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Introduction to Psychology PSY101

Lesson 15

SENSATION

Traditionally, the five special senses have been defined as taste, smell, sight, hearing and feeling. However,

touch is now considered to reflect the activity of the general senses, and equilibrium, or balance, can be

thought of as a new fifth special sense. In contrast to the general sensory receptors, most of which are

modified dendrites of sensory neurons, the special sensory receptors are distinct receptor cells. They are

either localized within complex sensory organs such as the eyes and ears, or within epithelial structures such

as the taste buds and olfactory epithelium.

The principle function of the special sensory receptors is to detect environmental stimuli and transfer their

energy into electrical impulses. These are then conveyed along sensory neurons to the central nervous

system, where they are integrated and processed, and a response is produced.

There is no firm agreement amongst neurologists as to exactly how many senses there are. The

disagreements stems from a lack of consensus as to what the definition of a sense should be. Although it is

still routinely taught that there are five senses (sight, hearing, touch, smell, taste; a classification first devised

by Aristotle, it is generally agreed that there are at least nine different senses in humans, and a minimum of

two more observed in other organisms.

A broadly acceptable definition of a sense would be "a system that consists of a sensory cell type (or

group of cell types) that respond to a specific kind of physical energy, and that correspond to a

defined region (or group of regions) within the brain where the signals are received and

interpreted". Where disputes arise is with regard to the exact classification of the various cell types and

their mapping to regions of the brain. It can also be defined as:

· Sensation is a process that makes possible, and facilitates our contact with reality

· `To sense' means to become aware of something

· All living organisms have sense organs. Sensation is the process by which our sense organs respond

to different stimuli

· It is the mechanism through which stimuli from outside or inside the body are received and felt by

different faculties e.g., hearing, sight, smell, touch, taste, and equilibrium.

In the process of sensation the incoming (afferent) nerve impulse is registered in that part of the brain,

which has a potential of such reception. The awareness of a stimulus results from the perception of the

sensory receptors

The five senses

·Vision

·Hearing/ Auditory sense or Audition; also associated with maintenance of bodily balance

·Smell/ Olfaction

·Taste

·The skin sensations/ Kinesthetic sense; touch, pressure, temperature, and pain

Vision/The visual Sensation

·The sensation that takes place through the function of eyes; eyes receive the visual messages or

stimulation, that is carried by nerves to the concerned part of the brain that processes the received

information.

The Human Eye

· The eye is a very complex, delicate, and vital structure, that is responsible for an organism's

interaction with the external world. It is the most important and influential sense organ.

· It receives information from the outside world in the form of light, and sends loads of information

to the brain all the time

· The human eye is a little less than one inch in diameter and almost spherical

· The eye has a very specific design or form, which captures and processes light coming from

outside...light reflected by the stimuli

Introduction to Psychology PSY101

Eyes function like a camera, which has its aperture, and a lens through which the light enters and

cells present in it process the received light just as do the intricate internal parts of the camera.

Anatomy/ Structure of the Eye

The anatomy of the eye is broadly divided into three parts along with its chambers

i. The external structure of the eye

ii. The immediate structure of the eye

iii. The internal structure of the eye

All structures are important in terms of their processing and functioning

The External Structure

The entire external structure of the eye is a "light-tight" box

Cornea

· A transparent

external surface, a

five-layered

membrane that

covers both the

pupil and the iris.

· It is the first and

most powerful

lens or layer of the

visual apparatus

that helps to form

the sharp image

on the retinal

photoreceptor

cells, along with

the crystalline lens

Sclera

· Outer walls of the eye are formed by a hard, white

substance called `sclera', hence sclerotic coat that covers

5/6th of the surface of the eye

· The outside of the eye is light-tight and its mechanism is

designed in such a manner that only small amount of

light can enter into a small opening that enables the

production of a clearer vision, because a smaller opening

also acting as the `aperture', creates a sharper image

The Immediate Part

Pupil

· A dark, adjustable opening in the center of the eye through which the light enters. It changes its size as

the amount of light entering the eye varies

· It looks dark and black in appearance, because of the absorbing pigments in the retina

Iris

Around the pupil of the eye, there is a ring of muscle tissue that controls the size of the pupil

opening, through its contraction and expansion

It contains the color pigments and thus gives color to the eye__ the color which the eyes possess

such as brown, black, green, blue etc are due to the iris muscles

Lens

The transparent part of the eye that is located behind the pupil that changes it shape in order to

focus images on the retina

Introduction to Psychology PSY101

The lens changes its own thickness in order to focus image properly on retina__ this ability of the

lens is called " accommodation"

The process of accommodation depends largely on the location of the object with respect to the

observer's body __ distant objects require a relatively flat lens and the muscles that are controlling it

are relaxing as compared to when focusing the nearer objects, when muscles contract, taking

tensions off the lens thus making the lens more round shaped

Fluids in the Chambers of the Eye

· Eye has three important layers or chambers:

Anterior layer that lies between the cornea and iris

ii.

Posterior layer that lies between iris and lens

Vitreous layer that lies between the lens and the retina

iii.

Anterior and posterior chambers are filled with aqueous humor

Whereas the vitreous chamber is filled with a more viscous fluid, the vitreous humor

The eye is filled with these two liquids that are important because they help maintain the

shape of the eye and provide nourishment to the cells present in the eye

The function of these fluids is the same as that of blood in other parts of the body; the

difference being that these liquids are nearly transparent, so that they can nourish the cells

of the eye without interfering with the light that enters in the eye

Choroid

It is the middle layer of the eye, and the lining of the 3/5th of the eyeball

· The choroid serves two important functions; i.e. nourishment and absorption

· The choroid carries blood to the retina and the humors in order to provide nourishment to the eye

· The other important function is that choroid absorbs any light that strikes on it__ this is extremely

important, because light that passes through the rods and cones does not reflect back. But, in any

case, if it (light) reflects back, the photo-sensitive cells would receive the light message twice, and

would percept wrong, that there was twice as much light as there really was.

Internal Structure of the Eye

· Includes the most important structure; "the retina".

· Retina is the light- sensitive inner surface or chamber of the eye that converts the electromagnetic

energy of the light into useful information for the brain.

· It contains about 130 million nerve cells.

· It contains the receptors rods and cones plus the neurons; these cells are very important as they

initiate the processing of visual information.

· The retina has an area of 5 square centimeters located at the back of the eye, which is a location

where all light detection takes place.

· The retina is a network of nerve cells connected with over 100 million photosensitive rod and

cones.The signals generated by these rods and cones are then sent, via the optic nerve, to the visual

areas in the brain

Plexiform Layer

· It is the top layer of the retina, which does not interpret the light striking this part

· A web of optical nerves that carry signals of rods and cones to the optic nerves

· It is located between the photosensitive cells and the vitreous humor so that the web's cells can be

nourished

· These cells are nearly transparent so only minimal interference takes place when light strikes on the

photosensitive cells

Introduction to Psychology PSY101

Rods and Cones

a. Rods

· Retinal receptors which are long, cylindrical, and light sensitive; that can only detect black, white and

gray; they functions well in dim light, and are largely insensitive to color and small details__ functions

when cones do not respond

· Rods are used for `peripheral vision', i.e. the objects that are outside the main center of focus, and for

night vision

b. Cones

· Retinal receptors, cone- shaped and light sensitive, concentrated near the center of the retina that is

concerned with sharp focusing, fine details and color sensation; they work well in well- lit conditions

i.e., bright or sufficient light

· Rods and cones are distributed unevenly throughout the retina

· There are fewer cones i.e. about seven million than rods i.e., 125 million

Important Regions of Retina; Fovea and Blind Spot

a. Fovea

· The very sensitive and important part of the retina that aids in focusing; it is the area of best vision.

· The largest concentration of cones is present in fovea

· There are no rods present in fovea

b. Blind Spot

· The area/ point where the optic nerve leaves the eye; no receptor cells are located

here, thus creating a " blind" spot__ area of no vision

The Human Eye

Rods

Cones

Transfer of Visual

Information from the Eye to

125-Million

6-7Million

the Brain

Number

· As the light strikes the

receptor cells i.e. rods

Periphery

Location in the

Center

and cones, the chain of

events begin, which

transforms the light

Sensitivity in dim

Low

High

impulses into the neural

impulses, which are

then sent to the brain

Color sensitive

Yes

via the optic nerve

· Before reaching the

brain, some initial transformation in the visual information takes place

· Rods contain "RHODOPSIN"__ a reddish- purple substance, in which chemical reaction/

changes occur when activated by light

· Substance in the cones is different from that of rods__ but their principles of functioning are

similar

· Functioning of the neural impulses begins when stimulation of nerve cells in the eye takes place__

information is transmitted to the brain by " bipolar cells" and

" ganglion cells"Bipolar

cells are the nerve cells leading to the brain__ triggered by the nerve cells in the eye; they receive

information directly from rods and cones and then that information is transferred to the ganglion

cells

Ganglion cells are the nerve cells that collect and summarize visual information from rods and

cones and then transfer it to the brain; the visual information is moved out from the eyeball

through the optic nerves

Introduction to Psychology PSY101

Optic nerve contains bundled ganglion axons; located at the back of the eye, optic nerve carries

information to the brain

When optic nerve leaves the eyeball, it does not take the direct route to the brain, instead, the optic

nerves of both eyes meet, or intersect, at a point called "optic chiasm"__ point where the optic

nerves are reversed and ` righted' in the brain

When the optic nerves split at this point, the nerve impulses from the right half of each retina go to

the right side of the brain and those from the left half to the left side of the brain

Processing of Visual Information

· When information reaches the brain, it passes through a number of stages

· At first, the ganglion cells are activated (summarize information from rods and cones)

· In some cases, ganglion cells are activated by light in the middle/ center, whereas, some ganglion cells are

activated when there is darkness in the center and light in the surrounding areas; the ultimate function is

to maximize the detection of variations in light and darkness

· The entire processing of visual information takes place in the visual cortex of the brain which is quite

complex

Feature detection occurs when the neurons in the visual cortex are activated on seeing the particular

shape/ pattern; discovered by the Nobel- prize winners David Hubel and Torsten Wiesel

· They found out that some cells are activated only on particular shape/ pattern, while only moving

stimuli activates other cells

Adaptation

· The process of the eye becoming used to a certain amount of light is called adaptation

· What will happen when you enter into the movie theatre/ a dark room and hardly see anything?

The isle, other people, or your seat. And what happens after a few moments? It is an example of

adaptation to darkness

· Dark Adaptation means a heightened sensitivity to light resulting from being in low level of light

for some duration

· On the contrary, you can see quite well in light after coming from the darkness__ dark adaptation

· The speed of processing from dark to light and light to dark adaptation is largely dependent on the

rate at which the chemical composition in the rods and cones takes place

Color Vision: Color, Effects and Uses

Color

· In reality, in the physical world, there exist no colors __ only light waves of different wavelengths are

what we perceive as colors

· When our eyes look at an object, we actually see the reflection of light after striking some object

· The normal human eye has the ability to differentiate/ distinguish among hundreds of such bands of

wavelengths as they are received by the cones (sensory cells) of the retina___ this ability makes it

possible to perceive the world in color

· Human eye has the ability to distinguish among hundreds of wavelengths but our ability is quite limited,

and the way we describe it is even more restricted

· A color name refers to attributing the colors of what we perceive in prototype of the range of different

wavelengths e.g. "red" refers to colors that have more red attributes than orange/ yellow attributes, and

the word "orange" refers to colors that have more orange attributes, so as the "yellow" as there really is

no clear-cut distinction

Perception of Color

· Depends on subjective experience

· The wavelengths referred to by two people using the same color name almost always differ

· Also, due to the biological basis of our color vision, there is a high degree of universality in the use

of color terms across cultures and languages

Introduction to Psychology PSY101

Is There Any Psychological Effects of Color

· Do colors have any effect/ impact on our emotions and behavior? ___ the answer is yes. But what

kind of effect and how it affects our emotions and behavior is not yet conclusive

· Our reactions to colors are affected by a combination of biological, physiological, psychological,

social and cultural factors

· Certain colors seem to be more universal in their effects on mood and behavior___ they are known

as "warm" and "cool" colors

Warm colors: include magenta, red, orange, yellow, and yellow-green mainly; these speed up our

perception of time and produce feelings that are warm, cozy, and inviting.

These colors are the associated with excitement, happiness and comfort

Cool Colors: include violet, blue, light blue, and sea green generally

Emotions associated with these colors range from soothing, calmness, and peacefulness to sadness,

withdrawal and repression

Different cultures and societies use specific colors for particular occasions, which symbolize their culture

and tradition. e.g. White color

The Use of Color and in professional and everyday life

Color is one of the most effective tools in

· Fashion

· Advertising

· Presentations etc

Psychologists believed that color impression could account for 60% or more of the acceptance or rejection

of that product or service

Color Vision

· Not only a exciting and pleasurable experience but it also helps in locating and identifying objects in the

visual scene that would be hard to be identified if only in shades of gray

· The ability to detect, differentiate and discern different colors (wavelengths) gives us more information

for detecting and identifying objects than would be provided solely by black and white vision

· Color vision is mediated by specialized nerve cells in the retina known as "cones" that function only in

bright light

· When light becomes dim, rods take over, and they provide neither color vision nor high acuity (ability

to detect fine detail, such as that needed for reading)

Color Vision Needs Several Different Pigments

· During the 1800's, visual pigments were discovered in the retina

· Both rods and cones possesses visual pigments

· This pigment molecule (embedded in the cell membrane of the photoreceptor), is a large protein

called an `opsin', and is coupled with a small molecule called a `chromophore' (a form of Vitamin

A) that absorbs light

· When the chromophore absorbs light, it changes its shape which, in turns, activates the large opsin

molecule___ leads to a molecular event that culminates in an electrical change in the cell membrane

· The electrical signal is, then send to the retinal ganglion cells, whose axons take the information

about light (electrical signals) to the brain

· Researches have shown that mainly three pigments (photoreceptor cells) with different

combinations could produce vision that is capable of detecting all colors of the visible spectrum

· Scientists found that the human retina has three types of cones, each containing a different visual

pigment. They are red, green, and blue cones, each containing a pigment similar to rhodopsin

(pigment found in rods)Cone cells detect primary colors, but our brain mixes these colors in

seemingly infinite variable proportions due to which we are able to perceive a wide range of colors

· Red cones absorb `longer wavelengths', green cones `medium wavelengths', and blue cones `shorter

wavelengths'

Introduction to Psychology PSY101

Theories of Color Vision

· A realistic and useful understanding about the structure and the function of the eye began in

around 17th century when the gross anatomy of the eye was firmly established

· The two most prominent German and French researchers Johannes Kepler and Rene' Descartes

made many discoveries/ advances in understanding vision

· Most of their work concentrated on the physical concepts of light rays and geometric optics in the

visual process

· Kepler was the first to propose that the lens of the eye focuses images onto the retina Kepler's

investigations were found correct when, after few decades, Descartes did experiments on this

· He postulated that the image was inverted as a result of being focused onto the retina by the eye's

lens

· In his landmark experiment, Descartes surgically removed an eye from an ox and scraped the back

of the eye to make it transparent. He then placed the eye on a window ledge as if the ox were

looking out of the window. He looked at the back of the eye and saw an inverted image of the

scenery outside__ he, then, correctly postulated that the image was inverted as a result of being

focused onto the retina by the eye's lens

Goethe's Theory of Color

· Writer and scientist

· Wrote 1400-page treatise on color __ published in 1810

· Goethe believed that the color sensations that reach our brain were shaped by our perception -- by

the mechanism of human vision and by the ways our brain processes information

· Therefore what we see of an object largely depends on the object, the lighting and our

perceptionStudied after-images, colored shadows and complementary colorsSought to derive laws

about

· Color harmony,

· Methods/ ways of characterizing physiological colors (how colors affect us) and

· Subjective visual phenomena

Thomas Young

· Thomas Young introduced the original theory of color vision around 1790, prior to the discovery

of the cone cells in the retina

· Young was the first to propose that the human eye sees only the three primary colors, red, blue and

yellow and that all of the other visible colors are combinations of these

· It is now known that color vision is more complicated than this, but Young's work formed the

foundation of color vision theory for the later scientists

Three Main Theories of Color Vision

· There are a number of theories related to color vision and color blindness, but the three very

important and famous theories are;

1. Parallel processing theory of vision

2. Trichromatic theory of color vision

3. Opponent- process theory of color vision

I. Parallel Processing Theory of Vision

· The brain's distinctive and natural way of processing information in which various aspects

of information are processed simultaneously__ includes many brain functions such as

vision; contrasts with the serial/ step- by- step processing of most computers and of

conscious problem solving

· Unlike most computers, which work/ processes information instantly through step- by-

step serial processing, humans do parallel processing___ means do several things at one

time

Introduction to Psychology PSY101

Our brain is capable of dividing visual information into various dimensions/ categories

such as color, depth, movement etc and works on each aspect/ part simultaneously

Visual information is processed in the following manner. i.e.

One looks at someone/ something

Information is sent to the brain

The component features are reassembled and,

In, still not a clearly and exactly known way, the scene is combined/ composed into the

consciously perceived meaningful image

Parallel Processing Theory of Vision

Environmental

Stimuli = scene

Retinal processing: receptors

rods and

bipolar cells

ganglion cells

cones

Feature detection

Brain's detector cells responds to elementary features__ bars, edges or

gradient of light

Abstraction

Brain's high-level cells respond to combined

Information from feature detector cells

Recognition

Brain matches the constructed image with

stored images

Trichromatic Theory

·Initially by Sir Thomas Young, and later developed as Young-Helmholtz Trichromatic Theory of Color

vision

·The retina contains three types of cones. Each type responds to specific range of wave length

Hering's Opponent- Process Theory

·The receptor cells are linked in pairs and they work in opposition to each other

·There is a black- white pairing, a blue- yellow pairing, and a red- green pairing

· If an object reflects light containing more green than red, it will stimulates the firing of the cells sensitive

to green, at the same time inhibiting the firing of red- sensitive cells; the object will thus appear green

·This theory explains the phenomenon of after- image very well

Color Vision Deficiencies: Congenital And Acquired

· Color vision deficiencies are classified into congenital and acquired

· Congenital deficiencies are due to the inborn abnormality of cone photoreceptor cells

· Acquired deficiencies results any time after birth___ occur due to the disorders that affect the eye

or/ and the brain

· In both of these deficiencies, people experience colors differently from those with normal

trichromatic color vision

Introduction to Psychology PSY101

Three main congenital color deficiencies are;

I.Dichromacy

II.Anomalous trichromacy

III.Rod monochromacy

I. Dichromacy

· Individuals with this deficiency are able to match all the colors using two primaries rather

than three

· Dichromatic miss one of the three cone types

There are the following three main types of dichromacy, which are named on the basis of cone photo

pigment that is missing. They are;

1. Protanopia

2. Deuteranopia

3. Tritanopia

· Both protanopia and deuteranopia are sex-linked i.e. inherited recessively from a gene located on the X

chromosome

· If this deficiency is inherited, males (XY) will manifest it because they have only one X

chromosome__females (XX) have two X chromosomes, so they will only be color deficient if the

deficiency is carried on to both X chromosomes. They can pass this condition to their male offspring

without being color deficient themselves ProtanopiaRed photo pigment is absent and is replaced by the

medium green photo pigment due to this; people with this deficiency confuse red and green color

· See shorter wavelengths such as blue

DeuteranopiaAlso confuse red and green

It is sex- linked deficiency and effect about 1% of males and .01% of females

Tritanopia

Basis of this deficiency is not yet clear__ may occur due to the absence of short wavelengths such as blue

People with this deficiency are insensitive to blues and yellows and see the world in red and green

A very rare condition and occur only in .002% of males and .001% of females

Anomalous trichromacy

· People with this deficiency mix the primaries in different proportions

· Like normal trichromats, these people also need three wave lengths___ but due to spectral sensitivities,

they often mingled them up

Rod monochromacy

· Extremely rare hereditary condition which occurs due to the absence of cones in the eye

· As vision is dependent on the rods photoreceptors, the acuity is very weak because of the absence of

cones

· Absence of cones are also responsible for poor fixation, erratic eye movements, visual fields defects and

serious refractive error

Color Blindness

·Inability to see specific colors

·25 of men and 2 out of 10,000 women are color blind

·Red- green color blindness; inability to distinguish red from green

·Yellow-blue; the person can not differentiate between yellow and blue

·Monochromatic color blindness; inability to perceive any color at all

·The Trichromatic theory explains this phenomenon very well

Introduction to Psychology PSY101

Every Day Experiences and Color Vision Deficiencies

Our daily routine is highly dependent on the efficient ability to discriminate among colors such as

selecting clothes, matching paint colors etc, that requires efficient color vision___ inability/ deficiency can

seriously effect individual's ability to learn, to work or to move freely from one place to another

Colors play an influential role in child's initial/ primary education because it is used to categorize the

educational materials

Good color judgment is also important as far as the occupation of the person is concerned__ essential for

the painter, pilots, surgeons, military men, safety officer, dermatologists, pharmacists, chemist, buyer of

textiles, food inspectors, electricians and marine navigator

Extensively used for communicating safety information__ color coded targets are easily noticed and

quickly detected

Red stands out for fire, danger, or stop

Yellow for amber

Blue indicates need for caution

Green to signal safety

Orange to communicate potential danger and

Purple to warn about for radiation hazards

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