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Chapter 4: Sensation

Introduction

To survive we must know the world around us because most objects in the world are charged with

meaning.

So “Where does knowledge come from?”

The simple answer is through our senses.

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The Origins of Knowledge

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The Origins of Knowledge

Empiricisms (experience) says that we collect information from our senses and transform that

experience into knowledge.

John Locke (1632-1704)

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John Locke says that human mind is like a blank tablet (tabula rasa) on which experience writes. Empiricists argued that all knowledge comes through stimuli that excite the senses.

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Stimuli

To get information, energy from objects or events (distal stimuli) impinges on the sensory surface

generating proximal stimuli.

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DistalStimulus Proximal

Stimulus5

Empiricism

However, there are problems with empiricism, e.g., how do

we perceive qualities like, depth, size, and shape, not

directly given in the proximal stimulus? The empiricists answer by asserting that

much of perception is built up through learning by

association.How is the hand closer than the

car, for they cast an image on the

retina that is similar in size.

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Empiricism

Empiricists argue that our perception of depth, size, and shape etc., are embedded in the

proximal stimulus, as cues. And the use of these cues are based on learning (association).

Henderson State University: Garrison Hall

Size of the pillars gets smaller as it recedes in the picture and

gives us the sense of depth. We learn to associate seeing the pillars get shorter as we

move in the corridor and thus perceive depth by associating

seeing with our previous learning.

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Nativism

Philosophers like Kant pointed out that humans do not perceive sensory information by simple

association only, but use innate mental categories

to perceive incoming stimuli.

Immanuel Kant (1724-1804)

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Categories

1. We perceive an object as a whole and not as bits of experience as the associationists claimed. The apple is a single object (category: unity) and not bits of texture, color, and brightness etc.

2. Likewise, perception of depth is also a unitary experience, not bits of objects perceived, that differ in size.

3. Kant suggested that perception was based on categories like these which were inborn (innate).

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Psychophysics

Psychophysics

The dispute between empiricists and nativists was a debate among philosophers… The charting of

relationship between physical stimuli and psychological experiences began with the field of

psychophysics founded by Gustav Fechner.

Psychophysics asks questions like, what changes in our perception of a sound as the frequency of the

sound waves change?

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Psychophysics and Dualism

Fechner realized that sensations and the stimuli that produce them belong

to two totally different realms, one to the mental realm (sensation) and the

other to the physical realm (stimuli). However

each realm could be measured separately.

Gustav Fechner (1801-1887)

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Two Realms

Physical Stimulus Registered Sensation

Sound pressure Auditory sensation

Light flash Visual sensation

Sucrose solution Sensation of sweetness

Hydrogen sulfide gas Rotten egg smell sensation

Sandpaper Course textural sensation

Measuring Sensory Intensity

1. How can we measure a strong smell from a weak one? How can we measure a loud sound from a faint one? Answers to such questions led to quantitative measurement of sensation.

2. How can we measure differences between sensations made by blue versus green light? How can we measure differences in sensations caused by sour versus sweet taste? Answers to such questions led to qualitative measurement of sensation.

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Difference Threshold

Before Fechner, Weber (1834) pointed out that

just noticeable difference (JND) between two

stimulus intensities was a constant ratio (k = δI/IS).

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Ernst Heinrich Weber (1795–1878)

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Weber Fraction

δI/IS = K, where δI = IC – IS

IC = intensity comparison Stimulus

IS = Intensity standard stimulus

(IC – IS)/IS = K

If IS = 50g and IC = 51g, Weber fraction K,

equals:

(51 -50)/50 = K

1/50 = K

or K = 0.02

How much weight would be required to notice a

difference if the weight of standard stimulus (IS ) was

150 gram?

(IC – IS)/IS = K

(IC - 150)/150 = 0.02

(IC - 150)= 0.02 X 150

(IC - 150) = 3

or IC = 153

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Weber Fraction Graphed

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153

204

255

306

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459

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Standard Stimulus (g) Intensity (IS)

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Modality Sensitivities

Based on his measurements, Weber concluded that there were different JNDs for different senses. Thus

senses differed in their sensitivities.

Sensory ModalityWeber Fraction

(δI/I)

Vision (brightness, white light) 1/60

Kinesthesis (lifted weights) 1/50

Pain (thermally aroused on skin) 1/30

Audition (tone, moderate loudness) 1/10

Pressure (cutaneous pressure “spot”) 1/7

Smell (odor of India rubber) 1/4

Taste (table salt) 1/3

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Psychophysical Scale

Fechner used Weber’s fraction to

arrive at a new formulation

(S = K log R) to develop the

psychophysical scale, that showed a

relationship between the physical and the

psychological realms.

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JNDs

1. JNDs became the basis to develop the psychological (sensation) scale. With a defined increment in the stimulus intensity the subject reported sensing a difference between two intensities.

2. However, new research shows that individual’s sensory systems are affected by internal and external noise. Sometimes they say they don’t detect a difference when there is a difference between stimulus intensities at other times they do when there is none.

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Signal Detection Theory (SDT)

SDT takes into account the sensitivity of the subject amidst noise and can tease apart the subject’s bias or

the ability to make a decision from his sensitivity.

Stimulus Present

Stimulus Absent

Decision “Yes” Hit False Alarm

Decision “No” MissCorrect

Rejection

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Uses of SDT

SDT has been successfully used in a number of practical fields like communication, medicine, and

law.

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1. How does the radar operator detects and decides whether the approaching plane is that of the enemy.

2. How does the histologist decides whether she can detect and make a decision about cancer in a patient.

3. And finally how can a jury decide whether someone is guilty or not.

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The Functioning of the Senses

The Functioning of the Senses

1. Seeing, hearing, tasting, smelling, and touching are 5 basic senses, including others, like vestibular, kinesthetic and skin sensations.

2. Each sensory cell transform physical energy of the stimulus into electrochemical process which the brain can understand. This is called transduction.

3. For each sense, stimulus energy can be coded for psychological intensity or quality.

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The Functioning of the Senses

4. This coding is achieved through changes in neural firing for stimulus intensity. Rapid firing more intense stimulus.

5. Different types of neurons fire to register stimulus quality. Specific neurons work as “labeled” lines to process stimulus quality. This idea is based on law specific nerve energies (Müller, 1826).

6. Stimulus quality may also be registered by patterns of neuronal firing.

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Vestibular Sense

Amongst senses, vestibular sense registers bodily acceleration in the

linear and the angular dimensions. It is

processed through the semicircular canals in the

inner ear.

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Hearing

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Hearing

To hear, the ear is sensitive to sound pressure, which can vary in intensity, frequency and timbre.

The intensity and frequency are represented below.

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Sound Intensities

Sound Intensity (dB)

Manned Spacecraft launching (150 ft) 180

Loudest rock band on record 160

Pain threshold (approximately) 140

Loud thunder 120

Shouting 100

Noisy automobile 80

Normal conversation 60

Quiet office 40

Whisper 20

Rustling of leaves 10

Threshold of hearing 0

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Sound Frequencies

Sound Frequency (Hz)

Top note of grand piano 4244

Top note of piccolo 3951

Top range of soprano voice 1152

Top range of alto voice 640

Middle C 256

Top range of baritone voice 96

Top range of bass voice 80

Bottom range of contra bassoon 29

Bottom range of grand piano 27

Bottom note of organ 16

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Timbre

Timbre or quality of sound is based on complex waves. They are essentially a summation of simple

sine waveforms.

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Simple vs. Complex Sounds

A simple sound frequency (or pure tone) is produced by a tuning fork. However most of the

sounds are complex, e.g., piano sound. Complexity of sound represents its timbre.

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The Ear

Sound waves pass through the ear canal to vibrate the eardrum adhered to three bones called

ossicles. Vibrations from ossicles are delivered on to the oval window of the cochlea to transduce sound signals to auditory impulses in neurons.

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Cochlea

The cochlea is a snail shaped organ in the inner ear contains the basilar membrane. Hair cells on this membrane deform by its movement caused by

fluid that runs through cochlear duct.

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Place Theory: Hearing

Originally proposed by Helmholtz (1857) and later confirmed by von

Békésy (1961), the basilar membrane deforms at

different places to gives rise to sensations of different frequencies

(pitch).

35Basilar membrane perturbed

Helmholtz (1821-1894) Von Békésy (18299-1972)

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Frequency Theory: Hearing

However, lower frequencies (50 Hz) cause the basilar membrane to vibrate equally over its entire surface,

and very high frequencies (20,000 Hz) cannot be registered by single neurons.

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So how can we perceive different sound frequencies? Frequency theory suggests

that pitch is determined by the firing rates (patterns) of neurons (Wever, 1965).

Ernest Wever (1902-1991)

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Sound: Further Processing

Hair cells in the cochlea transduce mechanical motion into neural signals which are processed by

nuclei in the midbrain, thalamus (medial geniculate nucleus) and the auditory area in the

temporal cortex.

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Sound: Further Processing

1. When a sound arrives in the brain, it must identify and recognize its quality. So a note on the piano needs to be differentiated from a note produced by a harpsichord (timbre).

2. Brain needs to identify a sound (my cell phone) from another sound (someone else's cell phone).

3. Brain must also localize sound in space, whether it is coming from the left or the right.

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Sound: Further Processing

Neurons in the auditory cortex have a frequency map (tonotopic map). Similar preferred pitches tend

to be located close to each other.

39Auditory cortex: cat

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Vision

Visual Sense

Visual sense is the most important sense for human beings. We trust this sense more than

other senses and it utilizes the largest area in the human brain compared to other senses.

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Eye: The Visual Organ

Light travels through the cornea and the lens to reach the retinal layer.

Iris controls the amount of light that goes

through. And the lens thickens and thins

(accommodation) to focus on near and far

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Retina: Photosensitive Layer

1. Photoreceptor Layer: Cones perceive color; are

concentrated in the fovea

Rods perceive light/dark; are completely absent in the fovea

2. Bipolar Cell Layer

3. Ganglion Cell Layer: axons converge to form optic nerve. Optic nerve exits eyeball forming blind spot.

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Photoreceptors

Rods:Peripheral retinaDetect black n’ whiteWork in twilightconditions(120 million)

Cones:Central retinaFine detail and colorWork in daylightor well-lit conditions (6 million)

Photoreceptor Pigments

1. Rods: Rhodopsin

Sensitive to light and breaks down easily when light present. Therefore signals ‘bright’ versus ‘dim’.

2. Cones: Three photopigments

Light chemically changes the photopigments. Different wavelengths cause different photopigments to react, helping us to perceive color.

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Visual System Processes

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If a stimulus remains unchanged sensitivity

to this stimulus decreases. Neurons

and other sensory cells fire more strongly to

novel input.

Visual System Processes

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In the same fashion, contrast is another factor that leads us to being sensitive to differences in stimulation.

Brightness, edges, and shapes are still other factors that require change in stimulation.

The gray on the whiter background looks darker than on a black background.

Contrast Effects: Illusions

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If you look at the grid on the right and focus in the middle, you see

gray spots at the intersections. This

illusory phenomenon is due to lateral inhibition

of brain cells in the visual system (see

below).

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Contrast Effects: Mach Bands

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Contrast effects do not simply separate bright from dark, but accentuates brightness differences, as seen in Mach bands. Each band is homogeneous gray strip however, at each edge left side looks darker and right

side lighter than the band’s gray tone.

Contrast Effects: Mach Bands

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Figure A shows how stimulus intensity

uniformly changes over Mach bands, however

perceived intensity (Figure B) is a different story. Brightness and

darkness on each band’s edge, are accentuated.

Contrast Effects: Physiology

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Cell C is inhibited by cells B and D

less strongly thus C sends a strong signal. Cell D is inhibited by its neighbors more

strongly thus sends a weaker signal. Thus the edge is

emphasized.

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Color

White light is a narrow band of electromagnetic radiation, composed of spectrum of wavelengths

ranging from 400nm (violet) to 700nm (red). Shorter wavelengths than violet are called

ultraviolet (seen by bees) and longer wavelengths than red are called infrared, sensed as heat.

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Hue or Color

The human eye can detect 7 million colors. Short wave lengths tend to be bluish in color and longer

wave lengths tend to be reddish in color.

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Hue

Brightness

Brightness is a dimension of color that differentiates black (low brightness) from white

(high brightness). Chromatic colors also have different brightness levels.

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Brightness

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Saturation

Saturation is simply purity of a color. Saturation determines to what extent a color is chromatic

(saturated) or achromatic (unsaturated).

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Saturation

Physiological Basis of Color

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What is the physiological basis of color? The answer lies in two mechanisms: 1. retina and, 2. the nervous

system.

Color Receptors

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Normal human vision depends on three different kinds of cones, thus our color vision is trichromatic. We have

short, medium and long-wavelength cones, each responding to different spectral colors, however there is

overlap in responsiveness (see below).

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Young-Helmholtz Theory

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Young (1801) and then Helmholtz (1866) suggested that human retina contains three receptors and that when long wavelength cones were activated we perceived redder hues and when short wavelength cones were

activated we perceived bluish tones. And all other colors were perceived as combination of activation of

cones.

Problems with Young-Helmholtz Theory

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1. Young-Helmholtz theory of color vision explains why red, green and blue have special status. They all are primary colors, not based on mixtures of other colors.

2. However, the theory does not explain why yellow looks like a primary color, when it is a mixture of red and green.

Problems with Young-Helmholtz Theory

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3. Another problem with Young-Helmholtz theory is that it cannot explain why colors come in pairs, e.g., simultaneous color contrast. The gray patch on the yellow square looks darker and bluer, and gray patch on blue square looks lighter and yellower.

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Problems with Young-Helmholtz Theory

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4. A clearer example of pair of colors can be appreciated when we study negative after images. Each color generates a negative after image. Red results in green after image and vice versa, and blue in yellow etc.

Stare in the middle of the flower for a

minute, blink twice and then look at

the black dot on the right.

Opponent-Process Theory

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Herring (1872) and later Hurvichand Jameson (1955) proposed opponent-process theory of

color vision, which suggests that red-green, blue-yellow and

black-white colors are paired at the next level in the nervous

system beyond retinal receptors.

Color Blindness

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Color blindness can be due to two defects in the visual system: 1. Defective opponent process,

2. Missing photopigment or both. Eight % men and .03%

women suffer from color blindness with red-green form

of color blindness as most common.

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Shape Perception

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We perceive shapes through cells called feature detectors. Cells respond differentially to various stimuli, like Straight: horizontal, vertical, lines, Corners, angles,

and even movement, velocities. With animal preparations (shown below) such feature detector

could be outlined.

Shape Perception

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Different kind of responses are elicited in the retina and the nervous system which are combined to form

feature detectors.

Shape Perception

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Feature detectors is not the only kinds of cells in the visual nervous system. There are cells that respond to

complex shapes like faces, hands, fingers etc.,

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Some Final Thoughts

Active Perceiver

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All scientific evidence suggests that we actively shape our sensations. And these include perception of shape, motion, and edges. The visual brain’s activity increases

as we process information even more deeply.

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Questions

1. The perspective that humans categorize and interpret incoming sensory information, offered as an alternative to the idea that human senses passively perceive the world, is known as:

a. transduction.

b

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nativism.

c. signal detection theory.

d. empiricism.

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Questions

2. The idea that different sensory qualities within a single sense (i.e., red versus green; sweet versus sour) are signaled by which neurons are firing more or less in response to the stimulus is known as ______________ theory.

a. specificity .

b. signal detection.

c. pattern.

d. frequency.

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Questions

3. Contrast-effect illusions, such as the Mach bands, are produced by what mechanism of visual processing?

a. lateral inhibition.

b. negative afterimage.

c. simultaneous color.

d. feature detection.

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Questions

4. The auditory receptors that transduce sound waves into neural firing are located in the:

a. cochlea.

b. outer ear.

c. auditory canal.

d. oval window.