6. Sensation

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PSY 1 GENERAL PSYCHOLOGYHandout # 4Sensation and Perception

Introduction to Sensation

Sensation is a conversion of physical stimulation to a neural impulse from thebody to the brain. Psychologists study the senses because we come to know ourworld primarily through them and what we sense often affects our behavior. Oursenses inform us of the presence of stimuli or any of change in a stimulus. The firstexperimental psychological techniques were developed for the study of sensation.

These early techniques were called psychophysical methods.

Psychophysics is an area of psychology that examines the relationshipbetween sensory stimuli and individual psychological or behavioral reactions tothese stimuli. Psychophysics has been traditionally concerned with detectingthresholds.

The smallest amount of a stimulus that can be detected or noticed at least50% of the time is called the absolute threshold. On the other hand, the change thatis needed to make a stimulus noticeable different is called the just noticeabledifference (jnd).

The study of the just noticeable threshold lead to Webers law which statesthat the amount of change needed to produce a jnd is a constant proportion of theoriginal stimulus. Webers law indicates that the more intense the stimulus, themore the stimulus intensity has to be increased before a change is noticed. Forexample, if music was being played softly, a small increase in sound would benoticeable. If music was being played loudly, it would require a much greaterincrease in sound to perceive a difference in volume. Stated mathematically,Webers Law asserts:

I = CI

Where: I = jndI = stimulus of Intensity IC = a constant

Another area of psychophysics is its study on how humans can adapt andassimilate to the sensations around them. For example, when you enter a roomwhich has a specific odor, you first sense this odor. However, as time passes by, youwont even notice the odor being there. This adjustment in a sensory capacity afterprolonged exposure to prolonged stimuli is called adaptation. Adaptation occurs as

people become accustomed to a stimulus and change their frame of reference. In asense, our brain mentally turns down the volume of stimulation it is experiencing.This decline in sensitivity to sensory stimuli is due to the inability of the sensorynerve receptors to fire off messages to the brain indefinitely. Because the receptorcells are most responsive to changes in stimulation, constant stimulation is noteffective in producing a sustained reaction.


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Vision

Light

Lightis the physical stimulus for vision. The visual spectrum (light that is visibleto the human eye) is made up of various wavelengths of light measured innanometers (nm). A nanometer is one-billionth of a meter, and the visual spectrumvaries from 400nm to 700nm.

The Visual System

The human visual system consists of the eyes, several parts of the humanbrain, and the pathways connecting them. The eye contains two systems, one forforming the image and the other for transducing the image into electrical impulses.

The critical parts of these systems are illustrated below:

An analogy is often made between an eye and a camera. Although thisanalogy is misleading for many aspects of the visual system, it is appropriate for theimage-forming system, which focuses light reflected from an object to form animage of the object on the retina, which is a thin layer of tissue at the back of theeyeball. The image-forming system itself consists of the cornea, the pupil, and the

lens. Without them, we could see light but not pattern.

The cornea is the transparent front surface of the eye: light enters here, andrays are bent inward by it to begin the formation of the image.

The lens completes the process of focusing the light on the retina. To focuson objects at different distances, the lens changes shape. It becomes morespherical for near objects and flatter for far ones. In the eyes of myopic(nearsighted) people, the lens does not become flat to bring far objects into focus,


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although it focuses well on near objects. In the eyes of hyperopic (farsighted)people, the lens does not become spherical enough to focus on the near objects,although it focuses well on far objects. As people with otherwise normal vision getolder (e.g. into their 40s), the lens loses much of its ability to change shape orfocus at all. Such optical defects can, of course, generally be corrected with eye-glasses or contact lenses.

The pupil, the third component of the image-forming system, is a circularopening whose diameter varies in response to the level of light present. It is largestin dim light and smallest in bright light, which helps to ensure that enough lightpasses through the lens to maintain image quality at different light levels.

All of these components focus the image on the retina. There thetransduction system takes over. The heart of the system is the receptors. There aretwo types of receptor cell, rods and cones, so called because of their distinctiveshapes. These receptors are specialized for different purposes. Rods are receptorcells designed for seeing at night; they operate at low intensities and lead to lowresolutions, colorless sensations. Cones are receptor cells best for seeing during theday; they respond to high intensities and result in high-resolution sensations thatinclude color. The retina also contains a network of neurons, along with supportcells and blood vessels.

An illustration of the rod and the cones and the complete visual system arepresented below:


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When we want to see the details of an object, we routinely move our eyes sothat the object is projected on to the fovea, which is a small region at the center ofthe retina. The reason we do this has to do with the distribution of receptors acrossthe retina. In the fovea, the receptors are plentiful and closely packed; outside thefovea, on the periphery of the retina, there are fewer receptors. More closelypacked receptors mean higher resolution.

As the light makes contacts with the receptors, the rods and the cones thenstart the process of transducing light into electrical impulses. Rod and conescontains photopigments, which are chemicals that absorb light. Absorption of lightby the photopigments starts a process that eventuates in a neural impulse. Oncethis transduction step is completed, the electrical impulses must make their way tothe brain via connecting neurons. The response of the rods and cones are firsttransmitted to bipolar cells and from there to other neurons called ganglion cells.

The long axons of the ganglion cells extend out of the eye to form the optic nerve ofthe brain. Where the optic nerve leaves the eye, there are no receptors, and we aretherefore blind to a stimulus in this region. We do not notice this hole in our visualfield known as the blind spot because the brain automatically fills it in throughthe process of completion. This process happens as the information around theblind spot to fill the gap in the retinal image. For example, when the visual systemdetects a straight bar going into one side of the blind spot and another leaving theother side, it fills the missing part for you.

Color Sensation

The three attributes used to describe color are hue (determined by thewavelength of the light), brightness (which is a function of the amplitude of thewavelength), and saturation (purity or richness of color).

Longer wavelengths of light appear red (around 700nm), middle wavelengthsappear green (500nm), and shorter wavelengths appear blue (470nm). Achromaticcolors (i.e. white and black) cannot be distinguished on the basis of hue. Onlychromatic colors differ in saturations.

When a single wavelength of light is perceived, the hue appears pure or

saturated. As other wavelengths are added, the hue becomes distilled and appearsgrayer or less saturated. A mixture of wavelengths in which all the wavelengthswere equally strong would appear gray. A deep red is a highly saturated red;whereas pink is a desaturated red. Saturation produces a red red or a greengreen.

As the intensity of the light increases, the light will appear brighter.Brightness is an aspect of both color (chromatic) and black-and-white (achromatic)vision. Being hueless, blacks and white do not differ quantitatively but only in their


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brightness value. The brightness that we perceived is greatly dependent not only onthe energy value of the light intensity but on the state of adaptation of the eye.Also, brightness varies according to wavelength for a given intensity of light; andconversely, the hue and saturation change somewhat if the brightness is altered.

The Young-Helmholtz Theory or Trichromatic Theory of color vision proposesthat there are three kinds of receptors in the cones of the eye, one for each of thethree primary colors. Physiological data has supported this hypothesis. Threedifferent kinds of cones have been discovered, one sensitive to red light, one togreen light, and one sensitive to blue light.

According to the Young-Helholtz Theory, when you look at a red object, thered cones are stimulated to sense a message to the brain so that you senseredness. All other colors are perceived as a result of the mixture of red, green, orblue cones being stimulated. A yellow object, for example, stimulates green andred cones to respond to it. The color white occurs when red, blue, and green conesare stimulated equally, and black results from no cone stimulation.

However, it was noted that certain kinds of color blindness were not wellexplained by the Young-Helmholtz theory. The most common form of colorblindness is red-green blindness. Individuals with red-green blindness find it difficultto sense red or green but have no trouble sensing yellow. This is does not agreewith the Young-Helmholtz Theory which implies that yellow is a mixture of red andgreen. It is argued that yellow was much as a primary color as red or green or blueand developed the Opponent-Process Theoryof color vision.

The Opponent-process theory states that there is a red-green receptor, ayellow and blue receptor, and a dark-light (black or white) receptor. Only onemember of a pair can respond either a red or green, yellow or blue, dark or light andnot red and green, or yellow and blue. If one member of a receptor pair isstimulated more than its opponent, the corresponding color will be seen. Forexample, if red is stimulated more than green, then red will be seen and vice versa.If both members of the pair are stimulated equally, they cancel each other out andthis leaves only gray. Members from non-opponent pairs may interact and bestimulated at the same time, resulting in colors such as yellow-red or blue-green.

The Young Helmholtz theory seems to be a good description of visualprocessing in the retina because cones have been found to be sensitive to red,green (and not to red-green or blue-yellow). The Opponent-process theory seems tobe a better explanation of color vision at higher levels within the brain (i.e. from the

optic nerve and beyond).

Hearing and Balance

We perceive different shapes and sizes of sound waves as different sounds.Hearing is affected by three properties of the sound wave: its amplitude, frequencyand purity. The height or amplitude, of the sound wave determines what weperceive as loudness. The taller the wave is, the louder the sound. The scale for asounds loudness id decibels (dB). The starts with zero, which is the threshold for


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normal human hearing. The scale has no upper limit, but sounds above 150-170dBare seldom registered anywhere. Markers for loudness include:

Whisper = 30dB

Regular Human Conversation = 55 60 dB

Jackhammer = 90 dB Loud Bar or Nightclub = 100 110 dB

Very loud rock concert = 110 -120 dB

Jet Airplane = 130 140 dB

*Hearing a sound at 160dB could make ones eardrum burst.

The frequencyof the sound wave or how many waves occur in a given periodof time, we perceive as the sounds pitch. Frequency is measured in units calledhertz (Hz), which is how many times the wave cycles per second. The higher thefrequency, the higher the pitch. The higher the keys on the piano those further tothe right are higher pitch than the lower keys, for example. The range for humanpitch perception is from 20Hz to about 20,000Hz but most people cannot hear

sounds at either extreme. Sounds below 20Hz are called subsonic and above20,000Hz are called ultrasonic. Most sounds we hear are in the range of 400 to4000Hz. The Human voice generally ranges 200 to 800Hz, and a piano plays notesranging from 30 to 4000Hz.

The thirty property of sound waves, purity, refers to the complexity of thewave. Some sound waves are pretty simple, made of only one frequency. Mosthowever, are almost always a mixture of frequencies and how much of a mixturedefines its purity. We perceive purity as timbre. Musicians often refer to timbre asthe color of the sound. Timbre allows us to distinguish a middle C (256Hz) asbeing from either a piano or from a violin. They both are 256Hz and may even be ofequal loudness, but we have no trouble telling them apart because they produce

waves of different purities.

The Auditory System

Although many of us think primarily of the outer ear when we speak of theear, that structure is only a simple part of the whole. The outer ear acts as a reversemegaphone, designed to collect and bring sounds into the internal portions of theear. The location of the outer ears on different sides of the head helps with soundlocalization, the process by which we identify the direction from which a sound iscoming. Wave patterns in the air enter each ear at a slightly different time, and thebrain uses the discrepancy as a clue to the sounds point of origin. In addition, thetwo outer ears delay or amplify sounds of particular frequencies to different

degrees.

An illustration of the auditory system is presented below:


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Sound is the movement of air molecules brought about by a source ofvibration. Sounds, arriving at the outer ear in the form of wavelike vibrations, arefunneled into the auditory canal, a tube-like passage that leads to the eardrum. Theear drum is aptly named because it operates like a miniature drum, vibrating whensound waves hit it. The more intense the sound, the more the eardrum vibrates.

These vibrations are then transferred into the middle ear, a tiny chamber containingthree bones (i.e. the hammer, the anvil, and the stirrup) that transmit vibrations tothe oval window, which is a thin membrane leading to the inner ear.

The inner ear is the portion of the ear that changes the sound vibrations into

a form in which they can be transmitted to the brain. When sound enters the innerear through the oval window, it moves into the cochlea, a coiled tube that lookssomething like a snail and is filled with fluid that vibrates in respond to sound.Inside the cochlea is the basilar membrane, a structure that runs through the centerof the cochlea, dividing it into an upper chamber and a lower chamber. The basilarmembrane is covered with hair cells. When the hair cells are bent by vibrationsentering the cochlea, the cells send a neural message to the brain.

When an auditory message leaves the ear, it is transmitted to the auditorycortex of the brain through a complex series of neural connections. As the messageis transmitted, it is communicated through neurons that respond to specific types ofsounds. Within the auditory cortex itself, there are neurons that respond selectively

to very specific sorts of sound features, such as clicks and whistles. Some neuronsrespond only to a specific pattern of sounds such as a steady tone but not anintermittent one. Furthermore, specific neurons transfer information about asounds location through their particular of firing.

Neighboring cells in the auditory cortex of the brain are responsive to similarfrequencies. The auditory cortex, then, provides us with a map of soundfrequencies, just as the visual cortex furnishes are representation of the field. In


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addition, because of the asymmetry in the two hemispheres of the brain, the leftand right ears process sound differently. The right ear reacts more to speech, whilethe left ear responds more to music.

Balance

Several structures of the ear are related more to our sense of balance thanour hearing. The Semicircular canals of the inner ear consists of three tubescontaining fluid that sloshes through them when the head moves, signalingrotational or angular movement to the brain. The pull on our bodies caused by theacceleration of forward, backward, or up-and-down motion, as well as the constantpull of gravity, is sensed by the otoliths, tiny motion-sensitive crystals in thesemicircular canals. When we move, these crystals shift like sands on the windybeach. The brains inexperience in interpreting messages from the weightless is thecause of the space sickness commonly experienced by two-thirds of all spacetravelers.

Olfaction

The volatile molecules given off by a substance are the stimuli for smell. Themolecules leave the substance, travel to the air, and enter the nasal passage. Themolecules must also be soluble in fat, because the receptors for smell are coveredwith a fatlike substance.

Olfactory System

The olfactory system composes the receptors in the nasal passage, certainregions of the brain, and interconnecting pathways. The receptors for smell arelocated high in the nasal cavity. When the cilia (i.e. hairlike structures) of thesereceptors come into contact with volatile molecules, an electrical impulse results.

This is the transduction process. This impulse travels along nerves fibers to theolfactory bulb, a region of the brain lies just below the frontal lobes. The olfactorybulb in turn is connected to the olfactory cortex on the inside of the temporal lobes.Interestingly, there is a direct connection between the olfactory bulb and the part ofthe cortex known to be involved in the formation of long-term memories; perhapsthis is related to the idea that a distinctive smell can be a powerful aid in retrievingold memory.

An illustration of the olfactory system is presented below:


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Sensing Intensity and Quality of Smell

Human sensitivity to the intensity of smell depends on the substanceinvolved. Absolute threshold can be as low as 1 part per 50 billion parts of air. Still,we are far less sensitive to other species. Dogs, for example, can detectconcentrations 100 time lower than those that can be detected by humans. Ourrelative lack of sensitivity is not due to our having less sensitive olfactory receptors.Rather, we just have fewer of them by about a factor of 100; roughly 10 millionreceptors for humans versus 1 billion for dogs.

Although we rely less on smell than do other species, we are capable of

sensing many different qualities of odor. Estimates vary, but a healthy personappears to be able to distinguish among 10,000 to 40,000 different odors, withwomen generally doing better than men. Professional perfumers and whiskeyblenders can probably do even better- discriminating among perhaps 100,000odors. Moreover, we know something about how the olfactory system codes thequality of odors at the biological level. The situation is most unlike the coding ofcolor in vision, for which three kinds of receptors suffice. In smell, many kinds ofreceptors an estimate of 1,000 kinds of olfactory receptors is not unreasonable seem to be involved. Rather than coding a specific odor, each kind of receptor mayrespond to many different odors. So quality may be partly coded by the pattern ofneural activity, even in this receptor-rich sensory modality.

Taste (Gustation)

The sense of taste (gestation) involves receptor cells that respond to fourbasic stimulus qualities: sweet, sour, salty, and bitter. Although the specialization ofthe receptor cells leads them to respond most strongly to a particular type of taste,they also are capable of responding to other tastes as well. Ultimately, every taste


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is simply a combination of the basic flavor qualities, in the same way that theprimary colors blend into hues.

The receptor cells for taste are located for roughly 10,000 taste buds, whichare distributed across the tongue and other parts of the mouth and the throat. Thetaste buds wear out and are replaced every 10 days or so. Thats a good thing,because if our taste buds werent constantly reproducing, wed lose the ability totaste after wed accidentally burned our tongues.

The sense of taste differs significantly from one person to another, largely asa result of genetic factors. Some people, dubbed supertasters, are highly sensitiveto taste, they have twice as many taste receptors as nontasters who are relativelyinsensitive to taste. Supertasters finds sweets sweeter, cream creamier, and spicydishes spicier, and weaker concentrations of flavor are enough to satisfy anycravings they may have. In contrast, because they arent so sensitive to taste,nontasters may seek out relatively sweeter and fattier foods in order to maximizethe taste. As a consequence, they may be prone to obesity.

An illustration of the olfactory system is presented below:

Bodily Senses (Somatosensation)

We feel things on our skin and in our bodily organs. The largest contact

surface area any sensory input has with our bodies is the skin, and it is carefullymapped in the somatosensory area cortex in the parietal lobe of the brain. Bodilysenses also include knowing where our body parts are. In addition, we also sensethings inside our bodies organ pain, levels of the heart rate, depth of breathing toname a few. These senses based in the skin, body, or any membrane surfaces areknown as the bodily senses. There are two basic bodily or somatic senses: touch,and pain.


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Touch

The top layers of the skin have receptor cells that are different to sensitive todifferent tactile qualities some to shape, some to grooves, some to vibrations andmovements. These receptor cells are known as mechanoreceptors, and they are likethe photoreceptors in the eye or the hair cells in the ear. There are in fact, fourdifferent kinds of mechanoreceptors, each of which has a unique profile ofsensitivity. Some of the mechanoreceptors are slow to change and others are fast tochange with variation in tactile stimulation. Some are sensitive to fine detailswhereas others are not sensitive (e.g. feeling the grooves of a coin). Somemechanoreceptors also sense movement and vibration, such as when someone runsfingers over your forearm.

The most basic mechanoreceptor are the free nerve endings of sensorynerves, which reach the skin and perceive touch stimuli, especially the painful ones.Other specialized receptors include the Vater-Pacini corpuscles, which can detectchanges in pressure and vibrations produced on the skin and its stretching;Meissner corpuscles, which respond to touch stimuli; Krause corpuscles, which aresensitive to cold and the Ruffini corpuscles, which are sensitive to heat.

It is important to point out, however, that different areas of skin havedifferent numbers of mechanoreceptors. If someone put a screwdriver and a penagainst your feet, for example, you might have trouble telling them apart. You havefar fewer mechanoreceptors on the soles of your feet than on your fingertips. This isa good thing since it would be difficult to have extremely sensitive soles.

Like photoreceptors in the eye, mechanoreceptors mark only the beginning ofthe journey from sensation to perception. The sensory qualities (i.e. shape, size,hardness, and temperature) of the screwdriver and pen stimulate different kinds ofmechanoreceptors in the skin, but the resulting sensory impulses must travel to thebrain to be processed and interpreted. When something touches our fingertips,forearm, or shoulder, a dedicated region of cortex becomes active and we perceivethe sensation of being touched.

Tactile sensations from our skin travel via sensory neurons to the spinal cordand up to the brain. The first major structure involved in processing bodilysensations is the thalamus, which relays the impulses to the somatosensory cortexin the parietal lobes.

Repeated sensory and motor tactile experience changes the amount of

cortex involved in processing that particular sensation or movement. The generallocation in the somatosensory cortex stays the same, but areas of the cortexdevoted to that experience or function grow. The more one body region is touchedand stimulated, the more sensory or motor cortex is used to process informationfrom the mechanoreceptors. For instance, musicians who play stringed instrumentssuch as a violin use the right hand to bow and the left hand to play the notes.

Researchers have found out that experienced violinists have largerrepresentations, or brain maps of the hand and finger regions of the


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somatosensory cortex than do nonmusicians. Athletes who practice the samemovement over and over, whether it is hitting a tennis ball or shooting a basketball,no doubt have similarly well developed sensory and motor cortexes. By rehearsingthe action learned, the cortical area that is responsible for that action grows andbecomes more developed.

Pain

Pain is no fun, but we need it to survive. People born with no perception canbe severely injured or killed, because they dont know they have been harmed. Painis a complex emotional and sensory experience associated with actual or potentialtissue image. It is usually very unpleasant, but people vary widely in theirexperiences of pain, what they think is painful, and whether they might even enjoypain. In fact, some people feel no pain during great injury (e.g. soldiers in battlesituations), and others fell pain when no tissue damage is present. The lattersituation occurs with the phantom limb pain, wherein people who have lost a limbfeel the pain in the missing arm or leg. Such cases dramatically show how pain isnot just a direct result of tissue damage, but an experience in the brain as well. Painis also enhanced by ones reaction to the injury. Often the emotional reaction topain creates as much suffering as the actual tissue damage. In fact, physical andemotional pains involve many of the same brain structures.

How do we sense and perceive pain? Its not merely touch gone too far. Infact, damage to the skin is only one kind of pain. Other forms include organ tissueand nerve damage as well as joint inflammation. Pain form the skin damage iscalled nociceptive pain. The skin has pain receptors that are sensitive to heat, cold,chemical irritation, and pressure, all of which are kinds of nociceptors. Heat,frostbite, chemical burns, and cutting or hitting your thumb with a hammer all hurtbecause these events stimulate nociceptors in our skin. The nociceptors sendsignals to the spinal cord and then to the brain, signaling that the damage hasoccurred. Your brain can then initiate an appropriate response, such as pulling yourhand away from the hot burner. You can now see why it is very dangerous not toexperience pain!

We now know that the spinal cord may actually play an active rather thanpassive role in pain perception. That is, the spinal cord does not simply relay in thepain messages from the sensory neurons to the brain; it also can enhance thosemessages. Most surprisingly, it is not neurons in the spinal cord that enhance thepain signals, but rather the glial cells wrapped around the axons. Once the painmessages get sent and even enhanced by the spinal cord, they move on to the

brain.

Many brain structures are involved in the perception of skin damage alone. Apartial list of brain structures activated by skin-based pain includes the thalamus,limbic system, insula, anterior cingulated cortex. A recent and somewhat surprisingfinding is that some of the brain regions activated when we experience physicalpain also are activated during emotional pain especially when we are rejected byothers or see others receive shocks. The brain regions that are active in bothphysical and emotional pain are the anterior cingulated cortex and the insula. Even


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more fascinating, is that when we observe a loved one being given a mild shock,only the anterior cingulated cortex and the insula become active, not thesomatosensory cortex. , which is activated when we ourselves are shocked. Sowhen we see someone we love hurt, the aspects of the pain circuit involved withemotion are active but not the entire circuit.

One of the more influential explanations of pain is the one proposed byRonald Melzack and Patrick Wall. Their gate-control theoryof pain proposes that thespinal cord regulates the experience of pain either by opening or closing neuralchannels, called gates, involved in pain sensations that get sent to the brain.Smaller neural channels are dedicated to pain sensations, and when they areactivated, pain messages get sent to the brain. Activation of larger neural channelsthat are involved in the sensation of pain can inhibit the transmission of painimpulses to the brain. This mechanism explains why certain kinds of stimulation -such as acupuncture or even rubbing ones skin can relieve sensations of pain.

The signals from acupuncture may override other, even more sensations of pain,such as chronic pain from injury.

What is most interesting about the gate control theory of pain is the idea thatinhibitory channels can actually come from the brain as well as the body. Messagessent by the brain itself can close channels in the spinal cord in pain sensations.

Thoughts, feelings, and beliefs can influence pain sensations, which is one reasonwhy people vary too much in their perception of pain. Different people experiencingthe same level of pain may have completely different experiences of their pain.

In addition to thoughts and feelings controlling the experience of pain, ourbodies have natural painkillers called endorphins. When we get hurt, our bodyresponds by releasing these substances. Endorphins work by stimulating the releaseof neurotransmitters that interfere with pain messages in the spinal cord and thebrain. Endorphin release may explain why people initially experience no pain after ahorrible injury from an accident. For example, soldiers and automobile accidentvictims often report no immediate sensations of pain. Only hours afterward ormaybe the next day while in a hospital does the pain begin. Endorphins also play arole in acupuncture-based pain relief.

If thoughts, feelings, and endorphins are not enough to control pain, thereare drug treatments. For small aches and pains, many people take aspirin,acetaminophen, ibuprofen and other similar drugs. These work to controlinflammation. For more severe pain, doctors may prescribe opioids. Opioids are aclass of drug known as analgesics, meaning without pain. Morphine, heroin,

oxycodone, and hydrocodone are all opioids. All but heroin are commonlyprescribed for pain relief. They work to deaden or lessen pain by blocking neuralactivity involved in pain perception. Morphine for example, is widely used beforeand after medical procedures and in the care of terminally ill patients. There is ahigh risk of dependency on opioids, so their use must be carefully monitored.


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Introduction to Perception

Consider Figure (A) for a moment, it is a picture of a vase. Or is it? Take

another look, and instead you may see the profiles of two people. Now that an

alternative interpretation has been pointed out, you will probably shift back and

forth between the two interpretations. Similarly, if you examine the shapes of

figures (B) and (C) long enough, you will probably experience a shift in what you are

seeing. The reason for these reversals is this: because each figure is two-

dimensional; the usual means we employ for distinguishing the figure (the object

being perceived) from the ground (the background or spaces within the object) do

not work.

(A) (B) (C)

The fact that we can look at the same figure in more than one way illustrates

an important point. We do not passively respond to visual stimuli that happen to fall

on our retinas. Instead, we actively try to organize and make sense of what we see.We turn now from a focus on the initial response to a stimulus (sensation) to what

our minds make of that stimulus, which is what perception is all about. Perception is

a constructive process by which we go beyond the stimuli that are presented to us

and attempt to construct a meaningful situation out of it.

The Difference between Sensation and Perception


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Sensation and perception are related because perception involves the

interpretation of sensory information. However, it is prudent to differentiate the two

concepts for better understanding.

The major senses of the human organism include seeing, hearing, taste,smell, and bodily sensation. For each sense, we have a sense organ that is affected

by a particular type of physical energy. The ear, for instance, is responsive to sound

waves. Within each sensory realm there are a number of qualities which recur in

varying combinations, such as with the sense of hearing loud, soft, shrill, resonant,

humming, screeching, to name just a few of the qualities that can be discerned by

the human ear. The different experiences that we are able to discern within each

sensory realm are called sensation.

The sensations are of tremendous variety, but can all be reduced to a few

basic physical variables. Though the eye can see thousands of different scenes,

they are all produced by light; though the ear can hear sounds from a siren to asymphony, they are all produced by differing intensity and frequency vibration of

sound waves.

Sensations provide us with basic elementary experiences which we further

interpret into meaningful events. This interpretation of sensations is a much more

complex process than simply registering and reflecting the external world. It

involves encoding, storage (memory) and organization of the sensations that are

received. It is this process that is calledperception.

A very general characteristic of all perceptual experience is to attend and

organize selectively that data that is provided by the sensory system. Precisely howthis is accomplished has intrigued scientists, and their research and

experimentation has provided an abundant theoretical basis from which the student

can explore the mysteries of how we can construct our sense-based reality. The

problem of explaining how and why we perceive the way we do is one of the most

controversial fields in psychology today.

Theories on Perception

1. Direct Perception Theory (Nativists) assert that perception is an innate

mechanism and is a function of biological organization.

2. Image Cue Theory(Empiricists) believe that perceptions are learned based

on past experiences.

3. Ecological View argues that perception is an automatic process that is a

function of information provided by the environment.

4. Constructionist View holds that we construct reality by putting together the

bits of information provided by our senses.


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**Perceptual Set a readiness to perceive a stimulus in a particular way.

Perceptual Organization

We tend to organize our sensations into meaningful perceptions. Some of the

most basic perceptual processes can be described by a series of principles that

focus on the ways we organize bits and pieces of information into meaningful

wholes. Known as the Gestalt laws of organization, these principles were set forth in

the early 1900s by a group of German psychologists who studied patterns, or

gestalts. Those psychologists discovered a number of important principles that are

valid for visual (as well as auditory) stimuli, as described below:

(A)

(B)

(C)

(D)

(E)

Figure (A) Illustrates the Principle ofClosure

We usually group elements to form enclosed or complete

figures rather than open ones. We tend to ignore the

breaks and concentrate on the overall form

Figure B Illustrates the Principle ofProximity

We perceive elements that are closer together as

grouped together. Instead of seeing six individual

circles, we see a vertical or a horizontal rectangular set.

Figure C illustrates the Principle ofSimilarity

Elements that are similar in appearance are perceivedas grouped together. Instead of seeing an alternate

horizontal pattern of squares and circles, we could see

columns of squares and circles

Figure (D) Illustrates the Principle of Simplicity

(Pragnanz)

When we observe a pattern, we perceive it in the

most basic, straightforward manner that we can.

Figure (E) Illustrates the Principle ofContinuity

We perceive things that provide more continuity than

others. In the example, the top branch is seen as

continuing the first segment of the line. This allows us

to see things as flowing smoothly without breaking

lines u into multi le arts.

Figure (F) Illustrates the Principle of Figure-ground

relations

We group some sensations into an object or figure

that stand out on a plain background. The figure is

the distinct shape with clearly defined edges and the

ground has no defined edges. Reversible or

ambiguous figures have no clearly defined figures

and backgrounds (the figure and background can be


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(F)

(G)

Although Gestalt psychology no longer plays a prominent role in

contemporary psychology, its legacy endures. One fundamental gestalt principle

that remains influential is that two objects considered together from a whole that is

different from simple combination of objects. Gestalt psychologists argued that the

perception of stimuli in our environment goes well beyond the individual elements

that we sense. Instead it represents an active, constructive process carried out

within the brain.

Top-Down and Bottom-Up Processing

Ca- yo- re-d t-is -en-en-e, w-ic- ha- ev-ry hi-d l-tt-r m-ss-ng? It probably wont

take you too long to figure out that it says, Can you read this sentence, which has

every third letter missing?

If perception were based primarily on breaking down a stimulus into its most

basic elements, understanding the sentence, as well as other ambiguous stimuli,

would not be possible. The fact that you were probably able to recognize such an

imprecise stimulus illustrates that perception proceeds along two different avenues,

called top-down processing and bottom-up processing.

In top-down processing, perception is guided by a higher-level knowledge,experience, expectations, and motivations. You were able to figure out the meaning

of the sentence with the missing letters because of your prior reading experience,

and because written English contains redundancies. Not every letter of each word is

necessary to decode its meaning. Moreover, your expectations played a role in your

being able to read the sentence.

Figure (G) Illustrates the Phi Phenomenon

The Phi Phenomenon can be best described as

perceived motion when the object is in fact

stationary. For example, as the light blinks

sequentially in a counter clockwise manner, it is

erceived as movin .
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However, top-down processing cannot occur on its own. Even though top-

down processing allows us to fill the gaps in ambiguous and out-of-context stimuli,

we would be unable to perceive the meaning of such stimuli without bottom-up

processing. Bottom-up processing consists of the progression of recognizing and

processing information from individual components of a stimulus and moving to theperception of the whole. We would make no headway in our recognition of the

sentence without being able to perceive the individual shapes that make up the

letters. Some perception, then, occurs at the level of the patterns and features of

each of the separate letters.

Top-down and bottom-up processing occur simultaneously, and interact with

each other, in our perception of the world around us. Bottom-up processing permits

us to process the fundamental characteristics of stimuli, whereas top-down

processing allows us to bring our experience to bear on perception. As we learn

more about the complex processes involved in perception we are developing a

better understanding of how the brain continually interprets information from the

senses and permits us to make responses appropriate to the environment.

Perceptual Constancies

Consider what happens when you finish a conversation with a friend and she

begins to walk away from you. As you watch her walk down the street, the image on

your retina becomes smaller and smaller. Do you wonder why she is shrinking?

Of course not. Despite the very real change in the size of the retinal image

you factor into your thinking the knowledge that your friend is moving farther away

from you because of perceptual constancy. Perceptual Constancy is a phenomenonin which physical objects are perceived as unvarying and consistent despite

changes in their appearance or in their physical movement. We are able to

recognize the same objects at a variety of angles, at various distances, and even

under different colored lighting because of perceptual constancies. Presented below

are examples of these constancies:

Size Constancy objects we are familiar with are perceived as their true size

despite changes in the distance between us and the objects

Shape Constancy objects appear to be the same shape despite changes in

their orientation toward the viewer.

Brightness or Lightness Constancy objects appear to stay the samebrightness despite changes in the amount of light falling to them.

Color Constancy the hue of an object appears to stay the same despitechanges in background lighting.

In some cases, though our application of perceptual constancy can mislead

us. On good example of this involves the rising moon. When the moon first appears

at night, close to the horizon, it seems to be huge much larger than when it is high


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in the sky later in the evening. You may have thought that the apparent change in

the size of the moon was caused by the moons being physically closer to the earth

when it first appears. In fact, though, this is not the case at all: the actual image of

the moon on our retina is the same whether it is low or high in the sky.

Instead, the moon appears to be larger when it is close to the horizon

primarily because of the phenomenon of perceptual constancy. When the moon is

near the horizon, the perceptual cues of intervening terrain and objects such as

trees on the horizon produce a misleading sense of distance. The phenomenon of

perceptual constancy leads us to take that assumed distance into account when we

view the moon, and it leads us to misperceive the moon as relatively large. In

contrast, when the moon is high in the sky, we see it by itself, and we dont try to

compensate for its distance from us. In this case, then, perceptual constancy leads

us to perceive it as relatively small.

Depth Perception

Depth perception involves the interpretation of visual cues in order to

determine how far away objects are. There is currently a debate as to whether

depth perception is an inborn ability or a learned response as a result of experience

(nature or nurture).

Gibson and Walk (1960) developed an apparatus they called the visual cliff

that is used to measure depth perception in infants and toddlers. The visual cliff

consists of an elevated glass platform divided into two sections. One section has a

surface that is textured with a checkerboard pattern of tile, while the other has a

clear glass surface with a checkerboard pattern several feet below it so it looks like

the floor drops off. Gibson and Walk hypothesized that if infants can perceive depth,

they should remain on the shallow side of the platform and avoid the cliff side,

even if coaxed to come across by parents.

Gibson and Walk found that infants

would crawl or walk to their mothers when the

mothers were on the shallow side of theplatform, but would refuse to cross the deep

side even with their mothers encouragements

to cross. The results of this and other visual

cliff studies still do not prove that depth

perception is innate because before infants

can be tested, they must be able to crawl and

may have already learned to avoid drop-offs.


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Two types of visual cues, binocular cues and monocular cues allow us to

perceive depth.

A. Binocular Cues

Binocular cues for depth require the use of both eyes. The two binocular cues

are convergence and retinal disparity.

Convergence involves the interpretation of muscular movements related to

how close or how far away an object is. For an object closer that approximately 25

feet, our eyes must converge (move inward) in order to perceive it as a single

object clearly in focus. Our perceptual system uses this muscular movement as a

cue for closeness. For an object farther than 25 feet, our eyes tend to focus on

infinity (little to no muscular movement required), and again, our perceptual system

uses this as a cue that the object must be far away.

Retinal Disparity is the difference in locations, on the retinas, of the

stimulation by a single object. This means that an object viewed by both eyes will

stimulate one spot on the right retina and a different spot on the left retina. This is

due to the fact that the object is at a different distance from each eye. Retinal

disparity is also used as a cue for depth because the eyes are set a certain distance

apart in the head and objects closer than 25 feet are sensed on significantly

different locations on each eyes retina. Viewing objects that are close causes

considerable retinal disparity (very different portions of each retina are stimulated)

and viewing objects at a distance create little retinal disparity (similar portions of

each retina are stimulated).

B. Monocular Cues

Monocular cues for depth require the use of only one eye. Two-dimensionalpresentations (e.g. photographs, television) also rely on monocular cues to indicatedepth. Monocular cues for depth include:

Linear Perspective parallel lines appear to converge on the horizon (e.g.

railroad tracks)

Relative Size closer objects appear larger; the larger of two figures will

always appear closer because the two objects will project retinal images ofdifferent sizes.

Overlap or interposition objects that are overlapped or partially concealedby other objects will appear farther away

Gradient of texture objects that are closer have greater detail or texture

than those far away

Aerial Perspective close objects are bright and sharp; distant objects are

pastel and hazy


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Relative Motion or Motion Parallax- when moving our head from side to side,

nearby objects appear to move more than distant objects; far objects appearto move slower than nearby objects

Height on a plane or height in a field objects that are closer appear to be

lower in the field than objects that are farther away. Looming Effect or Optical Expansion when we approach objects, objects

close to us appear to be moving towards us faster than those far away

Accommodation lens of the eye must bend or adjust to bring to focus

objects that are relatively close

Perceptual Illusions: The Deception of Perceptions

An illusion is an incorrect or inaccurate perception of the stimulus beingpresented. An illusion is not the same as a hallucination where there is no stimulusbeing presented.

There is evidence that learning might have an important bearing on theperception of illusions. There are individual differences in how and how stronglyillusions are perceived, and illusions tend to diminish in effect the more you observethem.

Psychologists study illusions because they help us understand underlyingperceptual processes. Illusions occur because of cues in the environment.Motivation, expectancy, and/or experiences trick us into perceiving thingsincorrectly. Some common visual illusions are presented below:

The size distance hypothesis is based on size constancy and states that if two

objects project the same retinal image but appear at different distances from the

viewer, the object that appears farther from the viewer will typically be perceived aslarger. Because the top line in the Ponzo Illusion looks farther away (because of thelinear perspective) than the bottom line, we perceive the distant line to be longer.

The Ponzo Illusion

In the Ponzo Illusion, the top horizontal lineappears to be longer when in fact it is identical

in length to the bottom line. Because the top line

appears farther away, the principles of size

constancy as well as the size distance

hypothesis help explain this illusion.
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The Mller-Lyer and the Judd Illusions

The center horizontal lines in both figures

of the Mller-Lyer Illusion are the same

length, but the one in the top figure is

perceived as longer. This is because the

apparent length of both straight lines is

distorted by the arrowheads added to the

ends. The same is true for the Judd illusion

where the line segments on either side of

the center dot are actually equal in length.

The Ebbinghaus and Delboeuf Illusions

The Ebbinghaus Illusion occurs because of

comparative size. The center circle of the

figure on the left looks smaller than the

corresponding center circle of the figure on

the right even though they are both the

same size. The center circles are perceived

incorrectly because of the comparisons

made relative to their surrounding circles.

The same is true for the Delboeuf Illusionwhere the outer circle of the right figure (C)

is actually identical to the inner circle of the
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6. Sensation

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