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Hearing Musical StreamsAuthor(s): Stephen McAdams and Albert BregmanSource: Computer Music Journal, Vol. 3, No. 4 (Dec., 1979), pp. 26-43+60Published by: The MIT PressStable URL: http://www.jstor.org/stable/4617866.

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2/21

Hearing

Musica l

Streams

Stephen

McAdams

Hearing

and

Speech

Sciences

Stanford

University

School of Medicine

Stanford,

California

94305

Albert

Bregman

Department

f

Psychology

McGill

University

Montreal,

Quebec,

Canada

Introduction

The

perceptual

ffectsof

a

soundare

dependent

pon

the musical ontext nwhich hatsounds imbedded. hat s,

a

given

ound's

erceived

itch,

imbre,

nd oudness re

n-

fluenced

by

the sounds hat

precede

t,

coincidewith

it,

and

even ollow

t in

time.

Thus,

hiscontext nfluences

he

way

a listener

will associate

he soundwith

various

melodic,

rhythmic,

ynamic,

armonic,

nd

timbral tructures ithin

the

musical

equence.

t

thus behooves he

composer

nd

interpreter

o

understand

he various

erceptual rganizing

principles

hataffect he

derivation

f musical ontext

rom

sequences

f

acoustic

vents.

We

nclude

he

interpreter

ere

because

everal

musical

dimensions,

uch

as

timbre,

attack

and

decay

transients,

nd

tempo,

are

often not

specified

exactlyby

the

composer

nd

are

controlled

y

the

performer.

In this articlewe shalldiscussprincipleshatdescribe

how

variousmusical

imensions

ffect he

perceived

ontinui-

ty

of music.

Leon

van

Noorden

as

stated hat "in

sequences

where

he tones follow

one another

n

quick

succession,

effectsare

observed

which ndicate

hat

the tones arenot

processed

ndividually

y

the

perception

ystem.

On

the one

hand

we

find various

ypes

of

mutual

nteraction etween

successive

ones,

such as

forward

nd

backward

masking,

loudness

nteractions

ndduration

nteractions. n

he other

hand,

a kind of connection

s

found

between

he

successive

perceived

ones."

[28,

p.1

].

As

for

simultaneousonic

events,

Bregman

5,

9]

has

suggested

hat

different

ounds

are

ex-

tracted

ccording

o

various

erceptual

nd

cognitive

rga-

nizational

mechanismsrom

he

superimposed

coustic ibra-

tions.While omeresearchers ould ike to attribute hese

phenomena

o

mechanisms

n the

peripheral

r

early

entral

nervous

ystem which

are

surely

nvolved o

some

extent,

see

[24,

33]),

prominent

members

f thismore

psychophys-

ically

oriented school"

re

beginning

o

think he

organiza-

tion

s

too

complex

o

be so

easily

explained.

However,

ather

than

delve

nto

theoretical

xplanations

f these

phenomena,

it will suffice

or

the

present

purpose

o describe

hem

n

general

nd o

briefly

quantify

ome

salient

parameters

hat

have

ntrigued

s

with

compositional

ossibilities.

his

artic

thuspresents,na tutorialashion, review f researchin-

cluding

ur

own)

whichhas direct

mplications

or musi-

cians,

specially

or

composers

orking

with

computer

musi

Inall of our

researchnd n

mostother

research

ited,

com-

puters

predominantly

DP-11

minicomputers)

ereused

o

synthesize

he

sounds

presented.

They

were

alsousedfor

presentation

f sound

timuli,

ollection

f

responses

rom h

listeners,

nd

analysis

f

the data.For

thorough

ummari

and heoretical

reatmentsf thisarea

of

research,

ee

[2,

5,

9.]

.

What

s An

Auditory

tream?

Auditory treamormationheory s concernedwith

how the

auditory

ystem

determines

hether

sequence

f

acoustic

vents

esults rom

one,

or

more

han

one,

"source

A

physical

source"

may

be

considered

s

some

sequence

f

acoustic vents

emanating

romone

location.

A

"stream"

is a

psychological

rganization

hat

mentally

epresents

uch

a

sequence

nd

displays

certain

nternal

onsistency,

r con

tinuity,

that allows

that

sequence

o be

interpreted

s a

"whole."

By way

of

example,

wo

possible

perceptual

r-

ganizations

f

a

repeating

ix-tone

sequence

re llustrated

n

Figure

1. Time

s

represented

n

the horizontal

xis and

frequency

s

represented

n

the

vertical

xis.

Thedotted

ine

connecting

he tones

n the

figure

ndicate

he stream

erce

In the first

configuration,

ix

tones

are

heardone

after

the

other n a continuouslyepeatingycle (Taped llustratio

la)l;

it is

easy

to

follow

the entire

melodic

pattern.

n

the

second

percept,

hough,

ne

might

hear wo

separate

hree-

tone

patterns

which

appear

o

have ittle

relationship

o

eachother

Taped

llustration

1b).

It is

difficult

n thiscase

to

follow

he

original

ix-tone

pattern.

Note

that

n

the

first

example

ne stream

s

heard,

nd n

the

second,

wo

are

hea

?

1979

by

Stephen

McAdams

nd

Albert

Bregman

(1)

A

tape

containing

ound

examples

s

available

rom

Mr.

McAdams.

escriptions

f

the

taped

llustration

are

ound

n

Appendix

.

Page

26

Computer

Music

Journal,

Box

E,

Menlo

Park,

CA

94025

Volume

3

Numb


3/21

D

D

D

B"B

/B

B'

F

F

%

F

-

"

I

I

.

O

ST

I

I

Cr

I

I

-

%

%

(a)

(b)

Time

-,

Time

One

Stream

Two Streams

Figure

1. A repeating6-tone sequence

composed

of

interspersed

high

and low tones can resultin differentpercepts.In Figure

l

with

high

and

low tones

alternating

at a

tempo

of 5

tones/sec.,

one

perceptual

tream

is

heard.

In

Figure

1

b,

at

a

tempo

of

10

tones/sec.,

the

high

tones

perceptually

egregate

rom the low tones

to form two streams

(cf.

Taped

Illustration

1).

D

4

F

t

t

D

o

0

L.

F

C

C

u

"=I

-U

A

,/

A

/

Time

-+

Time

-+

I : J J

J i

I : J J J :

II

U

High

Stream Low

Stream

High

Stream

Low Stream

Figure

2.

Due

to the

competition

among

stream

organizations,

one

F

may

be

perceived

as

belonging

o

either the

higher

stream

or

the

lower

stream

but

not to

both.

The

organization

of

the

streams

changes,

among

other

things,

the

perceived

rhythmic

structure,

as

indicatedunder

each

diagram

cf.

Taped

Illustration

2).

Stephen

McAdams

nd Albert

Bregman:

Hearing

Musical

Streams

Pag


4/21

In

the rest

of

this section we

will

discuss

some

of the

properties

exhibited

by

a

stream.

For

example,

it is

possible

to focus

one's

attention

on a stream and follow

it

through

time

[6,

28].

In the first

taped

example

one

can

follow the

six-tone

pattern

without

any

trouble.

Thus

a

stream

must

exhibit

a certaincoherence

over

time.

However,

n

the second

example,

it

is

difficult

to follow

the six-tone

pattern,

but

it is

easy

to

follow

either three-tone

pattern.

Notice that one

can

pay

attention

to either the

higher

or lower

stream,

switching

between

them

at

will,

but that it

is not

possible

to attend

to

both

simultaneously (Taped

Illustration

ib).

Indeed,

each

three-tone pattern n Figurelb constitutesa separate tream

and

maintains

ts own

temporal

coherence.

While

a listener

s

paying

attention

to

one

coherent

stream,

other acoustic

in-

formation

is

perceptually

relegated

to

the

background.

If

one

group

of

sounds is distinct

enough,

the

foreground

back-

ground

relation

may

be almost

involuntary

and it

may

require

a

great

deal

of attentional

effort to focus

on

streams

nitially

relegated

o the

background.

The

information-processing

ature

of the

stream

segre-

gation

process

s

suggested

by

the

observation

hat the

segre-

gation

of

a

sequence

nto

smaller treams

akes time

to

occur

[4,

13].

In

Figure

lb,

notice that

one

can

hear a

six-tone

pat-

tern for

the first

few

cycles

before

it

segregates

nto

two

separate treams TapedIllustration b). It thus appears hat

the

perceptual

system

assumes

things

are

coming

from

one

source

until it

acquires

enough

information

to

suggest

an

alternate

nterpretation.

When

temporal

coherence

is

lost in

a

sequence,

it

be-

comes

more difficult to order the

events of

that

sequence

n

time

[6,

11,

14,

28]

.

In

Figure

la,

it

would

be

easy

to tell the

orderof

tones

A,

B,

andC. But as this

larger

treambreaks

down into

the smaller

treamsof

Figure

b,

i.e.,

as

temporal

coherence

s

lost,

it

becomes

more difficult

to

judge

the order

of these tones. In such a case

one

might

notice

that tone

A

comes

before tone

C but

it

would be hard

to tell

whether

tone B

came before

A,

between

A

and

C,

or

after

C. This

statement

should be

qualifiedby noting

that in

the

tone se-

quences

mentioned

all

of the

tones are

of

equal

loudness

and

timbre.

The

tone

sequences

are furthermore

continually

recycling

and

are

faded

in

so that the

listener cannot label tone

A as

being

the first

tone in the

sequence.

Information n a

musical

context,

such

as

the

first

beat

being emphasized

as

a

downbeat,

may give

the

listener an

anchor

point

against

which

to relate

the

temporal

positions

of other

tones. Thus

one

can

judge

the order

of

events in

time

within

a

given

perceptual

streambut

not

necessarily

across

streams.

Accompanying

his

is the observation

hat different streamscan

appear

o

overlap

in

time even when

they

do

not.

Again

in

Figure

lb,

notice

that one can hear two three-tone

patterns apparently

going

on at

the same time even

though

the tones are

alternating

(TapedIllustration b).

If an

event

is

potentially

a memberof

more

than

one

"competing"

tream,

one

may perceive

t as

belonging

o one

stream

or another

but

not to both

simultaneously[3,

10,

11] .

This

is not

to

say

that

a musiciancannot

hear

several

simul-

taneous

lines. The

point

is that it is

impossible

o

use several

parsing

schemes at the

same time.

Figure

2 illustrates

the

effect

on

our second

example

of

moving

the

higher

triplet

into closer

frequency

proximity

to

the

lower

triplet.

We can

find an

intermediate

position

where

the lowest

tone,

tone

F,

can

group

with

either

the

higher

or lower

stream

(Taped

Illustration

2).

Notice

that

this

regrouping

esults n

a

rhyth-

mic transformation

as

shown under

each

configuration.

Some

non-musical

examples

of

this

phenomenon

are the

fac

vase illusion

and

the reversible

Necker

cubes;

Escher

and

Vasarely

have

produced

art works

based

on such

principle

of

perceptual

organization.

Finally,

this

brings

up

the

relationship

between

"sourc

and

"stream."

A stream

s

perceived

as

emanating

rom

a

sin

source.

So,

in

the

first

example,

the

pattern

was

fairly

con

tinuous

and was

easily

recognized

as

coming

from

one

sourc

but in Figure lb, the large frequencydistancebetween the

two

three-note

groups

introduced

a sort

of

discontinuity

that caused

the

perceptual

system

to

interpret

the

sequenc

as

resulting

rom

two sources.

Since

at

any given

moment

th

composite

pressure

variations

timulating

he ear

result from

several

ources,

the

auditory system

needs

a

battery

of heuri

tics to

parse,

or

segregate,

he

information

into

separate

streams.

It thus needs

to

build a

description

of the

acoustic

environment rom

separate

descriptions

of

the

various

stream

and

the

relationships

between

them

[2].

Factors

which the

perceptual

ystem

uses to

build

descriptions

of

streams,

and

subsequently

sources,

are

frequency,

rate

of occurrence

of

events

(or

tempo),

intensity,

timbre,

and

attack/decay

tran

sients.

In

the

rest

of the

paper

hese will be discussed

n

som

detail.

Of

course

it is obvious

that sounds

are

assigned

perce

tually

to different

sources

when

the

physical

sources

are at

different

spatial

positions.

In

this

case,

intensity, spectral,

and

temporal

cues

are

all

utilized to

parse

the

sound

into

separate

sources.

However,

we

will

primarily

confine

our

discussion

to the

illusion

of

many

sources which

occurs

due

to the

organizations

within

a

single

emanation

of sound

Frequency

and

Tempo

Consider

hat

a

repetitive

cycle

of tones

spread

over a

certain

frequency

range

may

be

temporally

coherent,

or in

tegrated,

at a

particular

empo.

It is

possible

to

gradually

n-

crease the

tempo

until certain tones

group

together

into

separate

treams

on the

basis

of

frequency,

as

discussed

abov

The faster

the

tempo,

the

greater

he

degree

of breakdown

o

decomposition

into narrower

treams

until

ultimately ever

given

frequency

might

be

beating along

in its own

stream.

This

last

possibility

is

dependent

upon

a

number

of other

factors which

will be

discussed

ater.

Figure

3 illustrates

the

possible

stages

of

perceptual

decomposition

or

a

recycling

six-tone

pattern

as

one

gradu

ly

increases

the

tempo (Taped

Illustration

3).

Note

that

streams

per

se are

not tracked

beyond

a

certain

point,

but

a

texture

or

timbre s

perceived

since

the

ability

to

temporally

resolve he

individual

ones

degenerates

altogether.

Of

cours

one couldhold the tempo constantandgradually xpandthe

frequency

relationships

o achieve

a

similar

streaming

effec

[3,

6,

14,

17],

but

the

musical

consequences

would

be

vastly

different

as

one can hear

n

Taped

Illustration

4.

Dowling [1

used

simple

melodies

to illustrate

this

frequency-based

streamingprinciple.

He interleaved

wo melodies

n

the

same

frequency

range

hereby making

t

very

difficult,

without

pr

knowledge

of the

melodies,

to

separate

them

perceptually

But as

they

were

pulled

apart

n

frequency,

.e.

when

all

the

tones

of one

of

the melodies

were

transposed

upward,

each

melody

became

apparent Taped

Illustration

5).

Page

28

Computer

Music

Journal,

Box

E,

Menlo

Park,

CA

94025 Volume 3 Numb


5/21

r

I

?

I

r

I

I

I

i I I

I

-

I

.

"

.

.2-.

,

,

=.

cc

L

L

0

0)0

cm

Q/)

%

r-

S

IE

0

oo

E

O

-------

-----,

--------

--o,)=----------

-

CC

u,

QI

----------------------?

-I

Time

Timbre/Texture

Figure

3.

This

figure

illustrates the

decomposition

of

an

acoustic

sequence

into

smallerand

smaller

perceptual

treams

as

the

frequency

separation

between the

tones or the

tempo

of

the

sequence

increases.In the

latter

case,

a

point

is

ultimately

reachedwhere

one can

no

longer perceive

individual onal

events;

a texture or

timbre is

heard

nstead

(cf.

Taped

Illustra-

tions

3

and

4).

The

requency

ange

withinwhich he

perceptual

ystem

groups

ones on the basisof

frequency

roximity

s

not

constant;he groupinganvarywith the particular attern

of

frequencies

resented.

or

example see Figure ),

two

tones,

A and

B,

are

arranged

ith

given

frequency

nd

temporal

eparations

uch

that

they

will

always

stream

together

whenno other ones are

present.

We can create

a

similar

rganization

ith

tonesX and

Y

in another

requency

range.

These wo

groups

will

each

form

a stream

s

can

be

heard n

Taped

llustration

a.

Bybringing

he

pairs

nto

the

same

requency ange,

new streams an be

formed,

uchas

A-X

and

B-Y,

on

the basisof

an

alternative

roximity

organization

3]

(Taped

llustration

b).

Thus,

he

particular

relationships

etween

requencies

n a

tonal

pattern,

ndnot

just

the

frequency

eparation

etween

adjacent

ones,

plays

a

vital

role n

the formation f

streams.

It

appears

romour

examples

hatthere s an

essentially

inverse

and

strictly

interdependent

elationship

etween

tempo

and

frequency

elationshipsmong

ndividual

ones

[26,

28]:

the

faster he

tonesfollowone

another,

he

smaller

the

frequency

eparation

t

which

hey

segregate

nto

separate

perceptual

treams.

Conversely,

he

greater

he

frequency

separation,

he slower

he

tempo

at

which

egregation

ccurs.

This llustrates

et

another

spect

of

music

which,

with the

aid

of

the

computer,

ancomeunder

he

composer's

ontrol.

Suppose

we

make

a

graph,

s in

Figure

,

which

relates

frequencyeparation

f

two

alternating

ones

on one axisto

A&B A&B

Isolated Absorbed

A

A

je.-"

x,0.1

4

S

O

O

Y

>

B

B

a)

(D

L

X

Time

Time-

Figure

4. This

figure

illustrates

he

effect

of

frequency

cont

as

opposed

to

frequency

separation

on

stream ormation.

In

the

first

part,

tones

A

and

B

form

one

perceptual

tream

wh

is unaffectedby the simultaneous treamcomposedof tone

and

Y.

When

ones

X

and

Y

are moved

into the

proximity

o

tones

A and

B,

which

remain

unchanged,

an

alternate

perceptual

nterpretation

esults

whereby

tones

A

and B ar

assigned

o

separate

treams

on

the

basis

of

a new

frequenc

context

(cf.

Taped

Illustration

).

the rate

of

alternation

f

the

tones

on

the

other

axis.

Note

thatthe horizontal xis ndicates

ncreasing

one

repetiti

time,

which

corresponds

o

decreasing

empo.

Onecandra

boundaries n

this

graph

ndicating

he

frequency-tem

regionsnwhich hetonescohere s a single tream nd ho

in

which

hey

segregate

nto

two

simultaneous

treams f

different

requencies.

here

are

two

such

boundaries.

eo

VanNoorden

28]

has

termed hese he

ission

boundary

temporal

oherence

oundary

ndnoted

hat

they

are

slig

different oreach

person.

n

Figure

the

upper

urve

s

the

temporal

oherence

oundary.

Above his

boundary

t is

impossible

o

integrate

he

two

alternating

ones into one

stream.Below

this

boundary

ies

the

temporal

oheren

region,

where t is

possible

o

integrate

vents nto

a

sing

perceptual

tream.

The lower

boundary

s the

fission

boundary.

elow his t

is

impossible

o

hearmore

hanon

stream.Note

that

the

regions

f

fission

and coherence ls

overlap, reating n ambiguousegionwhereeitherperce

may

be

heard.

As the

tempo

decreases

emporal

oherence

an b

maintained

t

greater

requency

eparations.

owever,

he

frequency

eparation equired

or

fission remains

airly

constant

with

decreasingempo.

The

emporal

oherence

fission boundaries bove

and

below

a

given

tone

are

symmetrical

ith

respect

o

pitch.

This

ndicates

hat

the

phenomena

f

temporal

oherence nd

fission

may

occur

depending

n the

absolutemusical

nterval

etween

he

ton

but

irrespective

f

the direction

f

pitchchange.

While

he

are substantial

uantitative

ifferencesn

theseboundar

Stephen

McAdams

nd

Albert

Bregman:

Hearing

Musical

treams

Pag


6/21

Tempo

(Tones/Sec)

20 10

5

3

o

15

Al

/

Alwaysays

c/

Segregated,

O

10 5

Tone

Repetition

TiAm

biguous

=

IRegion

Always

Fission

BoundaryCoen

0 50 100 150

200

250 300

350 400

Tone Repetition Time (msec)

Figure

5.

Boundaries of

temporal

coherence

(upper

curve)

and

fission

(lower

curve)

define three

perceptual regions

in

the

stream

relationship

between two

alternating

tones each

lasting

40

msec.

This

relationship

is

a

function of both

the

tempo

of

alternation

and

the

frequency separation

between the tones

(after [28]

).

between

listeners,

the

qualitative

rends

tend

to

be similar

[28].

Note that the difference between boundaries is

increasingly

ubstantial

for

tempi

below about

10

tones/sec.

(tone

repetition

time

=

100

msec.).

This

means that

at

very

slow

tempi

of, say,

five

tones/sec.,

a

separation

of more

than

a minor 10th is

necessary

o induce

streaming.

Below

this

it

becomes

virtually

impossible

to induce

streaming.

In the

limiting

case,

if the

temporal

distance

between tones

is too

great,

they

do

not

seem to

be

connected

at all but sound

as isolated events.

A

musically

relevant

aspect

of these

boundaries hould

be mentioned.

The

region

between

the

two

boundaries

may

be

considered

o be an

ambiguous egion

since

either

a

segregated

or an

integrated

percept

may

be heard.

The

primary

determining actorin this region s attention. In otherwords,

it is

possible

to shift one's

attention back

and forth

between

the two

percepts

when the

sequence

falls

in this

region.

For

example,

one

may

focus either on a whole

stream

percept

or

on

smaller ndividualstreams.

It

appears

that the closer

the

sequence

ies

to

one of

the

boundaries,

he

easier

t

is

to

focus

on the

percept

which

is

predominant

beyond

that

boundary.

Conversely,

t is

very

difficult in this

situation to shift

one's

attention back

to

the

other

percept.

Once

the

physical

values

go beyond

either of these

boundaries,

attaining

the

comple-

mentary

percept

may

be

considered

o be

impossible.

The

role

of

attention will be

discussed n more detail later.

FrequencyTrajectories

Another

frequency-based

effect involves

frequency

trajectories.

These are

important

on two

levels. The

first

involves

trajectories

between

tones

(see

Figure 6).

Bregma

and

Dannenbring

7]

have

found

that tones

that are

con-

nected

by

glissandi

are

much

less

likely

to

segregate

under

given

conditions

of

tempo

and

frequency

separation

than

those

which

make

abrupt requency

transitions. ntermedia

situations

beget

intermediate

results.

Yet even

when

using

sinewave for frequencymodulationof a sine tone, it is pos-

sible to discern

a sort

of

streaming

ffect

of the

higher

and

_I

I

Semi-

a I

D

I

I I

I

Ramped

I

I I

r- I

rapeady-

Tas

Sta

1

a.,

a

a

Discrete

aaTime-+

I __ _ __ _

Steady- Transition

Steady-

State State

Figure

6.

These

are

the 3

types

of

frequency

transitions

between

tones used

by

Bregman

and

Dannenbring

(1973).

In

the

top

section

the tones are

completely

connected

by

a

frequency

glissando.

In the

middle section

an

interrupted

glissando

is

directed towards

the

succeeding

tone.

No

frequency glide

occurs

in the bottom

section;

the

first tone

ends on

one

frequency

and

the next tone

begins

on

another

(after

[71 ).

The unconnected

tones

segregate

more

readily

than the

others.

lowerpeaksof the modulationat certainmodulationfreque

cies

(Taped

Illustration

7).

At

lower

modulating frequenci

one can

track

the

modulation,

but higher

modulating

fre-

quencies

result

n the

effect

of

a texture or timbre. This con

tinuum

is

found

for

modulation

involving

discrete

changes

in

frequency

as

well.

The second

type

of

trajectory

might

be

called

a melod

trajectory.

The

basic rule

goes: large

jumps

and

sudden

changes

n

direction

of a

melody produce

discontinuity

in

that

melody.

In

terms

of stream

formation,

one

or

two

tone

in

a

melody

that

are

removed

rom the

melodic

continuity

o

the

rest could

be

perceived

as

coming

from

a different sourc

Page

30

Computer

Music

Journal,

Box

E,

Menlo

Park,

CA

94025

Volume 3 Num


7/21

and

would not be

integrated

nto

the

melody

as

a

whole,

perhaps eaving

a

rhythmic

gap

in the

phrase

depending

on

the natureof the

main

sequence.

It

has been

reported,

how-

ever,

that

the

excluded tones

are

sometimes

noticed in the

background,

with their absence

having

ittle

effect

on

the

main

melody

line

[23].

Implied

polyphony

in a

solo

compound

melody

line,

as

in the suites

for

solo

instruments

by

Bach,

is a

compositional

use

of

this

principle.

Schouten

[30]

reported

that if

an

ascending

and

de-

scending

major

scale

fragment

played

with

sine

tones is

con-

tinually

repeated,

temporal

coherence

is

maintained

up

to

a

tempo of about 20 tones/sec. However, f these same tones

are

arranged

t

random,

he

maximum

tempo

at which

coher-

ence still occurs s

reducedto

about 5-10

tones/sec.

It

might

be inferred

that

the reduction

of

predictability

reduced the

pitch

boundary

within

which the

auditory

system

could

successfully

integrate

the

incoming

information. But

van

Noorden

[28]

found

that

previous

knowledge

of

the

order

of

tones

had

no

effect on

the

coherence

boundary.

It

might

be that the

small

frequency

umps

in

Schouten's

demonstra-

tion are effective

in

holding

the

stream

ogether.

Heise and

Miller

[23] investigated

our

melodic

con-

tours: a

V-shaped

contour,

an

inverted

V,

and

rising

and

fall-

ing

scale

patterns;

he

V-shaped

patterns

which

change

di-

rection

can be

thought

of

as

being

less

"predictable"

han

the scale

patterns

which move

in

only

one

direction.

Each

pattern

was

eleven tones

long

and

the

frequency

of

the

middle

tone was

variable.

They

found

that the

degree

to

which

the

variable

one

could

be

separated

n

frequency

from the

rest

of

the

pattern

before

it

segregated

nto its

own stream

was a

function

both of

the

shape

and of

the

steepness

of

the

pattern.

The

steepness

was varied

by

keeping

the

tempo

constant

and

varying

he

interval

between

successive

ones.

As

the rate

of

frequency change

for

the

entire

pattern

increases

over

time,

so does

the

amount of


of the

middle

tone

required

o

producesegregation.

Less

separation

of

the

middle

tone

is

required

to

produce

segregation

with

the

V-shaped patterns

than with

the

scale

patterns,

possibly

indicatingthat the perceptualsystem can follow

"predict-

able"

patterns

more

quickly

than

"unpredictable"

ones.

(However,

certain

anomalies

n

their

data,

which will not be

discussed

here,

might

suggest

other

interpretations.)

Van

Noorden

found

similarresults

for

patterns

with as

few

as

three

tones

[28]

.He

investigated

he relative

emporal

coherence of

so-called

linear

and

angular

three-tone se-

quences.

For

linear

sequences,

i.e.,

those

with two

tone

intervals

n the

same

direction,

the

temporal

coherence

bounda-

ry

occurs

at

faster

tempi

than were

found

with

angular

se-

quences

in

which the

melodic

pattern

changed

direction

at

the middle

tone. In

this latter

case

the

first

and third

tones

are more

contiguous

in

frequency

than are the first

and

second, or second andthirdtones, which facilitatesa loss of

temporal

coherence

similar o that found

by

Schouten. It

should be

noted

that while

these melodic

trajectory

effects

do

seem to

play

a role in

musical stream

ormation

beyond

the

simple

frequency

separation ffect,

they

are

certainly

con-

founded

by

other

contextual

organizations.

Both of

these

trajectory

examples

llustrate

a

principle

of

perceptual

organization

that uses

pattern

continuity,

in some form

or

another,

as a

criterion or

"source"

distinc-

tion.

Figure

7

illustrates,

however,

that

frequency proximity

may

sometimes

compete

with

trajectoryorganization;

Deutsch

found

that

simultaneous

ascending

and

descending

cale

pat-

terns

presented

to

opposite

ears

segregate

nto

upright

and

inverted

V-shaped

melodic

contours

[16].

Each

contour

is heard as if

being

presented

o one ear. The

same result

wa

found

by

Halpern

[22]

for simultaneous

ascending

and

de

scending

sine tone

glissandi,

as can be heard

n

Taped

Illust

tion

8;

in this case both

glissandi

were

presented

o both

ea

In

these two

examples

a

stream

boundary

s establishedat

the

pitch

where

the two lines

cross.

a

a

High I

\

Contour

I

t

1

--I

S

Low

.Contour

L

a

/ Low

'

_

Time-+

Figure

7. This

stimulus,

used

by

Deutsch

(1975),

is

com

posed

of

ascending

and

descending

scales

presented

sim

taneously

to

opposite

ears. Two

V-shaped

patterns

(hi

and low contours outlined

with

dashes)

are

perceived

rath

than the

complete ascending

and

descending

cale

pattern

Loudness

nd

Continuity

ffects

Fission

an be obtained

by alternating

equences

f

tonessimilar

n

frequency

nd

imbrebut

differing

n

inten

ity.

Figure

8

illustrates

he

range

of

percepts

achieved

as

the

amplitude

evelof toneA is varied elativeo thatof toneB

in the

alternating

equence

ABAB...

.

The reference

ev

for toneB used

by

vanNoorden

n these

experiments

28]

was35 db

SPL;

he

frequency

f both toneswas

1

KHz

an

each

tone

lasted40 msec.

If the levelof tone A is

below

the

auditory

hreshold

approximately

db

SPLat

1

KHz

only

the B streams heard

t half

tempo,

as

might

be

expec

(see

Figure a).

When

one A is loud

enough

o be heard

n

is at least

5

db below

tone

B,

two

separate

treams f

dif

ferent loudness can be

perceived,

each at half

tempo,

with

A

being

the softer stream

(see Figure

8b).

When

A is within

5

db of

B,

a

"pulsing"

tream

s heard and neither the

A

nor

the

B

streamcan be heard

ndependently,

.e.,

tempora

coherences inevitablen thisrangesee Figure c). As th

level

of

the A tone

s

increased bove

hatof the

B

tone,

th

different

percepts

may

result,

depending

n

the

alternati

tempo

of A andB. If this

tempo

s lessthanabout

13

tones

sec.,

fission s the next

percept

eard.This ime

B is the

sof

stream

see Figure

d).

Thusa certain

degree

of

loudness

difference llowsus to focus

on eitherstream

using

only

this

nformation.

f

the

tempo

s

greater

hanabout

12.5 1

tones/sec.,

he

percept

encountereds the "roll"

effect

discovered

y

vanNoorden. t

soundsas if stream

A,

the

louder

tream,

were

pulsing

t

half

tempo

as in the

fissio

percept,

ut the B

stream ounds

s f it were

pulsing

t

ful

Stephen

McAdams

nd Albert

Bregman:

Hearing

Musical

treams

Pag


8/21

A

Below Threshold

a

-K

A

B

(

B

time

Fission

b

fA

B

A

I

Coherence

c

A

B

A

B

Fission

d

A

B

A

B

Roll

e

A

B

A

B

Continuity

Masking

g

A

B,

A

I

Figure

8.

This

figure

illustrates

the

range

of

possible per-

cepts

found

by

van Noorden

(1975)

for

two

alternating

ine

tones

differing

in

each case

only

in

their

intensities.

Tone

B

was

kept

at 35 db SPL

throughout;

both tones were 40

msec.

long

and

had

a

frequency

of

1

KHz.

a)

The level

of

tone

A

is

below the

auditory

threshold.

b)

Tone A is

at

least

5

db below

tone B.

c)

Tone A

is

within

5 db

of

tone B.

d)

Tone

A

is

louder

than tone B with an

alternation

empo

less

than about

13

tones/sec.

e)

Tone A

is louder

than tone

B with more

than

13

tones/sec.

f)

Tone

A

is

about

18-30

db

louder

than

tone B

and

the

tempo

is still above

13

tones

/sec.

g)

Tone

A

is

more

than 30

db louder

than

tone

B.

The

arrows

indicate

the

percepts reported;

a more

complete

description

s

given

in

the

text.

tempo

(see

Figure

8e).

Thus

the A stream

may

be heard

n-

dependently,

but

not

the

B

stream.

In other

words,

t is as

if

the A tones consisted

of

two

parts:

one

that combines

with

the B

stream

to

give

a

full

tempo

roll,

and

another,

at half

tempo,

which can

be

perceived

separately.

At a

tempo

of

about

13

tones/sec.

another effect

emerges

when

the level

of A

is about

18-30 db above that

of

B.

This is the

continui

effect

shown

in

Figure

8f,

so

namedbecausetone

B

is

not

heardas

pulsing

but ratheras a

fairly

soft,

continuous

tone

under the

louder,

pulsing

A

stream;

this is an

example

of a

class of effects

that will be

discussed below.

Finally,

if the

levelof the A stream s incremented till further, his stream

completely

masks

the B

stream

Figure 8g).

Again,

this

set o

loudness-based

phenomena

exhibits an

ambiguous region

between the coherence and fission boundaries

where one

might pay

attention to either the A or B

streams

ndividuall

or

to the AB

stream as

a

whole.

There are

thus

three

per-

ceptual

regions

for

alternating

ones

at

the same

frequency

where the

tempo

is above about 12.5

tone/sec.:

the

roll

regi

the

continuity region,

and the

temporal

coherence

region.

Van Noorden made

quantitative

measurementsof

the

fission

boundary(see Figure 9).

For

tempi

of about

2.5

to 10

tones/sec.,

the

fission

boundary

is more or less

hori-

zonal,

i.e.,

the

intensity

difference

(AL) necessary

for a

segregatedpercept

does

not

change

with

tempo

over this

range,

but

lies about 2 to

4

db

on either

side of

the

referenc

tone level. For

tempi

less than 2.5

tones/sec.,

the minimum

level

difference

for

fission increaseswith

decreasing empo

(or

longer

inter-tone

intervals

of

silence)

and is

symmetric

about AL

=

0

(no

difference

in

level).

For

tempi

greater

than

10

tones/sec.

the level differenceat which fission occur

increases

with

increasing

empo

but

the situation s

not

sym

Tempo

(Tones/sec)

20

0

5

3 2

Masking

SContinuity

"

LA>

LB

20o

Fission

LA

>LB

08

.,

Roll

Inevitable

ev

le

--*

35 db

SP

_Coherence

20

Fission

LA

-

0

0.

mtTe

r

?

Time

Time

-

Figure

21. One result

of

McAdams'

tudy

(1977)

suggested

hat there

is

an

interaction

between

pitch "height"

and timbral

"sha

ness"

(see

text).

In

a

repeating

4-tone

sequence,

one of the

pairs

of

tones was

selectively

enriched

by adding

he

third

harmonic

A

greaterdegree

of

segregation

of

the

high

and low

streams

was found for the

formercase.

The

dashed ines

indicatethe two-

stream

percepts

and the

dotted

lines

indicate

a

potential

one-stream

percept.

The

vertical

solid lines

represent

he fusion and

timbre

of

the 2-tone

complex.

Page

38

Computer

Music

Journal,

Box

E,

Menlo

Park,

CA

94025

Volume 3

Numb


15/21

(or

frequency) organizations

and

simultaneous

(or

timbral)

organizations

n the

formation

of

auditory

streams.

The

stimulus used

by

Bregman

nd

Pinker

was a sine

tone

alter-

nating

with

a

two-tone

complex.

In

Figure

22,

tones

A and

B would

represent

he

sequential

organization

and

tones B

and

C would

represent

the

simultaneous

organization.

The

harmonicity

and

synchronicity

of

tones B

and

C

in

the

complex

were

variedaswas the


between

the sine

tone

A

and the

upper

component

B

of

the

complex.

The rationale or

varying

hese

two

parameters

was as

follows.

Tones with

frequency

relationships

derived

from

simple

ratios,i.e. those that exhibit "consonance,"should tend to

fuse

more

readily

than

combinations

considered

to

be

dis-

sonant. While

the

evidence for

this

was

very

weak

in the

Bregman

and

Pinker

study,

work

currently

n

progress

n

Bregman's

aboratory

strongly

suggests

that

this

is

indeed

the

case.

The

new

evidence

further

suggests

that

the

effect

of

harmonicity

tself

is

relatively

weak

and

may

be

over-

ridden

by

stronger

factors

such

as

frequency

contiguity

and

synchronicity

of

attack

and

decay.

Tones

with

synchronous

and

identically

shaped

attack

and

decay

ramps

are

more

likely

to

fuse

than

those

with

asynchronous

or

dissimilar

attacks

and

decays

[12, 15].

This

may

be

a

major

cue in

being

able

to

parse

out the

different

instruments

playing

together

in

an

orchestrasince they all have substantially

different attack characteristics.

n

addition,

there is

a

very

low

probability

of several

people precisely

synchronizing

he

attacks.

In

light

of this

work,

one

might

make the

following

predictions

or the

perception

of the stimuli

used

by

Bregm

and Pinker:

1) Sequential streaming

s favored

by

the

frequency

proximity

of tones

A and B

(as

we have

illustrate

in the

earlier

examples).

2)

The

simultaneous

or timbral)

fusion of tones

B and C is favored

by

the

synchrony

of

their

attacks.

3)

These two effects

"compete"

for

tone B's

memb

ship

in their

respective

perceptual

organizations.

4)

Finally

when tone B is "captured"by tone A, it is removed romth

timbral

structureand

tone C sounds less rich.

Thus,

it

is

reasoned hat if

the two

simultaneous ine

tones B and

C are

perceived

as

belonging

to

separate

streams,

they

should

be

heardas sine tones.

But if

they

are heard

as one

stream,

hey

should

sound like one

rich

tone.

It would

be

appropriate

o

introduce

the notion

of

"belongingness"

t this

point,

since

we talk of tones

belongi

to

streams,

and

of

frequency components

and the

timbre

resulting

rom their interaction

belonging

o a

perceived

on

event.

"Belongingness"

a

term used

in the

perceptual

itera

ture of Gestalt

psychology)

may

be

consideredas

a

principl

of

sensory organization

which serves to

reconstruct

physic

"units" nto perceptualeventsby grouping ensoryattribute

Stimulus

C B

LL

t

4-

Time

-

Percepts

:3

A

A

Cr

B

B

\

,

B B

C C C C

A &

B

Stream

A

&

B

Segregate

C

Pure

C

Rich

Time

-

Time

-

Figure

22.

The

competition

between

sequential

and simultaneous

organizations

n

the

formation

of

auditory

streams

s

shown

here.

Tone B

can

belong

either

to the

sequential

organization

with tone

A or to

the simultaneous

organization

with tone

C

but not to both at

the

same time.

Bregman

and Pinker

(1978)

varied

the

frequency

separations

between tones

A and

B

an

between

tones

B

and

C and also varied he relative

synchrony

of onset of tones B

and

C.

The

dotted

lines

in the

figure

indicat

the

stream

percepts

and the

vertical

solid

lines

represent

he fusion

of

tones

B and

C

(cf.

Taped

Illustration

15).

Stephen

McAdams

nd

Albert

Bregman:

Hearing

Musical treams

Pag


16/21

of

thoseevents nto

unified

percepts.

As

Bregman2]

points

out,

"belongingness

s a

necessary

utcomeof

any

process

which

decomposes

mixtures,

ince

any

sensory

ffect

must

be

assigned

o some

particular

ource." n

this

case,

when

the

simultaneous

ones

B

and

C are

segregated,

he

timbre

resulting

rom

heir

nteraction

till

exists

andcanbe

heard

f

one

istens

or

t,

but it

is not

perceptually

ssigned

o

either

of

the tones

B

or

C,

and

husdoes

not

affect he

perception

of them.The

nature

f a stream

s

such

hat ts

qualities

re

dueto the

perceptual

eatures

ssigned

o

it.

In

Taped

llustration

5

one

can

heara

case

n

which

A

is close o BinfrequencyndCisasynchronousith B.Then

a

case s

heard

where

A

is further

way

rom

B in

frequency

and

C

is

synchronous

ith

B.

Tones

B

and

C

have

he

same

frequencies

n

both

cases.Listen

or

both the

A-B

stream

nd

the richness f

tone

C.

The

istenersn

Bregman

nd

Pinker's

study

reported

erceiving

as

being

icher

when

B

and

C

were

synchronous,

nd

this

judged

ichness

ropped

ff

with an

increase

n

asynchrony,

.e.

as

C either

preceded

r

followed

B

by

29

or 58

msec.

As

the

frequency

eparation

etween

A and

B was

ncreased,

was

reportedly

erceived

s

being

increasingly

ich.

The

Role

of

Context

in

Determining

Timbre

These

indings

ndicate hat

the

perceived

omplexity

of a

moment

of

sound s

context-dependent

see [19]

as

another

xample

of

the

trend

toward

viewing

imbre

as

depending

n

context).

Context

may

be

supplied

y

a

number

of

alternative

rganizations

hat

compete

or

membership

f

elements

ot

yet

assigned.

imbres a

perceived

roperty f

a

stream

rganization

ather han

he

direct

esult

f

a

particular

waveform,

nd

is

thus

context-dependent.

n

other

words,

two

frequency

omponents

hose

synchronous

nd

harmonic

relationships

ould

cause

hem

o

fuse

under

solated

ondi-

tions

may

be

perceived

s

separate

ine

tones if

another

organizationresents

tronger

vidence

hat

they

belong

o

separate

equential

treams.

A

very

compelling

emonstration

f the

decomposit

of a timbre

organization

y

alternate

requency treamin

organizations

s illustrated

n

Figure

3. When

one A

is

presented

y

itself

t elicitsa

timbre,

enoted

TA.

f this

ton

is

preceded y

tone

B

eliciting

imbre

TB,

andsucceeded

y

tone

C

eliciting

imbre

TC,

one noticesthat

timbre

TA

completely

isappears

nd s

replaced y

timbres

TB

and

TC

Here

the

highest

and lowest

components

f tone

A

are

streamed

with those

of toneB and

subsequently

ssume

timbre

dentical

o thatof toneB.

Also,

he inner

ompone

of tone A

streamwiththose of tone

C andassume

like

timbreTaped llustration6).

An

important

uestion oncerning

he

assignment

f

timbre

and

pitch

to a tonaleventarises.

Bothtimbre

and

pitch

havebeen found

to be context

dependent10,

21,

respectively].

But each

may

be determined

y

different

ongoing

contextual-organizations;

s such

they may

be

considered

o

be

associated

erceptual

imensions

f a

soun

but

may

not be

inextricably

ound

o one another.How

rel

vant o music

heory,

hen,

arestudies hat

dealwiththe

pe

ceived

pitch

and imbre

f tones n isolation?

This

question

not meant o insinuate

hat

sensory

nd

psychophysicalxp

imentation reuseless.

Far rom t. If we

think n termsof

investigating

he

experience

f music

at

different

evels

of

processingFigure 4), we seehow important ll of these

areas f research

re n

building

he whole

picture.

The

raw

physical

nput

s

modified

by

the

limitsof the

sense

organ

whose

output

s stillfurthermodified

y

cognitive rocesse

But

by

studying teady-state,

r at least

relatively

imple

signals,

we can find

the limitsand

interactions f the

sens

organs

nd

peripheralrocesses,

uch

as

temporal

nd

spect

resolution,

ateral

nhibition,

nd

masking,

hich imits

affe

the final

percept.

Beyond

hese,

the

central

perceptual

processes

uch as

pitch

extraction,

imbre

buildup,

and

coherence

nd fission

modify

the initialneural

result

of

stimulation

f the

sensory

ystem.

Further

nteractions

n

higher

rain

processes

uchas

attentional

rocesses,memor

and

comparison

f

pitch,

imbre nd

oudness,

ontext

extr

One

Timbre

Two

Timbres

:TA

versus

I I :

1 2

11

A

B

A

C

Figure

23.

The

first

part

of

this

figure

shows

a

repeating

one

(A)

consisting

of

4

harmonics.

This

tone would

elicit

a certain

timbre

percept,

TA.

In

the

second

part

this

tone

is

preceded

by

tone

B,

consisting

of the

top

and bottom

harmonics,

and is

suc-

ceeded

by

tone C

consisting

of the

two

inner

harmonics.Tone

B

elicits timbre

TB

and

tone C

elicits

timbre

TC.

However,

due to

the

streaming

of tone

A's

components

with

those of

tones B

and

C,

TA

totally

disappears

nd

is

replacedby

TB

and

TC

(cf.

Taped

Illustration

16).

Page

40

Computer

Music

Journal,

Box

E,

Menlo

Park,

CA

94025

Volume3 Numb


17/21

Physical

Environment

Levels

of

Processing

Temporal

and

Spectral

Resolution

Lateral

nhibition

"Sensory"

Masking

etc.

Pitch

Extraction

Timbre

Buildup

"Perceptual"

Coherenceand FissionLimits

etc.

Attention

Memory

Context

Extraction

Form andTextureIntegration

etc.

Figure

24.

This block

diagram

uggests

a

possible

arrangement

of

the

processing

of

acoustic

information

at

different

inter-

connected levels

of

the

auditory

system.

tion,and ormand exturentegration,culpt hetransduced

information

nto

meaningful ercepts.

t is felt

thatall

of

these

levels

of

processing

eed

into each

other n a

sort

of

heter-

archical

as opposed

o

a

hierarchical)

ystem.

The

point

being

made

s that

in

the

framework

f

music

where

all of

these

complex

nteractionsre of

great

mportance,

he

context

that

s created

may

be

the

essential

eterminantf

the

musical

resultof

a

given

ound.

One

sound s

potentially erceivable

n

a

great

number

f

ways,

depending

n its

context.

Melody

This eads

us

to

believe

hat

the

fundamental

erceptual

element n musicmaybe the "melody" ather hanthe

isolated

one.

Or

n the

terminology

f

auditory

erception,

the fundamentaltructure

s the

auditory

tream.

This s

not,

of

course,

new

notion;

but an

empirical

pproachmay

allow

us to

clarify

and

delimit he

concept

o the

extent

that

we

may

predict

he

perceptual

esults.

That,

n

themindof the

first

author,

s the

primary

oncern f the

composer.

et

us,

then,

examine

melody

and ts

relation o

attention.

For our

purposes,

we can

think

of

melody

as a

connected nd

ordered uccession

f

tones

[28].

It

follows

thenthat

temporal

oherence

s

necessary

or

a

sequence

f

tones

to be

perceived

s

a

whole.

On the

other

hand,

a

sequence

f tones

may

segregate

nto two

or more

separa

streams

which

are

ndividually

oherent.

n

this

case,

we wo

perceive

everal imultaneousmelodiesrather

han

one.

If

such

an

operational

efinition

f

melody

s

tenable

we

must hen

question

ow t is thatsome

of the extension

elements

ther han

pitch

for

"melodic"

material

revali

perceptually.

or

example,

when

a

composer

ses timbre

thematic

material,

o

we

still

perceive

he

sequence

s main

taining

temporal ontinuity?

ometimes

oherence

s

mai

tained

by

sensitive

erformers

ndsometimes

t can

be

very

difficult

o

perceive.

Of

course,

we

have

not

included

th

elementswhichaffectthe perception f melody,suchas

underlying

armonic

tructure,

ut

we

are

only

attemptin

convey

the notion that

temporal

coherence

hould

be

considered

ssential

o

melody

ormation.

Conversely,

ne

may

use the

principles

f

fission

o

develop

ules

or

creati

polyphony

nd

counterpoint

n

sequences

f

acoustic

ven

Attention

ndMusical

tructure

It

has

become

apparent

o

the

first

author

hatmusic

structure,

sit is

perceived

n

real-time,

s

inextricably

ou

to

attentional

rocesses.

bit

of

introspection

illreveal h

there

are at least

two

kinds

of

attentional

rocesses,

whic

we

might

allactiveor willful

attention,

nd

passive

r

auto

matic

attention.One

mightwillfully

direct

one's

attention

some

object

or

sequence

of

events,

such

as

listening

o

particular

vents

within

a

piece

of

music.

Or,

someunusu

event

might

attract

ne's

attention

nexpectedly,

uchas

th

honking

orn

of

an

oncoming

ar

you

had

not

noticedas

yo

stepped

ntothe streetabsorbed

eep

n

thought.

Thathorn

demands

our

attentionand

n

all

probability

ets

t in a

hurry.

n

particular,

othkinds

of

attention

mayparticipa

in the

process

f

listening

o

auditory

treams.

or

nstance

in

Figure

when

a

sequence

f tones

ies

above

he

tempor

coherence

boundary,

o amount

of

activeattention

can

extract

he

percept

f

one

coherent tream.

Here

perceptio

is limited

by

passive

ttentional

rocesses 28].

Thishas

importantonsequencesorcomposers ho ntend o usefa

melodic

equences,

ince

t

suggests

hat there

are

tempi

a

which

he

listener

may

not

be able o

follow

as

a

melody

h

sequence

ou

have

constructed,egardless

f the attention

will

power

nvoked.An

example

f theseeffects

may

be

fou

in

the

sequences

resented

t different

ates n

Charles

od

Earths

Magnetic

Fields.

Without

pretending

o

know

the

composer's

ntent,

t can be

amusing

o listento

the same

sequence

decompose

nd

re-integrate

tself

during

variou

tempochanges.

n

a multi-streamed

equence

ne can rela

attentional

effort with the

result

that

attention

might

randomly

alternate

among

the available

treams.Or one

mig

selectively

focus

attention on

any

one of them

individual

and evenplay them againstone another.

Van Noorden

reported

that the

temporal

coherenc

boundary

(the

boundary

below which

all tones

may

belon

to

one

stream)

s not affected

by

previousknowledge

of the

sequence,

and considered

t to be a

function

of a

passive

attentional

mechanism,

given

that the listener

"wants to

hea

coherence."

However,

what

happens

between

the boundari

of

temporal

coherence

and fission

dependsupon

a number

factors,

such

as

context,

and seems

to

be under

the influenc

of attention.

Dowling [17],

for

example, reported

that if

listeners knew beforehand

whi:h

melodies were

being

inte

leaved,

they

could,

with

a

bit of

practice,

extract the

appro

Stephen

McAdams

nindAlh"rt

Rrtnmn-

o

Ra,;

...A--

Pag


18/21

priate melody

even at

very

small

separations

of

the

ranges

traversed

by

each

melody.

It is

currently

assumed

that this

ability

would

degenerate

at faster

tempi.

In

addition,

van

Noorden

found an

effect

at

very

fast

tempi

where t

was

virtually

mpossible

o

hear

sequences

within a

range

of two

or three

semitones as

other than

temporally

coherent.

At

very

fast

tempi

(about

12.5

tones/

sec.)

the

tones

of

such

narrow

patterns

are not heard

as

separate

membersof a

sequence,

but

actually

merge

nto

a

continuously

rippling

exture,

as one can

hear in

Taped

Illustration

17.

There

are

thus

attentional

limits

in

the

ability

of

the

auditory system

to tracka

sequence

of events. When events

occur too

quickly

in

succession,

the

systemusesthe variousorganizational ulesdiscussed n this

article

to

reorganize

he

events

into

smaller

groups.

It

may

then track

events

within

a

particular

group

if

the

listener

is

paying

attention

to

it,

but

this

narrowing

f

focus

necessarily

causesa

loss of

information.One result s

the

inability

to

make

fine

temporal

order

udgments

between

streams.

These

organi-

zational mechanisms eflect the

tendency

of the

auditory

sys-

tem to

simplify things

in

the face of

excessive

complexity.

In

the

example

where the

fast

sequence

of

tones

merges

nto a

continuous

"ripple,"

he

auditory system

is unable to

success-

fully

integrate

all of

the

incoming

nformation nto a

temporal

structureand

simplifies

the

situation

by

interpreting

t

as

texture

(see

also

[31]

).

Thus

the

auditory

system,

beyond

certaintempi,mayinterpret he sequenceasa singleevent and

assign

o it the

texture

or timbre

created

by

its

spectral

and

temporal

characteristics.

An

understanding

f

(or

intuition

about)

these

organi-

zational

processes

can

lead to new

dimensions

of

control

over

musical

structure.

For

example,

one

might

construct contra-

puntal

sequences

that

play

across various

stream

boundaries

and

through

different

borderline

regions

between

temporal

coherence

and

fission.

(It

may

be that

composers

such

as Bach

were

already

using perceptual

ambiguityconsciously

in their

work.)

Any

or

all

of

the

relevant

musical

parametersmight

be

used

to

accomplish

this.

Then,

an

appropriate

use of events

that

vie for or demand he

listener's

attention

can be

used

by

the

composer

to

"sculpt"

the

attentional

processes

of the

listener.Sincesomeeventsseemmore

striking

o some

persons

than to

others,

this

attentional

sculpture

n

time would lead

different

listeners

through

different

paths

of

auditory

experience.

Further,

perception

s

bound

to

vary

from

time

to

time within

a

single

person,

so

the

experience

would

be

different

with

each

listening.

A

composition

of

sufficient,

controlled

complexity

might

thus be

perceptually

nfinite

for

a

given

istener.

Conclusion

An

attempt

has

been

made here to

point

out that

composers

and

music theorists should

thoroughly

examine the

relationshipbetween the "musical"principlesthey use and

espouse,

and

the

principles

of

sensory,

perceptual,

and

cognitive organization

hat

operate

in the human

auditory

system. Many

of

the

principles

discussed n this

article extend

to

higher-level

perceptual

analysis

of

musical

context

and

structureand

may

well

represent

a

scientific

counterpart

o

some

extant music

theoretical

principles.

In other

cases,

though,

this

group

of

phenomenasuggests

perceptualorgani-

zations

which have

little

relation to

methods

currently

used

to construct

or

analyze

musical

structure. To

ignore

the

evidence

from

the real

life

system

in

developing

a

theory

of

music

or a musical

omposition

s to takethe chance

of

relegating

ne's

work o the realm f what

might

be

termed

"paper

music."

Acknowledgments

This

paper

s basedon a

workshop

ntitled

"The

Perceptual

actoring

f

Acoustic

Sequences

nto

Musica

Streams"

elivered

t the

1978 International

omputer

Music

Conference,

vanston,

llinois,

and

published

n

the

Proceedings

f

the

Conference.

he

authors

would

ike

to

thank

Dr. Leon

van

Noorden or

his

many

helpful

uggestio

andvaluable riticisms f themanuscript;igures , 8, 9,

10

and

20 were

redrawn,

with

permission,

rom

Dr. van

Noorden'shesis.

The

present

version f this artic

was

prepared

hileMr.

McAdams

as

a Graduate

ellow

of the

National

ScienceFoundation.

Dr.

Bregman's

esear

hasbeen

supported

y

grants

rom

he

NationalResearch

Council

f

Canada,

he

Quebec

Ministry

f

Education,

nd

the

McGill

University aculty

of

Graduate

tudies

and

Research.Muchof

the research sed

the

facilities f the

Computer-Based

aboratory

f the

McGill

University

Department

f

Psychology.

The

editors

of

Computer

Musi

Journalwould ike

to

thank

Mr.

McAdams

or

preparing

he

illustrations

n

this

article.

Appendix

Description

f

Taped

llustrations

1.

A

repeatingequence

f

three

high

ones

1600,

2000,

2500

Hz.)

s

interspersed

ith three

ow

tones

350,

430,

55

Hz.)

used

by

Bregman

nd

Campbell

1971). a)

At

a

tempo

o

five

tones/sec.

he

sequence

s

perceived

s one

stream f

alternating

igh

and ow tones

(cf.

Figure a).

b)

At

a

tempo

of

ten

tones/sec.

he

sequence

egregates

erceptually

nto

on

streamof

high

tones and one stream

of

low tones

(cf.

Figure

b).

2. Another

epeating

ix-tone

sequence

s

played

at

a

tempo

of ten

tones/sec.,

but the

higher

riplet

s closer n

frequency

o the

lower.Tone

F

may

be

perceived

s

belong

to either

he

high

stream r the

low

stream

epending

n the

listener's

ocus.

Note

that tone

F

cannot

belong

o both

streams t

once

(cf.

Figure

).

3.

A

repeating

ix-tone

sequence

tartsat

a

slow

tempo

As the

tempo

s

gradually

ncreased,

he

sequence

s

progres

sively

decomposed

nto

smaller

erceptual

treams

ntil

t is

no

longerpossible

o

follow

he tonaleventswhich

merge

n

the

percept

f

timbre

r

texture

cf

Figure ).

4.

Using

he same nitial

equence

s

n

Taped

llustratio

3,

the

frequencyeparation

etween

emporallydjacent

on

is

gradually

ncreased. he samesort of

decomposition

nto

smaller streams occurs. The limits in this example are

determined

ot

by temporal

esolution

ut

by

the

audible

frequency

ange

cf

Figure

).

5. The

tones of two familiarmelodies are interleaved n

th

same

frequency

mcadams bregman streams

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