imagine standing on a city street corner you might be struck by movement everywhere as cars and people go about their business by the sound of a street musician's melody or a horn honking in the distance by the smell of exhaust fumes or of food being sold by a nearby vendor and by the sensation of hard pavement under your feet we rely on our sensory systems to provide important information about our surroundings we use this information to successfully navigate and interact with our environment so that we can find nourishment seek shelter maintain social relationships and avoid potentially dangerous situations this chapter will provide an overview of how sensory information is received and processed by the nervous system and how that affects our conscious experience of the world we begin by learning the distinction between sensation and perception then we consider the physical properties of light and sound stimuli along with an overview of the basic structure and function of the major sensory systems the chapter will close with a discussion of a historically important theory of perception called gestalt learning objectives by the end of this section you will be able to distinguish between sensation and perception describe the concepts of absolute threshold and difference threshold discuss the roles attention motivation and sensory adaptation play in perception sensation what does it mean to sense something sensory receptors are specialized neurons that respond to specific types of stimuli when sensory information is detected by a sensory receptor sensation has occurred for example light that enters the eye causes chemical changes in cells that line the back of the eye these cells relay messages in the form of action potentials as you learned when studying biocsychology to the central nervous system the conversion from sensory stimulus energy to action potential is known as transduction you have probably known since elementary school that we have five senses vision hearing audition smell ulaction taste gustation and touch somato sensation it turns out that this notion of five senses is oversimplified we also have sensory systems that provide information about balance the vestibular sense body position and movement proprioception and kinesthesia pain noception and temperature thermosception the sensitivity of a given sensory system to the relevant stimuli can be expressed as an absolute threshold absolute threshold refers to the minimum amount of stimulus energy that must be present for the stimulus to be detected 50% of the time another way to think about this is by asking how dim can a light be or how soft can a sound be and still be detected half of the time the sensitivity of our sensory receptors can be quite amazing it has been estimated that on a clear night the most sensitive sensory cells in the back of the eye can detect a candle flame 30 m away okawa and samath 2007 under quiet conditions the hair cells the receptor cells of the inner ear can detect the tick of a clock 20 ft away gallanter 1962 it is also possible for us to get messages that are presented below the threshold for conscious awareness these are called subliminal messages a stimulus reaches a physiological threshold when it is strong enough to excite sensory receptors and send nerve impulses to the brain this is an absolute threshold a message below that threshold is said to be subliminal we receive it but we are not consciously aware of it over the years there has been a great deal of speculation about the use of subliminal messages in advertising rock music and self-help audio programs research evidence shows that in laboratory settings people can process and respond to information outside of awareness but this does not mean that we obey these messages like zombies in fact hidden messages have little effect on behavior outside the laboratory kunst wilson and zank 1980 renzinc 2004 nelson 2008 redell sarah lraine and gobanse 2009 lorsh derso and petty 2013 absolute thresholds are generally measured under incredibly controlled conditions in situations that are optimal for sensitivity sometimes we are more interested in how much difference in stimuli is required to detect a difference between them this is known as the just noticeable difference jn d or difference threshold unlike the absolute threshold the difference threshold changes depending on the stimulus intensity as an example imagine yourself in a very dark movie theater if an audience member were to receive a text message that caused the cell phone screen to light up chances are that many people would notice the change in illumination in the theater however if the same thing happened in a brightly lit arena during a basketball game very few people would notice the cell phone brightness does not change but its ability to be detected as a change in illumination varies dramatically between the two contexts ernst weber proposed this theory of change in difference threshold in the 1830s and it has become known as weber's law the difference threshold is a constant fraction of the original stimulus as the example illustrates perception while our sensory receptors are constantly collecting information from the environment it is ultimately how we interpret that information that affects how we interact with the world perception refers to the way sensory information is organized interpreted and consciously experienced perception involves both bottom up and top-down processing bottomup processing refers to sensory information from a stimulus in the environment driving a process and top-down processing refers to knowledge and expectancy driving a process as shown in figure 5.2 egith and yantis 1997 fine and minry 2009 yantis and egith 1999 figure 5.2 top down and bottom up are ways we process our perceptions imagine that you and some friends are sitting in a crowded restaurant eating lunch and talking it is very noisy and you are concentrating on your friend's face to hear what they are saying then the sound of breaking glass and clang of metal pans hitting the floor rings out the server dropped a large tray of food although you were attending to your meal and conversation that crashing sound would likely get through your attentional filters and capture your attention you would have no choice but to notice it that attentional capture would be caused by the sound from the environment it would be bottom up alternatively top- down processes are generally goal-directed slow deliberate effortful and under your control fine in minry 2009 miller and cohen 2001 miller and daspazto 2005 for instance if you misplaced your keys how would you look for them if you had a yellow key fob you would probably look for yellowess of a certain size in specific locations such as on the counter coffee table and other similar places you would not look for yellowess on your ceiling fan because you know keys are not normally lying on top of a ceiling fan that act of searching for a certain size of yellowess in some locations and not others would be top down under your control and based on your experience one way to think of this concept is that sensation is a physical process whereas perception is psychological for example upon walking into a kitchen and smelling the scent of baking cinnamon rolls the sensation is the scent receptors detecting the odor of cinnamon but the perception may be m this smells like the bread grandma used to bake when the family gathered for holidays although our perceptions are built from sensations not all sensations result in perception in fact we often don't perceive stimuli that remain relatively constant over prolonged periods of time this is known as sensory adaptation imagine going to a city that you have never visited you check into the hotel but when you get to your room there is a road construction sign with a bright flashing light outside your window unfortunately there are no other rooms available so you are stuck with a flashing light you decide to watch television to unwind the flashing light was extremely annoying when you first entered your room it was as if someone was continually turning a bright yellow spotlight on and off in your room but after watching television for a short while you no longer notice the light flashing the light is still flashing and filling your room with yellow light every few seconds and the photo receptors in your eyes still sense the light but you no longer perceive the rapid changes in lighting conditions that you no longer perceive the flashing light demonstrates sensory adaptation and shows that while closely associated sensation and perception are different there is another factor that affects sensation and perception attention attention plays a significant role in determining what is sensed versus what is perceived imagine you are at a party full of music chatter and laughter you get involved in an interesting conversation with a friend and you tune out all the background noise if someone interrupted you to ask what song had just finished playing you would probably be unable to answer that question one of the most interesting demonstrations of how important attention is in determining our perception of the environment occurred in a famous study conducted by daniel simons and christopher shabri 1999 in this study participants watched a video of people dressed in black and white passing basketballs participants were asked to count the number of times the team dressed in white passed the ball during the video a person dressed in a black gorilla costume walks among the two teams you would think that someone would notice the gorilla right nearly half of the people who watched the video didn't notice the gorilla at all despite the fact that he was clearly visible for 9 seconds because participants were so focused on the number of times the team dressed in white was passing the ball they completely tuned out other visual information inattentional blindness is the failure to notice something that is completely visible because the person was actively attending to something else and did not pay attention to other things mack androck 1998 simons and shabri 1999 in a similar experiment researchers tested inintentional blindness by asking participants to observe images moving across a computer screen they were instructed to focus on either white or black objects disregarding the other color when a red cross passed across the screen about onethird of subjects did not notice it figure 5.3 most simons schol and shabri 2000 figure 5.3 nearly one-third of participants in a study did not notice that a red cross passed on the screen because their attention was focused on the black or white figures credit corey zanker motivation can also affect perception have you ever been expecting a really important phone call and while taking a shower you think you hear the phone ringing only to discover that it is not if so then you have experienced how motivation to detect a meaningful stimulus can shift our ability to discriminate between a true sensory stimulus and background noise the ability to identify a stimulus when it is embedded in a distracting background is called signal detection theory this might also explain why a mother is awakened by a quiet murmur from her baby but not by other sounds that occur while she is asleep signal detection theory has practical applications such as increasing air traffic controller accuracy controllers need to be able to detect planes among many signals blips that appear on the radar screen and follow those planes as they move through the sky in fact the original work of the researcher who developed signal detection theory was focused on improving the sensitivity of air traffic controllers to plane blips sweats 1964 our perceptions can also be affected by our beliefs values prejudices expectations and life experiences as you will see later in this chapter individuals who are deprived of the experience of binocular vision during critical periods of development have trouble perceiving depth faucet wang and burch 2005 the shared experiences of people within a given cultural context can have pronounced effects on perception for example marshall seagal donald campbell and melville herskovitz 1963 published the results of a multinational study in which they demonstrated that individuals from western cultures were more prone to experience certain types of visual illusions than individuals from non-western cultures and vice versa one such illusion that westerners were more likely to experience was the muller liar illusion figure 5.4 the lines appear to be different lengths but they are actually the same length figure 5.4 in the muller liar illusion lines appear to be different lengths although they are identical a arrows at the ends of lines may make the line on the right appear longer although the lines are the same length b when applied to a three-dimensional image the line on the right again may appear longer although both black lines are the same length these perceptual differences were consistent with differences in the types of environmental features experienced on a regular basis by people in a given cultural context people in western cultures for example have a perceptual context of buildings with straight lines what seagal's study called a carpentered world seagal at all 1966 in contrast people from certain non-western cultures with an uncarpered view such as the zulu of south africa whose villages are made up of round huts arranged in circles are less susceptible to this illusion seagal at all 1999 it is not just vision that is affected by cultural factors indeed research has demonstrated that the ability to identify an odor and rate its pleasantness and its intensity varies cross-culturally ayab kanamura saiito distelle martinez gomez and hudson 1998 children described as thrillsekers are more likely to show taste preferences for intense sour flavors liam westerbeak walterink coke and degraphth 2004 which suggests that basic aspects of personality might affect perception furthermore individuals who hold positive attitudes toward reduced fat foods are more likely to rate foods labeled as reduced fat as tasting better than people who have less positive attitudes about these products aaron mela and evans 1994 learning objectives by the end of this section you will be able to describe important physical features of waveforms show how physical properties of light waves are associated with perceptual experience show how physical properties of sound waves are associated with perceptual experience visual and auditory stimuli both occur in the form of waves although the two stimuli are very different in terms of composition waveforms share similar characteristics that are especially important to our visual and auditory perceptions in this section we describe the physical properties of the waves as well as the perceptual experiences associated with them amplitude and wavelength two physical characteristics of a wave are amplitude and wavelength figure 5.5 the amplitude of a wave is the distance from the center line to the top point of the crest or the bottom point of the trough wavelength refers to the length of a wave from one peak to the next figure 5.5 the amplitude or height of a wave is measured from the peak to the trough the wavelength is measured from peak to peak wavelength is directly related to the frequency of a given waveform frequency refers to the number of waves that pass a given point in a given time period and is often expressed in terms of hertz hz or cycles per second longer wavelengths will have lower frequencies and shorter wavelengths will have higher frequencies figure 5.6 figure 5.6 this figure illustrates waves of differing wavelengths/frequencies at the top of the figure the red wave has a long wavelength/short frequency moving from top to bottom the wavelengths decrease and frequencies increase light waves the visible spectrum is the portion of the larger electromagnetic spectrum that we can see as figure 5.7 shows the electromagnetic spectrum encompasses all of the electromagnetic radiation that occurs in our environment and includes gamma rays x-rays ultraviolet light visible light infrared light microwaves and radio waves the visible spectrum in humans is associated with wavelengths that range from 380 to 740 nanome a very small distance since a nanometer nm is 1 billionth of a meter other species can detect other portions of the electromagnetic spectrum for instance honeybees can see light in the ultraviolet range wakua stenga and arakawa 2007 and some snakes can detect infrared radiation in addition to more traditional visual light cues chen deng broth ding and tang 2012 heartline cass and loop 1978 figure 5.7 light that is visible to humans makes up only a small portion of the electromagnetic spectrum in humans light wavelength is associated with perception of color figure 5.8 within the visible spectrum our experience of red is associated with longer wavelengths greens are intermediate and blues and violets are shorter in wavelength an easy way to remember this is the pneummonic roybiv red orange yellow green blue indigo violet the amplitude of light waves is associated with our experience of brightness or intensity of color with larger amplitudes appearing brighter figure 5.8 different wavelengths of light are associated with our perception of different colors credit modification of work by johannes alman sound waves like light waves the physical properties of sound waves are associated with various aspects of our perception of sound the frequency of a soundwave is associated with our perception of that sound's pitch highfrequency sound waves are perceived as high-pitched sounds while low frequency sound waves are perceived as low-pitched sounds the audible range of sound frequencies is between 20 and 20,000 hertz with greatest sensitivity to those frequencies that fall in the middle of this range as was the case with the visible spectrum other species show differences in their audible ranges for instance chickens have a very limited audible range from 125 to 2,000 hertz mice have an audible range from 1,00 to 91,000 hertz and the beluga whale's audible range is from 1,00 to 123,000 hertz our pet dogs and cats have audible ranges of about 70 to 45,000 hertz and 45 to 64,000 hertz respectively strain 2003 the loudness of a given sound is closely associated with the amplitude of the soundwave higher amplitudes are associated with louder sounds loudness is measured in terms of db db a logarithmic unit of sound intensity a typical conversation would correlate with 60 db a rock concert might check in at 120 db figure 5.9 a whisper 5 ft away or rustling leaves are at the low end of our hearing range sounds like a window air conditioner a normal conversation and even heavy traffic or a vacuum cleaner are within a tolerable range however there is the potential for hearing damage from about 80 db to 130 db these are sounds of a food processor power lawn mower heavy truck 25 ft away subway train 20 ft away live rock music and a jackhammer about onethird of all hearing loss is due to noise exposure and the louder the sound the shorter the exposure needed to cause hearing damage lei strapman leia and westerberg 2017 listening to music through earbuds at maximum volume around 100 to 105 db can cause noise induced hearing loss after 15 minutes of exposure although listening to music at maximum volume may not seem to cause damage it increases the risk of age related hearing loss kuawa and lieberman 2006 the threshold for pain is about 130 db a jet plane taking off or a revolver firing at close range dunl 1982 figure 5.9 this figure illustrates the loudness of common sounds although wave amplitude is generally associated with loudness there is some interaction between frequency and amplitude in our perception of loudness within the audible range for example a 10 herz soundwave is inaudible no matter the amplitude of the wave a 1,00 hz soundwave on the other hand would vary dramatically in terms of perceived loudness as the amplitude of the wave increased of course different musical instruments can play the same musical note at the same level of loudness yet they still sound quite different this is known as the tambber of a sound tambber refers to a sound's purity and it is affected by the complex interplay of frequency amplitude and timing of sound waves learning objectives by the end of this section you will be able to describe the basic anatomy of the visual system discuss how rods and cones contribute to different aspects of vision describe how moninocular and binocular cues are used in the perception of depth the visual system constructs a mental representation of the world around us figure 5.10 this contributes to our ability to successfully navigate through physical space and interact with important individuals and objects in our environments this section will provide an overview of the basic anatomy and function of the visual system in addition we will explore our ability to perceive color in depth figure 5.10 10 our eyes take in sensory information that helps us understand the world around us anatomy of the visual system the eye is the major sensory organ involved in vision figure 5.11 light waves are transmitted across the cornea and enter the eye through the pupil the cornea is the transparent covering over the eye it serves as a barrier between the inner eye and the outside world and it is involved in focusing light waves that enter the eye the pupil is the small opening in the eye through which light passes and the size of the pupil can change as a function of light levels as well as emotional arousal when light levels are low the pupil will become dilated or expanded to allow more light to enter the eye when light levels are high the pupil will constrict or become smaller to reduce the amount of light that enters the eye the pupil's size is controlled by muscles that are connected to the iris which is the colored portion of the eye figure 5.11 the anatomy of the eye is illustrated in this diagram after passing through the pupil light crosses the lens a curved transparent structure that serves to provide additional focus the lens is attached to muscles that can change its shape to aid in focusing light that is reflected from near or far objects in a normalsighted individual the lens will focus images perfectly on a small indentation in the back of the eye known as the fauva which is part of the retina the light sensitive lining of the eye the faua contains densely packed specialized photo receptor cells figure 5.12 these photo receptor cells known as cones are light detecting cells the cones are specialized types of photo receptors that work best in bright light conditions cones are very sensitive to acute detail and provide tremendous spatial resolution they are also directly involved in our ability to perceive color while cones are concentrated in the faua where images tend to be focused rods another type of photo receptor are located throughout the remainder of the retina rods are specialized photo receptors that work well in low light conditions and while they lack the spatial resolution and color function of the cones they are involved in our vision in dimly lit environments as well as in our perception of movement on the periphery of our visual field figure 5.12 the two types of photo receptors are shown in this image cones are colored green and rods are blue most of us have experienced the different sensitivities of rods and cones when making the transition from a brightly lit environment to a dimly lit environment imagine going to see a blockbuster movie on a clear summer day as you walk from the brightly lit lobby into the dark theater you notice that you immediately have difficulty seeing much of anything after a few minutes you begin to adjust to the darkness and can see the interior of the theater in the bright environment your vision was dominated primarily by cone activity as you move to the dark environment rod activity dominates but there is a delay in transitioning between the phases if your rods do not transform light into nerve impulses as easily and efficiently as they should you will have difficulty seeing in dim light a condition known as night blindness rods and cones are connected via several interneurons to retinal ganglen cells axons from the retinal ganglen cells converge and exit through the back of the eye to form the optic nerve the optic nerve carries visual information from the retina to the brain there is a point in the visual field called the blind spot even when light from a small object is focused on the blind spot we do not see it we are not consciously aware of our blind spots for two reasons first each eye gets a slightly different view of the visual field therefore the blind spots do not overlap second our visual system fills in the blind spot so that although we cannot respond to visual information that occurs in that portion of the visual field we are also not aware that information is missing the optic nerve from each eye merges just below the brain at a point called the optic cayazm as figure 5.13 shows the optic kayazm is an x-shaped structure that sits just below the cerebral cortex at the front of the brain at the point of the optic kayazm information from the right visual field which comes from both eyes is sent to the left side of the brain and information from the left visual field is sent to the right side of the brain figure 5.13 this illustration shows the optic cayazm at the front of the brain and the pathways to the occipital lobe at the back of the brain where visual sensations are processed into meaningful perceptions once inside the brain visual information is sent via a number of structures to the occipital lobe at the back of the brain for processing visual information might be processed in parallel pathways which can generally be described as the what pathway and the where how pathway the what pathway is involved in object recognition and identification while the where how pathway is involved with location in space and how one might interact with a particular visual stimulus milner and goodale 2008 lighter and haxby 1994 for example when you see a ball rolling down the street the what pathway identifies what the object is and the where how pathway identifies its location or movement in space what do you think the ethics of research using animals david hubil and torstston visel were awarded the nobel prize in medicine in 1981 for their research on the visual system they collaborated for more than 20 years and made significant discoveries about the neurology of visual perception hubil and vizzle 1959 1962 1963 1970 weasel and hubil 1963 they studied animals mostly cats and monkeys although they used several techniques they did considerable single unit recordings during which tiny electrodes were inserted in the animals brain to determine when a single cell was activated among their many discoveries they found that specific brain cells respond to lines with specific orientations called ocular dominance and they mapped the way those cells are arranged in areas of the visual cortex known as columns and hyper columns in some of their research they sutured one eye of newborn kittens closed and followed the development of the kitten's vision they discovered there was a critical period of development for vision if kittens were deprived of input from one eye other areas of their visual cortex filled in the area that was normally used by the eye that was sewn closed in other words neural connections that exist at birth can be lost if they are deprived of sensory input what do you think about sewing a kitten's eye closed for research to many animal advocates this would seem brutal abusive and unethical what if you could do research that would help ensure babies and children born with certain conditions could develop full vision instead of becoming blind would you want that research done would you conduct that research even if it meant causing some harm to cats would you think the same way if you were the parent of such a child what if you worked at the animal shelter like virtually every other industrialized nation the united states permits medical experimentation on animals with few limitations assuming sufficient scientific justification the goal of any laws that exist is not to ban such tests but rather to limit unnecessary animal suffering by establishing standards for the humane treatment and housing of animals in laboratories as explained by steven laam the director of the interdisciplinary center for bioeththics at yale 2012 possible legal and regulatory approaches to animal testing vary on a continuum from strong government regulation and monitoring of all experimentation at one end to a self-regulated approach that depends on the ethics of the researchers at the other end the united kingdom has the most significant regulatory scheme whereas japan uses the self-regulation approach the us approach is somewhere in the middle the result of a gradual blending of the two approaches there is no question that medical research is a valuable and important practice the question is whether the use of animals is a necessary or even best practice for producing the most reliable results alternatives include the use of patient drug databases virtual drug trials computer models and simulations and non-invasive imaging techniques such as magnetic resonance imaging and computed tomography scans animals in science alternatives nd other techniques such as micro dosing use humans not as test animals but as a means to improve the accuracy and reliability of test results in vitro methods based on human cell and tissue cultures stem cells and genetic testing methods are also increasingly available today at the local level any facility that uses animals and receives federal funding must have an institutional animal care and use committee that ensures that the nih guidelines are being followed the iau must include researchers administrators a veterinarian and at least one person with no ties to the institution that is a concerned citizen this committee also performs inspections of laboratories and protocols color and depth perception we do not see the world in black and white neither do we see it as two-dimensional 2d or flat just height and width no depth let's look at how color vision works and how we perceive three dimensions height width and depth color vision normal-sighted individuals have three different types of cones that mediate color vision each of these cone types is maximally sensitive to a slightly different wavelength of light according to the tri chromatic theory of color vision shown in figure 5.14 all colors in the spectrum can be produced by combining red green and blue the three types of cones are each receptive to one of the colors a graph is shown with sensitivity plotted on the y-axis and wavelength in nanometers plotted along the x-axis with measurements of 400 500 600 and 700 three lines in different colors move from the base to the peak of the y-axis and back to the base the blue line begins at 400 nmters and hits its peak of sensitivity around 455 nm before the sensitivity drops off at roughly the same rate at which it increased returning to the lowest sensitivity around 530 nm the green line begins at 400 nm and reaches its peak of sensitivity around 535 nm its sensitivity then decreases at roughly the same rate at which it increased returning to the lowest sensitivity around 650 nm the red line follows the same pattern as the first two beginning at 400 nm increasing and decreasing at the same rate and it hits its height of sensitivity around 580 nm below this graph is a horizontal bar showing the colors of the visible spectrum figure 5.14 this figure illustrates the different sensitivities for the three cone types found in a normal sighted individual credit modification of work by vanessa isawitz connect the concepts color blindness a personal story several years ago i dressed to go to a public function and walked into the kitchen where my seven-year-old daughter sat she looked up at me and in her most stern voice said "you can't wear that." i asked "why not?" and she informed me the colors of my clothes did not match she had complained frequently that i was bad at matching my shirts pants and ties but this time she sounded especially alarmed as a single father with no one else to ask at home i drove us to the nearest convenience store and asked the store clerk if my clothes matched she said my pants were a bright green color my shirt was a reddish orange and my tie was brown she looked at me quizzically and said "no way do your clothes match." over the next few days i started asking my co-workers and friends if my clothes matched after several days of being told that my co-workers just thought i had a really unique style i made an appointment with an eye doctor and was tested figure 5.15 it was then that i found out that i was color blind i cannot differentiate between most greens browns and reds fortunately other than unknowingly being badly dressed my color blindness rarely harms my day-to-day life figure 5.15 the ishihara test evaluates color perception by assessing whether individuals can discern numbers that appear in a circle of dots of varying colors and sizes some forms of color deficiency are rare seeing in grayscale only shades of black and white is extremely rare and people who do so only have rods which means they have very low visual acuity and cannot see very well the most common x- inherited abnormality is red green color blindness burch 2012 approximately 8% of males with white european descent 5% of asian males 4% of african males and less than 2% of indigenous american males australian males and polynesian males have red green color deficiency birch 2012 comparatively only about 0.4% in females from white european descent have red green color deficiency burch 2012 the tri chromatic theory of color vision is not the only theory another major theory of color vision is known as the opponent process theory according to this theory color is coded in opponent pairs black white yellow blue and green red the basic idea is that some cells of the visual system are excited by one of the opponent colors and inhibited by the other so a cell that was excited by wavelengths associated with green would be inhibited by wavelengths associated with red and vice versa one of the implications of opponent processing is that we do not experience greenish reds or yellowish blues as colors another implication is that this leads to the experience of negative after images an after image describes the continuation of a visual sensation after removal of the stimulus for example when you stare briefly at the sun and then look away from it you may still perceive a spot of light although the stimulus the sun has been removed when color is involved in the stimulus the color pairings identified in the opponent process theory lead to a negative after image you can test this concept using the flag in figure 5.16 figure 5.16 stare at the white dot for 30 to 60 seconds and then move your eyes to a blank piece of white paper what do you see this is known as a negative after image and it provides empirical support for the opponent process theory of color vision but these two theories the tri chromatic theory of color vision and the opponent process theory are not mutually exclusive research has shown that they just apply to different levels of the nervous system for visual processing on the retina triromatic theory applies the cones are responsive to three different wavelengths that represent red blue and green but once the signal moves past the retina on its way to the brain the cells respond in a way consistent with opponent process theory land 1959 kaiser 1997 depth perception our ability to perceive spatial relationships in three-dimensional 3d space is known as depth perception with depth perception we can describe things as being in front behind above below or to the side of other things our world is three-dimensional so it makes sense that our mental representation of the world has three-dimensional properties we use a variety of cues in a visual scene to establish our sense of depth some of these are binocular cues which means that they rely on the use of both eyes one example of a binocular depth cue is binocular disparity the slightly different view of the world that each of our eyes receives to experience this slightly different view do this simple exercise extend your arm fully and extend one of your fingers and focus on that finger now close your left eye without moving your head then open your left eye and close your right eye without moving your head you will notice that your finger seems to shift as you alternate between the two eyes because of these slightly different view each eye has of your finger a 3d movie works on the same principle the special glasses you wear allow the two slightly different images projected onto the screen to be seen separately by your left and your right eye as your brain processes these images you have the illusion that the leaping animal or running person is coming right toward you although we rely on binocular cues to experience depth in our 3d world we can also perceive depth in 2d arrays think about all the paintings and photographs you have seen generally you pick up on depth in these images even though the visual stimulus is 2d when we do this we are relying on a number of moninocular cues or cues that require only one eye if you think you can't see depth with one eye note that you don't bump into things when using only one eye while walking and in fact we have more moninocular cues than binocular cues an example of a moninocular cue would be what is known as linear perspective linear perspective refers to the fact that we perceive depth when we see two parallel lines that seem to converge in an image figure 5.17 some other moninocular depth cues are interposition the partial overlap of objects and the relative size and closeness of images to the horizon figure 5.17 we perceive depth in a two-dimensional figure like this one through the use of moninocular cues like linear perspective like the parallel lines converging as the road narrows in the distance credit mark dale moulder dig deeper stereo blindness bruce bridgeman was born with an extreme case of lazy eye that resulted in him being stereoblind or unable to respond to binocular cues of depth he relied heavily on moninocular depth cues but he never had a true appreciation of the 3d nature of the world around him this all changed one night in 2012 while bruce was seeing a movie with his wife the movie the couple was going to see was shot in 3d and even though he thought it was a waste of money bruce paid for the 3d glasses when he purchased his ticket as soon as the film began bruce put on the glasses and experienced something completely new for the first time in his life he appreciated the true depth of the world around him remarkably his ability to perceive depth persisted outside of the movie theater there are cells in the nervous system that respond to binocular depth cues normally these cells require activation during early development in order to persist so experts familiar with bruce's case and others like his assume that at some point in his development bruce must have experienced at least a fleeting moment of binocular vision it was enough to ensure the survival of the cells in the visual system tuned to binocular cues the mystery now is why it took bruce nearly 70 years to have these cells activated peek 2012 learning objectives by the end of this section you will be able to describe the basic anatomy and function of the auditory system explain how we encode and perceive pitch discuss how we localize sound our auditory system converts pressure waves into meaningful sounds this translates into our ability to hear the sounds of nature to appreciate the beauty of music and to communicate with one another through spoken language this section will provide an overview of the basic anatomy and function of the auditory system it will include a discussion of how the sensory stimulus is translated into neural impulses where in the brain that information is processed how we perceive pitch and how we know where sound is coming from anatomy of the auditory system the ear can be separated into multiple sections the outer ear includes the pa which is the visible part of the ear that protrudes from our heads the auditory canal and the tempanic membrane or eardrum the middle ear contains three tiny bones known as the oicles which are named the malas or hammer incas or anvil and the stapes or sturrup the inner ear contains the semic-ircular canals which are involved in balance and movement the vestibular sense and the cookia the cookia is a fluid-filled snail-shaped structure that contains the sensory receptor cells hair cells of the auditory system figure 5.18 figure 5.18 the ear is divided into outer pa and tempanic membrane middle the three oicles malus incas and stapes and inner clay and basil membrane divisions sound waves travel along the auditory canal and strike the tempanic membrane causing it to vibrate this vibration results in movement of the three oicles as the oicles move the stapes presses into a thin membrane of the cookia known as the oval window as the stapes presses into the oval window the fluid inside the cookia begins to move which in turn stimulates hair cells which are auditory receptor cells of the inner ear embedded in the basler membrane the basler membrane is a thin strip of tissue within the cookia the activation of hair cells is a mechanical process the stimulation of the hair cell ultimately leads to activation of the cell as hair cells become activated they generate neural impulses that travel along the auditory nerve to the brain auditory information is shuttled to the inferior caliculus the medial geniculate nucleus of the phalamus and finally to the auditory cortex in the temporal lobe of the brain for processing like the visual system there is also evidence suggesting that information about auditory recognition and localization is processed in parallel streams rouscher and tion 2000 reneier at all 2009 pitch perception different frequencies of sound waves are associated with differences in our perception of the pitch of those sounds low frequency sounds are lower pitched and high frequency sounds are higher pitched how does the auditory system differentiate among various pitches several theories have been proposed to account for pitch perception we'll discuss two of them here temporal theory and place theory the temporal theory of pitch perception asserts that frequency is coded by the activity level of a sensory neuron this would mean that a given hair cell would fire action potentials related to the frequency of the soundwave while this is a very intuitive explanation we detect such a broad range of frequencies 20 to 20,000 hertz that the frequency of action potentials fired by hair cells cannot account for the entire range because of properties related to sodium channels on the neuronal membrane that are involved in action potentials there is a point at which a cell cannot fire any faster shemma 2001 the place theory of pitch perception suggests that different portions of the basor membrane are sensitive to sounds of different frequencies more specifically the base of the basler membrane responds best to high frequencies and the tip of the basler membrane responds best to low frequencies therefore hair cells that are in the base portion would be labeled as high pitch receptors while those in the tip of basler membrane would be labeled as low pitch receptors shamma 2001 in reality both theories explain different aspects of pitch perception at frequencies up to about 4,000 hertz it is clear that both the rate of action potentials and place contribute to our perception of pitch however much higher frequency sounds can only be encoded using place cues shamma 2001 sound localization the ability to locate sound in our environments is an important part of hearing localizing sound could be considered similar to the way that we perceive depth in our visual fields like the moninocular and binocular cues that provided information about depth the auditory system uses both monural one-eared and binaural two-eared cues to localize sound each pa interacts with incoming sound waves differently depending on the sound source relative to our bodies this interaction provides a manoral cue that is helpful in locating sounds that occur above or below and in front or behind us the sound waves received by your two ears from sounds that come from directly above below in front or behind you would be identical therefore manural cues are essential growth pekka and macalpine 2010 binaural cues on the other hand provide information on the location of a sound along a horizontal axis by relying on differences in patterns of vibration of the eardrum between our two ears if a sound comes from an off-center location it creates two types of binaural cues interoral level differences and interoral timing differences interoral level difference refers to the fact that a sound coming from the right side of your body is more intense at your right ear than at your left ear because of the attenuation of the soundwave as it passes through your head interoral timing difference refers to the small difference in the time at which a given sound wave arrives at each ear figure 5.19 certain brain areas monitor these differences to construct where along a horizontal axis a sound originates growth at all 2010 figure 5.19 localizing sound involves the use of both monoral and binaural cues credit plane modification of work by max fandal hearing loss deafness is the partial or complete inability to hear some people are born without hearing which is known as congenital deafness other people suffer from conductive hearing loss which is due to a problem delivering sound energy to the cookia causes for conductive hearing loss include blockage of the ear canal a hole in the tempanic membrane problems with the oicles or fluid in the space between the eard drum and cookia another group of people suffer from sensor neural hearing loss which is the most common form of hearing loss sensory neural hearing loss can be caused by many factors such as aging head or acoustic trauma infections and diseases such as measles or mumps medications environmental effects such as noise exposure noise induced hearing loss as shown in figure 5.20 tumors and toxins such as those found in certain solvents and metals figure 5.20 environmental factors that can lead to sensory and neural hearing loss include regular exposure to loud music or construction equipment a musical performers and b construction workers are at risk for this type of hearing loss given the mechanical nature by which the soundwave stimulus is transmitted from the eardrum through the oicles to the oval window of the cookia some degree of hearing loss is inevitable with conductive hearing loss hearing problems are associated with a failure in the vibration of the eardrum and/or movement of the oicles these problems are often dealt with through devices like hearing aids that amplify incoming sound waves to make vibration of the eardrum and movement of the oicles more likely to occur when the hearing problem is associated with a failure to transmit neural signals from the cookia to the brain it is called sensory neural hearing loss one disease that results in sensory neural hearing loss is minè's disease although not well understood minè's disease results in a degeneration of inner ear structures that can lead to hearing loss tenitis constant ringing or buzzing vertigo a sense of spinning and an increase in pressure within the inner ear simon and majerian 2011 this kind of loss cannot be treated with hearing aids but some individuals might be candidates for a coclar implant as a treatment option coclear implants are electronic devices that consist of a microphone a speech processor and an electrode array the device receives incoming sound information and directly stimulates the auditory nerve to transmit information to the brain what do you think deaf culture in the united states and other places around the world deaf people have their own language schools and customs this is called deaf culture in the united states deaf individuals often communicate using american sign language asl asl has no verbal component and is based entirely on visual signs and gestures the primary mode of communication is signing one of the values of deaf culture is to continue traditions like using sign language rather than teaching deaf children to try to speak read lips or have colear implant surgery when a child is diagnosed as deaf parents have difficult decisions to make should the child be enrolled in mainstream schools and taught to verbalize and read lips or should the child be sent to a school for deaf children to learn asl and have significant exposure to deaf culture do you think there might be differences in the way that parents approach these decisions depending on whether or not they are also deaf learning objectives by the end of this section you will be able to describe the basic functions of the chemical senses explain the basic functions of the somataensory noceptive and thermoseptive sensory systems describe the basic functions of the vestibular proprioceptive and kinesthetic sensory systems vision and hearing have received an incredible amount of attention from researchers over the years while there is still much to be learned about how these sensory systems work we have a much better understanding of them than of our other sensory modalities in this section we will explore our chemical senses taste and smell and our body senses touch temperature pain balance and body position the chemical senses taste gustation and smell olfaction are called chemical senses because both have sensory receptors that respond to molecules in the food we eat or in the air we breathe there is a pronounced interaction between our chemical senses for example when we describe the flavor of a given food we are really referring to both gustatory and alactory properties of the food working in combination taste gustation you have learned since elementary school that there are four basic groupings of taste sweet salty sour and bitter research demonstrates however that we have at least six taste groupings umami is our fifth taste umami is actually a japanese word that roughly translates to yummy and it is associated with a taste for monosodium glutamate kinnaman and vandenbuk 2009 there is also a growing body of experimental evidence suggesting that we possess a taste for the fatty content of a given food mizushia ininoa and fushiki 2007 molecules from the food and beverages we consume dissolve in our saliva and interact with taste receptors on our tongue and in our mouth and throat taste buds are formed by groupings of taste receptor cells with hairlike extensions that protrude into the central pore of the taste bud figure 5.21 taste buds have a life cycle of 10 days to two weeks so even destroying some by burning your tongue won't have any long-term effect they just grow right back taste molecules bind to receptors on this extension and cause chemical changes within the sensory cell that result in neural impulses being transmitted to the brain via different nerves depending on where the receptor is located taste information is transmitted to the medulla phalamus and lybic system and to the gustatory cortex which is tucked underneath the overlap between the frontal and temporal loes maf haley and fontineini 2012 roper 2013 figure 5.21 21 a taste buds are composed of a number of individual taste receptor cells that transmit information to nerves b this microraph shows a close-up view of the tongue surface smell olaction olactory receptor cells are located in a mucous membrane at the top of the nose small hair-ike extensions from these receptors serve as the sites for odor molecules dissolved in the mucus to interact with chemical receptors located on these extensions figure 5.22 22 once an odor molecule has bound a given receptor chemical changes within the cell result in signals being sent to the olfactory bulb a bulb-like structure at the tip of the frontal lobe where the olfactory nerves begin from the olfactory bulb information is sent to regions of the lbic system and to the primary olfactory cortex which is located very near the gustatory cortex lovichi and belucio 2012 spores at all 2013 figure 5.22 22 olfactory receptors are the hair-like parts that extend from the olfactory bulb into the mucous membrane of the nasal cavity there is tremendous variation in the sensitivity of the olfactory systems of different species we often think of dogs as having far superior olfactory systems than our own and indeed dogs can do some remarkable things with their noses there is some evidence to suggest that dogs can smell dangerous drops in blood glucose levels as well as cancerous tumors wells 2010 dogs extraordinary alactory abilities may be due to the increased number of functional genes for alactory receptors between 800 and 1200 compared to the fewer than 400 observed in humans and other primates neimura and nay 2007 many species respond to chemical messages known as pherommones sent by another individual wasaki and prey 2004 pherommonal communication often involves providing information about the reproductive status of a potential mate so for example when a female rat is ready to mate it secretes pherommonal signals that draw attention from nearby male rats pherommonal activation is actually an important component in eliciting sexual behavior in the male rat furlow 1996 2012 pervvis and hannes 1972 saxs 1997 there has also been a good deal of research and controversy about pherommones in humans comfort 1971 russell 1976 wolf gang kimble 1992 weller 1998 touch thermosception and noception a number of receptors are distributed throughout the skin to respond to various touch related stimuli figure 5.23 these receptors include mner's corp pusles pacinian corp pusles merkel's discs and rafini corpusles mner's corp pussles respond to pressure and lower frequency vibrations and pinian corpusles detect transient pressure and higher frequency vibrations merkel's discs respond to light pressure while rafini corpusles detect stretch aera and genti 2013 in addition to the receptors located in the skin there are also a number of free nerve endings that serve sensory functions these nerve endings respond to a variety of different types of touchreated stimuli and serve as sensory receptors for both thermosception temperature perception and noception a signal indicating potential harm and maybe pain garland 2012 po and re 2012 spray 1986 sensory information collected from the receptors and free nerve endings travels up the spinal cord and is transmitted to regions of the medulla phalamus and ultimately to the somatoensory cortex which is located in the post central gyus of the parietal lobe pain perception pain is an unpleasant experience that involves both physical and psychological components feeling pain is quite adaptive because it makes us aware of an injury and it motivates us to remove ourselves from the cause of that injury in addition pain also makes us less likely to suffer additional injury because we will be gentler with our injured body parts generally speaking pain can be considered to be neuropathic or inflammatory in nature pain that signals some type of tissue damage is known as inflammatory pain in some situations pain results from damage to neurons of either the peripheral or central nervous system as a result pain signals that are sent to the brain get exaggerated this type of pain is known as neuropathic pain multiple treatment options for pain relief range from relaxation therapy to the use of analesic medications to deep brain stimulation the most effective treatment option for a given individual will depend on a number of considerations including the severity and persistence of the pain and any medical/schyological conditions some individuals are born without the ability to feel pain this very rare genetic disorder is known as congenital insensitivity to pain or congenital analesia while those with congenital analesia can detect differences in temperature and pressure they cannot experience pain as a result they often suffer significant injuries young children have serious mouth and tongue injuries because they have bitten themselves repeatedly not surprisingly individuals suffering from this disorder have much shorter life expectancies due to their injuries and secondary infections of injured sites us national library of medicine 2013 the vestibular sense proprioception and kinesthesia the vestibular sense contributes to our ability to maintain balance and body posture as figure 5.24 24 shows the major sensory organs utricle sacle and the three semic-ircular canals of this system are located next to the cookia in the inner ear the vestibular organs are fluid fil and have hair cells similar to the ones found in the auditory system which respond to movement of the head and gravitational forces when these hair cells are stimulated they send signals to the brain via the vestibular nerve although we may not be consciously aware of our vestibular systems sensory information under normal circumstances its importance is apparent when we experience motion sickness and/or dizziness related to infections of the inner ear khan and chang 2013 figure 5.24 the major sensory organs of the vestibular system are located next to the cookia in the inner ear these include the utricle sacule and the three semic-ircular canals posterior superior and horizontal in addition to maintaining balance the vestibular system collects information critical for controlling movement and the reflexes that move various parts of our bodies to compensate for changes in body position therefore both proprioception perception of body position and kinesthesia perception of the body's movement through space interact with information provided by the vestibular system these sensory systems also gather information from receptors that respond to stretch and tension in muscles joints skin and tendons lakner and dizio 2005 proski 2006 proski and gandavia 2012 proprioceptive and kinesthetic information travels to the brain via the spinal column several cortical regions in addition to the cerebellum receive information from and send information to the sensory organs of the proprioceptive and kinesthetic systems learning objectives by the end of this section you will be able to explain the figure ground relationship define gestalt principles of grouping describe how perceptual set is influenced by an individual's characteristics and mental state in the early part of the 20th century max worerimer published a paper demonstrating that individuals perceived motion in rapidly flickering static images an insight that came to him as he used a child's toy to chistocope worerimemer and his assistants wolf gang kohler and kurt kofka who later became his partners believed that perception involved more than simply combining sensory stimuli this belief led to a new movement within the field of psychology known as gestalt psychology the word gestalt literally means form or pattern but its use reflects the idea that the whole is different from the sum of its parts in other words the brain creates a perception that is more than simply the sum of available sensory inputs and it does so in predictable ways gestalt psychologists translated these predictable ways into principles by which we organize sensory information as a result gestalt psychology has been extremely influential in the area of sensation and perception rock and palmer 1990 one gestalt principle is the figure ground relationship according to this principle we tend to segment our visual world into figure and ground figure is the object or person that is the focus of the visual field while the ground is the background as figure 5.25 25 shows our perception can vary tremendously depending on what is perceived as figure and what is perceived as ground presumably our ability to interpret sensory information depends on what we label as figure and what we label as ground in any particular case although this assumption has been called into question peterson and gibson 1994 visera and o'reilly 1998 an illustration shows two identical black facelike shapes that face towards one another and one white vase-like shape that occupies all of the space in between them depending on which part of the illustration is focused on either the black shapes or the white shape may appear to be the object of the illustration leaving the others perceived as negative space figure 5.25 the concept of figure ground relationship explains why this image can be perceived either as a vase or as a pair of faces another gestalt principle for organizing sensory stimuli into meaningful perception is proximity this principle asserts that things that are close to one another tend to be grouped together as figure 5.26 illustrates figure 5.26 the gestalt principle of proximity suggests that you see a one block of dots on the left side and b three columns on the right side how we read something provides another illustration of the proximity concept for example we read this sentence like this not i heo r hat we group the letters of a given word together because there are no spaces between the letters and we perceive words because there are spaces between each word here are some more examples kenny omno featuring his sentence what do he des we might also use the principle of similarity to group things in our visual fields according to this principle things that are alike tend to be grouped together figure 5.27 for example when watching a football game we tend to group individuals based on the colors of their uniforms when watching an offensive drive we can get a sense of the two teams simply by grouping along this dimension two additional gestalt principles are the law of continuity or good continuation and closure the law of continuity suggests that we are more likely to perceive continuous smooth flowing lines rather than jagged broken lines figure 5.28 the principle of closure states that we organize our perceptions into complete objects rather than as a series of parts figure 5.29 figure 5.28 good continuation would suggest that we are more likely to perceive this as two overlapping lines rather than four lines meeting in the center figure 5.29 closure suggests that we will perceive a complete circle and rectangle rather than a series of segments according to gestalt theorists pattern perception or our ability to discriminate among different figures and shapes occurs by following the principles described above you probably feel fairly certain that your perception accurately matches the real world but this is not always the case our perceptions are based on perceptual hypotheses educated guesses that we make while interpreting sensory information these hypotheses are informed by a number of factors including our personalities experiences and expectations we use these hypotheses to generate our perceptual set for instance research has demonstrated that those who are given verbal priming produce a biased interpretation of complex ambiguous figures gulcation and woodbury 2010 dig deeper the depths of perception bias prejudice and cultural factors in this chapter you have learned that perception is a complex process built from sensations but influenced by our own experiences biases prejudices and cultures perceptions can be very different from person to person research suggests that implicit racial prejudice and stereotypes affect perception for instance several studies have demonstrated that non-black participants identify weapons faster and are more likely to identify non-weapons as weapons when the image of the weapon is paired with the image of a black person payne 2001 payne shimizu and jacobe 2005 furthermore white individuals decisions to shoot an armed target in a video game is made more quickly when the target is black corell park jud and wittenbrink 2002 corell erland and ido 2006 this research is important considering the number of very high-profile cases in the last few decades in which black people were killed by people who claimed to believe that the unarmed individuals were armed and or represented some threat to their personal safety