light, structures of eye from iris to

retina, and from retina to primary visual cortex. The visual system contains pathways that

transfer messages to human brain. Every part of the visual process is important to make sure

we can see a complete, clear image. People who having problem with anyone of the parts, they

will need the assistance of certain optical instruments.

Nowadays, optical technology has developed. There are many tools were invented to

enhance the visual abilities. Besides, many tools were also created to help people who have

visual problem. However, the topic that will be discussed further here later is all about the

original abilities of human visual system.

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1. Light Enters the Eye and Reaches the RetinaAccording to Pinel (2000), light first enters the eye through the cornea, a transparent

tissue devoid of blood vessels but abounding in nerve cells. Upon entering the eye at the air-

cornea interface, light undergoes a significant degree of bending. Light moves from the corneainto the anterior chamber. Iris controls the amount of light that enters. Light passes pupil. Two

sets of delicate muscles change the pupil size in response to light stimulation. When the

illumination is high and pupils are constricted, the image falling on each retina is sharper and

depth of focus is greater. When illumination is low, the pupil will dilate to let in more light, the

eye will hard to focus on detail of the objects and depth of focus is low too.

After that, light falls on the crystalline lens. The lens provides fine tuning that required

in the image formation process. The lens will change its own shape appropriately. The lens

shapes are controlled by the ciliary muscles. When the muscles are relaxed, the lens assumes

its flattest shape, provide the least refraction of incident light rays. In this state, the relaxed eye

is focus on distant object. When muscles are tensed, the lens bulges, become more curved and

provide increase refraction of light. In this strained state, the eye is focus on nearby objects.

After its final refraction by the crystalline lens, light enters the posterior chamber filled

with the vitreous humor. After that, light rays reach their terminus at the inner layer of the eye

which is the retina. The retina is dotted with an overlapping pattern of rods and cones, which

will discussed more detail in chapter 2.1 later.

Figure 1 A section of the human eye.

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2. The Retina And Translation Of Light Into Neural SignalsRetina has five different layers of cells (see Figure 2), is receptors, horizontal cells,

bipolar cells, amacrine cells and retinal ganglion cells. The amacrine cells and horizontal cells

are specialized for lateral communication that means communication across the major channelsof sensory input. The amacrine cells and bipolar cells, release the inhibitory neurotransmitter

GABA and the receptors and bipolar cells release the excitatory neurotransmitter glutamate.

This inside out arrangement creates two visual problems. One is that the incoming

light is distorted by the retinal tissue through which it must pass before reaching the receptors.

The other is that for the bundle of retinal ganglion cell axons to leave the eye, there must be a

gap in the receptor layer. This gap is called the blind spot. The visual system uses information

provided by the receptors around the blind spot to fill in the gaps in our retinal images. The first

or these two problems is minimized by the fovea. The fovea is an indentation about 0.33

centimeters in diameter, at the center of the retina. The fovea it is the area of the retina that is

specialized for high acuity vision. The thinning of the retinal ganglion cell layer at the fovea

reduces the distortion of incoming light.

Figure 2The cellular structure of the mammalian retina.

2.1 Cone and rod visionTwo different types of receptors in the human retina is cone and. Normally, cones active only in

the day and rods active only at night. The cones and rods related to duplexity the theory of

vision. The theory explains that cones and rods mediate different kinds of vision. Cones or

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photopic vision predominates and provides high acuity color information in good lighting and

rod or scotopic vision, high sensitivity and allowing for low acuity vision in dim light, but lacks

detail and color information. The differences between photopic and scotopic in the way the two

systems are wired.

There is a large difference between the two systems in convergence. The output of

several hundred rods may ultimately converge on a single retinal ganglion cells, whereas it is

not uncommon for a retinal ganglion cell to receive input from only a few cones and the effect

of dim light simultaneously stimulating many rods can summate to influence the firing of a

retinal ganglion cell onto the output of the stimulated rods converges, whereas the effect of the

same dim light applied to a sheet of cones cannot summate to the same degree and retinal

ganglion cells may not respond to the light and then, the convergent scotopic system pays for

its high degree of sensitivity with a low level of acuity. When a retinal ganglion cell that receives

input from hundreds of rods changes its firing, the brain has no way of knowing which portion

of the rods contributed to the changes. Figure 3 representations of the convergence of cones

and rods on retinal ganglion cells. There is a low degree of convergence in cone fed pathway

and high degree of convergence in rod - fed pathway.

Figure 3 A schematic representation of the convergence of cones and rods on retina

ganglion cells.

Cones and rods differ in their distribution on the retina. Only cones are present in the

fovea. At the boundaries of the foveal indentations, the proportion of cones declines markedly

and there is an increase in the number of rods. Rods in nasal hemiretina (the half of each retina

next to the nose) are more than in temporal hemiretina (the half next to the temples).

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2.2 Spectral sensitivity curveSpectral sensitivity curve is a graph of lights relative brightness of the same intensity

presented at different wavelengths. Human and other animals with both cones and rods have

two spectral sensitivity curves. They are photopic spectral sensitivity curve and a scotopicspectral sensitivity curve. The photopic spectral sensitivity of humans can be determined by

having subjects judge the relative brightness of different wavelengths of light shone on the

fovea. Scotopic spectral sensitivity an relative brightness of different wavelengths of light.

2.3 Eye movementOur eye will continually scans the visual field by making a series of brief fixation.

Saccades is a kind of very quick eye movement that connect the three fixation that occur every

second. The visual system integrates the foveal images from the preceding few fixations to

produce a wide angled, high acuaity, and richly colored perception.

After a few seconds of viewing, a simple stabilized retinal image disappears, leaving a

featureless gray field. The movements of the eyes then increases, presumably in an attempt to

bring the image back. Such movements are futile in this situation because the stabilized retinal

image simply moves with the eyes. In the few second, the stimulus pattern, or part of it

spontaneously reappears, only to disappear once again. This case happens because the

neurons of the visual system respond to change rather than to steady input. It respond only

weakly to a continuous, unchanging stimulus.

One function of eye movements is to keep the retinal image moving back and forth

across the receptors, thus ensuring that the receptors and the neurons to which they are

connected receive a continually changing pattern of stimulation. When a retinal image is

stabilized, parts of the visual system stop responding to the image and it disappears.

2.4 Visual transduction: The conversion of light to neural signalVisual transduction is the conversion of light to neural signals by the visual receptors.

Rhodopsin is a red pigment (a pigment is any substance that absorbs light) that extracted from

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the predominantly rod retina of the frog. This pigment had a curious property. The pigment

exposed to continuous intense light, it was bleached (lost its color) and lost its ability to absorb

light, but when it was returned to the dark, it regained both its redness and light absorbing

capacity.

Rhodopsin is a G protein linked receptor that responds to light rather than to

neurotransmitter molecules. Rhodopsin receptors initiate a cascade of intracellular chemical

events when they are activated. When rods are in darkness, an intracellular chemical called

cyclic GMP (guanosine monophosphate) keeps sodium channels partially open, thus keeping the

rods slightly depolarized and steady flow of excitatory glutamate neurotransmitter molecules

emanating from them. When rhodopsin receptors are bleached by light, the resulting cascade of

intracellular chemical events deactivates the cyclic GMP and in so doing, it closes the sodium

channels and reduces the release of glutamate. The transduction of light by rods makes an

important point: signals are often transmitted through neural systems by inhibition.

3. From Retina to Primary Visual CortexRetina-geniculate-striate pathway is a pathway in the brain that carries visual information.

This pathway conducts signals from the retina to the primary visual cortex via the lateral

geniculate nuclei of the thalamus. According to Valberg (2005), the primary visual cortex is also

known as Area 17, striate cortex or V1.

The geniculate body is about the size of a large pea and has six main layers of cells.

Each layer receives input from all parts of the contralateral visual field of one eye, there are no

binocular cells in the lateral geniculate nuclei.

Another characteristic of retina-geniculate-striate system is retinotopic. When two stimuli

are presented to adjacent areas of the retina, the adjacent neurons at all levels of the system

will be excited. There is a disproportionate representation of the fovea in the retinotopic layout

of primary visual cortex. Although the fovea is only small part of the retina, but it dedicated

about 25% to the analyse of primary visual cortexs input.

There are two pathways in the retina-geniculate-striate system. The P pathway runs

through the top four layers of each lateral geniculate-nucleus. They contain parvocellular cells6


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that are composed of neurons with small cell bodies. The P pathway receives inputs from the

midget ganglion cells of the retina and terminates in lower layer IV. Most of them are

particularly responsive to color, to fine pattern details, and to stationary or slowly moving

objects. Cones provide majority input to the P pathway.

The second pathway call M pathway. It runs through the bottom two layers of lateral

geniculate-nucleus that are known as magnocellular layers. The bottom two layers are

composed of neurons with large cell bodies. This pathways neurons are particularly responsive

to movement. The rods of the eye provide the majority of the input to the M pathways. The

magnocellular neurons terminate above the parvocellular neurons in lower layer IV.

Figure 4The retina-geniculate-striate system.

4. Seeing EdgesIn a sense, a visual edge is nothing; it is merely the place where two different areas

of a visual image meet. The perception of an edge is the perception of a contrast between

two adjacent areas of the visual field. Adjacent to each edge, the brighter stripe looks

brighter than it really is and the darker stripe looks darker than it really is. The non-existent

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stripes of brightness and darkness running adjacent to the edges are sometimes called

Mach Band; they enhance the contrast at each edge and make the edge easier to see. Our

perception of edge is better than the real thing.

4.1 Lateral inhibition and contrast enhancementLateral inhibition is the dominant feature of distributed sensory networks where each

individual receptor drives down each of its neighbors in proportion to its own excitation. The

strengths of these connections are fixed and are generally arranged as excitatory among

nearby receptors and inhibitory among farther receptors. All receptors in the network

receive a mixture of excitatory and inhibitory signals from other competitive receptors. As a

result, the competitive network structure a distinction between the receptor or a group of

receptors which have the strongest output and the receptors with weaker output become

larger. Weaker receptors might be suppressed. The response of the whole network can

vary. Machs study concluded that the brighter and darker contours are physiologically

provoked. There is a brightness enhancement at the region where the bright area becomes

darker and there is a darker band where the dark area becomes brighter (see Figure 5).

Mach Bands

Figure 5 Illustration of Mach Band.

4.2 Receptive fields of visual neuronsThe receptive fields of visual neurons are the area of the visual field within which it

is possible for a visual stimulus to influence the firing of that neuron. Visual system neurons

tend to be continually active, thus effective stimuli are those that either increase or

decrease the rate of firing. The final step in the method is to record the response of the

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neuron to various stimuli within its receptor field in order to characterize the types of stimuli

that most influence its activity. Then, the electrode is advanced slightly, and the entire

process of identifying and characterizing the receptive field properties is repeated for

another neuron, and then for another, and another, and so on.

4.3 Receptive fields : neurons of the retina-geniculate-striate pathwayHubel and Wiesel study the visual system neurons by recording from the three levels

of the retina-geniculate-striate pathway: first from retinal ganglion cells, then from lateral

geniculate neurons and finally from the striate neurons of lower layer IV, the terminus of

the way.

When a spot of white light was shone onto the various parts of the receptive fields

of neurons in the retina-geniculate-striate pathway, there were two different responses. The

neurons either displayed a burst of firing when the light was turned on (on firing) o it

displayed an inhibition of firing when the light was turned on and a burst of firing when it

was turned off (off firing). It depended on whether they were on-centre cells or off-centre

cells, as illustrated in Figure 6.

On-centre cells respond to lights shone in the central region of their receptive fields

with on firing and to lights shone in the periphery of their receptive fields with inhibition,

followed by off firing when the light is turned off. Off centre cells display the opposite

pattern, they respond with inhibition, followed by off firing in response to lights in the

centre of their receptive fields and with on firing to lights in the periphery of their

receptive fields. In effect, on-centre and off-centre cells respond best to contrast.

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Figure 6 The receptive fields of an on-centre cell and an off-centre cell.

5. Seeing ColorColor is one of the most obvious qualities of human visual experience. Color can divide

into two categories such as achromatic colors and chromatic colors. Each category has three

types of colors. Achromatic colors have black, white and gray. Meanwhile chromatic colors have

blue, green and yellow. Our eye can determines the color we perceive depends on the

wavelength of light that it reflects into eye. Sunlight is a most sources of artificial light contain

complex mixtures of most visible wavelength of light that strike them to varying degrees and

reflect the rest. When the wavelength of objects is mixed, it can influence our perception of

their color.

5.1 Component and opponent processingColor visions have two theories that we must know. First is component theory and

second is opponent process theory. The component theory or also known as trichromatic theory

is color vision was proposed by Thomas Young in year 1802 and refined by Hermann Von

Helmholtz in year 1852. In these theories, there are three different kinds of color receptor

(cones). Each receptor has different spectral sensitivity. These theories derived from the

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observation that any color of the visible spectrum can be matched by a mixing together or three

different wavelengths.

The second theory of color vision is opponent process theory and these theories were

proposed by Ewald Hering in year 1878. Hering suggested that there are two different classes

of cells in visual system for encoding color and another one for encoding brightness. Hering

hypothesized that each of three classes of cells encoded two complementary color perceptions.

Complementary colors are pair of color that produces white or gray when combined in equal

measure. Color vision has several behavioral observations based on Hering opponent process

theory.

Both of the theories have a negative perception because it was fueled more by the

adversarial predisposition of scientists than by the incompatibility of the two theories.Meanwhile, many research subsequently proved that both color coding mechanisms coexist in

the same visual systems.

6. Cortical Mechanism of VisionDamage to an area of the primary visual cortex (refer figure 7) produces a scotoma

(local areas of blindness). Scotoma is an area of blindness in the corresponding area of

contralateral visual field of both eyes. A man who have scotoma might notice a black spot at

the corner of the eye which impedes peripheral vision. It is important to seek care from an

ophthalmologist if a scotoma appears, because it can indicate a serious problem. Treatment can

be used to prevent the spot from growing larger, and to address the underlying issue which led

to the development of the scotoma.

Figure 7 The visual areas of the human cerebral cortex.

6.1 Scotoma: Completion11


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Many patients with extensive scotomas are unaware of their deficits. One of the factors

that contribute to this lack of awareness is the phenomenon of completion. Completion is the

visual systems automatic use of information obtained from receptors around a scotoma to

create a perception of the missing portion of the retinal image. A patient with scotoma who

looks at a complex figure, parts of which lies in the scotoma often reports seeing a complete

image. In some cases this completion may depend on residual visual capacities in the scotoma,

however completion also occurs in cases in which this explanation can be ruled out.

6.2 Scotoma: BlindsightBlindsight is another phenomenon displayed by patients with scotoma resulting from

damage to primary visual cortex. Blindsight is the ability of such patients to respond to visual

stimuli in their scotomas even though they have no conscious awareness of the stimuli. The

existence of blindsight has an important theoretical implication and it suggest that not all visual

information is funneled into cortical circuit through the primary visual cortex. If all visual signals

were funneled through the primary cortex, damage to a portion of this retinotopically laid out

structure should produce total blindness in the associated area of the visual field. The existence

of the blindsight suggests that some information is being conducted via the parallel pathways

directly into secondary visual cortex. So, one such parallel pathway goes from the superior

coliculus to the pulvinar nucleus of the thalamus to the prestriate cortex.

6.3 Functional areas of secondary and association visual cortexSecondary visual cortex and the portions of association cortex that are involved in visual

analysis are both composed of different areas, each specialized for a particular type of visual

analysis. The neurons in each functional area respond most vigorously to different aspects of

visual stimuli for example is to their color, movement or shape. Selective lesions to the different

areas produce different visual losses and there are often subtle anatomical differences among

the areas.

6.4 Dorsal and ventral streams

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Many pathways that conduct information from the primary visual cortex through various

specialized areas of secondary and association cortex are parts of two major streams that is the

dorsal stream and the ventral stream. The dorsal stream flows from the primary visual cortex to

the infer temporal cortex. The dorsal stream is involved in the perception of where objects are

and the ventral stream is involved in the perception of what objects are. The major

implication of the where versus what theory and other parallel processing theories of vision

is that damage to some areas of cortex may abolish certain aspect of vision while leaving others

unaffected. Indeed the most convincing support for the influencing where versus what

theory has come from the comparison of the specific effects of damage to the dorsal and

ventral stream. The function of the dorsal stream is to direct behavioral interaction with object,

whereas the function of the ventral stream is to mediate the conscious perception of objects

that is the control of behavior versus conscious perception theory. Then, the control of

behavior versus conscious perception theory can readily explain the two major

neuropsychological finding that are the foundation of the where versus what theory.

6.5 ProsopagnosiaProsopagnosia is visual agnosia for face. Agnosia is a failure of recognation that is not

attributable to a sensory deficit or to verbal or intellectual impairment, whereas visual agnosia is

a specific agnosia for visual stimuli. Visual agnosics can see visual stimuli but they dont what

they are. Visual agnosia themselves are often specific to a particular aspect of visual input and

are named accordingly for example is movement agnosia, object agnosia, and color agnosia are

difficulties in recognizing movement, objects and color respectively. It is presumed that each

specific visual agnosia result from damage to an area of secondary visual cortex that mediate

the recognition of that particular attribute.

Prosopagnosics are visual agnosia with a particular difficulty in telling one face from

another. In extreme cases, prosopagnosics cannot recognize themselves. Prosopagnosia was

initially assumed to result from bilateral damage to a particular area of cortex dedicated to therecognition of face.

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CONCLUSIONThe course of Psychology Physiology aimed to study the functions of our body.

Therefore, the visual system is one of the must to study in this course.

This assignment let us understand that we can see objects around us not just because

of the eye. We appreciate the opportunity to learn the connection between internal function of

eye and brain. We realize the complexity in the process of how we see an object. We also

found that other than blind and color blind, there are many other visual problems that we never

heard before.

The information we receive along the process of complete this assignment let us

become more aware of the amazing abilities of our visual system. We can explain to our friends

more detail in this area during the presentation. What we gain from this study also can increase

our strength when we study on our schools subjects and also working on our profession in the

future.

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REFERENCES1. Goldstein, E. B. (2002). Sensation and Perception. US: Wodsworth, Thomson.

2. Pinel, J. P. J. (2000). Biopsychology. Massachusetts: Ally & Bacon.

3. Pedrotti, L. S. (1999). Optics and Vision. New Jersey: Prentice-Hall, Inc.

4. Santrock, J. W. (2000). Psychology. USA: Mcgraw-Hill.

5. Valberg, A. (2005). Light Vision Color. England: John Wiley & Sons Ltd.

6. Zimbardo, P. G., Johnson, R. L., & Hamilton, V. M. (2003). Psychology: Core Concepts

(7thEdition). USA: Allyn & Bacon.

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how we see 2nd

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