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Chapter 6

Chapter 6. Sensation and Perception.docx

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Trent University
PSYC 1020H
Wolfgang Lehmann

Sensation and Perception  Sensation: the detection of physical energy emitted or reflected by physical objects  Cells doing the detecting are located in the sense organs (eyes, ears, tongue, nose, skin), and these sensory processes produce immediate awareness of sound, colour, form, and other building blocks of consciousness  Perception: a set of mental operations that organizes sensory impulses into meaningful patterns  Helps make sense of the world impinging on our senses  Sensation and perception are the foundation of learning, thinking, and acting  There are more than just the 5 senses: vision (eyes), hearing (ears), taste (tongue), touch (skin), and smell (nose) o Skin also senses heat, cold, pain, and itching and tickling o Ear is also a receptor that accounts for a sense of balance o Skeletal muscles are responsible for a sense of bodily movement The Riddle of Separate Sensations  Sensations starts with the sense receptors: cells located in the sense organs that convert physical energy in the environment or body into electrical energy  Upon detecting a stimulus (such as light), the sense receptors covert the stimulus energy into electrical impulses that travel along the brain and transmit what they learn to the sensory neurons (field officers) in the peripheral nervous system  The sensory nerves all use neural impulses to communicate  We can experience many different kinds of sensations because the nervous system encodes the messages  Anatomical code was described in 1826 by Johannes Muller in his doctrine of specific nerve energies: the principle that different sensory modalities exist because signals received by the sense organs stimulate different nerve pathways leading to different areas of the brain; implies that what we know about the world ultimately reduces to what we know about the state of our own nervous system o Signals from the eye cause impulses to travel along the optic nerve to the visual cortex o Signals from the ear cause impulses to travel along the auditory nerve to the auditory cortex o Light and sound waves produce different due to these anatomical differences  Sensory crossover can occur and is called synesthesia: a condition in which stimulation of one sense also evokes another o The person actually experiences the second sensation o Ex. Vladimir Nabokov said the letter ‘B’ actually made him see burnt sienna, and T made him see pistachio green  Synesthesia runs in families and is more common in females  Functional codes rely on the fact that sensory nerves and neurons fire, or are inhibited from firing, only in the presence of specific stimuli; at any given time only specific cells in the nervous system are firing when others are not Measuring The Senses  Psychophysics: the study of the physical properties of stimuli are related to our psychological experiences with them 1. Absolute Thresholds  Is the smallest quantity of physical energy that can be reliably detected by an observer  Is one method of how to find out how sensitive the senses are, which in this method, is done by showing people a series of signals that vary in intensity and asking them which signals they can detect  ‘reliable’ detection can occur when a person detects a signal 50% of the time  Has allowed psychologists to find that our senses are very sharp i. Ex. With normal sensory abilities, you can see a candle flame on a clear dark night almost 50km away  However, our senses are only tuned in to a narrow band of physical energies i. Ex. We are visually sensitive to a tiny fraction of the electromagnetic energy surrounding us  There are other species that can pick up signals that we cannot 2. Difference Thresholds (Just noticeable difference)  The smallest difference in stimulation that can be reliably detected by an observer when two stimuli are compared  When comparing stimuli A and B, the difference threshold will depend on the intensity or size of A: the larger or more intense A is, the greater the change must be before you are able to detect a difference 3. Signal Detection Theory  The first 2 theories have serious limitations: measurements can be affected by a person’s general tendency, follow habits of saying yes or no, or want to impress the observer creating a bias  This theory states that an observer’s response in a detection ask can be divided into a sensory process, which depends on the intensity of the stimulus, and a decision process, which is influenced by the observer’s response bias  A researcher may be able to include some trials in which no stimulus is present and others where a weak stimulus is present  This would allow 4 kinds of responses from the person: i. They would detect a signal that was present (a hit) ii. They would say the signal was there when it wasn’t (a false alarm) iii. They would fail to detect the signal when it was present (a miss) iv. They would correctly say the signal was absent when it was absent (a correct rejection)  The information can be inserted into a mathematical formula yielding separate estimates of a person’s response bias and sensory capacity  Signal detection theory assumes there is no single threshold Sensory Adaptation  When a stimulus is unchanged or repetitive, sensation fades or disappears which results in sensory adaptation  Sensory adaptation: the reduction or disappearance of sensory responsiveness when stimulation is unchanging or repetitious  We never fully adapt to extremely intense stimuli (a terrible toothache, ammonia odour, heat of the desert sun), as well as we rarely adapt to visual stimuli whether they are weak or intense  Sensory deprivation: the absence of normal levels of sensory stimulation  University of British Columbia: The notion of sensory deprivation as being unpleasant or dangerous is an oversimplification as the experimental procedures themselves probably aroused anxiety  The human brain requires a minimum amount of sensory stimulation to function normally Sensing Without Perceiving  Too much stimulation can also be bad, as it can lead to fatigue and mental confusion  Our capacity for selective attention (the ability to focus on some parts of the environment and block out others) protects us from being overwhelmed by countless sensory signals that are constantly impinging upon our sense receptors  Competing sensory messages all enter the nervous system and get processed which enables the person to pick up important information, such as our own name being spoken  It also causes us to miss much that is going on around us as a result, which means our conscious awareness of our environment is not complete  Inattentional blindness: failure to consciously perceive something you are looking at because you are not attending to it  Common example: It is disastrous if you are listening to someone on your cellphone while driving and fail to see a pedestrian crossing the street in front of you. This is an example of selective attention Vision  The stimulus for vision is light  Visible light comes from the sun and other stars, from light bulbs, and is reflected off objects  Light travels in waves, and the physical characteristics of the waves affect the psychological dimensions of our visual world: hue, brightness, and saturation  Hue: the dimension specified by colour names, and is related to the wavelength of light o Shorter waves are seen as violet and blue, longer ones as orange and red o There are many factors affecting colour perception  Brightness: lightness or luminance, dimension related to the amount of light emitted from or reflected by an object o Intensity corresponds to the amplitude (maximum height) of the wave o Generally, the more light an object reflects, the brighter it appears  Saturation: vividness or purity of colour, dimension related to the complexity of light waves o Light containing only a single wavelength is said to be ‘pure’ and the resulting colour is completely saturated o The other extreme is white light, lacking any colour and is completely unsaturated The Eye  The front part of the eye is covered by the transparent cornea  Cornea: protects the eye and bends incoming light rays toward a lens located behind it  The eye lens works by subtly changing its shape, becoming more or less curved to focus light from objects that are close or far away  Iris: the part of the eye that gives it colour, surrounds the pupil  Pupil: the round opening of the eye, allows to let more or less light in to adjust to the environment  Visual receptors are located in the retina  Retina: the back of the eye, neural tissue lining the back of the eyeball’s interior which contains the receptors for vision  The brain interprets the upside-down pattern of stimulation as something that is right side up  About 120 -125 million receptors in the retina are long and narrow and are called rods  Rods: visual receptors that respond to dim light  There are about 70-80 million cone-shaped receptors called cones: visual receptors involved in colour vision  Rods cannot distinguish different wavelengths of light so they are not sensitive to colour making it difficult to clearly identify colours in dim light  The process of dark adaptation (a process where visual receptors become maximally sensitive to dim light) involves chemical changes in the rods and cones  The cones adapt within around 10 minutes but never become very sensitive, and the rods adapt within around 20 minutes or longer but are much more sensitive  Rods and cones are connected by synapses to bipolar neurons, which communicate with neurons known as ganglion cells: neurons in the retina that gather information from receptor cells and that have axons that make up the optic nerve  Optic nerve: carries information out through the back of the eye and on to the brain  We are normally unaware of a blind spot because: o The image projected on the spot is hitting a different ‘non-blind’ spot in the other eye o Our eyes move so fast that we can pick up the complete image o The brain fills in the gap Why The Visual System Is Not A Camera  The eye is not like a camera because it does not passively record the external world  Neurons in the visual system actively build up a picture of the world by detecting its meaningful features  Feature detector cells: cells in the visual cortex that are sensitive to specific features of the environment  These detector cells respond to more complex features, and were demonstrated by David Hubel and Torsten Wiesel, who recorded impulses from individual cells in the brains of cats and monkeys  They found that different neurons were sensitive to different patterns projected on a screen in front of the animal’s eyes  Scientists have found that some cells in the visual system have more complex specialities o Ex. Some cells in the right temporal lobe appear to respond maximally to faces.  Scientists have concluded that evolution has given us an innate face module that could help explain why infants prefer to look at faces instead of images scrambling the facial features  Generally, the brain’s job is to take fragmentary information about edges, angles, shapes, motion, brightness, texture, and patterns How We See Colours 1. The Tri-Chromatic Theory  It is a theory that proposes three mechanisms in the visual system, each sensitive to a certain range of wavelengths  Their interactions are assumed to produce all the different experiences of hue  Applies to the first level of processing, and occurs in the retina; with the retina producing three types of cones: i. One type responds maximally to blue (or to wavelengths near the short end of the spectrum which give rise to blue) ii. A second type responds maximally to green iii. A third type responds maximally to red  Total colour blindness is due to a genetic variation causing the retina to be absent or non-functional: the visual world consists of black, white, and shades of grey  Most ‘colour blind’ people are actually ‘colour deficient’: the person is usually unable to distinguish red and green, and the world is painted in shades of blue, yellow, brown, and grey  Colour blindness/deficiency is rare in women because of the way the condition is inherited 2. The Opponent-Process Theory  It is a theory that assumes that the visual system treats pairs of colours as opposing or antagonistic  Applies to the second stage of colour processing, which occurs in the ganglion cells in the retina and in neurons  Those cells are referred to as opponent-process cells: which either respond to short wavelengths bit are inhibited from firing by long wavelengths, or vice versa  Some opponent cells fire in response to one and turn off in response to the other i. Ex. Some fire in response to red, others in response to green. Others fire in opposite fashion to yellow and blue.  This produces a colour code that is passed to the higher visual centers  The code treats red and green, and blue and yellow, as antagonistic which allows us to describe a colour as being ‘bluish green’, ‘yellowish green’ but not ‘reddish green’ or ‘yellowish blue’  This theory was first predicted by Ewald Hering, but did not specify these cells were in the brain also  Cells inhibited by a particular colour produce a burst of firing when the colour is remov
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