(PDF) The Attention System of the Human Brain

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The

Attention

System

of

the

Human

Brain

12.

PERSONAL

ALTi..ORM5

Michael

I.

Posner

and

Steven

E.

Petersen

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The

concept

of

attention

as

central

to

human

performance

extends

back

to

the

start

of

experimental

psychology

(James,

1890j,

yet

even

a

few

years

ago,

it

would

not

have

been

possible

to

outline

in

even

a

preliminary

form

a

functional

anatomy

of

the

human

attentional

system.

New

developments

in

newrmscience

(+H-lfnrd

&

Picton,

4987;

Raichle,

1983;

Wurtz,

Goldberg

&

Robinson,

1980)

have

opened

the

study

of

highcr

cognition

to

physiological

analysis,

and

have

revealed

a

system

of

anatomical areas

that

appear

to

be

basic

to

the

selection

of

information

for

focal

(conscious)

processing.

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Annals

of

Neuroscience,

1989)

The

Attention

System

of

the

Human

Brain

ONR

Technical

Report

#89-1

Michael

I.

Posner

Department

of

Psychology

University

of

Oregon

Eugene,

OR

97403

and

Steven

E.

Petersen

Department

of

Neurology

and

Neurological

Surgery

Washington

University

School

of

Medicine

660

S.

Euclid,

Box

8111

St.

Louis,

MO

63110

B

-n

-n-o

Availability

Codes

;

-Avai'l

and/or

IDist

Special

/

INTRODUCTION

-

lrhe

concept

of

attention

as

central

to

human

performance

extends

back

to

the

start

of

experimental psychology,

(,&mes,-44W,90

yet

even

a

few

years

ago,

it

would

not have

been

possible

to

outline

in

even

a

preliminary

form

a

functional

anatomy

of

the

human

attentional

system.

New

developments

in

neuroscience

'-'.

2

-Picroa,--1-9S7,-

Raichk,-t983;

Wurtz-Goldberg-&

R

-

have

opened

the

study

of

higher

cognition

to

physiological

analysis,

and

have

revealed

a

system

of

anatomical

areas

that

appear

to

be

basic

to

the

selection

of information

for

focal

(conscious)

processing.

The

importance

of

attention

is

its

unique

role

in

connecting

the

mental

level

of

description

of

processes

used

in

cognitive

science

with

the

anatomical

level

common

in

nearoscience.

Sperry

tV98

'describes

the

central

role that

mental

concepts

play

in

understanding

brain

function.

as-

foHows:

"control

from below upward

is

retained

but

is

claimed

to

not

furnish

the

whole

story.

The

full

explanaticn

requires

that

one

take

into

account

new,

previously

nonexistent,

emergent

properties,

including

the

mental,

that interact

causally

at

their

own

higher

level

and

also

exert

causal

control

from

above

downward."

(p.

609)

If

there

is

hope

of

exploring causal

control of

brain systems

by

mental

states,

it

must

lie

through

an

understanding

of

how

voluntary

control

is

exerted

over

more

automatic

brain

systems.

We

argue

that

this

can

be

approached

through

understanding

the human

attentional

system

at

the

levels

of

both

cognitive

operations

and

of

neuronal

activity.

As

is

the

case

for

sensory

and

motor

systems

of

the brain,

our

knowledge

of

the

anatomy

of

attention

is

incomplete.

Nevertheless,

we

can

now

begin

to

identify

some

principles

of

organization

that

allow

attention

to

function

as

a

unified

system

for the

control

of

mental

processing.

Although

many

of

our

points

are

still

speculative

and

controversial,

we

believe

they

constitute

a

basis

for

more

detailed

studies

of attention

from

a

cognitive-neuroscience

viewpoint.

Perhaps

even

more

important

for

furthering

future

studies,

multiple

methods

of

mental

chronometry,

brain

lesions,

electrophysiology,

and

several

types

of

neuroimaging

have

converged

on

common

findings.

.

Three

fundamental

findings

are

basic

to

this

chapter.

First,

the

attention

system

of

the

brain

is

anatomically

separate

from

the

data

processing

systems

that

2

perform

operations

on

specific

inputs even

when

attention

is

oriented

elsewhere.

In

this

sense,

the

attention

system

is

like

other

sensory

and

motor systems.

It

interacts

with

other

parts

of

the

brain,

but

maintains

its

own

identity.

SeconC,

attention

is

carried

out

by

a

network of

anatomical

areas.

It

is

neither

the

property

of

a

single

center,

nor

is

it

a

general

function

of

the

brain operating

as

a

whole

(Mesulam,

1981;

Rizzolatti,

Gentilucci

&

Matelli,

1985).

Third,

the areas

involved

in

attention carry

out

different

functions,

and

these

specific

computations

can

be

specified

in

cognitive

terms

(Posner,

Petersen,

Fox

&

Raichle,

1988).

To

illustrate

these

principles,

it

is

important

to

divide

the

attenDti

systC

i-z

subsystems

that perform

different

but

interrelated

functions.

In

this

chapter,

we

consider

three

major

functions

that

have

been

prominent

in

cognitive

accounts

of

attention

(Posner

&

Boies,

1971):

(1)

orienting

to

sensory

events;

(2)

detecting

signals

for

focal

(conscious)

processing,

and

(3)

the

maintenance

of

a

vigilant

or

alert

state.

For

each

of

these subsystems,

we

adopt

an

approach that organizes

the

known

information

around

a

particular

example.

For orienting,

we use

visual locations

as

the

model,

because

of

the

large amount

of

work

done

with this

system.

For

detecting,

we

focus

on

reporting

the

presence

of

a

target

event.

We

think

this

system

is

a

general

one

that

is

important for

detection

of

information

from

sensory

processing

systems

as

well

as

information stored

in

memory.

However

the

extant

data

concern

primarily

the

detection

of

visual

locations

and

processing

of

auditory

and

visual

words.

For

alerting,

we

discuss

situations

in

which

one

is

required

to

prepare

for

processing

of

high

priority

target

events

(Posner,

1978).

For

the

subsystems

of

orienting,

detecting

and

alerting,

we

review

the

known

anatomy,

the

operations

performed,

and

the

relationship of attention

to

data

processing

systems

(e.g.

visual

word

forms,

semantic

memory)

upon

which

that

attentional

subsystem

is

thought

to

operate. Thus

for

orienting,

we

review

the

visual

attention

system

in

relationship

to

the

data

processing

systems

of

the

ventral

occipital

lobe.

For

detecting,

we

examine

an

system

in

relationship

to

networks

that subserve

semantic

associations.

For

alerting,

we

examine

arousal

systems

in

relationship

to

the

selective

aspects

of

attention.

Insofar

as

possible,

we

draw

together evidence

from

a

wide

variety

of

methods,

rather

than

arguing for the

primacy

of

a

particular

method.

ORIENTING

Visual

Loations

3

Visual

orienting

is

usually

defined

in

terms

of

the

foveation

of

a

stimulus

(overt).

Foveating

a

stimulus

improves

efficiency

of

processing

targets

in

terms

of

acuity,

but

it

is

also

possible

to

change

the

priority

given

a

stimulus

by

attending

to

its

location covertly

without

any

change

in

eye

or

head

position

(Posner,

1988).

If

a

person

or monkey

attends

to

a

location,

events

occurring

at

that

location

are

responded

to

more

rapidly

(Eriksen

&

Hoffman,

1972;

Posner,

1988),

give

rise

to

enhanced

scalp

electrical activity

(Mangoun

&

Hillyard,

1987),

and

can

be

reported

at

a

lower

threshold (Bashinski

&

Bachrach,

1980;

Downing,

1988).

This

improvement

in

efficiency

is

found

within

the

first

150

millisec after

an

event

occurs

at

the

attended

location.

Similarly,

if

people

are

asked

to

move

their

eyes

to

a

target,

an

improvement

in

efficiency

at

the

target

location

begins well

before

the eyes

move

(Remington,

1980).

This

covert

shift

of

attention

appears

to

function

as

a

way

of

guiding

the

eye

to an

appropriate

area

of

the

visual

field

(Fischer

&

Breitmeyer,

1987;

Posner

&

Cohen,

1984).

The

sensory

responses

of

neurons

in

several

areas

of

the

brain

have

been

shown

to

have

a

greater

discharge

rate

when

a

monkey

attends

to

the

location

of

the

stimulus,

than

when

the monkey

attends

to

some

other

spatial

location.

Three

areas

particularly

identified

with

this

enhancement

effect

are

the

posterior

parietal

lobe

(Mountcastle,

1978;

Wurtz,

Goldberg

&

Robinson,

1980),

the

lateral

pulvinar

nucleus

of

the

postereolateral

thalamus

(Petersen,

Robinson

&

Morris,

1987) and the

superior

colliculus.

Similar

effects

in

the

parietal

cortex

have

been

shown

in

normal

humans

using

positron

emission

tomography

(Petersen,

Fox

&

Raichle,

1988).

While

brain

injuries

to

any

of

these

three

areas

in

human

subjects

will

cause

a

reduction

in

the

ability

to

shift

attention covertly

(Posner,

1988),

each

area

seems

to

produce

a

somewhat

different

type

of

deficit.

Damage

to

the

posterior

parietal

lobe

has

its

greatest

effect

on

the

ability

to

disengage

from

an

attentional

focus

to

a

target

located

in

a

direction opposite

to

the

side

of

the

lesion

(Posner,

1988).

Patients

with

a

progressive

deterioration

in

the

superior

colliculus

and/or

surrounding

areas

also show

a

deficit

in

the

ability

to

shift

attention.

In

this

case,

the

shift

is

slowed

whether

or

not

attention

is

first

engaged

elsewhere.

This finding

suggests

that

a

computation

involved

in

moving

attention

to

the

target

is

impaired.

Patients

with this

damage

also

return

to

former target

locations

as

readily

as

to

fresh

locations

that

have

not

recently

been

attended.

Normal

subjects

and

patients

with

parietal

and

other cortical

lesions

have

a

reduced

probability

of

returning

attention

to

already

examined

locations (Posner,

1988;

Posner

&

Cohen,

1984).

These

two

4

deficits

appear

to

be

those

most

closely

tied

to

the

mechanisms

involved

with

saccadic

eye

movements.

Patients

with

lesions

of

the thalamus

and

monkeys

with

chemical

injections

into

the

lateral pulvinar

also

show

difficulty

in

covert

orienting

(Petersen,

Robinson

&

Morris,

1987;

Posner,

1988).

This

difficulty

appears

to

be

in

engaging attention

on

a

target

on

the side

opposite

the lesion

so

as

to

avoid

being

distracted

by

events

at

other

locations.

A

study

of

patients

with

unilateral

thalamic

lesions

showed slowing

of

responses

to

a

cued

target

on

the

side

opposite

the

lesion

even

when

the subject

had

plenty

of

time

to

orient there.

This

contrasted

with

the

results

found

with

parietal

and

midbrain

lesions,

where

responses

are

nearly

normal

on

both

sides

once

attention

has

been

cued

to

that

location.

Alert

monkeys

with chemical

lesions

of

this

area

made

faster

than

normal

responses

when

cued

to

the

side

opposite

the

lesion

and

given

a

target

on

the

s:de

of

the

lesion,

as

though

the

contralateral

cue

was

not

effective

in

engaging

their

attention

(Petersen,

Robinson

&

Morris,

1987). They

were

also

worse

than

normal

when

given

a

target

on

the

side

opposite

the

lesion,

irrespective

of

the

side

of

the

cue.

It

appears

difficult

for

tbalamic lesioned

animals

to

respond

to

a

contralateral

target

when

another

competing

event

is

also

present

in

the

ipsilateral

field

(R.

Desimone,

personal

communication).

Data

from normal

human

subjects

required

to

filter

out irrelevancies,

showed

selective

metabolic

increases

in

the

pulvinar

contralateral

to

the

field

required

to

do

the

filtering

(LaBerge

&

Buchsbaum.

1988).

Thalamic

lesions

appear

to

give

problems

in

engaging

the

target

location

in

a

way

that allows

responding

to

be

fully

selective.

These

findings

make

two

important points.

First,

they

confirm

the

idea

that

anatomical

areas carry

out

quite

specific

cognitive

operations.

Second,

they suggest

a

hypothesis

about

the

circuitry

involved

in

covert

visual

attention

shifts

to

spatial

locations.

The

parietal

lobe

first

disengages

attention

from

its

present

focus,

then the

midbrain

area

acts to move

the

index

of

attention

to

the area

of

the

target

and the

pulvinar

is

involved

in

reading

out

data

from

the

indexed

locations.

Further

studies

of alert

monkeys

should

provide

ways

of

testing

and

modifying

this

hypothesis.

Hemispheric

Differences

The

most

accepted

form

of

cognitive

localization,

resultiag

from

studies

of

split

brain

patients

(Gazzaniga,

1970),

is

the

view

that

the

two

hemispheres

perform

different

functions.

Unfortunately,

in

the

absence

of

methods

to

study

more

detailed

localization,

the

literature

has

tended

to

divide

cognition

into

various

dichotomies

assigning

one

to

each

hemisphere.

As

we

develop

a

better

understanding

of

how

5

cognitive

systems

(e.g.

attention)

are

localized,

hemispheric

dominance

may

be

treated

in

a

more

differentiated

manner.

Just

as

we

can

attend

to

locations

in

visual

space,

it

is

also

possible

to

concentrate attention

on

a

narrow

area

or

to

spread

it

over

a

wider

area

(Eriksen

&

Yeh,

1985).

To

study

this

issue,

Navon

(1978)

formed

large

letters

out

of

smaller

ones.

It

has

been

found

in

many

studies

that

one

can

concentrate

attention

on

either

the

small

or

large

letters

and

that

the

attended

stimulus

controls

the

output

even

though

the

unattended

letter still

influences

performance.

The

use

of

small

and

large

letters

as

a

method

of

directing

local

and

global

attention

turns

out

to

be

allocation

of

visual

channels

to

different

spatial

frequencies.

Shulman

&

Wilson.

(1987)

showed

that

when

attending

to

the

large

letters,

subjects

are

relatively

more

accurate

in

the

perception

of

probe

grating

of

low

spatial

frequency

and

this

reverses

when

attending

to

the small

letters.

There

is

evidence

from the

study

of

patients

that

the

right

hemisphere

is

biased toward

global

processing

(low

spatial

frequencies)

and

the

left

for

local

processing

(high

spatial

frequencies)

(Robertson

&

Delis,

1986;

Sergent,

1987).

Right

hemisphere patients

may

copy

the

small

letters

but miss

the

overall

form,

while

those

with

left

hemisphere lesions

copy

the

overall

form

but

miscopy

the

constituent

small

letters.

Detailed

chronometric

studies

of

parietal

patients

reveal

difficulties

in

attentional

allocation

so

that

right hemisphere

patients attend poorly

to

the

global

aspects

and

left

hemisphere

to

the local

aspects

(Robertson,

Lamb

&

Knight,

1988).

These

studies

support

a

form

of

hemispheric

specialization within

the

overall

structure

of

the

attention

system.

The

left

and

right hemisphere

both

carry

out

the

operations

needed

for

shifts

of

attention

in

the

contralateral

direction,

but

they have

more

specialized

functions

in

the

level

of

detail

to

which

attention

is

allocated.

It

is

likely

that

these

hemispheric

specializations

are not

absolute

nor

innate.

Rather

they

are

probably

relative

and

emerge

over

time,

perhaps

in

conjunction

with

the

development

of

literacy.

The

general

anatomy

of

the

attention

system

that

we

have

been

describing

lies

in

the

dorsal visual

pathway

that

has its

primary

cortical

projection

area

in

V1

and

extends

into

the

parietal

lobe (see

figure

1).

A

major

aspect

of

the

study

INSERT

FIG

1

of

attention

is

to

see

how

attention

could

influence

the

operations

of

other

cognitive

systems

such

as

are

involved

in

the

recognition

of

visual

patterns.

The

visual

6

pattern

recognition

system

is

thought

to

involve

a

ventral

pathway,

stretching

from

V1

to

th:

infratemporal

cortex. Anatomically,

these

two

areas

of

the

brain

can

be

coordinated

through

the

thalamus

(pulvinar)

(Petersen,

Robinson

&

Morris,

1987),

or

through

other

pathways

(Zeki

&

Shipp,

1988).

Functionally,

attention

might

be

involved

in

various

levels

of

pattern recognition,

from

the

initial

registration

of

the

features

to

the

storage

of

new

visual

patterns.

Pattern

Recoanition

VISUAL

SEARCH

All

neurons

are

selective

in

the

range

of

activation

to

which

they

will respond.

The

role

of

the

attention

system

is

to

modulate

this

selection

for

those

types

of

stimuli

that

might

be

most

important

at

a

given

moment.

To

understand

how

this

form

of

modulation

operates,

it

is

important

to

know how

a

stimulus

would

be

processed without

the

special

effects

of

attention.

In

cognition, unattended

processing

is

called

"automatic"

to

distinguish

it from

the

special

processing

that

becomes

available

with

attention.

We

have

learned

quite

a

bit

about

the automatic

processing

that

occurs

in

humans along

the

ventral

pathway

during recognition

of

visual

objects

(Posner,

1988;

Treisman

&

Gormican,

1988).

Treisman

has

shown

that search

of

complex

visual

displays

for

single

features

can

take

place

in

parallel

with

relatively little

effect

of

the

number

of

distractors.

When

a

target

is

defined

as

a

conjunction

of

attributes

shared with

distractors that

are

themselves

heterogeneous

(Duncan

&

Humph

reys,

in

press),

the

search

process

becomes

slow,

effortful

and

serial.

We

know

from

cognitive studies

(LaBerge

&

Brown,

1988;

Treisman

&

Gormican,

1988)

that

cueing

people

to

locations

influences

a

number

of

aspects

of

visual

perception.

Treisman

has

shown

that

when

attempting

to

conjoin features,

subjects

use

focal

attention,

and

it has

also

been

shown

that

spreading

focal

attention

among

several

objects

leads

to

a

tendency

for

misconjoining

features

within

those

objects,

regardless

of

the

physical

distance

between

them (Cohen

&

Ivry,

1989).

Thus,

attention

not

only

provides

a

high

priority

to

attended

features,

but

does

so

in

a

way

that

overrides

even

the

physical

distance

between

objects within

the

focus

of

attention.

While

these

reaction

time

results

are

by

no

means

definitive

markers

of

attention,

there

is

also

evidence

from

studies

with brain

lesioned

patients

that

support

a

role

of

the

visual

spatial

attention

system.

These

clinical studies

examine

the

ability

of

patients

to

bisect

lines

(Riddoch

&

Humphreys,

1983),

search

complex

visual

patterns

(Riddoch

&

Humphreys,

1987),

or

report

strings

of

letters

(Friedrich,

7

Valker

&

Posner,

1985;

Sieroff, Pollatsek

&

Posner,

1988).

Damage

to

the

posterior

)arietal

lobe

appears

to

have

specific influences

on

these

tasks.

Patients

with

right

)arietal

lesions

frequently

bisect

lines

too

far

to

the

right

and

fail

to

report

the

left

nost

letters

of

a

random

letter

string

(Sieroff,

Pollatsek

&

Posner,

1988).

However,

hese

effects

are

attentional

not

in

the

recognition

process

itself.

Evidence for

this

is

hat

they

can

frequently

be

corrected

by

cueing

the

person

to

attend

covertly

to

the

,eglected

side

(Riddoch

&

Humphreys,

1983;

Sieroff,

Pollatsek

&

Posner,

1988).

The

:ues

appear

to

provide

time

for

the

damaged

parietal

lobe

to

disengage

attention

and

thus

compensates

for

the

damage.

It

is

also

possible

to

compensate

by

substituting

a

word

for

a

random

letter

string.

Patients

who

fail

to

report

the

left

most

letters

of

a

random

string

will

often

do

so

correctly

when

it

makes

a

word. If

cues work

by

directing

attention,

they

should

also

influence

normal

performance.

Cues

presented

prior

to

a

letter

string

do

improve

the

performance of

normals for

nearby

letters,

but

cues

have

little

or

no

influence

on

the

report

of

letters

making

words

(Sieroff

&

Posner,

1988).

Blood

flow

studies

of

normal

humans

show

that

an

area

of

the

left

ventral occipital

lobe

is

unique

to

strings

of letters

that

are

either

words

or

orthographically

regular

nonwords

(A.

Snyder,

S.E.

Petersen,

P.T.

Fox,

&

M.E.

Raichle,

personal

communication).

This

visual

word form area

(see

Figure

1)

appears

to

operate

without

attention,

and

this confirms

other

data

that

recognition

of

a

word

may

be

so

automated

as

not

to

require

spatial attention,

while

the

of

searching

for

a

single

letter,

forming

a

conjunction,

or

reporting

letters

from

a

random

string

do

appear

to

rely

upon

attention.

Studies

of

recording

from

individual

cells

in

aiert

monkeys

confirm

that

attention

can

play

a

role

in

the

operation

of

the

ventral

pattern

recognition system

(Wise

&

Desimone,

1988).

It

appears

likely

that

the

pathway

by

which

the

posterior

a.:ention

system

interacts

with

the

pattern

recognition

systew

is

through

the

thalamus

(Petersen,

Robinson,

&

Morris,

1987).

This

interaction

appears

to

require

about

90

millisec since

cells

in

V4

begin

to

respond

to

unattended

ltermq

within their

receptive

field, but

shut

these

unattended

areas

off

after

90

msec

(Wise

&

Desimone,

1988).

Detailed

models

of

the

nature of

the

interaction

between

attention

and

pattern recognition

are

just

beginning

to

appear

(Crick,

1984;

LaBerge

&

Brown,

1988).

IMAGERY

In

most

studies

of

pattern recognition,

the

sensory

event begins

the

process.

However,

it

is

possible

to

instruct

human

subjects

to

take

information

from

their

long

term

memories

and

construct

a

visual

representation

(image)

that

they

might then

inspect

(Kosslyn,

1988).

This

higher

level

visual

function

is

called

imagery.

The

importance

of

imagery

as

a

means

of

studying

mechanisms

of

high

level

vision

has

not

been

well

recognized

in

neuroscience.

When

imagery

can

be

8

iployed

as

a

means

of

studying vision,

it

allows

more

direct

access

to

the

higher

vels

of

information processing

without

contamination

from lower

levels.

There

is

now

considerable

evidence

that

some

of

the

same

anatomical

mechanisms

are

ed

in

imagery

as

are

involved

in

some

aspects

of

pattern

recognition

(Farah,

1988;

3sslyn,

1988).

Patients

with

right

parietal

lesions,

who show

deficits

in

visual

ienting

of

the

type

that

we

have

described

above,

also

fail

to

report

the

intralesional

side

of

visual

images

(Bisiach,

Luzzatti

&

Perani,

1981). When

asked

imagine

a

familiar

scene, they

make

elaborate reports

of

the

right

side

but

not

the

ft. The

parts

of

the

image

that

are

reported

when

facing

in

one

direction

are

-glected

when

facing

in

the

other.

This

suggests

that

the

deficit

arises

at

the

time

scanning

the

image.

When

normal

subjects

imagine

themselves

walking

on

a

familiar

route,

blood

ow

studies

show

activation

of

the

superior parietal

lobe

on

both

sides

(Roland,

?85).

Although

many

other

areas

of

the

brain

are

also

active

in

this

study,

most

of

tern

are

common

to

other verbal

and

arithmetical

thoughts,

but

activation

of

the

iperior

parietal

lobe

seems more

unique

to

imagery.

As

discussed

previously,

the

arietal

lobe seems

to

be

central

to

spatial

attention

to

external

locations.

Thus,

it

ppears

likely

that

the

neural

systems

involved

in

attending

to

an

external

location

re

closely related

to

those

used

when

subjects

scan

a

visual

image.

,NTERIOR

ATTENTION

SYSTEM

In

her

paper

on

the

topography

of

cognition,

Goldman-Rakic

(1988)

describes

le

strong

connectionb

between the

posterior parietal

lobe

and

areas

of

the

lateral

ad

medial

frontal

cortex.

This

anatomical

organization

is

appealing

as

a

basis

for

elating

what

has

been

called

involuntary

orienting

by

Luria

(1973),

and

what

we

ave

called

the

posterior

attention

system,

to

focal

or

conscious

attention.

Cognitive

studies

of

attention

have

often

shown

that

detecting

a

target

roduces widespread

interference

with

most

other cognitive

operations

(Posner,

978).

It

has been

shown

that

monitoring

many

spatial

locations

or

modalities

roduces

little

or

no

interference

over

monitoring

a

single

modality,

unless

a

target

ccurs

(Duncan,

1980).

This

finding

supports

the

distinction

between

a

general

alert

Late,

and

one

in

which

attention

is

clearly

oriented

and

engaged

in

processing

,formation.

In

the

alert

but

disengaged

state,

any

target

of

sufficient

intensity

has

ttle

trouble

in

summoning

the

mechanisms

that

produce

detection.

Thus

monitoring

iultiple

modalities

or locations

produce only small

amounts

of

interference.

The

nportance

of

engaging

the

focal

attention

system

in

the

production

of

widespread

iterference

between

signals

supports

the

idea

that there

is

a

unified system

9

involved

in

detection

of

signals

regardless

of

their

source.

As

a

consequence

of

detection

of

a

signal

by

this

system,

we

can

produce

a

wide

range

of

arbitrary

responses

to

it.

We

take

this

ability

to

produce

arbitrary

responses

as

evidence

that

the

person

is

aware

of

the

signal.

Evidence

that there

are

attentional

systems

common

to

spatial orienting

as

well

as

orienting

to

language

comes

from

studies

of

cerebral

blood

flow

during cognitive

tasks.

Roland

(1985)

has

reported

a

lateral

superior frontal

area which

is

active

both

during

tasks

involving

language

and

in

spatial

imagery

tasks.

However,

these

studies

do

not

provide

any

clear

evidence

that

such

common

areas

are

part

of

an

attentional

system.

More

compelling

is

evidence

that

midline

frontal

areas,

including

the

gyrus

and

the

supplementary

motor area,

are

active

during

semantic

processing

of

words

(Petersen,

et

al,

1988),

and

that

the

degree

of

blood

flow

in

the

anterior cingulate

increases

as

the number

of

targets

to

be

detected

increases (Posner

et

al,

1988).

Thus,

the

seems

to

be

particularly

sensitive

to

the

operations

involved

in

target

detection.

The

anterior cingulate

gyrus

is

one

of

the

areas

reported

by

Goldman-Rakic

(1988)

to

have

alternating

bands

of

cells

that

are

labelled

by

injections

into

the

posterior

parietal

lobe

and

the

dorsolateral

prefrontal cortex.

These

findings

suggest

that

the

anterior cingulate

should

be

shown

to

be

important

in

tasks

requiring

the

posterior

attention

system

as

well

as

in

language

tasks.

It

has

often

been

argued

from

lesion

data

that

the

anterior cingulate

plays

an

important

role

in

aspects

of

attention

including

neglect

(Mesulam,

1980;

Mirsky,

1987).

Does

attention

involve

a

single unified

system,

or

should

we

think

of

its

functioning

as

being

executed

by

separate

independent

systems?

One

way

to

test

this

idea

is

to

determine whether

attention

in

one

domain

(e.g. language)

affects

the

ability

of

mechanisms

in

another

domain

(e.g.

orienting

toward

a

visual location).

If

the

system

is

important

in

both

domains,

there

should

be

a

specific

interaction

between

even remote domains

such

as

these

two.

Studies

of

patients

with

parietal

lesions

(Posner,

Inhoff,

Freidrich

&

Cohen,

1987)

showed

that

when

they were

required

to

monitor

a

stream

of

auditory

information

for

a

sound,

they

were

slowed

in

their

ability

to

orient

toward

a

visual

cue.

The

effect of

the

language

task

was

rather

different

than

engaging

attention

at

a

visual

location

because

its

effects

were

bilateral rather

than being

mainly

on

the

side

opposite

the

lesion. Thus,

the

language task appeared

to

involve

some

but

not all

of

the

same

mechanisms that

were

used

in

visual

orienting.

10

This

result

is

compatible

with the view

that

visual

orienting

involves

separate

ut

interconnected systems

with those

used

for

language

processing.

A

similar

result

as

found with

normal

subjects

when

they

were

given

visual

cues while

shadowing

a

auditory

message

(Posner,

Sandson,

Dhawan

&

Shulman,

1989).

Here,

the

effects

f

the

language

task

were most marked

for

cues

in

the

right

visual

field,

as

though

ie

common

system might

have

involved

lateralized

mechanisms

of

the

left

emisphere. These

findings fit

with

the

close

anatomical

links between

the

and

the

posterior

parietal

lobe on

the

one

hand

and

language

areas

of

the

iferal

frontal

lobe

on

the

other.

They

suggest

to

us

a

possible hierarchy

of

attention

ystems

in

which

the

can

pass

control

to

the

posterior

system

when

is

not occupied with

processing

other

material.

A

spotlight

analogy

has

often

been used

to

describe

the

selection

of

information

om

the

ventral

pattern

recognition

system

by

the

posterior

attention

system

rreisman

&

Gormican,

1988).

A

spotlight

is

a

very

crude

analogy

but

it

does

apture

some

of

the

dynamics

involved

in

disengaging,

moving

and

engaging

ttention.

This

analogy

can

be

stretched

still

further

to

consider

aspects

of

^he

iteraction

between

the

system

and

the

associative

network

hown

to

be

active

during processing

of

semantic

associates

and

The Attention System of the Human Brain

Abstract