Hineline - Psych 50 - Spring, 1999

WEEK 2: 31 Jan – 3 Feb (Second week on Exp Psych)

Reading assignments: From Chapters 3 & 4 (note sequence)

Pp. : 100-110, 122-129, 110-122, 169-177

Note any topics that I did not get to last week, which may turn up on the exam.

III. SENSATION & PERCEPTION -- or -- SENSITIVITY & DISCRIMINATION

(Like Brain & Behavior, this topic gets 3-4 weeks in Psych 51,

and then has its own follow-up course, Psych 111)

Concerns our sensitivity to various dimensions of our environment (“psychophysics” )

and the nature of our discriminations among those events.

With limited time, I'll emphasize aspects related to the other topics that

I am covering, and will deal only with Audition & Vision

omitting taste, smell, kinesthetic, vestibular, & skin senses.

A) FIRST, SOME CONCEPTS THAT APPLY TO ALL SENSORY SYSTEMS

1) The neural activity of different sensory systems seems not to differ in kind,

but different groups of neurons are involved in different sensory modalities.

In general, SENSORY CODING entails different groups of neurons for

stimuli we judge as qualitatively different,

and different firing rates for stimuli of differing intensity.

At a couple of points, I shall illustrate how neurons combine

and constitute differential reactions WITHIN modalities.

2) TRANSDUCER: changes one form of energy to another (photocell, light bulb, electromagnet)

RECEPTOR (typically a single cell) - a transducer that stimulates neurons

We categorize the Various types of receptors on the basis of

the types of physical stimulation they react to.

Thus, photo-, chemo-, mechano-, thermo-

EFFECTOR - structure that, stimulated by neurons, produces an effect on the environment

(muscle, gland)

Note: while not emphasized here, effectors are transducers too.

The receptor/effector distinction

is analogous to that of afferent vs. efferent neurons.

SENSE ORGAN - structure that partly determines the way in which the energy

affects individual receptors. (e.g. eye, ear, taste buds, etc.)

A sense organ may contain many receptors.

3) THRESHOLDS: These are measures of the limits of Sensitivity

ABSOLUTE THRESHOLD- the simplest measure of sensitivity.

The smallest stimulus intensity that enables consistent discrimination

of presence vs. absence of the stimulus.

DIFFERENCE THRESHOLD (JND = just noticeable difference)

The smallest difference between two stimuli that enables

consistent discrimination between them.

WEBER'S LAW - Within a given sensory system, the size of the JND for intensity

is a constant proportion of the stimulus magnitude.

e.g. judging weights (based on tension receptors in muscles & tendons)

10 lb. vs other weights - we would have observed a JND of 0.2 lb.

if 10.2 or 9.8 were barely discriminated from 10.0

50 lb. vs other weights? Weber's law predicts JND of 1.0 lb.

"WEBER FRACTION" = JND/absolute intensity = 1/50 = .02 in this example

In both the auditory and visual systems, the Weber fraction is

fairly constant over a million‑fold range of intensities.

This would be like using the same scale to weigh both air-letters and automobiles.

SIGNAL-DETECTION THEORY

Deals with the fact that measures of threshold reflect not only the observer's sensitivity,

but also the consequences of detecting the stimulus.

e.g. What if you were an inspector of fabrics in a dye factory:

"If you fail to reject a defective lot, you're fired."

vs.

"We've been losing a lot of money lately, and the

customers are not likely to notice minor flaws."

"Response Bias," then, is affected by the consequences of detecting or not detecting.

The relevant principles regarding this aspect of discriminating

will be covered next week.

4) SENSORY ADAPTATION VS. HABITUATION

(the latter is not mentioned until a later chapter, on infant behavior)

ADAPTATION - Changed sensitivity resulting from changed stimulation.

can be increase or decrease; occurs at receptor).

Can Illustrate adaptation with respect to temperature,

using three buckets of water,

Or taking a shower with a friend.

HABITUATION: is different; is a simple form of learning

Decreased reactivity with continued stimulation

Unlike adaptation, it can be reversed by verbal prompting,

or by suddenly reducing stimulus intensity.

(cf. air conditioner)

Thus, it has to do with selective attention

((regarding mechanism, remember the reticular formation?)

One more principle that applies to most sensory systems

5) LATERAL INHIBITION: an important basic principle that shows how some types of distinctions

get made within sensory systems.

Definition: an active unit opposes the activity of its neighbors.

Among other things, it accounts for the fact that

your teeth appear whiter if your skin is darker.

And for the fact that you feel mainly the edges of the elastic on your underwear.

The principle:

Activity in a given neural unit, not only has excitatory effects down the line,

but also inhibitory effects on neighboring units.

An analogy:

A long line of people treading water, and pressing down on each others' shoulders.

(Everybody's more or less even)

Now, those on half the line press less forcibly.

Consider the effect at the boundary. XXXXXXXXXXxxxxxxxxx

One is being pushed down, without pushing down in return.

(that person will be lowest in the water)

One is pushing down, without being pushed down in return.

(that person will be highest in the water)

Now, replicate the line many times to get "field effect"

XXXXXXXXXXxxxxxxxxx

My reason for including lateral inhibition even though

the textbook does not:

It illustrates how distinctions get made

THROUGH THE INTERACTIONS AMONG GROUPS OF NEURONS

rather than by independent actions of individual neurons.

AUDITION (hearing) The book begins with vision;

I begin with audition, because it is more tangible

A couple of opening questions:

1) "Do dogs have more sensitive ears than humans do?" yes -- and no

If our ears were much more sensitive, we would hear

the air molecules randomly bouncing our eardrums.

"Then how is it that dogs can hear events that we cannot?"

We'll get to that.

2) "If a tree were to fall on a deserted island, would it make a sound?

No. "Why?"

Taking the latter one first (falling tree on deserted island):

SOUND IS HEARING -- one of our reactions to events in the world.

An ACOUSTIC STIMULUS is an event which we react to by hearing,

but the stimulus is not the hearing.

Vibrations: typically, changes of air pressure at the ear.

can also be vibrations of skull

(cf. own voice via tape recording)

I have an audio speaker producing acoustic stimulation right now,

but the pulsations are too weak for you to hear them.

Making them more intense

does not change them from non-existing to existing

The term, "ultrasound" is a misnomer, for

The type of physical energy is the same as that for sound,

but it is a frequency to which we are not sensitive.

Thus, there are two basic, crucial dimensions: frequency (Hertz)

amplitude (Decibels)

3) Auditory sensitivity: reviewing, we measure it via ABSOLUTE THRESHOLD

lower threshold = greater sensitivity)

What is the least intense acoustic stimulus that we can discriminate?

Absolute threshold - discrimination of presence vs absence. (50% of the time).

As I said before, if we were any more sensitive,

we'd hear randomly bouncing air molecules.

I should have said this is true only under ideal conditions

(2,000 Hz, isolated room ...

As a graduate student, I accidentally verified this when trying to study in an anechoic chamber:

(blood pulsing; tinnitus; swallowing)

We can measure and describe how the Absolute threshold is affected by the frequency of the stimulus:

"Low-fidelity ears?" greatest sensitivity at 1K-4K

We require much more energy at both low and high frequencies.

Illustrate w. oscillator: |

(3 frequencies) | . .

I T | . .

70 Hz N H | . .

2000 Hz T R | . .

17000 Hz E E | . .

N A | . .

S H | . .

I O | . .

T L | . . .

Y D |______________________________________

20 200 2 K 10 K 20K

CYCLES PER SECOND (HZ)

Now, about that dog: his/her range of greatest sensitivity is much wider. (15-50,000 Hz)

Thus, the dog hears both lower (earthquakes) and higher (whistles)

but at our best frequencies, we do just as well.

Auditory Sensitivity in Non-ideal, ordinary conditions --

MASKING effects of "ambient noise" -- we can measure it

as change of threshold: higher threshold = less sensitive

4) PITCH, LOUDNESS, TIMBRE

Illustrate the first two, via oscillator

Fixed intensity, vary frequency

Fixed frequency, vary intensity

Pitch is most affected by frequency, but it can change slightly with intensity (e.g. fog horn)

Loudness - strongly affected by intensity, but also by frequency.

an intensity that you barely hear at 16K Hz

will be loud indeed, at 4K Hz.

(Illustrate via Sweep w intermediate intensity)

Note, if time permitted, I could demonstrate several special effects

re discriminating between simultaneously present tones.

Complex tones: OVERTONES (HARMONICS) - even multiples presented together,

may be heard as changed quality of the sound (timbre)

rather than as separate sounds.

Other complex sounds -- WHITE NOISE, vowels & consonants, etc.

We are amazingly capable of discriminating several

simultaneous acoustic stimuli superimposed on the eardrum.

NEURAL BASES FOR FREQUENCY DISCRIMINATION:

PLACE THEORY different loci within the inner ear

Describes our functioning with respect to high frequencies

vs. FREQUENCY THEORY - how does counting occur?

Describes our functioning with respect to low frequencies

Hineline Psych 50, begin Wednesday-Thursday of Second Week

[ On Board: IN ADDITION TO VISUAL THRESHOLD CURVES ]

[ REPEAT THE SPECIFICATIONS OF READING ASSIGNMENT

Begin w. leftovers from last time, e.g. combinations of acoustic frequencies?

place-theory vs. frequency theory?

Then:

C) VISION considered in a way analogous to the way we covered audition

1) The type of energy - ELECTROMAGNETIC RADIATION

the same kind as radio waves, x-rays, a vibrating magnet.

(Note, again, the importance of distinguishing between the stimulus and our reaction to it.)

Propagated at 186,000 miles per second;

higher frequency results in shorter wavelength (nanometers). (10^-⁹ meters)

Easier to measure wavelength than frequency,

The two are inversely related -

thus, specifications are in terms of wavelength.

2) Visual Receptors -- ROD & CONE shaped cells in the RETINA.

(note - you will not be tested on anatomy that I don't mention)

TRANSDUCTION - via absorption of light by PHOTOPIGMENTS

result is stimulation of neurons.

Changes of wavelength result in our describing mainly changes of color.

However, brightness also changes at the same time..

3) Measurement of visual sensitivity is made difficult by adaptation

(this occurs very little in audition, where habituation is more common)

But adaptation also tells us something about ORGANIZATION OF THE RETINA:

Indeed, it provided the first indication of what rods & cones do.

DARK ADAPTATION -- Pre-expose to constant light intensity

Measure threshold w brief flashes,

as function of time in dark.

Large-area stimulus containing broad range of wavelengths

|. gives curve with two portions:

[ Adjust I T | .

[ ordinate N H | .

[ to match T R | . .

[ threshold E A E | .

[ curves N T S | .

[ already on S H | .

[ the board I O | .

T L | .

Y D | .

|__________________________________________

0 5 10 20 30

MINUTES IN DARK

Evidence for two distinct systems (the textbook barely acknowledges this, on p. 112)

If we stimulate only the FOVEA, (explain what is)

we get only the first part.

And if we use LONG WAVELENGTHS (red) we also get only the first part

Also, depending on wavelength used,

different colors will be seen during these measurements.

Examine that part of the retina w. microscope -- one finds only cones

The corresponding system is called the PHOTOPIC system

[book mentions day & night vision, without these terms]

Outside of but near the fovea, both parts of the curve are found, but

If we stimulate far out in the PERIPHERY,

only the second part of the dark adaptation curve is found.

and this identifies the SCOTOPIC SYSTEM

The lights will appear bluish white, irrespective of wavelength,

and thresholds are, of course, much lower than photopic.

Another interesting point on the retina is the BLIND SPOT,

which arises from the bundling together of axons

that form the optic nerve.

The textbook describes on p. 113 how to verify your own blind spot.

VISUAL THRESHOLD CURVES AS A FUNCTION OF WAVELENGTH.

(text shows spectrum on p. 116, but doesn't show these curves)

Dark adapt before measuring: brief flashes of light,

vary intensity at each of many wavelengths

to measure threshold at each wavelength

p p

| s p s p

I T | s p s p

N H | s p s p

T R | s p s p

E A E | s p s p

N T S | s p s p

S H | s p s p

I O | s p s p

T L | s s p = photopic (cones)

Y D | s s s = scotopic (rods)

| s s

|_______________________________________________

400 500 600 700

BLUE GREEN YELLOW ORANGE RED

VIOLET

Note: the differing minima of the two functions

correspond to the minima of the two parts of dark adapt. curve

A practical application of this: sodium-vapor street lights

instead of mercury-vapor street lights. -- wavelength close to 550.

Other practical applications -- to detect a dim star ...

red goggles for fighter pilots

5) ANALOGOUS DIMENSIONS OF VISUAL & AUDITORY EXPERIENCE:

Brightness, Hue, Saturation vs. Loudness, Pitch, Timbre

The third item in each set, concerns Mixtures of stimuli:

Auditory Stimulus of single frequency: "pure tone"

like a tuning fork (or the oscillator we heard)

To a large extent, the ear is differentially sensitive to

simultaneously presented stimuli of differing frequencies.

(as already described)

Thus, all the acoustic stimulation in this room

is superimposed on the same eardrum.

However: different frequencies maximally stimulate different

locations within the auditory mechanism.

Combinations of visual wavelengths are a very different matter:

Visual stimulus of single wavelength -- "spectral color"

most easily obtained with a prism

If multiple wavelengths of light strike one part of the retina,

we see only one color -- changing the combination of wavelengths changes the color

Additive vs subtractive color mixture, etc. (pp. 117-118 of book)

Most details of color mixture indicate TRICHROMATIC SYSTEM

(three types of cones)

But a few aspects of color mixture indicate OPPONENT PROCESSES

COMPLEMENTARY COLORS combine to give white

also evident in afterimages & color-shadows

These apparently arise via interactions between neurons,

in the retina and elsewhere.

Given our time limitations, shall not cover phenomena of color in detail.

nor will you be examined on them in detail (wait for Psych 51 & 111)

They’re important, though – e.g. current work on design of high-definition TV

6) SPECIALIZED FEATURE‑SENSITIVE CELLS IN THE VISUAL SYSTEM

Hubel & Wiesel's experiments on RECEPTIVE FIELDS – (see text, pp. 120-121)

the area and types of stimuli to which a given cell is sensitive.

Many of them can be accounted for in terms of LATERAL INHIBITION

Some (ganglion cells in retina) respond differentially to light areas surrounded by dark

others to dark areas surrounded by light

Still others, in visual areas of cortex:

Edges with particular orientations at particular locations

Edges with particular orientations over wide areas

Combinations of edges (corners, etc)

Other areas of cortex seem to react to combinations of these,

e.g. sensitivity to movement , or combined reactions to shape, movement & color.

Still more complicated feature-sensitivities: e.g. "bug detectors" in frog's eye.

"monkey-paw" or "monkey-face" detectors in monkey's cortex

7) OTHER TYPES OF DIFFERENTIAL SENSITIVITY

ACUITY -- spatial separation within visual patterns (familiar eye tests)

DEPTH PERCEPTION ("Distance Discrimination") [in perception chapter]

MONOCULAR CUES: INTERPOSITION