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Common features of sensation and perception
The chemical senses: Smell and taste
The tactile or cutaneous senses: Touch, pressure, pain, vibration
The auditory sense: Hearing
The visual sense: Sight
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Sensation—the act of using our sensory systems to detect environmental stimuli
 Perception— recognizing and identifying sensory stimulus
Raw Sensory Data
Vision Light waves
Hearing Sound waves
Smell Airborne chemicals
Taste Food chemicals
Touch Pressure
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What do we need to do to the raw sensory data so that our brain can understand it?
Sensory receptor cells—specialized cells that convert a specific form of environmental stimuli into neural impulses.
Sensory transduction—the process of converting a specific form of sensory data into a neural impulse that our brain can read
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Absolute threshold is the smallest amount of a stimulus that one can detect
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For example, what is the dimmest light you can see?
 Difference threshold (or JND)—the minimal difference needed to notice a difference between two stimuli
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For example, when do you perceive a difference in change of volume in your ear?
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Constant stimulation decreases the number of sensory messages sent to the brain, which causes decreased sensation. Why? We can’t afford to waste attention on unchanging stimuli
Examples:
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A crying baby will wake us, but not a thunderstorm that might be even louder
You notice how tight your pants are when you first put them on but over the course of the day they seem looser
(you don’t register the sensation of the tightness)
You do not notice how much perfume you put on; at first it seems good but then you do not smell it so you put on more and more…
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Bottom-up processing—the raw sensory data is sent to the brain and your brain uses all of that data to build a perception
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You take 1,000s of data points of visual stimuli and put them together to create an image of your mother
Top-down processing—you use previously learned information to help recognize and interpret the data coming into your brain
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You recognize some of those data points and immediately match them to your previous knowledge about your mother’s face
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 Perceptual set is the readiness to interpret a certain stimulus in a certain way
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Chemicals, called odorants, are carried through the air and reach the 5 million receptor cells located at the top of each nasal cavity
Olfactory receptor neurons o
The receptor cells turn that molecule into a neural impulse (transduction) and send that impulse to the olfactory bulb (smell centre in the brain)
Lock-and-key binding
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Chemical substances in the food we eat dissolve in saliva and fall into the crevices between the bumps
(papillae) of the tongue
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The bumps hold our taste buds
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Each taste bud contains 60 to 100 sensory receptor cells for taste
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Taste buds are our receptor cells for taste
The taste buds translate the chemical message into a neural impulse and send that impulse to the thalamus and, eventually, the cerebral cortex
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Sweet
Sour
Bitter
Salt
Umami—the taste of monosodium glutamate
(MSG)
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Anosmia—inability to smell
Ageusia—inability to taste
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Both are usually caused by head trauma
Other conditions:
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Reflex epilepsy—a seizure occurs only after exposure to a specific odour
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Migraine headaches—specific odours can trigger migraines
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 The tactile or somatosensory system is a combination of skin senses:
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Pressure, touch, temperature, vibration, pain
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The tactile senses rely on a variety of receptors located in different parts of the skin
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Free nerve endings
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Located near the surface of the skin
Function: Detect touch, pressure, or pain
Meissner’s corpuscles
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Located in fingertips, lips, and palms (hairless skin areas)
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Function: Transduce information about sensitive touch
Merkel’s discs
Located near the surface of the skin
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Function: Transduce information about light to moderate pressure against the skin
Ruffini’s end organs
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Located deep in the skin
Function: Register heavy pressure and movement of the joints
Pacinian corpuscles
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Located deep in the skin
Function: Respond to vibrations and heavy pressure.
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Sensory neurons register pressure
Information sent to the spinal cord, then the thalamus
(telephone operator)
Information is then sent to the somatosensory cortex that registers the sensation
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Tactile information is processed contralaterally
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Fast pathway pain (myelinated pathway)—sharp, localized pain is felt quicker because it travels along myelinated neurons to the brain
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Slow burn (unmyelinated pathway)—nagging, burning pain is slower to be felt because it travels along unmyelinated pathways
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Gate control theory—patterns of neural activity can actually close a “gate” that prevents messages from reaching parts of the brain where they are perceived as pain
Pain threshold
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Chronic pain is the most common abnormality associated with the somatosensory system
Endorphins and enkephalins are naturally occurring pain-killing chemicals in the brain
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They can be found in opiates such as morphine or heroine
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They are produced naturally through intense physical activity, sex, or intense stress (endogenous opiates)
Cingulotomy—destruction of the cingulate cortex
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An extreme form of neurosurgery to relieve chronic pain
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Familial dysautonomia—rare genetic condition associated with an inability to detect pain or temperature
Phantom limb sensations—tactile hallucinations of touch, pressure, vibration, and pain in the body part that no longer exists
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“Mirror box” therapy
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 Sound waves—vibrations of the air in the frequency of hearing
 Frequency—the number of cycles per second in a wave
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Determines pitch of sound
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Measured in units called Hertz (Hz), which represent cycles per second
 Amplitude—the magnitude (height of a wave)
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Determines loudness
Measured in units called decibels (dB)
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Sound waves enter the outer ear
Hits the eardrum (tympanic membrane)
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Thin membrane that moves with sound waves
Passes into the middle ear
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Contains the 3 smallest bones
(or ossicles) in the human body: maleus (hammer), incus (anvil), and stapes
(stirrup)
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Stapes hits the oval window and creates vibrations that move fluid in the cochlea
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Oval window—membrane separating the ossicles and the inner ear
The vibrations move the basilar membrane (in the cochlea), which is covered with sensory receptors called hair cells
As hair cells move, neural impulses are created and sent to the brain
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Place theory
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Vibration of the basilar membrane (BM) at different places results in different pitches/frequencies
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Near the oval window (where BM is thinner)—higher frequencies; lower frequencies occur farther from the oval window
Frequency theory
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Different sound frequencies are converted into different rates of action potentials or firing
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High-frequency sounds produce a more rapid firing than do low-frequency sounds
Different firing rates contribute to sound perception of low frequency tones
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General loudness—louder sounds seem closer
Loudness in each ear—the ear closer to the sound hears a louder noise than the ear farther from the sound
Timing—sound waves will reach the ear closer to the source of the sound before they reach the ear farther away
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 Cochlea  brainstem  thalamus  auditory cortex
 auditory association areas in the cortex
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Tonotopic map—information transmitted from different parts of the cochlea is projected to specific parts of the auditory cortex
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Deafness
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Can be genetic or caused by infection, physical trauma, or exposure to toxins, including overdose of common medications such as Aspirin
Tinnitus—ringing in the ear
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Usually due to abnormalities in the ear
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One of every 200 people experiences tinnitus
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Light waves enter the cornea
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Cornea is protective outer layer
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Passes through the pupil
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The pupil is a small opening in the eye
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Passes through the lens
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The lens focuses the light waves
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Projected onto the retina
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Retina contains all of the receptor cells (rods and cones)
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The rods and cones transduce the light waves into a neural impulse.
Rods
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Used for periphery and night vision
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Not as acute as cones (i.e., fuzzy vision)
Many more rods than cones (over
100 million)
Cones
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Used for central and colour vision
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Very acute (i.e., very clear)
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The fovea in the centre of the eye is all cones
Not as many cones (4–5 million)
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The neural impulses are sent to the optic nerve
• The optic nerve contains the axons of 1 million ganglion cells that extend through the wall of the retina and go to the brain.
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The optic nerve carries messages from each eye to the visual cortex (occipital lobe)
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Trichomatic theory—There are three different sensors for colour and each type responds to a different range of wavelengths of light
 We see more than three colours, which is he variety of colours arise from combining the three colours
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The activation of one cone (at retinal level) inhibits another cone
This theory explains colour vision at the level of the ganglion cells
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Ganglion cells are arranged in opposing cells: redgreen, yellow-blue, black-white
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• Opponent-process theory may explain colour afterimages: continual viewing of red weakens the ability to inhibit green; remove red and you see green
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• Most people who are colour blind cannot distinguish between red and green; they would see only a random pattern of dots in this figure.
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Visual agnosia—damage to the “what” pathway; cannot recognize objects
Prosopagnosia—a form of visual agnosia in which people cannot recognize faces
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 Hemi-neglect—damage to the “where” pathway; people ignore one side of their visual field
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Examples of effects: apply makeup to only one side of face, shave only one side of face, eat food on only one side of plate
People with damage to the right side of their “where” pathways neglect the left side of their visual field
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The process by which we organize small pieces of sensory experience into meaningful wholes
Gestalt principles
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Visual information is organized into coherent images
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Gestalt means whole or totality
The whole is more than the sum of its parts
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Proximity—stimuli near to one another tend to be grouped together
Similarity—stimuli resembling one another tend to be grouped together
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Continuity—stimuli falling along the same plane tend to be grouped together
Good form—stimuli forming a shape tend to be grouped together while those that do not remain ungrouped
Closure—we tend to fill in small gaps in objects so that they are still perceived as whole objects
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 Proximity—we group nearby figures together
• Similarity—figures similar to each other are grouped together
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 Closure—tendency to perceive incomplete figures as whole and complete
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 Binocular cues are cues from both eyes
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Convergence—the tendency of the eyes to move toward each other as we focus on objects up close
Binocular disparity—different images of objects are cast on the retinas of each eye
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When images are very different, we perceive object is close by
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When images are similar, we perceive object is farther away
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Interposition—when one object blocks part of another from our view, we see the blocked object as farther away.
Elevation—we see objects that are higher in our visual plane as farther away than those that are lower.
Texture gradient—we can see more details of textured surfaces, such as the wood grain on a restaurant table, that are closer to us.
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Linear perspective—parallel lines seem to converge in the distance.
Shading—we are accustomed to light, such as sunlight, coming from above us. We use differences in the shading of light from the top to the bottom of our field of view to judge size and distance of objects.
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Clarity or arial perspective—we tend to see closer objects with more clarity than objects that are further away. Realizing that this is sometimes referred to as a fog or smog effect will clarify what we mean by it.
Familiar size—once we have learned the sizes of objects, such as people or restaurant plates, we assume that they stay the same size, so objects that look smaller than usual must be farther away than usual
Relative size—when we look at two objects we know are about the same size, if one seems smaller than the other, we see it as farther away than the other.
Motion Parallax—if we look out the side window of a moving car or train, objects that are closer to us appear to move past us more quickly than do objects that are further away (and very distant objects (like mountains) sometimes do not appear to move at all).
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 Size constancy—we perceive objects as the same size regardless of the distance from which it is viewed
 Shape constancy—we see an object as the same shape no matter from what angle it is viewed
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Blindness—About 278,000 people in Canada suffer from visual impairments, of which about 108,000 are legally blind
Amblyopia—a loss of visual abilities in a weaker eye
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Caused by abnormal development of the brain’s visual cortex due to a failure to receive visual stimulation from both eyes by the age of six
Strabismus—lack of coordinated movement of both eyes; can lead to amblyopia; affects about 2 percent of the population
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