Sensation and Perception

 Without sensation and perception, you could not read this book, find food, communicate with others, or experience any of the joys or challenges of life.  With no vision, no hearing, no smell, no taste, no touch, or movement, you would not be able to make sense of the world outside of your body.

 Once we take information in through our senses, we do something with it to interpret it. What we do depends on many factors like our mood, the circumstances, or even our cultural background.  Your nervous system sorts through all of this incoming information and analyzes the stimuli entering through your many sensory systems. It is automatic.

 The analysis that takes place during this processing is part of perception, our interpretation of the incoming sensory information. Perception reflects learning, expectations, and attitudes.  Stimulation of the senses and the ways in which people interpret that stimulation is affected by several concepts. These concepts include absolute threshold, difference threshold, signal-detection theory, and sensory adaptation.

 Absolute Threshold – the weakest amount on stimulus that can be seen.  Difference Threshold – the ability to detect the changing levels of a stimulus.  Signal-Detection Theory – a method of distinguishing sensory stimuli that takes into account not only their strengths but also such elements as the setting, your physical state, your mood, and your attitudes.

 People are quicker to detect a signal among noise when:  they expect the signal  it is important that the signal is detected  they are alert  Experience matters in detecting signals  10 hours of playing an action video game increased novice players’ signal detection skills.

 Sensory Adaptation – the process by which we become more sensitive to weak stimuli and less sensitive to unchanging stimuli.  VISION - Many of us consider vision to be the most essential of our senses. No other sense allows us to gather so much information from nearby and distant sources.

 Light enters the eye as waves of electromagnetic energy, part of a broad, ever-present spectrum of electromagnetic radiation in our environment.  The light is projected onto a surface. The amount of light that enters is determined by the size of the opening in the colored part of the eye, the pupil.

 The pupil admits light into the inner portion of the eye, where it encounters the lens which adjusts to the distance of objects by changing its thickness.  These changes project a clear image of the object onto the retina, which is the part of the eye that acts like the film in a camera.

 The retina’s photoreceptors hands off the image to a nerve that carries the visual input to the brain – the visual area called the occipital lobe.  We have a blind spot where the optic nerve leaves the eye because this area lacks photoreceptors.  There are two different types of photoreceptor cells – rods and cones – that differ in both appearance and in function.

 Rods can detect only black, white, and shades of gray because they are sensitive to bright light. Cones provide color vision, but only if there is enough light.  Adaptation to bright light happens much more quickly than adaptation to dark light. Dark adaptation begins within a few minutes and continues to improve for up to 45 minutes.

 Bright light adaptation may hurt initially, but takes place within a couple of minutes.  Visual acuity is determined by the ability to see visual details.  The Snellen Chart is used by eye doctors to test your vision. As people age, the lenses in the eyes become more brittle and make focusing more difficult. Thus, they become far-sighted. Usually this starts around middle age.

 The wavelength of light determines the color. People with normal color vision see any color in the spectrum of visible light.  Cones, one of the two types of photoreceptors in the retina of the eye, enable us to perceive color.  Some are sensitive to blue, some to green, and some to red. When more than one kind of cone is stimulated at the same time, we perceive other colors of the spectrum.

 Afterimages occur after staring at an image for a short period of time and then looking away. The image will be in a complimentary color.  People who are color blind are partially or totally unable to distinguish color due to an absence or, or malfunction, in the cones. Really, they are just limited in the number or colors that they see.

 Objects do not “possess” color (in a sense, the tomato isn’t red, it’s everything but red...)  long wavelengths of red light are “rejected” (reflected) from the tomato  The rays are not colored.  Color is a product of our brains’ transduction of light waves.  JND so low that we can discriminate between over 7 million colors.

 Generally, color blind people lack either the red or green receptors and have trouble telling the difference between the two.  This is an inherited condition and is much more common among males than females. Many times, the person does not even realize that it exists.

 HEARING - Sound comes in waves that are produced by vibration. Each of these vibrations is called a cycle or a sound wave. Every sound has its own pitch and loudness.  The length of the wave determines the pitch of the sound. The more cycles per second (frequency), the higher the pitch of a sound.

 The human ear can hear sound waves that vary from 20 to 20,000 cycles per second (frequency). Many animals hear sounds well beyond that limit.  The height (amplitude) of a wave determines its loudness. Loudness is measured in decibels. Zero decibels (dB) is considered the threshold of hearing.

 Any prolonged sound over 85 dB can produce hearing loss.  The ear is shaped to capture sound waves. What we normally think of as the ear is actually the outer ear.  The middle ear is a thin membrane that vibrates when sound waves strike it. As it vibrates, it transmits the sound to three small bones – the hammer, the anvil, and the stirrup.

 These bones vibrate and transmit sound to the inner ear, where the cochlea’s neurons move in response to the vibrations of its fluids.  The movement generates neural impulses that are transmitted to the brain via the auditory nerve. Auditory input in projected onto the hearing areas of the cerebral cortex.

 How do we locate where a sound is coming from? Step one is to determine which ear hears the sound first. Sound travels through air at about 750 miles per hour.  Step two is to determine which ear hears the louder, more intense sound.

 About 2 million Americans experience deafness, which can be inherited or caused by disease, injury, or old age.  Conductive deafness occurs because of damage to the middle ear and can usually be helped by using hearing aids.  Sensorineural deafness is caused by damage to the inner ear. If the auditory nerve is not damaged, some help might be obtained from a cochlear implant.

 SMELL - People do not have as strong a sense of smell as many animals. Smell is a chemical sense.  Molecules given off by many substances circulate in the air.  When these molecules reach the upper nasal passages, olfactory cells that project from the brain can process them.

 By triggering various combinations of olfactory cells, thousands of different odors are detectable.  These cells do not regenerate when damaged, so many elderly individuals have a noticeably decreased sense of smell.

 TASTE - Taste is also a chemical sense. You have receptor cells located on the surface of the tongue that respond to the chemical structure of the foods you eat. These cells can detect four different tastes: salty, sweet, sour, and bitter.

 Taste buds that are responsive to sweetness are located at the tip of the tongue. Receptors for bitterness are at the back of the tongue.  Sourness is sensed along the sides of the tongue, and saltiness overlaps the areas sensitive to sweetness and sourness.

 Taste buds are composed of groups of between 50 and 150 columnar taste receptor cells bundled together like a cluster of bananas.  The taste receptor cells within a bud are arranged such that their tips form a small taste pore, and through this pore extend microvilli from the taste cells. The microvilli of the taste cells bear taste receptors.

 Among humans, there is substantial difference in taste sensitivity. Roughly one in four people is a "supertaster" that is several times more sensitive to bitter and other tastes than those that taste poorly.  Taste receptors can be damaged by heat. Fortunately, they do replace themselves within a few days.

 It is also clear that the sense of smell profoundly affects the sensation of taste. Think about how tastes are blunted and sometimes different when your sense of smell is disrupted due to a cold.

 SKIN SENSES - Touch is a combination of pressure, temperature, and pain. It is our physical connection with the outside world.  The four basic skin senses are pain, warmth, cold, and pressure. Your experience of other skin sensations flows from various combinations of these four basic senses.

 Different parts of the body are more sensitive to pressure than others. The fingertips, lips, nose, and cheeks are more sensitive than the shoulders, thighs, and calves.  The receptors are just beneath the skin. When skin temperature increases because you touch something warm, receptors for warmth fire. Decreases in skin temperature cause receptors for cold to fire.

 The pain message is sent from the point of contact to the spinal cord to the thalamus in the brain. Then it is projected to the cerebral cortex, where the person registers the location and severity of the brain.  Chemicals called prostaglandins help the body transmit pain messages to the brain.

 The Gate Theory suggests that only a certain amount of information can be processed by the nervous system at a time.  Rubbing, scratching, or icing and injured area partially blocks the sensation of pain by competing with the pain message for space.

 Phantom limb pain appears to reflect the activity of the neurons in the brain that store memories connected with the missing limb.  BODY SENSES - Without other body senses, you would have to pay attention just to keep your balance, lift your leg to climb stairs, or even to get food into your mouth.

 The vestibular sense, located in your inner ear, provides information about your overall orientation. It enables you to keep your balance, tells you whether you are upside-down, and lets you know if you are falling.  Kinesthesis is the sense that informs people about the position and motion of parts of their bodies. It relies on receptor cells located throughout your muscles, tendons, and joints.

 PERCEPTION - Perception is the way that we organize or make sense of our sensory impressions.  Gestalt psychologists used the following rules of perceptual organization to make sense of sensory information:  1. Closure – the tendency to perceive a complete or whole figure even when there are gaps in what your senses tell you.

 2. Figure-Ground Perception – the perception of figures against a background. We experience this every day. Objects that draw your attention will be nearer to the center of the visual field than the edge. It may be moving, and it will often be fairly large and colorful.

3. Proximity – nearness. If objects are close together, we place them in the same group. 4. Similarity – people think of similar objects as belonging together. 5. Continuity – your brain assumes movement continues in a particular direction unchanged. People prefer smooth, continuous patterns.

6. Common Fate – things moving together must belong together or have a common purpose. To be able to sense movement, humans need to see an object change its position relative to other objects. (We don’t feel earth move.) You can look for objects that are stable, like buildings. You can also close your eyes and try to sense motion.

 Psychologists have also studied illusions of movement, such as stroboscopic motion. This is produced by showing the rapid progression of images or objects that are not moving at all.  Movies work in this way. Because of the law of continuity, humans prefer to see things as one continuous image. Perception smoothes over the interruptions and fills in the gaps.

 Depth perception is very important.  Depth in this case means distance away. You would not be able to drive a car, catch a ball, shoot a free throw, thread a needle, or place clean dishes on a shelf without it.  You use both monocular and binocular cues.

 Monocular cues need only one eye to be perceived. They cause certain objects to appear more distant from the viewer by using perspective, clearness, overlapping, shadow, and texture gradient.  1. Perspective – objects that are farther away will stimulate a smaller area of your retina than the one that is near. We also use our experience to judge its distance.

 2. Clearness – nearby objects appear to be clearer, and we see more details, thus they appear to be closer to us.  3. Overlapping – this is the placing of one object over another. Experience teaches us to perceive partly covered objects as being farther away than the objects that block them from view.

 4. Shadows – give us information about objects’ three dimensional shapes and where they are placed in relation to the source of light.  5. Texture Gradient – texture is the surface quality and appearance of an object, while gradient is a progressive change. Closer objects are perceived a having a more varied texture than objects that are farther away.

 6. Motion Parallex – the most complex of monocular cues. It involves the image of something as the viewer moves. Objects seem to move forward or backward depending on how far away they are from the viewer.

 Binocular cues require both eyes for depth perception.  1. Retinal disparity – serves as a cue for depth only for objects that are within a few feet of us. It is the difference between the images you see with the retinas in your left and right eyes.

 2. Convergence – translates tension in the muscles that control your eyeballs into information about distance. To maintain your focus on an object as it moves closer to you, your eyes have to swing inward, or converge on it, giving us a cross-eyed look.

 Perceptual constancies require us to use our experiences.  1. Size – objects further away seem smaller and do not take up much room on our retina. Our perspective teaches us that objects’ size remain constant.  2. Color – objects maintain their color, even in different light.

 3. Brightness – the tendency to perceive an object as being equally bright, even when the intensity of the light around it changes.  4. Shape – the knowledge that an item only has one shape no matter what angle you view the item from.  Visual illusions are the result of your brain’s use of perceptual constancies.