<s>
The	O
N400	B-Algorithm
is	O
a	O
component	O
of	O
time-locked	O
EEG	B-Application
signals	O
known	O
as	O
event-related	B-Algorithm
potentials	I-Algorithm
(	O
ERP	O
)	O
.	O
</s>
<s>
The	O
N400	B-Algorithm
is	O
part	O
of	O
the	O
normal	O
brain	O
response	O
to	O
words	O
and	O
other	O
meaningful	O
(	O
or	O
potentially	O
meaningful	O
)	O
stimuli	O
,	O
including	O
visual	O
and	O
auditory	O
words	O
,	O
sign	O
language	O
signs	O
,	O
pictures	O
,	O
faces	O
,	O
environmental	O
sounds	O
,	O
and	O
smells	O
.	O
</s>
<s>
The	O
N400	B-Algorithm
was	O
first	O
discovered	O
by	O
Marta	O
Kutas	O
and	O
Steven	O
Hillyard	O
in	O
1980	O
.	O
</s>
<s>
They	O
conducted	O
the	O
first	O
experiment	O
looking	O
at	O
the	O
response	O
to	O
unexpected	O
words	O
in	O
read	O
sentences	O
,	O
expecting	O
to	O
elicit	O
a	O
P300	B-Algorithm
component	O
.	O
</s>
<s>
The	O
P300	B-Algorithm
had	O
previously	O
been	O
shown	O
to	O
be	O
elicited	O
by	O
unexpected	O
stimuli	O
.	O
</s>
<s>
Kutas	O
and	O
Hillyard	O
therefore	O
used	O
sentences	O
with	O
anomalous	O
endings	O
(	O
i.e.I	O
take	O
coffee	O
with	O
cream	O
and	O
dog	O
)	O
,	O
expecting	O
to	O
see	O
a	O
P300	B-Algorithm
to	O
the	O
unexpected	O
sentence-final	O
words	O
.	O
</s>
<s>
She	O
put	O
on	O
her	O
high-heeled	O
SHOES	O
)	O
elicited	O
a	O
P300	B-Algorithm
instead	O
of	O
negativity	O
in	O
the	O
N400	B-Algorithm
window	O
.	O
</s>
<s>
This	O
finding	O
showed	O
that	O
the	O
N400	B-Algorithm
is	O
related	O
to	O
semantic	O
processing	O
,	O
and	O
is	O
not	O
just	O
a	O
response	O
to	O
unexpected	O
words	O
.	O
</s>
<s>
The	O
N400	B-Algorithm
is	O
characterized	O
by	O
a	O
distinct	O
pattern	O
of	O
electrical	O
activity	O
that	O
can	O
be	O
observed	O
at	O
the	O
scalp	O
.	O
</s>
<s>
This	O
latency	O
(	O
delay	O
between	O
stimulus	O
and	O
response	O
)	O
is	O
very	O
stable	O
across	O
tasks	O
—	O
little	O
else	O
besides	O
age	B-Operating_System
affects	O
the	O
timing	O
of	O
its	O
peak	O
.	O
</s>
<s>
The	O
N400	B-Algorithm
is	O
a	O
negative	O
component	O
,	O
relative	O
to	O
reference	O
electrodes	O
placed	O
on	O
the	O
mastoid	O
processes	O
(	O
the	O
bony	O
ridge	O
behind	O
the	O
ear	O
)	O
,	O
and	O
relative	O
to	O
a	O
100	O
ms	O
pre-stimulus	O
baseline	O
.	O
</s>
<s>
Its	O
amplitude	B-Application
can	O
range	O
from	O
-5	O
to	O
5	O
microvolts	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
important	O
to	O
note	O
that	O
in	O
studies	O
using	O
the	O
N400	B-Algorithm
as	O
a	O
dependent	O
measure	O
,	O
the	O
relative	O
amplitude	B-Application
of	O
the	O
waveform	O
compared	O
to	O
another	O
experimental	O
condition	O
(	O
the	O
"	O
N400	B-Algorithm
effect	O
"	O
)	O
is	O
more	O
important	O
than	O
its	O
absolute	O
amplitude	B-Application
.	O
</s>
<s>
The	O
N400	B-Algorithm
itself	O
is	O
not	O
always	O
negative	O
—	O
it	O
is	O
just	O
a	O
more	O
negative-going	O
deflection	O
than	O
that	O
seen	O
to	O
other	O
conditions	O
.	O
</s>
<s>
A	O
typical	O
experiment	O
designed	O
to	O
study	O
the	O
N400	B-Algorithm
will	O
usually	O
involve	O
the	O
visual	O
presentation	O
of	O
words	O
,	O
either	O
in	O
sentence	O
or	O
list	O
contexts	O
.	O
</s>
<s>
In	O
a	O
typical	O
visual	O
N400	B-Algorithm
experiment	O
,	O
for	O
example	O
,	O
subjects	O
will	O
be	O
seated	O
in	O
front	O
of	O
a	O
computer	B-Device
monitor	I-Device
while	O
words	O
are	O
presented	O
one-by-one	O
at	O
a	O
central	O
screen	O
location	O
.	O
</s>
<s>
Stimuli	O
must	O
be	O
presented	O
centrally	O
because	O
eye	O
movements	O
will	O
generate	O
large	O
amounts	O
of	O
electrical	O
noise	O
that	O
will	O
mask	O
the	O
relatively	O
small	O
N400	B-Algorithm
component	O
.	O
</s>
<s>
Note	O
,	O
however	O
,	O
that	O
overt	O
responses	O
by	O
the	O
subject	O
are	O
not	O
required	O
to	O
elicit	O
the	O
N400	B-Algorithm
—	O
passively	O
viewing	O
stimuli	O
will	O
still	O
evoke	O
this	O
response	O
.	O
</s>
<s>
An	O
example	O
of	O
an	O
experimental	O
task	O
used	O
to	O
study	O
the	O
N400	B-Algorithm
is	O
a	O
priming	O
paradigm	O
.	O
</s>
<s>
The	O
N400	B-Algorithm
amplitude	B-Application
seen	O
to	O
the	O
target	O
word	O
(	O
honey	O
)	O
will	O
be	O
reduced	O
upon	O
repetition	O
due	O
to	O
semantic	O
priming	O
.	O
</s>
<s>
The	O
amount	O
of	O
reduction	O
in	O
amplitude	B-Application
can	O
be	O
used	O
to	O
measure	O
the	O
degree	O
of	O
relatedness	O
between	O
the	O
words	O
.	O
</s>
<s>
Another	O
widely	O
used	O
experimental	O
task	O
used	O
to	O
study	O
the	O
N400	B-Algorithm
is	O
sentence	O
reading	O
.	O
</s>
<s>
Experimenters	O
can	O
choose	O
to	O
manipulate	O
various	O
linguistic	O
characteristics	O
of	O
the	O
sentences	O
,	O
including	O
contextual	O
constraint	O
or	O
the	O
cloze	O
probability	O
of	O
the	O
sentence-final	O
word	O
(	O
see	O
below	O
for	O
a	O
definition	O
of	O
cloze	O
probability	O
)	O
to	O
observe	O
how	O
these	O
changes	O
affect	O
the	O
waveform	O
's	O
amplitude	B-Application
.	O
</s>
<s>
As	O
previously	O
mentioned	O
,	O
the	O
N400	B-Algorithm
response	O
is	O
seen	O
to	O
all	O
meaningful	O
,	O
or	O
potentially	O
meaningful	O
,	O
stimuli	O
.	O
</s>
<s>
Experiments	O
involving	O
the	O
presentation	O
of	O
spoken	O
words	O
,	O
acronyms	O
,	O
pictures	O
embedded	O
at	O
the	O
end	O
of	O
sentences	O
,	O
music	O
,	O
words	O
related	O
to	O
current	O
context	O
or	O
orientation	O
and	O
videos	O
of	O
real-word	O
events	O
,	O
have	O
all	O
been	O
used	O
to	O
study	O
the	O
N400	B-Algorithm
,	O
just	O
to	O
name	O
a	O
few	O
.	O
</s>
<s>
Extensive	O
research	O
has	O
been	O
carried	O
out	O
to	O
better	O
understand	O
what	O
kinds	O
of	O
experimental	O
manipulations	O
do	O
and	O
do	O
not	O
affect	O
the	O
N400	B-Algorithm
.	O
</s>
<s>
The	O
frequency	O
of	O
a	O
word	O
's	O
usage	O
is	O
known	O
to	O
affect	O
the	O
amplitude	B-Application
of	O
the	O
N400	B-Algorithm
.	O
</s>
<s>
With	O
all	O
else	O
being	O
constant	O
,	O
highly	O
frequent	O
words	O
elicit	O
reduced	O
N400s	B-Algorithm
relative	O
to	O
infrequent	O
words	O
.	O
</s>
<s>
As	O
previously	O
mentioned	O
,	O
N400	B-Algorithm
amplitude	B-Application
is	O
also	O
reduced	O
by	O
repetition	O
,	O
such	O
that	O
a	O
word	O
's	O
second	O
presentation	O
exhibits	O
a	O
more	O
positive	O
response	O
when	O
repeated	O
in	O
context	O
.	O
</s>
<s>
These	O
findings	O
suggest	O
that	O
when	O
a	O
word	O
is	O
highly	O
frequent	O
or	O
has	O
recently	O
appeared	O
in	O
context	O
,	O
it	O
eases	O
the	O
semantic	O
processing	O
thought	O
to	O
be	O
indexed	O
by	O
the	O
N400	B-Algorithm
,	O
reducing	O
its	O
amplitude	B-Application
.	O
</s>
<s>
N400	B-Algorithm
amplitude	B-Application
is	O
also	O
sensitive	O
to	O
a	O
word	O
's	O
orthographic	O
neighborhood	O
size	O
,	O
or	O
how	O
many	O
other	O
words	O
differ	O
from	O
it	O
by	O
only	O
one	O
letter	O
(	O
e.g.	O
</s>
<s>
Words	O
with	O
large	O
neighborhoods	O
(	O
that	O
have	O
many	O
other	O
physically	O
similar	O
items	O
)	O
elicit	O
larger	O
N400	B-Algorithm
amplitudes	B-Application
than	O
do	O
words	O
with	O
small	O
neighborhoods	O
.	O
</s>
<s>
This	O
has	O
been	O
taken	O
as	O
evidence	O
that	O
the	O
N400	B-Algorithm
reflects	O
general	O
activation	O
in	O
the	O
comprehension	O
network	O
,	O
such	O
that	O
an	O
item	O
that	O
looks	O
like	O
many	O
words	O
(	O
regardless	O
of	O
whether	O
it	O
itself	O
is	O
a	O
word	O
)	O
partially	O
activates	O
the	O
representations	O
of	O
similar-looking	O
words	O
,	O
producing	O
a	O
more	O
negative	O
N400	B-Algorithm
.	O
</s>
<s>
The	O
N400	B-Algorithm
is	O
sensitive	O
to	O
priming	O
:	O
in	O
other	O
words	O
,	O
its	O
amplitude	B-Application
is	O
reduced	O
when	O
a	O
target	O
word	O
is	O
preceded	O
by	O
a	O
word	O
that	O
is	O
semantically	O
,	O
morphologically	O
,	O
or	O
orthographically	O
related	O
to	O
it	O
.	O
</s>
<s>
In	O
a	O
sentence	O
context	O
,	O
an	O
important	O
determinant	O
of	O
N400	B-Algorithm
amplitude	B-Application
elicited	O
by	O
a	O
word	O
is	O
its	O
cloze	O
probability	O
.	O
</s>
<s>
Kutas	O
and	O
Hillyard	O
(	O
1984	O
)	O
found	O
that	O
a	O
word	O
's	O
N400	B-Algorithm
amplitude	B-Application
has	O
a	O
nearly	O
inverse	O
linear	O
relationship	O
with	O
its	O
cloze	O
probability	O
.	O
</s>
<s>
That	O
is	O
,	O
as	O
a	O
word	O
becomes	O
less	O
expected	O
in	O
context	O
,	O
its	O
N400	B-Algorithm
amplitude	B-Application
is	O
increased	O
relative	O
to	O
more	O
expected	O
words	O
.	O
</s>
<s>
Words	O
that	O
are	O
incongruent	O
with	O
a	O
context	O
(	O
and	O
thus	O
have	O
a	O
cloze	O
probability	O
of	O
0	O
)	O
elicit	O
large	O
N400	B-Algorithm
amplitudes	B-Application
as	O
well	O
(	O
although	O
the	O
amplitude	B-Application
of	O
the	O
N400	B-Algorithm
for	O
incongruent	O
words	O
is	O
also	O
modulated	O
by	O
the	O
cloze	O
probability	O
of	O
the	O
congruent	O
word	O
that	O
would	O
have	O
been	O
expected	O
in	O
its	O
place	O
Relatedly	O
,	O
the	O
N400	B-Algorithm
amplitude	B-Application
elicited	O
by	O
open-class	O
words	O
(	O
i.e.	O
</s>
<s>
Taken	O
together	O
,	O
these	O
findings	O
suggest	O
that	O
when	O
the	O
prior	O
context	O
builds	O
up	O
meaning	O
,	O
it	O
makes	O
the	O
processing	O
of	O
upcoming	O
words	O
that	O
fit	O
with	O
that	O
context	O
easier	O
,	O
reducing	O
the	O
N400	B-Algorithm
amplitude	B-Application
they	O
elicit	O
.	O
</s>
<s>
While	O
the	O
N400	B-Algorithm
is	O
larger	O
to	O
unexpected	O
items	O
at	O
the	O
end	O
of	O
a	O
sentence	O
,	O
its	O
amplitude	B-Application
is	O
generally	O
unaffected	O
by	O
negation	O
that	O
causes	O
the	O
last	O
word	O
to	O
be	O
untrue	O
and	O
thus	O
anomalous	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
sentence	O
A	O
sparrow	O
is	O
a	O
building	O
,	O
the	O
N400	B-Algorithm
response	O
to	O
building	O
is	O
more	O
negative	O
than	O
the	O
N400	B-Algorithm
response	O
to	O
bird	O
in	O
the	O
sentence	O
A	O
sparrow	O
is	O
a	O
bird	O
.	O
</s>
<s>
A	O
sparrow	O
is	O
not	O
a	O
building	O
and	O
A	O
sparrow	O
is	O
not	O
a	O
bird	O
)	O
,	O
the	O
N400	B-Algorithm
amplitude	B-Application
to	O
building	O
will	O
still	O
be	O
more	O
negative	O
than	O
that	O
seen	O
to	O
bird	O
.	O
</s>
<s>
This	O
suggests	O
that	O
the	O
N400	B-Algorithm
responds	O
to	O
the	O
relationship	O
between	O
words	O
in	O
context	O
,	O
but	O
is	O
not	O
necessarily	O
sensitive	O
to	O
the	O
sentence	O
's	O
truth	O
value	O
.	O
</s>
<s>
More	O
recent	O
research	O
,	O
however	O
,	O
has	O
demonstrated	O
that	O
the	O
N400	B-Algorithm
can	O
sometimes	O
be	O
modulated	O
by	O
quantifiers	O
or	O
adjectives	O
that	O
serve	O
negation-like	O
purposes	O
,	O
or	O
by	O
pragmatically	O
licensed	O
negation	O
.	O
</s>
<s>
Additionally	O
,	O
grammatical	O
violations	O
do	O
not	O
elicit	O
a	O
large	O
N400	B-Algorithm
response	O
.	O
</s>
<s>
Rather	O
,	O
these	O
types	O
of	O
violations	O
show	O
a	O
large	O
positivity	O
from	O
about	O
500-1000	O
ms	O
after	O
stimulus	O
onset	O
,	O
known	O
as	O
the	O
P600	B-Algorithm
.	O
</s>
<s>
A	O
striking	O
feature	O
of	O
the	O
N400	B-Algorithm
is	O
the	O
general	O
invariance	O
of	O
its	O
peak	O
latency	O
.	O
</s>
<s>
Although	O
many	O
different	O
experimental	O
manipulations	O
affect	O
the	O
amplitude	B-Application
of	O
the	O
N400	B-Algorithm
,	O
few	O
factors	O
(	O
aging	O
and	O
disease	O
states	O
and	O
language	O
proficiency	O
being	O
rare	O
examples	O
)	O
alter	O
the	O
time	O
it	O
takes	O
for	O
the	O
N400	B-Algorithm
component	O
to	O
reach	O
a	O
peak	O
amplitude	B-Application
.	O
</s>
