<s>
EEG	B-Algorithm
microstates	I-Algorithm
are	O
transient	O
,	O
patterned	O
,	O
quasi-stable	O
states	O
or	O
patterns	O
of	O
an	O
electroencephalogram	B-Application
.	O
</s>
<s>
Microstate	O
estimation	O
and	O
analysis	O
was	O
originally	O
done	O
using	O
alpha	O
band	O
activity	O
,	O
though	O
broader	O
bandwidth	O
EEG	B-Application
bands	O
are	O
now	O
typically	O
used	O
.	O
</s>
<s>
The	O
quasi-stability	O
of	O
microstates	O
means	O
that	O
the	O
"	O
global	O
 [ EEG ] 	O
topography	O
is	O
fixed	O
,	O
but	O
strength	O
might	O
vary	O
and	O
polarity	O
invert.	O
"	O
</s>
<s>
Thomas	O
Koenig	O
(	O
University	O
Hospital	O
of	O
Psychiatry	O
,	O
Switzerland	O
)	O
and	O
Dietrich	O
Lehmann	O
(	O
KEY	O
Institute	O
for	O
Brain-Mind	O
Research	O
,	O
Switzerland	O
)	O
are	O
often	O
credited	O
as	O
the	O
pioneers	O
of	O
EEG	B-Application
Microstate	O
analysis	O
.	O
</s>
<s>
In	O
their	O
1999	O
paper	O
in	O
the	O
European	O
Archives	O
of	O
Psychiatry	O
and	O
Clinical	O
Neuroscience	O
,	O
Koenig	O
and	O
Lehmann	O
had	O
been	O
analyzing	O
the	O
EEGs	B-Application
of	O
those	O
with	O
schizophrenia	B-Application
,	O
in	O
order	O
to	O
investigate	O
the	O
potential	O
basic	O
cognitive	O
roots	O
of	O
the	O
disorder	O
.	O
</s>
<s>
They	O
began	O
to	O
turn	O
their	O
attention	O
to	O
the	O
EEGs	B-Application
on	O
a	O
millisecond	O
scale	O
.	O
</s>
<s>
They	O
determined	O
that	O
both	O
normal	O
subjects	O
and	O
those	O
with	O
schizophrenia	B-Application
shared	O
these	O
microstates	O
,	O
but	O
they	O
varied	O
in	O
characteristics	O
between	O
the	O
two	O
groups	O
,	O
and	O
concluded	O
that	O
:	O
</s>
<s>
Isolating	O
and	O
analyzing	O
one	O
's	O
EEG	B-Application
microstate	O
sequence	O
is	O
a	O
post-hoc	O
operation	O
that	O
typically	O
utilizes	O
several	O
averaging	O
and	O
filtering	O
steps	O
.	O
</s>
<s>
When	O
Koenig	O
and	O
Lehman	O
ran	O
their	O
experiment	O
in	O
1999	O
they	O
constructed	O
these	O
sequences	O
by	O
starting	O
from	O
a	O
subject	O
's	O
eyes-closed	O
resting	B-Algorithm
state	I-Algorithm
EEG	B-Application
.	O
</s>
<s>
The	O
first	O
several	O
event-free	O
minutes	O
of	O
the	O
EEG	B-Application
were	O
isolated	O
,	O
then	O
periods	O
of	O
around	O
2	O
seconds	O
each	O
were	O
refiltered	O
(	O
Band-pass	O
≈	O
2	O
–	O
20Hz	O
)	O
.	O
</s>
<s>
Once	O
the	O
epochs	O
were	O
filtered	O
,	O
these	O
microstates	O
were	O
analytically	O
clustered	O
into	O
mean	O
classes	O
via	O
k-means	B-Algorithm
clustering	I-Algorithm
,	O
post	O
hoc	O
.	O
</s>
<s>
Since	O
the	O
brain	O
goes	O
through	O
so	O
many	O
transformations	O
in	O
such	O
short	O
time	O
scales	O
,	O
microstate	O
analysis	O
is	O
essentially	O
an	O
analysis	O
of	O
average	O
EEG	B-Application
states	O
.	O
</s>
<s>
Koenig	O
and	O
Lehmann	O
set	O
the	O
standard	O
for	O
creating	O
classes	O
,	O
or	O
recurrent	O
averaged	O
EEG	B-Application
configurations	O
.	O
</s>
<s>
Once	O
all	O
the	O
EEG	B-Application
data	O
is	O
collected	O
,	O
a	O
"	O
prototype	O
"	O
EEG	B-Application
segment	O
is	O
chosen	O
,	O
with	O
which	O
to	O
compare	O
all	O
other	O
collected	O
microstates	O
.	O
</s>
<s>
"	O
Similarity	O
of	O
EEG	B-Application
spatial	O
configuration	O
of	O
each	O
prototype	O
map	O
with	O
each	O
of	O
the	O
10	O
maps	O
is	O
computed	O
using	O
the	O
coefficient	O
of	O
determination	O
to	O
omit	O
the	O
maps	O
 '	O
polarities	O
.	O
</s>
<s>
Separately	O
...	O
for	O
each	O
class	O
the	O
prototype	O
maps	O
are	O
updated	O
combining	O
all	O
assigned	O
maps	O
by	O
computing	O
the	O
first	O
spatial	O
principal	B-Application
component	I-Application
of	O
the	O
maps	O
and	O
thereby	O
maximizing	O
the	O
common	O
variance	O
while	O
disregarding	O
the	O
map	O
polarity.	O
"	O
</s>
<s>
However	O
,	O
many	O
studies	O
have	O
also	O
found	O
other	O
EEG	B-Application
microstate	O
template	O
maps	O
that	O
are	O
likely	O
to	O
be	O
meaningful	O
.	O
</s>
<s>
It	O
is	O
the	O
current	O
hypothesis	O
that	O
EEG	B-Algorithm
Microstates	I-Algorithm
represent	O
the	O
basic	O
steps	O
of	O
cognition	O
and	O
neural	O
information	O
processing	O
in	O
the	O
brain	O
,	O
but	O
there	O
is	O
still	O
much	O
research	O
that	O
needs	O
to	O
be	O
done	O
to	O
cement	O
this	O
theory	O
.	O
</s>
<s>
This	O
study	O
investigated	O
EEG	B-Application
Microstate	O
variance	O
across	O
normal	O
humans	O
of	O
varying	O
ages	O
.	O
</s>
<s>
Assuming	O
that	O
asymmetric	O
microstates	O
result	O
from	O
predominantly	O
unilateral	O
brain	B-Application
activity	I-Application
,	O
while	O
symmetric	O
microstates	O
indicate	O
predominantly	O
bilateral	O
activity	O
,	O
the	O
observed	O
effects	O
may	O
be	O
related	O
to	O
the	O
growth	O
of	O
the	O
corpus	O
callosum	O
,	O
which	O
continues	O
until	O
late	O
adolescence	O
(	O
e.g.	O
,	O
Giedd	O
et	O
al.	O
,	O
1999	O
)	O
.	O
"	O
</s>
<s>
In	O
a	O
study	O
conducted	O
by	O
researchers	O
in	O
Geneva	O
,	O
the	O
temporal	O
dynamics	O
and	O
possible	O
fractal	B-Application
properties	O
of	O
EEG	B-Algorithm
microstates	I-Algorithm
were	O
analyzed	O
in	O
normal	O
human	O
subjects	O
.	O
</s>
<s>
Since	O
microstates	O
are	O
a	O
global	O
topography	O
,	O
but	O
occur	O
on	O
such	O
small	O
time	O
scales	O
and	O
change	O
so	O
rapidly	O
,	O
Van	O
De	O
Ville	O
,	O
Britz	O
,	O
and	O
Michel	O
hypothesized	O
that	O
these	O
"	O
atoms	O
of	O
thoughts	O
"	O
are	O
fractal-like	O
in	O
the	O
temporal	O
dimension	O
.	O
</s>
<s>
That	O
is	O
,	O
whether	O
scaled-up	O
or	O
scaled-down	O
,	O
an	O
EEG	B-Application
is	O
itself	O
a	O
composition	O
of	O
microstates	O
.	O
</s>
<s>
This	O
hypothesis	O
was	O
initially	O
illuminated	O
by	O
the	O
strong	O
correlation	O
between	O
the	O
rapid	O
time	O
scale	O
and	O
transience	O
of	O
EEG	B-Algorithm
microstates	I-Algorithm
and	O
the	O
much	O
slower	O
signals	O
of	O
a	O
resting	B-Algorithm
state	I-Algorithm
fMRI	I-Algorithm
.	O
</s>
<s>
"	O
The	O
connection	O
between	O
EEG	B-Algorithm
microstates	I-Algorithm
and	O
fMRI	B-Algorithm
resting	B-Algorithm
state	I-Algorithm
networks	O
(	O
RSNs	O
)	O
was	O
established	O
by	O
convolving	O
the	O
time	O
courses	O
of	O
the	O
occurrence	O
of	O
the	O
different	O
EEG	B-Algorithm
microstates	I-Algorithm
with	O
the	O
hemodynamic	O
response	O
function	O
(	O
HRF	O
)	O
and	O
then	O
using	O
these	O
as	O
regressors	O
in	O
a	O
general	O
linear	O
model	O
for	O
conventional	O
fMRI	B-Algorithm
analysis	O
.	O
</s>
<s>
Because	O
the	O
HRF	O
acts	O
as	O
a	O
strong	O
temporal	O
smoothing	O
filter	O
on	O
the	O
rapid	O
EEG-based	O
signal	O
,	O
it	O
is	O
remarkable	O
that	O
statistically	O
significant	O
correlations	O
can	O
be	O
found	O
.	O
</s>
<s>
The	O
fact	O
that	O
this	O
smoothing	O
did	O
not	O
remove	O
any	O
information-carrying	O
signal	O
from	O
the	O
microstate	O
sequence	O
and	O
that	O
furthermore	O
the	O
original	O
microstate	O
sequences	O
and	O
the	O
regressors	O
show	O
the	O
same	O
relative	O
behavior	O
at	O
temporal	O
scales	O
about	O
two	O
orders	O
of	O
magnitude	O
apart	O
suggests	O
that	O
the	O
time	O
courses	O
of	O
the	O
EEG	B-Algorithm
microstates	I-Algorithm
are	O
scale	O
invariant.	O
"	O
</s>
<s>
This	O
scale-invariant	O
dynamic	O
is	O
the	O
strongest	O
characteristic	O
of	O
a	O
fractal	B-Application
,	O
and	O
since	O
microstates	O
are	O
indicative	O
of	O
global	O
neuronal	O
networks	O
,	O
it	O
is	O
justifiable	O
to	O
conclude	O
that	O
these	O
microstates	O
exhibit	O
temporally	O
monofractal	O
(	O
one-dimensional	O
fractal	B-Application
)	O
behavior	O
.	O
</s>
<s>
From	O
here	O
we	O
can	O
see	O
the	O
possibility	O
that	O
fMRI	B-Algorithm
,	O
which	O
is	O
also	O
a	O
global	O
topography	O
measure	O
,	O
is	O
possibly	O
just	O
a	O
scaled-up	O
manifestation	O
of	O
its	O
microstates	O
,	O
and	O
thus	O
further	O
supports	O
the	O
hypothesis	O
that	O
EEG	B-Algorithm
microstates	I-Algorithm
are	O
the	O
fundamental	O
unit	O
of	O
one	O
's	O
global	O
cognitive	O
processing	O
.	O
</s>
<s>
Comparing	O
the	O
EEG	B-Application
microstate	O
classes	O
between	O
controls	O
and	O
those	O
with	O
psychosis	B-Application
has	O
yielded	O
important	O
results	O
,	O
suggesting	O
that	O
the	O
basic	O
resting-state	O
of	O
those	O
with	O
psychosis	B-Application
is	O
irregular	O
.	O
</s>
<s>
Numerous	O
studies	O
have	O
investigated	O
the	O
temporal	O
dynamics	O
of	O
EEG	B-Algorithm
microstates	I-Algorithm
in	O
people	O
with	O
schizophrenia	B-Application
.	O
</s>
<s>
In	O
the	O
first	O
study	O
comparing	O
the	O
temporal	O
dynamics	O
of	O
the	O
EEG	B-Algorithm
microstates	I-Algorithm
in	O
those	O
with	O
schizophrenia	B-Application
against	O
healthy	O
controls	O
,	O
Koenig	O
and	O
Lehmann	O
reported	O
that	O
those	O
with	O
schizophrenia	B-Application
tend	O
to	O
spend	O
too	O
much	O
time	O
in	O
microstate	O
Class	O
A	O
compared	O
to	O
controls	O
.	O
</s>
<s>
However	O
,	O
other	O
studies	O
in	O
schizophrenia	B-Application
research	O
have	O
suggested	O
a	O
different	O
picture	O
.	O
</s>
<s>
A	O
meta-analysis	O
comprising	O
studies	O
from	O
1999	O
to	O
2015	O
revealed	O
that	O
microstate	O
class	O
C	O
occurred	O
more	O
frequently	O
and	O
for	O
longer	O
durations	O
in	O
those	O
with	O
schizophrenia	B-Application
than	O
controls	O
,	O
while	O
microstate	O
class	O
D	O
occurred	O
less	O
frequently	O
and	O
for	O
shorter	O
durations	O
.	O
</s>
<s>
Similar	O
abnormalities	O
were	O
reported	O
in	O
a	O
study	O
with	O
adolescents	O
with	O
22q11.2	O
deletion	O
syndrome	O
,	O
a	O
population	O
that	O
has	O
a	O
30%	O
risk	O
of	O
developing	O
psychosis	B-Application
.	O
</s>
<s>
Classes	O
C	O
and	O
D	O
abnormalities	O
were	O
also	O
found	O
in	O
unaffected	O
siblings	O
of	O
those	O
with	O
schizophrenia	B-Application
,	O
which	O
prompted	O
the	O
authors	O
to	O
suggest	O
that	O
dynamics	O
of	O
microstates	O
C	O
and	O
D	O
are	O
a	O
candidate	O
endophenotype	O
for	O
schizophrenia	B-Application
.	O
</s>
<s>
They	O
found	O
that	O
these	O
people	O
spent	O
too	O
much	O
time	O
in	O
the	O
same	O
right-anterior	O
to	O
left-posterior	O
microstate	O
as	O
in	O
the	O
schizophrenia	B-Application
studies	O
.	O
</s>
<s>
This	O
suggests	O
temporal	O
lobe	O
malfunction	O
,	O
which	O
has	O
been	O
reported	O
in	O
fMRI	B-Algorithm
studies	O
of	O
those	O
with	O
PD	O
;	O
they	O
spent	O
an	O
average	O
of	O
9.26	O
milliseconds	O
longer	O
in	O
this	O
microstate	O
than	O
did	O
control	O
subjects	O
.	O
</s>
<s>
These	O
aberrant	O
microstate	O
sequences	O
are	O
very	O
similar	O
to	O
those	O
in	O
the	O
schizophrenia	B-Application
study	O
,	O
and	O
as	O
anxiety	O
is	O
commonly	O
found	O
in	O
schizophrenia	B-Application
,	O
it	O
may	O
indicate	O
a	O
strong	O
correlation	O
between	O
different	O
severities	O
of	O
neurological	O
pathologies	O
and	O
a	O
person	O
's	O
microstate	O
sequence	O
.	O
</s>
<s>
This	O
study	O
illuminates	O
the	O
complexity	O
of	O
brain	B-Application
activity	I-Application
and	O
EEG	B-Application
dynamics	O
.	O
</s>
<s>
Another	O
claim	O
is	O
that	O
longer	O
periods	O
of	O
stable	O
brain	B-Application
activity	I-Application
may	O
be	O
handling	O
smaller	O
amounts	O
of	O
information	O
processing	O
,	O
and	O
thus	O
few	O
changes	O
in	O
microstates	O
,	O
while	O
shorter	O
,	O
less-stable	O
brain	B-Application
activity	I-Application
may	O
reflect	O
large	O
amounts	O
of	O
different	O
information	O
to	O
process	O
,	O
and	O
thus	O
more	O
microstate	O
changes	O
.	O
</s>
