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A number of recent papers and books discuss theoretical efforts toward a scientific understanding of consciousness. Progress in imaging networks of brain areas active when people perform simple tasks may provide a useful empirical background for distinguishing conscious and unconscious information processing. Attentional networks include those involved in orienting to sensory stimuli, activating ideas from memory, and maintaining the alert state. This paper reviews recent findings in relation to classical issues in the study of attention and anatomical and physical theories of the nature of consciousness.
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Proc.
Nati.
Acad.
Sci.
USA
Vol.
91,
pp.
7398-7403,
August
1994
Review
Attention:
The
mechanisms
of
consciousness
(psychology/awareness/seection/vual
search)
Michael
I.
Posner
Instite
of
Cognitive
and
Decision
Scencess,
University
of
Oregon,
Eugene,
OR
97403
ABSTRACT
A
number
of
recent
pa-
pers
and
books
discuss
theoretical
efforts
toward
a
scientific
understanding
of
con-
sciousness.
Progress
in
imaging
networks
of
brain
areas
active
when
people
perform
simple
tasks
may
provide
a
useful
empir-
ical
background
for
distinguishing
con-
scious
and
unconscious
information
pro-
cessing.
Attentional
networks
include
those
involved
in
orienting
to
sensory
stim-
uli,
activating
ideas
from
memory,
and
maintaining
the
alert
state.
This
paper
reviews
recent
findings
in
relation
to
clas-
sical
issues
in
the
study
of
attention
and
anatomical
and
physical
theories
of
the
nature
of
consciousness.
What
is
it
to
be
conscious?
This
has
become
a
central
question
in
many
seri-
ous
scientific
circles
(1-6).
Proposals
range
from
the
anatomical,
for
example,
locating
consciousness
in
the
thalamus
(6)
or
in
thalamic-cortical
interactions
(2,
6),
to
the
physical,
for
example,
the
pro-
posal
that
consciousness
must
rest
on
quantum
principles
(1, 4).
Some
propos-
als
combine
physical
and
anatomical
rea-
soning.
For
example,
Beck
and
Eccles
(1)
argue
that
conscious
processing
acts
to
increase
the
probability
of
quantal
dis-
charge
at
the
synapse.
In
this
paper
I
propose
to
discuss the
issue
of
consciousness
in
light
of
recent
findings
about
attentional
networks
of
the
human
brain
that
lead
to
selection
of
sensory
information,
activate
ideas
stored
in
memory,
and
maintain
the
alert
state.
I
don't
believe
that
any
of
these
mechanisms
are
"consciousness"
itself,
just
as
DNA
is
not
"life,"
but
I
do
believe
that
an
understanding
of
consciousness
must
rest
on
an
appreciation
of
the
brain
networks
that
subserve
attention,
in
much
the
same
way
as
a
scientific
anal-
ysis
of
life
without
consideration
of
the
structure
of
DNA
would
seem
vacuous.
Attention
The
study
of
attention
has
a
long
history
within
psychology.
William
James
(7)
wrote
at
the
turn
of
the
century,
"Every-
one
knows
what
attention
is.
It
is
the
taking
possession
by
the
mind
in
clear
and
vivid
form
of
one
out
of
what
seem
several
simultaneous
objects
or
trains
of
thought."
The
dominance
of
behavioral
psychol-
ogy
postponed
research
into
the
internal
mechanisms
of
selective
attention
in
the
first
half
of
this
century.
The
finding
that
the
integrity
of
the brain
stem
reticular
formation
was
a
necessity
to
maintain
the
alert
state
provided
some
anatomical
re-
ality
to
the
study
of
an
aspect
of
attention
(8).
The
quest
for
information-processing
mechanisms
to
support
the
more
selec-
tive
aspect
of
attention
began
after
World
War
II
with
studies
of
selective
listening.
A
filter
was
proposed
that
was
limited
for
information
(in
the
formal
sense
of
infor-
mation
theory)
and
located
between
highly
parallel
sensory
systems
and
a
limited-capacity
perceptual
system
(9).
Selective
listening
experiments
sup-
ported
a
view
of
attention
that
suggested
early
selection
of
relevant
message,
with
nonselective
information
being
lost
to
conscious
processing.
However,
on
some
occasions
it
was
clear
that
unat-
tended
information
was
processed
to
a
high
level
because
there
was
evidence
that
an
important
message
on
the
unat-
tended
channel
might
interfere
with
the
selected
channel.
In
the
1970s
psychologists
began
to
distinguish
between
automatic
and
con-
trolled
processes.
It
was
found
that
words
could
activate
other
words
similar
in
meaning
(their
semantic
associates),
even
when
the
person
had
no
awareness
of
the
words'
presence.
These
studies
indicated
that
the
parallel
organization
found
for
sensory
information
extended
to
semantic
processing.
Thus,
selecting
a
word
meaning
for
active
attention
ap-
peared
to
suppress
the
availability
of
other
word
meanings.
Attention
was
viewed
less
as
an
early
sensory
bottle-
neck
and
more
as
a
system
for
providing
priority
for
motor
acts,
consciousness,
and
memory
(10).
Another
approach
to
problems
of
se-
lectivity
arose
in
work
on
the
orienting
reflex
(11).
The
use
of
slow
autonomic
systems
(e.g.,
skin
conductance
as
mea-
sures
of
orienting)
made
it
difficult
to
analyze
the
cognitive
components
and
neural
systems
underlying
orienting.
During
the
last
15
years
there
has
been
a
steady
advancement
in
our
understand-
ing
of
the
neural
systems
related
to
visual
orienting
from
studies
using
single-cell
recording
in
alert
monkeys
(12).
This
work
showed
a
relatively
restricted
num-
ber of
areas
in
which
the
firing
rates
of
neurons
were
enhanced
selectively
when
monkeys
were
trained
to
attend
to
a
location,
at
the
level
of
the
superior
col-
liculus
(i.e.,
the
midbrain),
selective
en-
hancement
could
only
be
obtained
when
eye
movement
was
involved,
but
in
the
posterior
parietal
lobe
of
the
cerebral
cortex,
selective
enhancement
occurred
even
when
the
animal
maintained
fixa-
tion.
An
area
of
the
thalamus,
the
lateral
pulvinar,
was
similar
to
the
parietal
lobe
in
containing
cells
with
the
property
of
selective
enhancement.
Until
recently,
there
has
been
a
sepa-
ration
between
human
information
pro-
cessing
and
neuroscience
approaches
to
attention
using
nonhuman
animals.
The
former
tended
to
describe
attention,
ei-
ther
in
terms
of
a
bottleneck
that
pre-
vented
limited-capacity
central
systems
from
overload
or
as
a
resource
that
could
be
allocated
to
various
processing
sys-
tems
in
a
way
analogous
to
the
use
of
the
term
in
economics.
On
the
other
hand,
neuroscience
views
emphasized
several
separate
neural
mechanisms
that
might
be
involved
in
orienting
and
maintaining
alertness.
Currently,
there
is
an
attempt
to
integrate
these
two
within
a
cognitive
neuroscience
of
attention.
An
impressive
aspect
of
current
developments
in
this
field
is
the
convergence
of
evidence
from
various
methods
of
study.
These
include
performance
studies
using
reaction
time,
dual-task
performance
studies,
recording
from
scalp
electrodes,
and
lesions
in
hu-
mans
and
animals,
as
well
as
various
methods
for
imaging
and
recording
from
restricted
brain
areas,
including
individ-
ual
cells
(13).
Current
progress
in
the
anatomy
of
the
attention
system
rests
most
heavily
on
two
important
methodological
develop-
ments.
(i)
The
use
of
microelectrodes
with
alert
animals
allowed
evidence
for
the
increased
activity
of
cell
populations
with
attention
(12).
(ii)
Anatomical
(e.g.,
computerized
tomography
or
magnetic
resonance
imaging)
and
physiological
[e.g.,
positron
emission
tomography
Abbreviation:
PET,
positron
emission
tomog-
raphy.
7398
Proc.
Natl.
Acad.
Sci.
USA
91
(1994)
7399
(PET)
and
functional
magnetic
resonanc
imagery]
methods
of
studying
parts
of
ti
brain
allowed
more
meaningful
invest
gations
of
localization
of
cognitive
fun
tion
in
normal
people
(13).
The
futum
should
see
the
use
of
localizing
method
together
with
methods
of
tracing
the
tim
course
of
brain
activity
in
the
huma
subject.
This
combination
should
provid
a
convenient
way
to
trace
the
rapid
time
dynamic
changes
that
occur
in
the
cours
of
human
information
processing.
Three
fundamental
working
hypothe
ses
characterize
the
current
state
of
el
forts
to
develop
a
combined
cognitive
neuroscience
of
attention.
(i)
There
ex
ists
an
attentional
system
of
the
brain
tha
is
anatomically
separate
from
variou
data-processing
systems
that
can
be
ac
tivated
passively
by
visual
and
auditor
input.
(ii)
Attention
is
carried
out
by;
network
of
anatomical
areas.
It
is
neither
the
property
of
a
single
brain
area
nor
is
it
a
collective
function
of
the
brain
work
ing
as
a
whole.
(iii)
The
brain
area
involved
in
attention
do
not
carry
out
the
same
function,
but
specific
computations
are
assigned
to
different
areas
(13).
It
is
not
possible
to
specify
the
com-
plete
attentional
system
of
the
brain,
but
something
is
known
about
the
areas
that
carry
on
three
major
attentional
func-
tions:
orienting
to
sensory
stimuli,
par-
ticularly
locations
in
visual
space;
detect-
ing
target
events,
including
ideas
stored
in
memory;
and
maintaining
the
alert
state.
Each
of
these
areas
of
research
provides
information
that
relate
to
the
theories
of
consciousness
discussed
in
my
first
paragraph,
but
as
will
be
appar-
ent
from
the
discussion
below,
each
re-
lates
only
partially
to
common
definitions
of
conscious
processing.
Visual
Orienting
How
can
one
study
attention?
Crick
ar-
gues
for
the
selection
of
a
model
system
that
involves
consciousness
in
a
limited
domain.
His
choice
is
awareness
during
visual
search.
Visual
search
has
also
been
a
traditional
vehicle
for
the
study
of
attention.
For
example,
if
you
are
asked
to
report
a
vertical
rectangle
in
Fig.
1,
the
time
to
do
so
is
linearly
related
to
the
number
of
distractors.
This
situation
is
thought
to
occur
because
one
has
to
orient
attention
to
each
location.
An
in-
dividual
may
do
so
by
making
an
eye
movement,
but
it
is
also
clear
that
the
eyes
can
remain
fixed
and
each
position
examined
covertly.
We
now
know
quite
a
lot
about
the
mechanism
that
performs
this
covert
operation
from
studies
of
nor-
mal
subjects,
brain-lesioned
patients,
and
monkeys
(13).
When
subjects
switch
attention
from
location
to
location,
they
activate
pro-
cesses
in
the
parietal
lobe
of
the
opposite
hemisphere
(14).
These
cortical
areas
are
I
E_
Ok
I_
-0l_
~
|
_
@
_~
-o0
..0±0
_
w
_~
|~
_ _
FIG.
1.
Searching
this
display
for
a
vertical
rectangle
requires
attention
to
successive
locations
because
the
nontargets
include
both
vertical
(ellipses)
and
rectangles
(horizontal).
Reaction
times
in
this
task
are
linearly
related
to
the
number
of
distractors.
[This
figure
was
reproduced
with
permission
from
ref.
37
(copyright
Elsevier,
Cambridge,
U.K.).]
involved
in
programming
the
switch
of
attention
in
the
opposite
visual
field.
The
t
mechanisms
of
the
parietal
lobe
are
not
symmetric.
PET
evidence
shows
that
the
right
parietal
lobe
is
involved
with
atten-
tion
shifts
in
both
visual
fields,
whereas
the
left
parietal
lobe
seems
restricted
to
rightward
shifts
of
attention
(14).
There
is
a
further
hemispheric
specialization
as
well.
The
left
hemisphere
seems
to
be
[
more
involved
when
attending
to
local
information
that
might
be
important
in
recognizing
objects,
whereas
the
right
hemisphere
seems
more
involved
when
global
features
are
involved,
such
as
in
general
navigation
in
an
environment
(15).
These
hemispheric
differences
are
es-
pecially
important
in
understanding
how
brain
damage
influences
our
awareness
of
the
visual
world.
In
normal
people
the
two
hemispheres
operate
as
a
unit
in
visual
search,
so
that
it
does
not
matter
if
all
of
the
distractors
are
located
in
one
visual
field
or
if
they
are
distributed
across
the
visual
fields.
However,
if
the
two
hemispheres
are
separated
surgically
by
cutting
the
fiber
tract
that
runs
be-
tween
them,
this
unity
no
longer
applies,
and
each
hemisphere's
system
operates
separately,
yielding
a
search
rate
that
is
twice
as
fast
(16).
Yet
the
patient
is
not
really
aware
that
a
change
has
taken
place
due
to
the
operation.
If
there
is
damage
to
the
right
parietal
lobe,
subjects
will
be
very
likely
to
neglect
targets
in
search
tasks
like
those
of
Fig.
1.
In
other
words,
they
can
be
unconscious
of
infor-
mation
on
the
side
of
space
opposite
the
lesion.
These
same
mechanisms
are
used
when
these
neglect
patients
recall
infor-
f
mation
from
memory,
and
they
neglect
the
left
side
of
visual
images
(17).
We
know
something
of
the
route
by
which
the
parietal
mechanism
influences
information
about
the
target
identity.
The
most
prominent
hypothesis
about
this
route
is
that
it
involves
the
pulvinar
nu-
cleus
of
the
thalamus.
There
is
good
evidence
of
activity
in
this
general
region
when
subjects
must
pull
out
a
target
from
surrounding
clutter
(18).
Thus
the
inter-
action
of
thalamus
and
the
cortex
plays
an
important
role
in
our
consciousness
of
the
target.
This
finding
fits
well
with
the
general
idea
outlined
by
Crick.
What
are
the
consequences
of
attend-
ing
to
a
visual
object?
We
know
from
cellular
recording
studies
in
monkeys
(12)
and
from
neuroimaging
studies
in
people
(13)
that
attention
provides
a
rel-
ative
increase
of
neural
activity
when
compared
with
comparable
unattended
information.
For
example,
an
instruction
to
attend
to
the
color,
form,
or
motion
of
visual
input
increases
neural
activity
in
the
regions
that
process
this
information
(19).
This
finding
is
certainly
consonant
with
the
proposal
that
mental
events
work
on
the
rate
of
transmission
of
neural
impulses
that
has
been
suggested
by
sev-
eral
theories
of
consciousness
(1,
3).
However,
the
principle
of
relative
am-
plification
by
attention
is
a
very
general
one
(13).
It
is
found
in
all
areas
of
the
brain
that
have
been
studied.
Attention
to
sensory
information
amplifies
brain
areas
used
to
processes
that
modality;
simi-
larly,
attention
to
motor
output
activates
brain
areas
used
to
generate
the
move-
ment.
This
principle
also
appears
to
ap-
ply
to
higher-level
cognitive
processes
(13).
For
example,
creating
a
visual
im-
Review:
Posner
Proc.
Natl.
Acad.
Sci.
USA
91
(1994)
age
from
a
verbal
instruction
is
now
known
to
produce
activation
in
visually
specific
areas
of
the
cortex
that
would
be
used
to
process
visual
input
of
the
same
type.
What
is
still
not
known
is
exactly
how
this
amplification
is
accomplished.
There
are
theories
at
the
neural
level
(20)
and
cellular
observations
that
suggest
a
role
for
suppression
of
the
unattended
pro-
cesses
(21,
22),
but
the
detailed
cellular
mechanisms
remain
to
be
clarified.
Are
the
thalamic-visual
cortex
inter-
actions
we
have
been
describing
"con-
sciousness"
as
suggested
by
Crick?
One
definition
of
consciousness
involves
awareness
of
the
outside
world,
and
the
interactions
of
thalamic
areas
with
the
visual
cortex
are
certainly
important
for
achieving
focal
awareness.
By
focal
awareness
I
mean
the
type
of
recognition
that
one
has
of
the
target
in
Fig.
1.
To
locate
the
target
one
must
effortfully
en-
gage
different
locations
to
be
certain
that
they
contain
the
constellation
of
features
(form
and
orientation)
required.
How-
ever,
it
would
not
be
appropriate
to
say
you
are
unaware
of
other
objects
in
the
field
that
are
not
the
focus
of
attention.
One
is
roughly
aware
of
the
density
of
nonattended
objects-their
extent
and
basic
format.
It
is
possible
to
distinguish
the
type
of
focal
awareness
needed
to
locate
the
target
from
a
more
general
awareness
of
the
background
(23).
Dam-
age
to
the
attention
network
involving
the
parietal
lobe
and
associated
thalamic
areas
produces
a
kind
of
loss
of
focal
awareness
on
one
side
of
the
world.
Ne-
glect
induced
by
parietal
lesion
may
leave
the
patient
unconscious
of
this
lack
of
awareness,
just
as
the
split
brain
person
is
unaware
that
search
of
the
visual
world
has
lost
integration.
However,
patients
with
parietal
lesions
usually
recover
awareness
of
information
opposite
the
lesion
but
show
a
permanent
loss
of
the
ability
of
stimuli
arising
there
to
produce
orienting
away
from
already
attended
events.
This
may
lead
to
a
failure
to
see
objects
on
the
left
side
of
Fig.
1,
but
if
cued
to
that
location,
they
become
con-
scious
of
them.
In
addition,
normal
sub-
jects
can
attend
to
aspects
of
visual
stim-
uli,
such
as
their
color
or
form,
without
activation
of
the
parietal
mechanisms
we
have
been
describing.
These
facts
seem
to
argue
that
the
parietal-thalamic
sys-
tem
represents
an
important
pathway
by
which
conscious
processing
is
achieved,
rather
than
consciousness
itself.
None-
theless,
the
clarification
of
neglect
that
has
arisen
from
recent
studies
has
helped
us
grasp
some
important
elements
of
achieving
focal
awareness.
search
all
the
target
locations
or
can
you
confine
your
search
to
the
vertical
ob-
jects
only?
There
is
considerable
evi-
dence
that
search
can
be
guided
by
in-
formation
about
the
color
or
orientation
or
other
nonlocational
features
of
the
target
(24).
How
is
this
implemented
in
the
brain?
It
seems
that
the
recruitment
and
control
of
posterior
brain
areas,
in
this
case,
is
supervised
by
an
anatomi-
cally
distinct
system
that
involves
more
anterior
structures
that
have
sometimes
been
called
an
executive
network
(ref.
13,
pp.
168-174)
(Fig.
2).
In
the
study
of
attentional
amplification
of
color,
form,
or
motion
mentioned
above
there
was
evidence
for
activation
of
a
frontal
atten-
tional
system,
but
no
parietal
activation
was
found
(19).
It
thus
appears
that
two
different
attentional
systems
serve
as
sources
of
activation
for
color
or
form
(frontal
areas)
and
for
location
(parietal),
although
both
may
enter
via
the
thalamus
to
amplify
activity
within
the
visual
sys-
tem
at
the
same
site
(e.g.,
V4).
In
guided
search,
selection
by
location
and
selection
by
color
or
form
occur
simultaneously
with
relatively
little
inter-
ference
(24),
unlike
the
situation
for
lo-
cation
when
the
corpus
collosum
is
in-
tact,
in
which
attention
cannot
be
shared
between
the
two
fields
(16).
One
specu-
lative
possibility
would
be
that
time
shar-
ing
is
possible
when
two
anatomically
distinct
attentional
sources
are
involved.
The
frontal
areas
that
serve
to
guide
search
appear
to
involve
a
network
that
includes
at
least
portions
of
the
basal
ganglia
and
of
the
anterior
cingulate
gy-
rus
(ref.
13,
pp.
168-174).
The
anterior
portion
of
the
cingulate
gyrus
appears
to
be
involved
in
a
wide
range
of
activities
that
have been
termed
collectively
"ex-
ecutive
function"
(25).
In
PET
language
studies,
when
subjects
were
required
to
name
the
use
of
familiar
nouns
(e.g.,
pound
to
hammer)
activation
of
the
an-
terior
cingulate
along
with
left
lateral
cortex
language
areas
was
most
promi-
nent.
When
subjects
were
required
to
respond
to
the ink
color
in
which
a
con-
flicting
color
name
was
presented
(Stroop
effect),
there
was
strong
activa-
tion
of
the
anterior
cingulate
along
with
prestriate
color
areas
(13).
The
detection
of
multiple
color
form
or
motion
targets
in
comparison
with
passive
viewing
of
the
same
stimuli
also
activated
the
ante-
rior
cingulate
(19).
All
of
these
situations
involve
selection
of
targets
from
compet-
ing
inputs,
which
is
considered
a
tradi-
tional
role
of
attention.
In
the
case
of
this
executive
attentional
network,
the
nature
of
the
target
does
not
seem
to
matter
very
much.
The
term
"executive"
suggests
two
important
overall
functions.
(i)
An
exec-
utive
is
informed
about
the
processes
taking
place
within
the
organization.
A
system
that
would
be
related
to
our
sub-
jective
experience
of
focal
attention
would
clearly
play
this
function
for
a
subset
of
current
(sensory)
and
stored
(memory)
information.
There
are
reasons
for
relating
anterior
cingulate
function
to
focal
awareness
of
the
target
(13).
For
example,
the
intensity
of
cingulate
activ-
ity
tends
to
increase
with
number
of
targets
in
a
set
of
stimuli
and
decreases
with
practice
on
any
single
stimulus
set.
These
findings
correspond
to
cognitive
theories
linking
focal
attention
to
number
and
difficulty
of
target
detection.
(ii)
A
second
function
of
an
executive
is
to
exercise
some
control
over
the
sys-
tem.
The
anatomy
of
the
anterior
cingu-
late
provides
pathways
for
connecting
it
to
both
the
posterior
parietal
area
and
to
anterior
areas
active
during
language
tasks
(26).
Working
memory
is
generally
thought
to
involve
both
a
representation
of
past
events
and
an
executive
system
involved
in
sustaining
and
transforming
this
representation
(27).
Recent
PET
(28,
29)
and
neurophysiological
(30,
31)
stud-
ies
show
that
lateral
areas
of
the
prefron-
tal
cortex
play
a
key
role
in
holding
on-line
a
representation
of
past
events.
-s~
p
Ta
"t
rx14
I
r\-k
rArI
...I
Attending
to
Ideas
Suppose
you
are
asked
to
locate
the
vertical
rectangle
in
Fig.
1.
Must
you
FIG.
2.
The
executive
attentional
system
involves
frontal
structures,
including
the
anterior
cingulate,
and
acts
upon
many
different
brain
areas
[reproduced
with
permission
from
ref.
13
(copyright
Freeman,
New
York)].
7400
Review:
Posner
---
[IL
II
7
t.
1
.T
_-
'.
7,1
:,.
--I.
;,''.
-1
't
'.
Proc.
Natl.
Acad.
Sci.
USA
91
(1994)
7401
Cellular
recordings
in
the
awake
monkey
indicate
that
cells
within
the
dorsolateral
prefrontal
cortex
maintain
a
representa-
tion
of
the
spatial
environment
when
monkeys
have
to
hold
in
mind
a
location
to
which
to
move
their
eyes
after
the
stimulus
disappears
(30).
Other
cells
within
the
inferior
convexity
hold
a
rep-
resentation
of
the
identity
of
past
stimuli
(31).
Lateral
areas
of
the
left
frontal
and
posterior
cortex
are
also