Chapter 4: Sensation andperception

4.1 The visual system:essential of sight

*Identify the three properties oflight, and describe the role of key eye structures in vision.

Thethree properties of light are amplitude, wavelength and purity. Amplitude isdefined as the strength of light, which affects the brightness of light. Aslight is perceived as a kind of wave in physics, wavelength is defined as thedistance of the wave in a period and the wave processes the same in therepeated periods. The wavelength of light affects the color of light weperceive. The purity of light is obviously a property to define how richness orsaturate we perceive light. The more purity of light, the less whiteness weperceive.

Theeye is the first instrument in our vision perception. The light comes into theeye and then a primary image is formed on the retina. The image here is alwaysupside down, different from the object or scene in the real world. The imagethen fires the neural cells on the retina and the is processed into neuralsignals which then is transported to the brain to form the final visionperception. Vision perception is complex and the eye is no exception.

Althoughthe eye is complex too, the principal information processing in the keystructures of the eye is relatively easy to understand, set aside the otherstructures. The lens and the cornea are responsible for focusing the light. Thecornea is a thin layer on the eyeball, in front of the lens, and can’t adjustto different curvature. The lens could adjust to different curvature with themuscles attach on it. When we focus on near objects, the lens becomes fatterbecause the light doesn’t need to blend so much to fall on the retina. Theprocess of the lens’ adjusting to curvature is called accommodation. The muscleattached on the lens is also important. If we focus on the nearer objects orscene, the muscle adjusts to the lens to a thinner layer and this process lastsfor a long time, it is likely that the capability of the muscle adjustmentdecreases. This could result in nearsightedness, in which close objects areseen clearly but distant objects are vague. As we grow old, it is likely thatthe muscle gets used to a fatter layer pattern, this is called farsightedness,in which distant objects are seen clearly but close objects are vague. Bothvisual problems could be solved by wearing a pair of glasses to help change thecurvature of light to correctly fall on the retina. The retina is a neuraltissue at the back of the eye, from which the signals are processed into neuralsignals. The retina is called the brain’s envoy in the eye. There are two kindsof visual receptors on the retina, one is the rod and the other is the cone. Thecone is sensitive to light and detailed information while the rod functionsbetter in the low illumination. These two cells distribute differently on theretina. The cones are concentrated in the center of the retina and the numberdecreases towards the sides. The fovea is the center area of the retina wherethere is only one kind of visual receptor, the cone. Therefore, the fovea isspecialized for the detailed information of the objects. When the environmentchanges from dark to light or from light to dark, the two cells get triggeredcorresponded to ensure maximum amount of information we get. The signals thenflow to the ganglion cells which then concentrate as a stream to the nextinstrument. 

*Trace the routing of signals fromthe eye to the brain, and explain the brain’s role in visual informationprocessing.

Therouting of signals in the eye has been mentioned above. The signals then flowinto the optic nerves at the back of each eye, and then travel to the opticchiasm. The optic chiasm is a cross section at which part of the visualinformation is processed into the contrary side of the hemisphere. 90% of theaxons from the retinas synapse in an area called LGN and 10% in the midbrain.The signals in the midbrain is believed to cooperate with other sensoryinformation. The signals in LGN then process to the occipital lobe and finallyin the primary visual cortex.

Thecells in the visual cortex is highly specialized to recognize the specificfeatures of an objects. They have been characterized as feature detectors. Thesignals then travel through two pathways, namely what and where pathways, wherethe information of what and where is perceived.

*Distinguish two types of colormixing and compare the trichromatic and opponent process theories of colorvision.

Thetwo types of color mixing are subtractive color mixing and additive colormixing. These two types of color mixing could be distinguished by whether lightis removed or imposed. Subtractive color mixing works by removing somewavelengths of light, leaving less light than was originally there. Additivecolor mixing works by superimposing lights, putting more light in the mixturethan exists in any one light by itself. One example for subtractive colormixing is that when we mix yellow and blue, we get green. And one example foradditive color mixing is that when we shine red, green and blue, we get white.It has been revealed that human processes of color perception parallel additivemixing much more closely than subtractive mixing.

Thetrichromatic theory of color vision says that there are three kinds of colorreceptors in the eye that are specialized for three types of colors, and thesethree colors are red, green and blue. It is believed that all colors are thedifferent proportion mixing of these three colors. This theory is supported bythe fact that any light is a mixing of these three primary colors. The opponentprocess theory of color vision holds that there are three pairs of colors eachof which could be mixed into gray, and different color receptors could detectthe other color in pairs. Both theories then reconcile to form the currenttheory for color vision. There are three types of cones on the retina, and thethalamus and the visual cortex response in the opposite color in pair.

4.2 The visualsystem: perceptual processes

*Discuss the subjectivity of formperception, inattentional blindness, and the concept of feature analysis.

 The same visual inputs could result indifferent visual perception as a result of subjectivity. As it is showed byreversible figure, people could see different figures in the same picture. Inthe process of interpreting sensory input, a perceptual set is formed accordingto the outside information. This perceptual set varies among people as a resultof previous perception experience.

Inattentionalblindness is a result of focusing on some parts of the sensory input and setaside the other. It happens when you highly focus on particular selection ofsensory input and fail to notice other information. This is one of the reasonswhy traffic accidents happen.

Featureanalysis is the process of identifying specific features of an objects andaccordingly form a more complex information. The bottom-up processing model isthe assumption of feature analysis, as the information comes from the outsideworld and then progress to the whole.

*Understand Gestalt principles of

visual perception and the role of perceptual hypothesis in form perception.

 Gestalt psychology holds the view that thewhole can be greater than the sum of its parts. The Gestalt principles arelisted below. 1.Figure and ground. This is a fundamental process when peopledecide which part of the scene to perceive as figure and as ground. Usually,the smaller, the higher, the more symmetrical, the more likely the object isseen as the figure. In the example of reversible figure, when people changewhich part to perceive as the figure and which as the ground, different contentis revealed. 2.Proximity. Things that are nearer one another tend to be groupedtogether as a whole. 3.Closure. Things that could be grouped to form a closureare perceived as a whole. 4.Similarity. Things that share similar features tendto be grouped together. 5.Simplicity. The law of Pragnanz holds the view thatsignals tend to be perceived as simple as possible in a good form.5.Continuity. People tend to perceive things in a continuous direction or form.

Aperceptual hypothesis is an inference about what form could be responsible fora pattern of sensory stimulation. When people see something that is somehowdistorted, perceptual hypotheses are one kind of explanation for the accuracycapture of information. People are constantly making and testing hypothesesabout what’s in the real world. The context is an important element thatinfluences perceptual hypotheses.

*Identify the monocular andbinocular cues used in depth perception, and discuss cultural variations indepth perception.

Themonocular cues used in depth perception could be divided into two kinds. Onekind is the result of active use of the eye in viewing the world, which meansthe eye actively adjust to better accommodate the outside world. The other kindof monocular cues are pictorial depth cues. This kind of cues are aboutdistance that could be reflected in a flat picture. The cues include linearperspective, texture gradients, interposition, relative size, height in plane,light and shadow. Linear perspective indicates that lines converge in thedistance. Texture gradients reflects that when the surface of an object isnearer, the texture is usually more obvious. When one object is between anotherobject and you, it is obvious that the object between is nearer. This cue iscalled interposition. Relative size is based on the assumption that the closeran object, the larger it is. Height in plane reflects that the distant objectsappear to be higher. Light and shadow are also useful in judging distance.

Theprincipal binocular depth cue is retinal disparity. This is a fact that when anobject is within 25 feet, the pattern of visual information on the right andleft retinas are slightly different. Another binocular cue is vergence of thetwo eyes. When we see a near object, the two eye vergence more to focus on thenearer object. This signal is unconsciously processed to judging the distant inthe eye.

Itis showed by Hudson that different cultural groups have slightly differentunderstanding of the depth cues in the same picture. Cultural variationsinfluence depth perception mainly because different experience in understanding2D pictures corresponding with 3D world.

*Describe perceptual constancies,

and discuss what visual illusions reveal about perception.

Perceptualconstancies are a phenomenon that we experience a stable perception in the faceof continually changing sensory input. For example, when we see an objectapproaching us, we don’t perceive the object as something that is growingbigger and bigger. Rather, we know that it is an object coming towards us, andits size remains the same.

Visualillusions could be strong evidence to demonstrate processes of visualperception. The Muller-Lyer illusion reveals that the context influences ourvisual perception and may be due largely to a combination of size constancyprocesses and misperception of depth. The Ponzo illusion suggests linearperspective. The Ames room reveals the perceptual hypothesis that the room isvertically and horizontally rectangular. The moon illusion could be the resultof size constancy and the misperception of distance.

4.3 Theauditory system: hearing

*Identify the three properties of sound,and summarize information on human hearing capacities.

Thethree properties of sound are amplitude, frequency and purity. The amplitudeaffects the loudness of sound, the frequency affects the pitch and the purityaffects the timber. Notice that, the pitch depends mainly on the frequency ofsound, and the amplitude also have some influence on it.

Humancan hear sound with frequency ranging from 20 Hz to 20000 Hz. Notice that theability to hear high frequency sound decline as we grow old. The amplitude ofsound we can hear is measured in decibels, dB in short. It is difficult todescribe the relationship between the amplitude and the loudness. A rough ruleis that when the amplitude increases by 6-10 dBs, the loudness would double.Very loud sounds could lead to hear loss, especially as a result of hearingloud music using earphones. People are also sensitive to the timber of soundsas we could distinguish two different sounds easily.

*Describe how sensory processingoccurs in the ear, compare the place and frequency theories of pitchperception, and discuss factors in auditory localization.

 The ear could be divided into three parts,namely the inner ear, the middle ear and the outer ear. The pinna is astructure of the outer ear which is responsible for collecting sounds for thesurroundings. The pinna is important for the elevational information of sound.The outer ear transforms information through the vibration of air molecules. Thesound enters the ear and travels along the auditory canal toward the eardrum,where the vibration of air molecules starts to be transformed into thevibration of movable bones. The middle ear is made up of three tiny bones,which is also the tiniest bones in our body, namely the hammer, the anvil andthe stirrup. The aim for the bones’ moving is to convert relatively largemovements with little force into smaller motions with greater force. In otherwords, the aim is to amplify tiny changes in air pressure. The inner ear isimportant for auditory perception with the structure called the cochlea. Thecochlea is where the auditory receptors are. The signals passing through theinner ear will then be routed through the thalamus and to the auditory cortexto form auditory perception.

Placetheory and frequency theory are the two theory to explain how sound waves aretranslated into auditory perception. Place theory holds the view that haircells located on the basilar membrane is independently responsible for certainfrequencies. The auditory perception depends on which hair cells are triggeredand therefore which frequency is the sound. The perception of pitch is matchedwith the vibration of different portions, places, along the basilar membrane.Frequency theory holds the view that the perception of pitch corresponds to therate, or frequency, at which the entire basilar membrane vibrates. Thedifference is that hair cells work together but not independently here. Boththeories reconcile to form a better understanding of the transformation. Theplace theory is correct, however, hair cells works together as the frequencytheory suggests. The wave travels along the basilar membrane and peaks at aparticular place, depending on the frequency of the sound wave.

Thereare mainly two cues for auditory localization perception. The intensity orloudness indicates the distance of sound as near source of sound tend to have agreater intensity. Notice that, there may be loss of sound intensity if thesound comes from distant area. Another phenomenon is that when the sound comesfrom one side (the left or the right) of the head, the head may cast the soundwave and therefore an area of “shadow” is created. This will result in thedifference of sound intensity in the two ears. Similarly, if the sound comesfrom one side of the head, the sound arrives at the two ears slightly atdifferent time. This timing difference is a useful information to detect thelocation of the sound.

4.4 The othersenses: taste, smell, and touch

*Describe the stimulus andreceptors for taste, and review research on individual differences in tastesensitivity.

    The physical stimulus for taste is chemicalsubstances which are dissolvable in water. The receptors of taste are clustersof taste cells located in the taste buds that line the trenches around tinybumps on the tongue. Researches have shown that people response differently tocertain tastes. According to the taste sensitivity, people can be divided intothree groups, namely nontasters, medium tasters and supertasters. It seems thatsupertasters are more sensitive to sweet and bitter substances and thereforesupertasters are less likely to be fond of sweets and high-fat foods, as wellas alcohol and nicotine, which benefits health. One disadvantage ofsupertasters is that they are more likely to reject vegetables.

*Describe the stimulus andreceptors for smell, and evaluate human’s olfactory capabilities.

The

physical stimulus for smell is chemical substances which dissolved in water, or

to be more precise, in the mucus in the nose. The receptors for smell are

olfactory cilia, which is hair-like and located in the upper portion of the

nasal passages. It is proved that human have about 350 different types of

olfactory receptors that could identify over 10000 kinds of odors. However, the

problem is that people could not identify the location of the smell and it is

hard to describe and name a specific odor.

*Describe the stimulus andreceptors for touch, and explain what is known about pain perception.

Thephysical stimulus for touch is mechanical, thermal, and chemical energy whichcomes into contact with the skin. Human have at least six kinds of sensoryreceptors on the skin. These six types of skin are specialized for different sensorytasks. However, the distinctions are not so clear as other sensation. Althoughpain is unpleasant, it is one of the vital information for survival, whichcould be seen as a result of evolution. Too much pain and no pain couldinfluence our daily life in a great extent. Chronic pain affects many Americansand exerts negative impacts on their life quality. On the other hand, peoplewho could not perceive are constantly in the danger of unnoticeable damage thatcould easily end their life. Pain perception is highly subjective, as isrevealed by the placebo effects, and is affected by beliefs, expectations,personality, mood, and other factors. Pain signals can be blocked, and thisphenomenon could be explained by Gate-control theory which indicates that incomingpain sensations must pass through a “gate” in the spinal cord that can beclosed, thus blocking ascending pain signals.