THE LONG STRANGE TRIP FROM SENSATION TO PERCEPTION
· The sense organs transform information from its physical form into a nerve impulse and transmit it to the brain, which organizes that information, interprets it, and then initiates a response.
· Sensation is the stimulation of our sense organs by the outer world.
o Our sense organs detect different features of our surroundings: eyes are sensitive to light waves, ears to sounds, skin to touch and pressure, tongues to tastes, and noses to odors.
· Perception is the act of organizing and interpreting sensory experience.
o It is how our psychological world represents our physical world.
· Before brains can create meaning from sensory information, our sense organs transform physical stimuli from the outer world to a form that the brain can use – action potentials.
Basic Sensory Processes
· Sensory adaptation refers to our diminished sensitivity to a constant stimulation. It ensures that we notice changes in stimulation more than stimulation itself.
· Once we know that a physical stimulus is something to attend to, the sense organs convert it into action potentials. This conversion of physical into neural information is called transduction.
o This occurs when cells in the retina change light waves to neural energy; when hair cells in the inner ear change sound waves to neural energy; when chemicals in the air bind to receptors in the nose; when food chemicals stimulate taste buds on the tongue; and when pressure and temperature stimulate nerve cells in the skin.
· Psychophysics is the study of how people psychologically perceive physical stimuli such as light and sound waves and touch.
o An absolute threshold is the lowest intensity level of a stimulus we can detect half of the time.
o Under ideal laboratory conditions, an average person on a very clear night could detect a single candle from 30 miles away or could detect 1 teaspoon of sugar in two gallons of water as compared to two gallons of pure water.
o One problem with using absolute thresholds is that detecting sensations is not only a matter of intensity of the stimulus, but also the decision-making process of the person in a particular context.
o Signal detection theory takes into account both stimulus intensity and the decision-making processes people use when saying whether they detect a stimulus.
o In signal detection research a low-intensity stimulus is presented on some occasions and not presented on other occasions. Instead of having a 50% detection line, signal detection presents only a single low-intensity stimulus.
§ There are four possible outcomes:
· A hit is correctly detecting a stimulus that is there.
· A miss is failing to detect a stimulus that is there.
· A false alarm is saying that a stimulus exists when it does not.
· A correct rejection is not reporting a stimulus that is not there.
§ The participant’s responses create a profile of hits, misses, false alarms, and correct rejections.
§ CONNECTION: Attention helps prevent sensory overload by filtering out sensory stimuli that aren’t important (see Chapter 6).
o Difference threshold (also known as the just noticeable difference [JND]) is the smallest amount of change between two stimuli that a person can detect half of the time.
o Weber’s law says that the size of the JND is a constant fraction of the intensity of the stimulus (e.g., 3% for weight perception).
o Our frame of mind can impact how we perceive things – this is known as our perceptual set.
§ Examples: mood, health, knowledge of how the world works, and cultural upbringing.
· Humans rely more on our sense of sight than other sense information.
Sensing Visual Stimuli
· The eye bends light, converts light energy to neural energy, and sends that information to the brain for further processing.
· The eye is the gateway to vision, but very little of what we experience as vision actually happens in the eye – it happens in the brain.
Vision and the Eye
o Light enters the eye at the cornea – a hard covering that protects the lens.
o It then passes through liquid until it reaches a hole called the pupil. Then it goes.
o The colored part of the eye, the iris, adjusts the pupil to control the amount of light entering the eye (for example, when light is very bright, the iris shrinks the pupil until you can adjust).
o The light then passes through the lens, which bends the light rays.
o Muscles around the lens alter its shape, depending on the distance of an object, to allow it to focus light on the retina – a thin layer of nerve tissue that lines the back of the eye. The process by which the muscles control the shape of the lens to adjust to viewing objects at different distances is known as accommodation.
o The retina consists of several layers of cells. The light that hits the retina travels through several cell layers before processing begins.
§ The deepest layer of cells, where processing of light energy begins, is the layer of photoreceptors, which convert light energy into nerve energy. In other words, they are transducers.
· Rods are most responsive to dark and light contrast. These very sensitive cells work well at low illumination and the dark.
o The process of adjustment to seeing in the dark is known as dark adaptation.
· Cones are responsible for color vision and are most functional in conditions of bright light. They act much more quickly than rods.
o The fovea—located on the back of the retina—contains the highest concentration of cones in the retina. As such, when images are projected onto the fovea we have the greatest visual acuity (i.e., clearest vision). In other words, our ability to see clearly depends on our cones.
Vision and the Brain
o After transduction at the photoreceptor layer, the visual information is processed by different layers of cells in the retina.
o Axons of the ganglion cells make up the optic nerve, which transmits the signals from the eye to the brain.
o The point at which the optic nerve exits the eye is the blind spot of the retina because at this point there are no receptor cells (so nothing is seen).
o When light enters the eye, the lens bends the light in such a way that the image is upside down compared to the orientation of the object in the outside world. The brain reorients the inverted image so that our world is right-side up.
o In people with normal vision the lens projects the image to hit just on the retina.
o In people who are nearsighted (people who can see things close to them but have problems with distance) the image focuses slightly in front of the retina.
o In people who are farsighted (they can see things far away but not up close) the image actually focuses behind the retina.
§ Age-related farsightedness occurs because, as we age, the lens becomes less flexible, making it more likely for images to be focused behind the retina.
o The optic nerve carries impulses to the thalamus (the brain’s sensory relay station) which then sends the message to the visual cortex of the occipital lobes.
o The information from the left visual field is processed in the brain’s right hemisphere, and the information from the right visual field is processed in the brain’s left hemisphere.
§ In each eye, each half of the retina sends out its own axons. So each optic nerve has two strands. One strand from each eye contains axons that travel from the retina to the thalamus and on to the visual cortex of the same side of the brain as the eye from which the axons come. The other strands cross to the opposite side of the brain in an area called the optic chiasm.
· CONNECTION: People whose optic chiasms have been severed respond to images projected to only one visual field (see Chapter 3).
o A cluster of the neuron cell bodies in the thalamus form the lateral geniculate nucleus (LGN). Visual information creates a point-by-point representation on the tissue of the LGN, meaning that patterns of neural firing that correspond to the shape projected on a specific region of retina affect a similar layout of cells in here. In other words, the retina and the LGN represent visual information in similar ways.
o Fibers from the LGN in the thalamus then travel to the visual cortex in the occipital lobes. Neurons in the visual cortex analyze the retinal image in terms of its various patterns, contrasts, lines, and edges.
§ Different cortical cells handle different aspects of this analysis.
Breaking New Ground: Specific Functions of Individual Neurons in Vision
· See separate section for detailed explanation.
Perceiving Visual Stimuli
o Feature detectors play a role in how we perceive movement and form. We perceive movement when an image moves across the retina. Simple and complex cells respond to either the orientation or direction of moving images.
o Several factors contribute to how we perceive movement.
§ One factor is the background against which an object moves and another factor is the size of the object. When an object moves across a complex background, it appears to move faster than when it moves across a simple background.
§ The size of the object affects perception of movement. All things being equal, smaller objects appear to move faster than larger objects.
§ We can also be fooled into thinking something is moving when it is not. We refer to this illusion as apparent motion because our brains interpret images that move across our retinas as movement.
· Movement neurons respond only when the image itself moves and not when the eye itself moves. This is one way the brain can determine the difference between real and false movement.
o Depth perception allows for the discrimination between what is near and far from us.
§ Binocular depth cues rely on input from both eyes.
· Binocular disparity comes from the fact that the eyes are separated by a few inches, so the image from each eye will provide slightly different viewpoints.
· Convergence occurs when the eyes move inward as an object moves closer to you. The muscles that move the eyeball contract and the brain makes use of the feedback from these muscles to perceive distance. This is the most effective as a depth cue for stimuli that are within 10 feet of us.
§ Monocular depth cues rely on input from one eye.
· Linear perspective involves parallel lines that converge or come together the further away they are from the viewer. The more they converge, the greater distance we perceive. A good example of this is the Müller-Lyer illusion.
· Texture gradient happens when the texture of a surface becomes more tightly packed together and denser as the surface moves to the background. These changes in textural information help us judge depth.
· Atmospheric perspective comes from looking across a vast space into the distance in the outdoors. Objects farther away appear more blurred and bluish as a result.
· Interposition happens when objects closer to the viewer often overlap with those farther away.
o The image on our retinas changes shape and size as objects move through space. The ability of the brain to preserve perception of such objects in spite of the changes in retinal image is known as perceptual constancy.
Size Constancy: We see things as the same size regardless of
the changing size of the image on the retina, because we know what the size of the
object is. A good example here is
§ Shape Constancy: The brain uses its knowledge of shapes to override changing retinal images that might make the world very confusing.
Organizing Visual Information: Gestalt Laws of Grouping
o Gestalt psychologists recognized that often we perceive wholes as more than merely the sum of their parts.
o Max Wertheimer, Kurt Koffka, and Wolfgang Köhler studied visual perception in the early 20th Century and described a set of principles or laws by which people organize elements of figures or scenes into whole objects.
o The law of similarity is the tendency to group like objects together.
o The law of continuity is the tendency to see points or lines in such a way that they follow a continuous path.
o The law of proximity says that we tend to group together objects that are near one another.
§ CONNECTION: The Gestalt law of proximity makes use of the short-term memory technique called “chunking” (see Chapter 7).
o The law of closure occurs when we perceive a whole object in the absence of complete information.
o Figure vs. ground: The figure is the thing that stands in front of a somewhat unformed background (i.e., the ground). Perhaps the most famous example of figure-ground effects is Rubin’s (1915) face-vase figure (see Suggested Websites).
o The Müller-Lyer illusion results from our tendency to see the right line as the inside corner of a room and the left one as the outside corner of a room or building, making use of the monocular depth cue of linear perspective.
Visual Perception: Bottom-Up or Top-Down?
o Bottom-up processing is the process of building a visual experience from smaller pieces. We put the pieces together, and then we “see” the whole (e.g., reading).
o Top-down processing occurs when the perception of the whole guides perception of smaller elemental features (e.g., facial recognition).
o Which process we use depends on the nature of the information being processed.
The Perception of Color
o The perception of color varies depending on our photoreceptors, our brains, and the physical characteristics of the stimulus at which we look.
§ Color perception is partly determined by wavelength, measured in billionths of a meter or nanometers (abbreviated nm). The spectrum of color visible to humans ranges from 400 nm, which most of us perceive to be the color blue, to 700 nm, which most of us perceive as red. Light that we perceive as green is at 550 nm.
· Two Theories of Color Vision
o Young and Helmholtz’s trichromatic color theory says that there are three kinds of cones: red, green, and blue, and all color we experience must result from a mixing of these three colors of light. This mixing occurs inside the eye in terms of how different kinds of cones respond to different wavelengths of light.
§ The human retina does contain three kinds of receptor cones, each sensitive to different wavelengths of light. The red cones fire in response to longer wavelength light. Green cones respond to medium wavelength light, and blue cones respond to shorter wavelength light. Different patterns of firing of these various kinds of photoreceptors combine to help create our experience of a wide array of colors. How much each cone is stimulated determines the color we will see.
o Hering (1878) proposed opponent process theory, which says that cones are linked together in three opposing color pairs: blue/yellow, red/green, and black/white. The members of the color pairs oppose one another, whereby activation of one member of the pair inhibits activity in the other.
§ This theory does can account for afterimages – visual images that remain after removal of the stimulus (see Suggested Activities).
§ This theory helps to explain some types of color blindness, and why we never experience some colors, such as reddish-green or yellowish-blue.
o Current research indicates that both theories account for how human color vision works.
§ The trichromatic theory explains processing at the retina or cone, of which there are three types.
§ Opponent process theory explains more about how cells in the LGN of the thalamus and visual cortex process color information.
· Deficiencies in Color Vision
o There are many types of color blindness. It generally refers to a weakness or deficiency in perception of certain colors.
o Usually results from an inherited pigment deficiency in the photoreceptors and generally occurs in men and boys.
o The most common form of color blindness results from a deficiency in red (long wavelength light) and green (medium wavelength light) sensitive cones.
§ People with this disorder have trouble distinguishing some shades of green from red, may see green and brown as similar, or might have difficulty distinguishing blue and purple (see Suggested Activities).
· Hearing begins when we sense sound waves. Sound waves must travel through some medium (fluid or, more commonly, the air) for us to hear them.
· Sound waves travel much slower than light waves, which is why you hear thunder after you have seen lightning.
· Hearing is affected by three physical properties of the sound wave: its amplitude, frequency, and purity.
o The height, or amplitude, of the sound wave determines what we perceive as loudness.
§ The taller the wave is, the louder the sound.
§ The scale for a sound’s loudness is decibels (dB).
o The frequency of the sound wave, or how many waves occur in a given period of time, we perceive as the sound’s pitch.
§ Frequency is measured in units called hertz (Hz), which is how many times the wave cycles per second. The higher the frequency, the higher the pitch.
· Most sounds we hear are in the 400 to 4,000 Hz range.
· Sounds below 20 Hz are called subsonic.
· Sounds above 20,000 Hz are called ultrasonic.
o Purity refers to the complexity of the wave. Most sound waves are pretty simple, made of only one frequency. They are almost always a mixture of frequencies and how much of a mixture defines its purity.
§ We perceive purity as timbre.
· The Outer Ear
o The structures on the sides of our head (pinnae) collect and funnel sounds into the passage called the auditory canal.
o Once inside this canal, sound vibrations travel to the eardrum, or tympanic membrane.
· The Middle Ear
o The sound waves on the tympanic membrane set into motion the bones of the middle ear: the hammer, anvil, and stirrup. These bones do more than just vibrate: they amplify the waves more than 20 times the energy they had entering the ear.
§ The hammer hits the anvil and the anvil moves the stirrup. The vibration of the stirrup, in turn, sets into motion a series of important changes in the inner ear.
· The Inner Ear
o The semi-circular canals play a key role in maintaining a sense of balance.
§ As the stirrup vibrates, it moves a membrane that covers the inner ear, called the oval window. The vibrations on the oval window send movement through the fluid-filled cavity of the cochlea.
o The cochlea is a bony tube, curled like a snail’s shell, and filled with fluid.
o The basilar membrane runs through the cochlea. Within the basilar membrane of the cochlea are hair cells, which are the sensory receptors for sound.
o As the vibrations move through the cochlear fluid, the basilar membrane vibrates, and this makes the hair cells bend. As they bend, the hair cells transduce the sound vibrations into electrical impulses, which may generate an action potential in the auditory nerve.
o Hair cells vary in size depending on where in the cochlea they are. The smallest hair cells are nearest the oval window and the largest hair cells are in the coiled-up center part of the cochlea.
§ There is a one-to-one connection between size of hair cell and its sensitivity to different frequency of sounds. The smallest cells are sensitive to the highest frequencies and the largest hair cells are sensitive to the lowest frequencies.
§ The louder the sound, the bigger the vibration in the cochlear fluid, the more stimulation of the hair cells, the faster the rate of action potentials in the auditory nerve, and the louder the sound we perceive.
o If the hair cells in the inner ear become damaged, as can happen when a person is exposed to very loud noises once or moderately loud noises (such as machines) over long periods of time, the person can suffer irreparable hearing loss.
· After the sound energy is changed to neural energy in the cochlea, the hair cells synapse with auditory neurons that transmit the sound impulses to the thalamus in the brain.
· From there, the neural impulses get relayed to various parts of the brain, including the brain stem, the thalamus, and the temporal lobes, home of the auditory cortex.
· The auditory pathways go from the cochlea to the inferior colliculus in the brain stem and from there to the medial (middle) geniculate nucleus (MGN). This is where we organize and interpret sounds from the outside world (i.e., hear).
· The auditory cortex receives inputs from several other cortical regions, including the visual cortex and regions involved in perceiving speech.
· There are also hemispheric differences in auditory perception:
o the right auditory cortex is more active in processing non-verbal stimuli
the left auditory cortex is more active in processing speech and language
Psychology in the Real World: Hearing Loss in the Age of the iPod
· Studies often divide the causes of hearing loss into age-related and noise-exposure, but in fact, these two are related.
o Being exposed to loud noise levels over long periods of time leads to a loss of hearing after 10 to 15 years.
· Noise often leads to age-related hearing loss, especially in the high-frequency range of 5,000-15,000 Hz.
o Factory or machine workers exposed to 90 dB level noise for 8 hours a day, 5 days a week, suffer permanent hearing loss after 10 years on the job (Lutman & Spencer, 1991; Taylor et al., 1965).
o Rock musicians tested before and after concerts, who were exposed to noise levels from 95 dB to 107 dB, showed both temporary and permanent hearing loss (Gunderson et al., 1997).
· Cell phone users, especially young students during class, have discovered a way to hear calls that their older teachers cannot - they use a ring tone that is a higher frequency that most older people cannot hear.
o The most well-known high-pitched ring tone is called mosquito.
§ Mosquito technology was actually invented by a company to disperse young people in a crowd (because they find it annoying) while leaving the older people unaffected (they cannot hear it).
§ The irony is that some younger people copied the tone and turned it into a ring tone for their cell phone that they can hear, but which many older people are unable to hear. (Still, some 30- and 40-year-olds can still hear it.)
· MP3 players, including the iPod, have maximum decibel levels of around 115-120 dB, about the loudness of a jet airplane. In 2001 researchers at the Centers for Disease Control and Prevention reported hearing loss from loud noise in nearly 13 percent of Americans between the ages of 6 and 19.
THE BODILY SENSES
· The senses based in the skin, body, or any membrane surfaces are known as the bodily senses. There are at least six distinct bodily or somatic senses: touch, temperature, pain, position/motion, balance, and interoception (perception of bodily sensations). Of these six senses, we will discuss touch and pain.
· The largest contact surface area any sensory input has with our bodies is the skin, and it is carefully mapped in the somatosensory cortex in the parietal lobe of the brain.
· Bodily senses also include knowing where our body parts are.
· We also sense things inside our bodies (e.g., organ pain, levels of heart rate, depth of breathing, etc.).
· CONNECTION: Figure 3.14 shows how the somatosensory cortex maps to specific regions of the body (see Chapter 3).
· The top layers of skin have receptor cells (mechanoreceptors) that are sensitive to different tactile qualities – some to shape, some to grooves, some to vibrations and movements.
o There are four different kinds of mechanoreceptors, each of which has a unique profile of sensitivity.
§ Some of the mechanoreceptors are slow to change and others are fast to change with variations in tactile stimulation.
§ Some are sensitive to fine details, whereas others are not sensitive to fine details.
§ Some sense movement and vibration.
o Different areas of skin have different numbers of mechanoreceptors (e.g., there are fewer mechanoreceptors on the soles of your feet than on your fingertips).
· The sensory qualities of shape, size, hardness, and temperature stimulate different kinds of mechanoreceptors in the skin but those sensory impulses must travel to the brain to be processed and interpreted.
o When our fingertips, forearm, or shoulder gets touched, a dedicated region of cortex becomes active and we perceive the sensation of being touched.
o Tactile sensations from our skin travel via sensory neurons to the spinal cord and up to the brain.
o The first major structure involved in processing bodily sensations is the thalamus, which relays the impulses to the somatosensory cortex in the parietal lobes.
o Repeated sensory and motor tactile experience changes the amount of cortex involved in processing that particular sensation or movement. The general location in the somatosensory cortex stays the same, but areas of the cortex devoted to that experience or function grow.
o The more one body region is touched or stimulated, the more sensory or motor cortex gets called into duty in processing that information.
Researchers have found that experienced
violinists have larger representations, or brain
maps, of the hand and finger regions of the somatosensory cortex than
non-musicians (Pantev, Engelien,
§ CONNECTION: What are the benefits of touch for premature and low-birth-weight newborns? (See Chapter 5.)
· We need pain to survive. People born with no pain receptors can get severely injured or killed, because they don’t know they have been harmed (Watkins & Maier, 2003).
· Pain is a complex emotional and sensory experience associated with actual or potential tissue damage.
· People vary in their experience of pain but it is needed to survive (if you can’t detect pain then you may not know if you have been injured).
· The experience of pain in limb or tissue that is missing is called phantom limb pain.
· Pain also is enhanced by one’s reaction to the injury. The emotional reaction to pain can create as much suffering as the actual tissue damage.
o Damage to the skin is only one kind of pain. Other forms include organ tissue and nerve damage as well as joint inflammation.
o Pain from skin damage is called nociceptive pain. The skin has pain receptors that are sensitive to heat, cold, chemical irritation, and pressure, all of which are kinds of nociceptors.
§ The nociceptors send signals to the spinal cord and then to the brain, signaling that damage has happened.
§ The brain can then initiate an appropriate response.
o The spinal cord may actually play an active rather than passive role in pain perception.
§ The spinal cord relays and, in some cases, enhances the pain messages from the sensory neurons to the brain.
§ This seems to be a function of the glial cells wrapped around the axons.
o Once the pain messages get sent and even enhanced by the spinal cord, they move on to the brain.
§ Some of the same brain regions activated when we experience physical pain also are activated during emotional pain (especially rejection and seeing others receive shocks).
§ The brain regions active in both physical and emotional pain are the anterior cingulate cortex (ACC) and the insula.
o Gate control theory of pain proposes that the spinal cord regulates the experience of pain by either opening or closing neural channels, called gates, involved in pain sensations that get sent to the brain.
o Smaller neural channels are dedicated to pain sensations and when they are activated, pain messages get sent to the brain.
o Larger neural channels are involved in non-pain sensations and when they are activated, they can inhibit or close the pain impulses sent to the brain. That is, they override pain messages.
o This theory explains why certain kinds of stimulation (e.g., acupuncture or rubbing a hurt area) can relieve sensations of pain.
o Inhibitory channels can actually come from the brain as well as the body. Messages sent by the brain itself (i.e., thoughts, feelings, and beliefs) can close channels in the spinal cord involved in pain sensations.
§ This is one reason why people vary so much in their perception of pain.
o Our bodies have natural painkillers called endorphins. When we are injured they are released and interfere with pain messages in the spinal cord and brain.
§ Endorphin release may explain such odd phenomena as why people initially experience no pain after a horrible injury from an accident.
o Drugs such as aspirin, acetaminophen, and ibuprofen, can help with everyday aches and pains by controlling inflammation.
o For more severe pain, doctors may prescribe opioids – a class of drug known as analgesics. Morphine, heroin, oxycodone, and hydrocodone are all opioids, and all but heroin are commonly prescribed for pain relief.
§ They work to deaden or lessen pain by blocking neural activity involved in pain perception.
§ There is a high risk of dependency on opioids, so their use must be carefully monitored.
§ CONNECTION: Why do opioids have a high potential for abuse? (See Chapter 6.)
THE CHEMICAL SENSE: SMELL AND TASTE
· Smell and taste are chemical senses because they respond to contact with molecules from objects we encounter in the world.
· Smell and taste are very important survival-related senses, as they govern our choices about what we take into our bodies.
o This is why these senses are very sensitive, are heightened during pregnancy, and can trigger emotional reactions.
· Unlike other senses, receptors for chemical molecules are regularly replaced every few weeks because of their constant exposure to dirt and bacteria that can impair function.
· Our receptors for smell reside high up in the nose.
· A small area high in the lining of the nasal cavity contains the olfactory sensory neurons – the receptors for smell.
o These neurons contain hair-like projections called cilia, which are similar to the hair cells in the inner ear. The cilia convert chemical information in odor molecules to neural impulses.
· When chemicals come in contact with the cilia, transduction occurs, and the olfactory message travels to the olfactory bulb in the forebrain.
· The olfactory bulb sends information either directly to the smell processing areas in the cortex or indirectly to the cortex by way of the thalamus.
o The primary olfactory cortex resides in the temporal lobe.
o The secondary olfactory cortex is in the frontal lobe near the eyes.
· Some fibers from the olfactory bulb go directly to the amygdala, which sends smell information to the hypothalamus, thalamus, and frontal cortex.
o These connections may explain why smells can instantly evoke an emotional memory.
· There may be as many as 1,000 different olfactory sensory receptors. Greater concentrations of odors will stimulate a greater number of sensory neurons, and this can lead us to perceive the same odor presented at different concentrations as being an entirely different smell.
· People differ considerably in their ability to sense odors. Some people lose the ability to sense smell with infection or injury, but usually this is short-term.
· Textured structures on the tongue are called papillae. They contain about 10,000 taste buds. The cells on the buds that process taste information are called taste cells.
· There are dozens of taste cells in each taste bud.
· Human experience of taste results from stimulation of taste buds on the front, sides, and rear of tongue. When chemicals from food or liquid come into contact with the tips of these taste buds, a chain of events unfolds that leads to the experience of taste.
o Different tastes use different mechanisms to stimulate an impulse in a taste cell. In general, chemicals alter the membranes of taste cells in ways that make them more likely to generate action potentials.
o Such signals from taste cells in various regions of the tongue then travel down fibers to the brainstem.
o From the brainstem, taste information travels to the thalamus and frontal lobe. Neurons from the thalamus project taste information to the taste cortex in the insula and other regions of the frontal-parietal cortex.
· Humans distinguish five basic taste qualities: bitter, sweet, salty, sour, and savory. Specific receptors exist for each type of taste.
· The experience of flavor results from the combination of taste plus smell.
· The region of the brain most involved in flavor perception is the orbitofrontal cortex (OFC). It receives inputs from brain areas involved in olfaction and taste, as well as touch and vision perception areas.
· Synesthesia occurs when a person experiences sensations in one sense when a different sense is stimulated. In short, synesthesia occurs when the senses get mixed up rather than stay separate.
· The most common form of synesthesia is one in which people experience numbers or sometimes letters as colors.
o Synesthesia may result from a cross-wiring or cross-activation of sensory neurons in various parts of the brain.
§ Cross-activation occurs when two areas of the brain, normally kept separate, get activated at the same time by the same stimulus.
o The OFC in the frontal lobes has many so-called bimodal neurons. Bimodal neurons respond to more than one sense—such as taste, smell, touch, and vision—and may become cross-activated in synesthesia.
o Certain hallucinogenic drugs can temporarily create synesthetic experiences.
Making Connections in Sensation and Perception: Differences Across Cultures
See separate section for detailed explanation.