Chapter 5: Sensation and Perception. Chapter Outline 1. Common features of sensation and perception...

Chapter 5: Sensation and Perception

Chapter Outline

1. Common features of sensation and perception2. The chemical senses: Smell and taste3. The tactile or cutaneous senses: Touch,

pressure, pain, vibration4. The auditory sense: Hearing5. The visual sense: Sight

© John Wiley & Sons Canada, Ltd.

Basic Definitions

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


Common Features of Sensation and Perception

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


Sensory Receptor Cells


Thresholds: Testing the Limits

Absolute threshold is the smallest amount of a stimulus that one can detect 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 For example, when do you perceive a

difference in change of volume in your ear?


Sensory Adaptation

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: 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…


Processing Sensory Information

Bottom-up processing—the raw sensory data is sent to the brain and your brain uses all of that data to build a perception 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 You recognize some of those data points and

immediately match them to your previous knowledge about your mother’s face


Perceptual Set

Perceptual set is the readiness to interpret a certain stimulus in a certain way


How Do We Smell?

Chemicals, called odorants, are carried through the air and reach the 5 million receptor cells located at the top of each nasal cavityo Olfactory receptor neurons

The receptor cells turn that molecule into a neural impulse (transduction) and send that impulse to the olfactory bulb (smell centre in the brain)o Lock-and-key binding


Human Olfactory System


How Do We Taste?

Chemical substances in the food we eat dissolve in saliva and fall into the crevices between the bumps (papillae) of the tongue The bumps hold our taste buds Each taste bud contains 60 to 100 sensory

receptor cells for taste 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


A Taste Bud


Five Taste Receptors

1. Sweet 2. Sour 3. Bitter 4. Salt 5. Umami—the taste of monosodium glutamate

(MSG)


Eating Is More than Taste and Smell


Smell and Taste Disorders

Anosmia—inability to smell Ageusia—inability to taste

Both are usually caused by head traumaOther conditions:

Reflex epilepsy—a seizure occurs only after exposure to a specific odour

Migraine headaches—specific odours can trigger migraines


Tactile or Cutaneous Senses

The tactile or somatosensory system is a combination of skin senses: Pressure, touch, temperature, vibration,

pain The tactile senses rely on a variety of

receptors located in different parts of the skin


Sensory Receptors in the Skin


Different Sensory Receptors

Free nerve endings Located near the surface of the skin Function: Detect touch, pressure, or pain

Meissner’s corpuscles Located in fingertips, lips, and palms (hairless skin areas) Function: Transduce information about sensitive touch

Merkel’s discs Located near the surface of the skin Function: Transduce information about light to moderate pressure

against the skin Ruffini’s end organs

Located deep in the skin Function: Register heavy pressure and movement of the joints

Pacinian corpuscles Located deep in the skin Function: Respond to vibrations and heavy pressure.


Steps to Perceiving Touch

1. Sensory neurons register pressure

2. Information sent to the spinal cord, then the thalamus (telephone operator)

3. Information is then sent to the somatosensory cortex that registers the sensation1. Tactile information is

processed contralaterally


Two Pathways of Pain

Fast pathway pain (myelinated pathway)—sharp, localized pain is felt quicker because it travels along myelinated neurons to the brain

Slow burn (unmyelinated pathway)—nagging, burning pain is slower to be felt because it travels along unmyelinated pathways


Why Do We Feel or Not Feel Pain?

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


Chronic Pain

Chronic pain is the most common abnormality associated with the somatosensory system

Endorphins and enkephalins are naturally occurring pain-killing chemicals in the brain They can be found in opiates such as morphine or

heroine They are produced naturally through intense physical

activity, sex, or intense stress (endogenous opiates) Cingulotomy—destruction of the cingulate cortex

An extreme form of neurosurgery to relieve chronic pain


Pain Abnormalities

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 “Mirror box” therapy


Properties of Sound

Sound waves—vibrations of the air in the frequency of hearing

Frequency—the number of cycles per second in a wave Determines pitch of sound Measured in units called Hertz (Hz), which

represent cycles per second

Amplitude—the magnitude (height of a wave) Determines loudness Measured in units called decibels (dB)


How the Ear Hears


Hearing

1. Sound waves enter the outer ear

2. Hits the eardrum (tympanic membrane) Thin membrane that

moves with sound waves

3. Passes into the middle earContains the 3 smallest

bones (or ossicles) in the human body: maleus (hammer), incus (anvil), and stapes (stirrup)


Hearing

4. Stapes hits the oval window and creates vibrations that move fluid in the cochlea Oval window—membrane

separating the ossicles and the inner ear

5. The vibrations move the basilar membrane (in the cochlea), which is covered with sensory receptors called hair cells

6. As hair cells move, neural impulses are created and sent to the brain


Different Theories of Hearing

Place theory Vibration of the basilar membrane (BM) at different

places results in different pitches/frequencies Near the oval window (where BM is thinner)—higher

frequencies; lower frequencies occur farther from the oval window

Frequency theory Different sound frequencies are converted into different

rates of action potentials or firing High-frequency sounds produce a more rapid firing than

do low-frequency sounds Different firing rates contribute to sound perception of

low frequency tones


Sound Localization

General loudness—louder sounds seem closerLoudness 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


Hearing and the Brain

Cochlea brainstem thalamus auditory cortex auditory association areas in the cortex Tonotopic map—information transmitted

from different parts of the cochlea is projected to specific parts of the auditory cortex


Auditory Abnormalities

Deafness 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 Usually due to abnormalities in the ear One of every 200 people experiences

tinnitus


Stimulus for Vision


Steps Involved in Vision

1. Light waves enter the corneaCornea is protective outer

layer

2. Passes through the pupil The pupil is a small opening

in the eye

3. Passes through the lens The lens focuses the light

waves

4. Projected onto the retinaRetina contains all of the

receptor cells (rods and cones)



5. The rods and cones transduce the light waves into a neural impulse.

Rods Used for periphery and night

vision Not as acute as cones (i.e., fuzzy

vision) Many more rods than cones (over

100 million)

Cones Used for central and colour

vision Very acute (i.e., very clear)

The fovea in the centre of the eye is all cones

Not as many cones (4–5 million)



6. 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.

7. The optic nerve carries messages from each eye to the visual cortex (occipital lobe)


Why Do We See in Colour?

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


Opponent-Process Theory

The activation of one cone (at retinal level) inhibits another cone

This theory explains colour vision at the level of the ganglion cells Ganglion cells are arranged in opposing

cells: red-green, yellow-blue, black-white


Afterimages

• Opponent-process theory may explain colour afterimages: continual viewing of red weakens the ability to inhibit green; remove red and you see green


Afterimages (blank page to see effect)


Colour Blindness

• Most people who are colour blind cannot distinguish between red and green; they would see only a random pattern of dots in this figure.


“What” and “Where” Pathways


“What” Pathway

Visual agnosia—damage to the “what” pathway; cannot recognize objects

Prosopagnosia—a form of visual agnosia in which people cannot recognize faces


“Where” Pathway

Hemi-neglect—damage to the “where” pathway; people ignore one side of their visual field 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


Top-Down Processing

The process by which we organize small pieces of sensory experience into meaningful wholes

Gestalt principles Visual information is organized into coherent images Gestalt means whole or totality The whole is more than the sum of its parts

Proximity—stimuli near to one another tend to be grouped together

Similarity—stimuli resembling one another tend to be grouped together

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


Impossible Figure


Grouping Rules

Proximity—we group nearby figures together

• Similarity—figures similar to each other are grouped together


Closure

Closure—tendency to perceive incomplete figures as whole and complete


Depth Perception

Binocular cues are cues from both eyes 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 When images are very different, we perceive object is

close by When images are similar, we perceive object is

farther away


Monocular Cues

Monocular cues are cues from one eye 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.

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.


Monocular Cues

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).


Monocular Cues and Illusions


Pavement Patty


Perceptual Constancy

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


Shape and Size Constancy: The Ames Room


Magnetic Hill, Moncton


Visual Abnormalities

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 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


Copyright

Copyright © 2012 John Wiley & Sons Canada, Ltd. All rights reserved. Reproduction or translation of this work beyond that permitted by Access Copyright (The Canadian Copyright Licensing Agency) is unlawful. Requests for further information should be addressed to the Permissions Department, John Wiley & Sons Canada, Ltd. The purchaser may make back-up copies for his or her own use only and not for distribution or resale. The author and the publisher assume no responsibility for errors, omissions, or damages caused by the use of these programs or from the use of the information contained herein.

Chapter 5: Sensation and Perception. Chapter Outline 1. Common features of sensation and perception...

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