<s>
N2pc	B-Algorithm
refers	O
to	O
an	O
ERP	B-Algorithm
component	O
linked	O
to	O
selective	O
attention	O
.	O
</s>
<s>
The	O
N2pc	B-Algorithm
appears	O
over	O
visual	O
cortex	O
contralateral	O
to	O
the	O
location	O
in	O
space	O
to	O
which	O
subjects	O
are	O
attending	O
;	O
if	O
subjects	O
pay	O
attention	O
to	O
the	O
left	O
side	O
of	O
the	O
visual	O
field	O
,	O
the	O
N2pc	B-Algorithm
appears	O
in	O
the	O
right	O
hemisphere	O
of	O
the	O
brain	O
,	O
and	O
vice	O
versa	O
.	O
</s>
<s>
This	O
characteristic	O
makes	O
it	O
a	O
useful	O
tool	O
for	O
directly	O
measuring	O
the	O
general	O
direction	O
of	O
a	O
person	O
's	O
attention	O
(	O
either	O
left	O
or	O
right	O
)	O
with	O
fine-grained	O
temporal	B-General_Concept
resolution	I-General_Concept
.	O
</s>
<s>
Luck	O
and	O
Hillyard	O
(	O
1990	O
)	O
first	O
observed	O
the	O
N2pc	B-Algorithm
while	O
seeking	O
to	O
document	O
electrophysiological	B-Application
correlates	O
of	O
focused	O
attention	O
during	O
visual	O
search	O
using	O
ERPs	B-Algorithm
.	O
</s>
<s>
Compared	O
to	O
the	O
waveform	O
over	O
cortex	O
ipsilateral	O
to	O
the	O
target	O
,	O
experimenters	O
observed	O
a	O
consistently	O
greater	O
negative	O
deflection	O
of	O
the	O
ERP	B-Algorithm
waveform	O
at	O
approximately	O
200	O
ms	O
after	O
the	O
stimulus	O
at	O
posterior	O
sites	O
(	O
i.e.	O
,	O
over	O
visual	O
cortex	O
)	O
contralateral	O
to	O
the	O
side	O
of	O
the	O
screen	O
subjects	O
attended	O
.	O
</s>
<s>
The	O
N2pc	B-Algorithm
first	O
received	O
its	O
name	O
from	O
Luck	O
and	O
Hillyard	O
(	O
1994	O
)	O
,	O
who	O
named	O
the	O
component	O
after	O
its	O
characteristic	O
features	O
.	O
</s>
<s>
The	O
experimenters	O
explored	O
what	O
factors	O
would	O
modulate	O
the	O
N2pc	B-Algorithm
using	O
a	O
visual	O
search	O
paradigm	O
in	O
which	O
subjects	O
had	O
to	O
report	O
the	O
presence	O
of	O
a	O
target	O
object	O
in	O
a	O
display	O
(	O
e.g.	O
,	O
a	O
green	O
box	O
or	O
a	O
horizontal	O
bar	O
)	O
.	O
</s>
<s>
They	O
confirmed	O
that	O
the	O
N2pc	B-Algorithm
appeared	O
contralateral	O
to	O
attended	O
stimuli	O
,	O
and	O
furthermore	O
found	O
that	O
it	O
did	O
not	O
appear	O
when	O
subjects	O
saw	O
only	O
one	O
object	O
at	O
a	O
time	O
or	O
had	O
to	O
spread	O
their	O
attention	O
over	O
all	O
the	O
items	O
in	O
the	O
display	O
.	O
</s>
<s>
These	O
data	O
led	O
the	O
experimenters	O
to	O
believe	O
the	O
N2pc	B-Algorithm
corresponds	O
to	O
a	O
filtering	O
process	O
that	O
occurs	O
whenever	O
people	O
focus	O
attention	O
on	O
one	O
object	O
while	O
ignoring	O
others	O
.	O
</s>
<s>
The	O
component	O
's	O
name	O
,	O
N2pc	B-Algorithm
,	O
abbreviates	O
its	O
characteristics	O
.	O
</s>
<s>
The	O
component	O
belongs	O
to	O
the	O
family	O
of	O
N2	B-Algorithm
ERP	B-Algorithm
components	O
,	O
a	O
negative	O
deflection	O
in	O
the	O
ERP	B-Algorithm
waveform	O
at	O
a	O
latency	O
of	O
approximately	O
200-300	O
ms	O
following	O
a	O
stimulus	O
.	O
</s>
<s>
For	O
example	O
,	O
when	O
a	O
person	O
pays	O
attention	O
to	O
something	O
in	O
the	O
left	O
side	O
of	O
the	O
visual	O
field	O
,	O
an	O
N2pc	B-Algorithm
appears	O
as	O
a	O
greater	O
negativity	O
over	O
the	O
right	O
posterior	O
areas	O
of	O
the	O
brain	O
than	O
the	O
left	O
.	O
</s>
<s>
MEG	O
has	O
been	O
used	O
to	O
localize	O
the	O
N2pc	B-Algorithm
primarily	O
to	O
lateral	O
extrastriate	O
cortex	O
and	O
inferotemporal	O
visual	O
areas	O
,	O
such	O
as	O
V4	O
.	O
</s>
<s>
The	O
N2pc	B-Algorithm
can	O
be	O
used	O
flexibly	O
in	O
nearly	O
any	O
task	O
in	O
which	O
one	O
would	O
like	O
to	O
study	O
the	O
direction	O
and	O
time	O
course	O
of	O
selective	O
attention	O
.	O
</s>
<s>
However	O
,	O
researchers	O
have	O
primarily	O
used	O
the	O
N2pc	B-Algorithm
in	O
visual	O
search	O
paradigms	O
to	O
study	O
the	O
deployment	O
of	O
attention	O
over	O
time	O
and	O
test	O
hypotheses	O
of	O
parallel	O
and	O
serial	O
models	O
of	O
visual	O
search	O
.	O
</s>
<s>
The	O
first	O
experiments	O
to	O
investigate	O
specifically	O
the	O
N2pc	B-Algorithm
used	O
a	O
visual	O
search	O
paradigm	O
in	O
which	O
subjects	O
reported	O
the	O
presence	O
or	O
absence	O
of	O
a	O
pre-defined	O
target	O
(	O
e.g.	O
,	O
a	O
green	O
rectangle	O
)	O
in	O
a	O
display	O
containing	O
one	O
"	O
oddball	O
"	O
stimulus	O
that	O
differed	O
on	O
a	O
single	O
feature	O
from	O
a	O
uniform	O
background	O
of	O
items	O
(	O
e.g.	O
,	O
a	O
green	O
square	O
among	O
blue	O
squares	O
)	O
.	O
</s>
<s>
The	O
pop-out	O
oddball	O
would	O
generate	O
an	O
N2pc	B-Algorithm
,	O
as	O
it	O
received	O
focused	O
attention	O
,	O
while	O
stimulus	O
characteristics	O
modulated	O
the	O
amplitude	O
and	O
latency	O
of	O
the	O
component	O
.	O
</s>
<s>
Subsequent	O
investigation	O
into	O
the	O
N2pc	B-Algorithm
manipulated	O
the	O
number	O
of	O
items	O
in	O
the	O
array	O
and	O
found	O
that	O
a	O
display	O
with	O
as	O
few	O
as	O
two	O
objects	O
elicits	O
the	O
component	O
.	O
</s>
<s>
Because	O
an	O
object	O
cannot	O
"	O
pop	O
out	O
"	O
and	O
attract	O
attention	O
in	O
a	O
two-item	O
display	O
,	O
experimenters	O
concluded	O
that	O
the	O
N2pc	B-Algorithm
must	O
reflect	O
top-down	O
,	O
controlled	O
processes	O
of	O
directing	O
attention	O
.	O
</s>
<s>
The	O
same	O
study	O
also	O
demonstrated	O
that	O
the	O
N2pc	B-Algorithm
does	O
not	O
only	O
occur	O
when	O
attending	O
to	O
visual	O
features	O
,	O
but	O
semantic	O
features	O
as	O
well	O
.	O
</s>
<s>
Even	O
in	O
this	O
case	O
of	O
semantically	O
defined	O
targets	O
,	O
subjects	O
demonstrated	O
an	O
N2pc	B-Algorithm
contralateral	O
to	O
the	O
target	O
word	O
.	O
</s>
<s>
Together	O
,	O
these	O
results	O
have	O
provided	O
strong	O
evidence	O
that	O
the	O
N2pc	B-Algorithm
reflects	O
the	O
location	O
of	O
covert	O
,	O
consciously	O
directed	O
attention	O
.	O
</s>
<s>
The	O
prototypical	O
visual	O
search	O
paradigm	O
for	O
eliciting	O
an	O
N2pc	B-Algorithm
component	O
has	O
subjects	O
attend	O
and	O
respond	O
to	O
a	O
target	O
stimulus	O
to	O
the	O
left	O
or	O
right	O
of	O
fixation	O
.	O
</s>
<s>
Unlike	O
regular	O
visual	O
search	O
experiments	O
,	O
however	O
,	O
two	O
major	O
criteria	O
most	O
hold	O
when	O
attempting	O
to	O
measure	O
N2pc	B-Algorithm
response	O
.	O
</s>
<s>
First	O
,	O
the	O
stimuli	O
should	O
be	O
identical	O
in	O
all	O
conditions	O
,	O
and	O
the	O
experimenter	O
should	O
only	O
manipulate	O
instructions	O
for	O
directing	O
attention	O
across	O
conditions	O
;	O
this	O
precludes	O
the	O
possibility	O
that	O
stimulus	O
features	O
drive	O
ERP	B-Algorithm
effects	O
rather	O
than	O
focused	O
attention	O
.	O
</s>
<s>
The	O
goal	O
is	O
to	O
minimize	O
the	O
variability	O
in	O
search	O
times	O
and	O
N2pc	B-Algorithm
latency	O
,	O
resulting	O
in	O
a	O
much	O
clearer	O
signal	O
when	O
the	O
waveforms	O
are	O
averaged	O
together	O
over	O
multiple	O
trials	O
.	O
</s>
<s>
An	O
example	O
experiment	O
for	O
eliciting	O
the	O
N2pc	B-Algorithm
that	O
follows	O
the	O
critical	O
principles	O
above	O
might	O
proceed	O
as	O
follows	O
:	O
Subjects	O
see	O
an	O
array	O
of	O
upright	O
and	O
inverted	O
T	O
's	O
.	O
</s>
<s>
We	O
should	O
expect	O
to	O
see	O
an	O
N2pc	B-Algorithm
contralateral	O
to	O
the	O
side	O
of	O
the	O
screen	O
the	O
attended	O
letter	O
appeared	O
.	O
</s>
<s>
The	O
N2pc	B-Algorithm
is	O
primarily	O
sensitive	O
to	O
the	O
directional	O
focus	O
of	O
attention	O
over	O
time	O
.	O
</s>
<s>
However	O
,	O
research	O
has	O
found	O
a	O
variety	O
of	O
factors	O
that	O
modulate	O
N2pc	B-Algorithm
response	O
.	O
</s>
<s>
N2pc	B-Algorithm
amplitude	O
is	O
sensitive	O
to	O
factors	O
related	O
to	O
increasing	O
demands	O
on	O
focused	O
attention	O
.	O
</s>
<s>
When	O
targets	O
are	O
defined	O
by	O
a	O
conjunction	O
of	O
features	O
(	O
e.g.	O
,	O
blue	O
,	O
horizontal	O
bar	O
)	O
rather	O
than	O
a	O
single	O
feature	O
(	O
e.g.	O
,	O
blue	O
bar	O
)	O
,	O
they	O
elicit	O
a	O
larger	O
N2pc	B-Algorithm
,	O
which	O
may	O
reflect	O
a	O
greater	O
demand	O
on	O
attention	O
to	O
identify	O
the	O
target	O
.	O
</s>
<s>
In	O
a	O
dual-task	O
situation	O
where	O
subjects	O
focus	O
on	O
a	O
demanding	O
primary	O
task	O
while	O
performing	O
target	O
detection	O
as	O
a	O
secondary	O
task	O
,	O
the	O
N2pc	B-Algorithm
only	O
appears	O
in	O
response	O
to	O
detecting	O
targets	O
defined	O
by	O
conjunctions	O
of	O
features	O
.	O
</s>
<s>
N2pc	B-Algorithm
amplitude	O
increases	O
as	O
distractors	O
appear	O
closer	O
to	O
the	O
target	O
,	O
which	O
increases	O
the	O
need	O
to	O
focus	O
on	O
the	O
target	O
while	O
filtering	O
the	O
distractors	O
(	O
but	O
also	O
see	O
Mazza	O
et	O
al.	O
,	O
2009	O
,	O
who	O
found	O
conflicting	O
results	O
)	O
.	O
</s>
<s>
Certain	O
experimental	O
conditions	O
can	O
eliminate	O
the	O
N2pc	B-Algorithm
entirely	O
.	O
</s>
<s>
These	O
results	O
have	O
been	O
used	O
to	O
argue	O
for	O
a	O
spatial	O
filtering	O
hypothesis	O
,	O
which	O
proposes	O
that	O
the	O
N2pc	B-Algorithm
reflects	O
the	O
process	O
of	O
ignoring	O
task-irrelevant	O
(	O
i.e.	O
,	O
non-target	O
)	O
stimuli	O
.	O
</s>
<s>
Early	O
investigations	O
of	O
the	O
N2pc	B-Algorithm
critically	O
found	O
that	O
the	O
component	O
was	O
sensitive	O
to	O
the	O
presence	O
of	O
distractors	O
,	O
appearing	O
only	O
when	O
distractors	O
accompanied	O
a	O
target	O
stimulus	O
.	O
</s>
<s>
Furthermore	O
,	O
N2pc	B-Algorithm
amplitude	O
increases	O
with	O
the	O
number	O
of	O
distractors	O
in	O
the	O
display	O
.	O
</s>
<s>
The	O
N2pc	B-Algorithm
also	O
disappears	O
when	O
targets	O
in	O
the	O
visual	O
search	O
task	O
are	O
defined	O
as	O
"	O
any	O
oddball	O
object	O
"	O
rather	O
than	O
by	O
one	O
or	O
more	O
specific	O
features	O
.	O
</s>
<s>
Consequently	O
,	O
the	O
spatial	O
filtering	O
process	O
is	O
discouraged	O
,	O
and	O
the	O
N2pc	B-Algorithm
therefore	O
does	O
not	O
appear	O
.	O
</s>
<s>
The	O
N2pc	B-Algorithm
literature	O
agrees	O
on	O
a	O
few	O
functional	O
characteristics	O
of	O
the	O
N2pc	B-Algorithm
.	O
</s>
<s>
First	O
,	O
the	O
N2pc	B-Algorithm
appears	O
whenever	O
a	O
person	O
focuses	O
attention	O
on	O
an	O
object	O
.	O
</s>
<s>
Finally	O
,	O
the	O
N2pc	B-Algorithm
is	O
generally	O
believed	O
to	O
be	O
tied	O
to	O
a	O
spatial	O
filtering	O
hypothesis	O
(	O
see	O
above	O
:	O
"	O
Eliminating	O
the	O
N2pc	B-Algorithm
"	O
)	O
.	O
</s>
<s>
The	O
last	O
point	O
regarding	O
the	O
functional	O
significance	O
of	O
the	O
N2pc	B-Algorithm
,	O
however	O
,	O
has	O
been	O
challenged	O
.	O
</s>
<s>
Some	O
have	O
contested	O
the	O
spatial	O
filtering	O
hypothesis	O
,	O
arguing	O
that	O
the	O
N2pc	B-Algorithm
reflects	O
an	O
enhancement	O
of	O
task-relevant	O
stimulus	O
processing	O
rather	O
than	O
a	O
suppression	O
of	O
irrelevant	O
stimuli	O
.	O
</s>
<s>
Other	O
work	O
has	O
explored	O
further	O
cognitive	O
processes	O
that	O
could	O
be	O
linked	O
to	O
the	O
N2pc	B-Algorithm
.	O
</s>
<s>
For	O
instance	O
,	O
the	O
classic	O
visual	O
search	O
paradigm	O
that	O
elicits	O
the	O
N2pc	B-Algorithm
could	O
be	O
broken	O
down	O
further	O
into	O
processes	O
of	O
shifting	O
attention	O
,	O
and	O
spatially	O
based	O
processing	O
of	O
non-target	O
locations	O
.	O
</s>
<s>
When	O
combining	O
the	O
visual	O
search	O
task	O
with	O
visual	O
cues	O
that	O
drew	O
attention	O
to	O
spatial	O
locations	O
in	O
the	O
display	O
,	O
experimenters	O
found	O
that	O
while	O
the	O
N2pc	B-Algorithm
may	O
not	O
reflect	O
shifts	O
of	O
attention	O
,	O
it	O
may	O
still	O
reflect	O
processing	O
of	O
a	O
location	O
in	O
space	O
that	O
may	O
or	O
may	O
not	O
contain	O
a	O
target	O
.	O
</s>
