Hemodynamic changes in response to the stimulated visual quadrants : A study with 24-channel near-infrared spectroscopy(<Special Edition>Brain Imaging and Psychonomic Science)

TheJbPaneseJouraatefPsychenemicScience

2009,Vol.2S,Nn.1,35-43

Original

Artiele

Hemodynamic

changes

A

study

within

response

to

the

stimulated

visual

_quadrants

24-channel

near-infrared

spectroscopy

Shuichiro

TAyA*,

Goro

MAEHARA*2,

and

Haruyuki

KoJiMA*3

Universit):

of Surrey*,

McCill

Uitiversdy,*2

and Kanazawa

U)ziverisly*3

Near-infrared

spectroscopy

(NIRS)

is

attracting _grewing

interest

as a _powerful tool

for

monitQring cortical activation associated with various _{psychological phenomena.}

Many

NIRS

studies have aimed toexplore brain

functions

associated with visual _perception. However, how

NIRS

can rnonitor hemodynamic responses inthe visual cortex corresponding _tostirnulation of

each visual quadrant

is

not well

known.

Here

we measured changes

in

concentration of

oxygen-ated hernoglobin and

deoxygenated

hemoglebin

'in

the

human

visual certex with a 24-ch

NIRS

instrumcnt.

Through

individual

stimulatjon of visual _quadrants we found that NIRS could

differentiallymonitor activation of theleftand right hemisphere when the Lower visual fieldwas

stimulated,

but

hardly

detected

activation of

both

hemispheres when the upper visual fieldwas stimulated.

The

result of

fMRI

scans using the same stimuli suggests

that

the

different

measure-ment responses toupper- and lower visual fieldstimulation are caused by the differencesinthe

depth

from

the sca]p of the region representing each visual fieid.

On

the

basis

of the _present resu]ts, we

discuss

the

limitations

and potential of

NIRS

measurements.

Key

words:near-infrared spectroscopy, visual cortex,retinotopy,

fMRJ,

brain

imaging

Introduction

Near-infrared spectroscopy

(NIRS)

isa relatively

new

brain

imaging

technique, which measures

rela-tivechanges inthe hemoglobin concentratjon

(for

a review, see VMringer & Chance, 1997; Strangman,

Boas,

&

Sutton,

2002},

The

use of

NIRS

has

scveral

disti,nct

ad vantages compared with other brain

imag-ining

techniques. such as

functional

magnetic

reso-nance

irnaging

(iMRI)

and _positron emission

tomo-graphy

(PET),

Firstly,NIRS can separately rneasure

the changes

in

the concentration of oxygenated

he-moglobin

(oxy-Hb)

and deoxygenated hemoglobin

{deoxy-Hb),

whereas

fMRI

and

PET

cannot

distin-guish between thesetwo indices.Secondty, the NIRS

* _{Department of}

_Psychology,

_University

_of

Surrey, Guildford,

Surrey

GU2 7XH, UK s.taya

@surrey.ac.uk

*2 _{McGM Vision Research, McGi]t University,}

MontreaL

Quebec

II3A IAI, Canada, goro.

maehara@rnai].mcgilLca

*3 _{Department of Psychology,}

_Kanazawa

sity, Kanazawa, Ishikawa 920-1192,

Japan,

hkojima@kenroku,kanazawa-u.ac.jp

equipment isrelatively compact and

its

measure-ment

is

relatively non-invasive, which enab]es moni-toring of human cortical activity in a variety of experimentai tasks,such as those requiring

bodily

movement or with participants who mjght not be

applicable forother imaging techniques, sueh as

in-fants

or people with

developmental

disordcrs.

These

advantages might encourage _{psychologists} to

con-duct

experiments using

NIRS,

possibly combined

with _{psychophysical} methods. Those researchers

could

be

interested in the measurement of

brain

functioning

associated with visual perception, The scope of thjsarticle isto_provide them with _practica]

data

of

NIRS

measurements

in

response to visual stimulation.

A number of studies have used NIRS to monitor

braln activation of the visual cortex of adults

{Kato,

KameL Takashima, & Ozaki, 1993;Takahashi, Ogata,

Atsumi, Yamamoto, Shiotsuka, Maki, Yamashita,

Ya-mamoto,

Koizumi,

Hirasawa,

&

Igawa,

2000;

Colier,

Quareslma,

Wenzel,

van

der

Sluijs,

Oeseburg,

Fer-rari,

&

Villringer,

2eOl; Maehara, Taya,

&

Kojima,

NII-Electronic Library Service

36

The

Japanese

Journal

of Psychonomic Science VoL 28,No, 1

2005; Schroctgr. Bttcheler,MUIIer, Uludag,

Obrig,

Lohrnann,

Tittgemeyer,

Villringer,

&

von

Cramon,

2004) and infants

CMeek,

Elwell,

Khan,

Romaya,

Wyatt, Delpy,

&

ZekL 1995; Taga, Asakawa, Maki,

Konishi,

&

KoizumL

2003).

These

studies were mainly concerned with the temporal characteristics of

NIRS

measurements

in

the occipital region. Ithas

been

reported that oxy-Hb

increases

and

dieoxy-Hb

decreases

with a

lag

of

2-4

safter theonset of visual stLmulation

(Colier,

et al. 2001; Schroeter,et al,,2004;

Taga, et al.

2003].

Spatial

characteristics of NIRS

measurement

in

the occipital corLex,

however,

are

lesswe!1 understood. The retinotopic organization, L

e. the _{point-to-point}mapping of thc visual

'field

onto

the cortex,

is

one of the

fundamental

features

of the

visual cortex and

has

been

vastly examined with

fMRI

and PET

(DeYoe,

Carman,

BandettinL

GIick-man,

Wieser,

Cox,

Miller,

&

Neitz,1996; Dougherty,

Kock,

Brewcr,

Fischer,

Modersitzki,

&

Wandell,

2003;

EngeL

GIover,

& WandelL 1997; Shipp, Watson,

Frackowiak,

&

Zeki,

1995),

The goal of the _present study isto clarify the

mea$urable and immeasurable area

in

thevisual

cor-tex with NIRS.

The

measurable cortical depth with

NIRS depends on the distance between the

near-jnfrared

(NR)

light emitting-optode and

detecting-optode; i.e.the larger the interoptodc distance,the

deeper the measurable

clepth.

The optode

distance

is

about 30 mm inmost NIRS

instruments

because

this

distance

is

optirnal formeasurernents of cortica]

sur-face.

Due

tothistechnicalrestriction,

NIRS

can only rmeasure regions

located

20-30 mrn

belo"r

the scalp

(e.g.

Chance,

Zhuang, UnAh, Alter,& Lipton, 1993,

but also see ZefC

White,

Dehghani,

Schlaggar,

&

Cul-ver, 2007). However,

it

is

highly

likely

that some

parts

of the visuai cortex are

located

far

deeper

than

the NIRS sensitive depth, because the visual cortex

extends within and around the ca]carine sulcus.

Therefore,

it

t･vould

be

important

for future NIRS studies to clarify which _part of the yisual fieidis represented inthese

immeasurable

cortical areas.

1'lere

we measured the activity of theadult

human

visual cortex with a 24-channel NIRS equipment.

We monitored the changes inoxy-Hb concentration

and deoxy-Hb concentration

during

the

individual

A

B

x'

C

l-9

_cm-1

･/'

9

rt

if

2

{g)

3

ge

i

@8O9eertOOO

" 12 13 140

Olsee

ri60t7as

B

""-･.

k'228232ge'242o'

i

O

emitter

op

detector

Figure

1,

(A)

An

examp]e of a stimulus,

(B)

Stimulated visual fields.

(C)

The

mcnt of the 16 _photodiodes and the location

of the

24

measurernent channels.

White

and _gray circ]cs show the NR lighternitters

and the detectors,respectively. The digi'ts

indicate

thenumber of channels,

stimulation ofcach of the fourvisuakquadrants with a

high-contrast

flickeringchecker-pattern. In

addi-tion,we conducted

fMRI

rncasurernent with thesame

visual stimuli tolocatecortical regions

correspond-ing

to each quadrant.

Comparing

the activatjon

maps obtained with these two imaging techniques,

we will

discuss

possibilities and

lirnitations

of

NIRS,

Methods

Participants.

Twenty

healthy

adults.

including

the three authors,

(ten

males and ten femaLes;mean

age:

24.0

±

4.9

_years,range:

19-･41

years)participated

inthe expcriment. All had nermal or corrccted-to-normal vision. AII_{participant.s}were informed about

NIRS

and the _purpose of the experiment and

in-formed consent was obtaincd

from

allparticipants.

The experiments were conducted

fo]lowing

the

Dec-larationof IIelsinki.

Visual stimulation. Stimu]i were prescnted on a

22-inch

CRT

moniter with a _pixelresolution of 1024 ×768 and a vertical refresh rate of 60 Hz. The time course of the stimulus _presentation was controlled

by

a

PC.

The

participantsobserved

the

stimuli at a

S.

TAyA

et al.:

Hemodynamic

changes

in

response to thestimulatedvisual _quadrants

37

viewing distanceof 1OO cm inadark room with their

head

on a chin-rest.

The

visual stimulus was a

high-contrast

radial

checker-pa'ttern _presented against a _gray

(51

cd/m2)

background

{Figure

1A).

The

dtameter

of the check-er-pattern was

12

deg

in

visual angle. The white and

black

areas of

the

_pattern

(104

cd/m2 and

2

cd!m2, respectively) were reversed at a temporal frequency

of 7.5Hz. The checker-pattern was divided intofour sector

iorms

and appeared at

individual

quadrants of

the visual field

CFigure

IB). The sectors were sepa-rated

by

a

1

deg

gap

from

each other to prevent

stimulation of cells located of the border of the

left

and right visual

fields,

and reception of signals from adjacent quadrants

(e.g.

Fukuda,

Sawai,

Watanabe,

Wakakuwa, & Morigiwa, 1989). A red-lined O.5deg

square was _presented as a

fixation

_pointat thecenter of themonitor,

Procedure.

One

measurement session consisted of

an initial30-sresting _period and fiverepetitiens of a

stimulus sequence, comprising of a

15-s

stimulating

periodfqllowed by a 30-sresting period, Durjng the

stirnulating _period,both thechecker-pattern and the

fixation

point were _presented.

In

the resting _peried,

only the fixationpoint was presented, Participants

were instructed tomaintain fixationfrom the

begin-ning totheend of each session.

The

visual

stimula-tion was _given on one of the four _quadrants of the

same visual

field

for

each session.

The

order of visual

field

stimulation was randomized

for

each

partici-pant

Two

sessions

{10

repetitions of

the

stimulation

period

in

total)were carried eut

for

each quaclrant.

NIRS recording. A 24-channel NIRS instrument

(ETG-4000,

Hitachi Medical Co.)_generated two

wave-lengths

of

NR

light

(635

and

830

nm) and measured

temporal changes

in

the concentra'Lion of oxy-Hb and

deoxy-Hb with a ternporalresolution of

O.1

s,

We

used a

4X4

matrix of photodiodes consisting of eight

light

emitters and eight

detectors

for

the

measurement

(Figure

IC).

The

bLood

oxygen level

was measured at the

30-mm

area between each

emit-terand detector pair. The 16 photodiodes composed

24 measurement channels. These _photodiodes were attached toa

flexible

silicon

frame

and _placedon

the

participants'occipital area so that ch 23

{the

center

of the bottom row of the 4×4 matrix) was _placed

O,5

cm above the

inion.

The

photodiodes covered a

9

×

9

crn area of the scalp,

inc]uding

Ol

and

02

following

the international IO120 system. According to

Oka-moto et aL

{2004),

who

investjgated

the

cranio-cerebral correlation based on the internationa],1O120 system,

the

monitored cortical area with thisoptode arrangement would include the visual cortex

(Vl-V3) inboth hemispheres.

Data

analysis,

Prior

toperforming the statistical analysis, we corrected the raw data with the

fQllow-ing

procedures.

First,

the raw

data

were

digitalLy

low-pass filteredat O,1Hz to remove measurement

noise

<i.e.

an abrupt rise and

fall

ofmeasured values),

Next, a

baseline

correction was performed toremove

the linear trend in hemoglobin concentratiQn. We

fitted

a

linear

function

to

the

data

_points sarnpled

during 1O-s_periods beforeand after theonset of each

sttmulation period,

After

this,

we subtracted values

of the baselinefunction

from

data obtained foreach

stimulus sequence.

Since

raw data of NIRS are relative values, we

cannot directlycompare them among _participantsor

channels.

Therefore,

the

data

were normalized

by

calculating

`effect

size'

for

each channel within

par-ticipants

_(Matsuda

&

Hiraki,2006;

Otsuka,

Nakato,

Kanazawa,

Yamaguchi,

Watanabe,

&

Kakigl,

2007;

Schroeter,

Zysset, Kruggel, & von

Cramon,

2003).

The

effect sizes

(d)

were calculated with the

foLlow-ing

equation:

d=(ml-m2)ls

with

`m

1'as themean values

in

a stimulation _period,

and 'm2' and `sZ respectively, as the mean and

the

standard

deviation

of the values sampled

during

the

10s

period

before

the stimulation. We used the

effect size value for_the ]ateranalysis.

For

the

statistical analysis, we conducted a

two-tailed,one sarnple t-testagainst zero performed on

the means of the effect sizes obtained from each channel and averaged over the 10 stimulation

peri-ods, Since theeffect sizes represent the standardtzed

Hb-Ieveldifferencebetween thestimulation and

rest-ing _period, this analysis reveals the channels that

were significantly activated

by

the

visual stimula-tion. The statistical threshold was setatp<.05 with

NII-Electronic Library Service

38 TheJapanese

_Journal

of

Psy'chonomic

Science

VoL 28,No. 1

the Bonfferoni correction.

fMRI scanning. We scanned three of the 20

par-ticipants

wlth fMRL

The

configuration and

presen-tation time course of the stimuli forthisfMRI

sean-njng were

the

same as

in

the

NIRS

measurement,

Functional images were acquired using a3.0Tesla

MR

scanner

(Trio,

Siemens,

Erlangen,

Germany).

For

functional imaging during the experiment,

T2*-weighted _gradient echo-planar irnaging

CEPI)

se-quences was used

to

_produce

30

slices

{TR

==

2000

_ms,

TE=30ms,

FA=76dcg,

field

of view

{FOV)=192

mm, voxel size--3.0 ×3.0X2.0mm). A

high-resolu-tion anatomical Tl-weighted

image

was also

ac-quired

by

magnetization-prepared rapid-acquisition

gradient-echo

(MP-RAGE)

imaging

<TR=2000ms,

TE=4.38 rns, FA=8deg, FOV=240mm, and voxel

size=O,9X

O.9

×

O.9

mm)

for

each _participant.

The data were analyzed using statistical

paramet-ric mapping

5

(SPM5;

Wellcorne

Department of

Cog-nitive

Neurology,

London,

UK,

www.fi1.ion.ucLac.uk! spm). The

first

five

volumes of each fMRI session were

discarded

toallow forstabilization of the mag-netization, and the remaining 705 volumes were used

foranalysis. Head motion was cerrectcd using

the

realignment program of

SPM5.

Inaddition, thedata

were spatially smeothed inthreedimensions using a

4-mm

full-width

half-maximum

Gaussian

kernel,

O.6

Generalized

linear model

(Friston,

Helmes

Worsley, Poline, Frith, & Frackowiak, l995) was

adopted to assess the

BOLD

(BIood

Oxygen

Level

Dependant> signal contingent with the neural

activ-ity.

As

explanatory variables of

GLM,

we prepared

the model of the neural activation related to the

stimulation of each visual _quadrant

by

convolving

the Box-car function with the hemodynamic

re-sponse function. Based on this model, the effects of each explanatory variable

(beta)

and t-value were ca]culated

for

each voxel. The neura] activation associated with each visual _quadrant was

defined

by

the

difference

between the effect of targetquadrant

and the totaleffects oi three other _quadrants

(e.g.

effect of UR=3UR--(UL fBR+BL}). The statistical

threshold was set at

P<,05

with a correct'ion based

on the

false

discovery

rate

(Genovese

et al.,

20e2).

Results

Temporal

change.

In

accordance with prevjous studies

(Colier

et al. 2001;

Takahashl

et aL

2000;

Taga et aL,

2003L

we

found

an

increase

ef oxy-Hb

and a decrease of deoxy-Hb

during

the stimulation

period,

Figure

2 shows an example of

the

raw

data

after

low-pass

filtering.

As

this

figure

shows, oxy-Hb

increased

_(and

deoxy-Hb decreased) _gradually 2-4 s

after the stimulus onset.

Also,

it

can

be

seen thatthe

O O.5 o

2

o.4

"E o.3

･E.

o.2

g

o.t c ru o =

8

-o.i

Z

-o.2

-O.3

O 20 40 60 80 100 "2e 140 160 180 200 220 240

Time

(sec)

Figure 2. An example of the time course of the changes in the IIb concentration, The unit of thc

ordinate

is

a relative value, mm*mo].

The

figure

represents the tirnecourse of the

Hb

changes

measured at ch

l7

of a participant

during

one session

in

which the

lower

left _quadrant was

stimulated. The changes

in

the oxy-Hb concentration are

indicated

by

the gray lineand those fortbe

deoxy-Hb concentration are indicated

by

the black Iine.

S.TAyA et al:Hemodynamic changes inresponse to thestimulated visual quadrants

39

Figure

3.

Topographjc

rnaps

for

the

Hb

changes obtained with stimulation of each visual _quadrant.

Thc orientation and ehannel

location

of these rnaps correspond to those

in

Fig

IC.

The maps

represent the results from stimulation of the upper left,upper right, lower ]eft,and lower right visual

fieldquadrant

('from

Ieftto right). The upper row shows the maps for oxy-Hb and the

lower

row

shows _those for deoxy-Hb, The lighter and darker areas indicate the increase and decrease of

hemoglobin

concentrations, respectively.

The

dots

and circles on each map represent the

locations

of

the measurement channels.

These

topographic maps were made

by

interpolating

the mean effect size

obtajned at each channe] through spline-fitting.

dqcrease

of

deoxy-Hb

reached itspeak slight]y

later

than the oxy-Hb

increase.

These

time courses of

Hb

change were in_good agreement with _previous NIRS studies

(Takahashi

et al.

2000;

Taga

et al,,

2003),

The

figurealso shows that thehemoglobin conccntration

gradually

increased

or

decreased

throughout an

ex-perimental sessjon.

The

source and mcaning of this

]ongitudinal

driftof the

hemoglobin

concentration are stillunclear,

but

they are assumingly' caused

by

"physiological

effects such as change$

in

respiratory or cardiac activities and body movements" or

"meas-urement

instability

such as unstab]e contacts

be-tween the opticul _probe and the head and the

unsta-ble

power of

NIR

Iight"

(Taga

et al.,2003). As noted

in

the

Data

analysis section, we removed the

linear

trend by performing thebaseline correction,

Topographic

map. Figure 3 topographically

rep-resents the changes

in

regiona] concentrations of

oxy- and deoxy-Hb for20_{participants.}The channels

which detected significant Hb changes are indicated

by circles inthis figure

_(two-tai]ed

one sample _t-test,

p<.05

with

the

Bonfferoni correction}.

The

results

show thatNIRS can detect_the neuro-vascular

activa-tion concordant with the contra-lateral structure of

Lhevisual cortex. The lower-right_quadrant stimulus

produced significant

hemodynamic

changes at the

leftoccipital area, whercas the lowcr-leltquadrant stimulus _produced

these

at the right occipital area

(right

panels

in

Figure

3).

On

theother

hand,

none of

the channels detected significant hemodynamic

changes when stimuli were _presented to the upper visual

field

(left

panels),

Significant

increase

of oxy-Hb concentration was

found

on the contra-Lateral side of the stimulated visual hemifield. The changes of

deoxy-Hb

concen-tratton,however, werc more wide-spread

{Figure

3).

There

werc significant changes

in

deoxy-Hb

concen-tration not on]y on thecontra-lateral side, but also on the

ipsi-latcraL

side of the stimulated visual

hemi-fie]d. For example, ch ]4 and ch 21 detected a

significant decrease of deoxy-Hb concentration with

right visual

hemificld

stimulation, although these

channe]s were

located

on theright side of the

NII-Electronic Library Service

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The

Japanese

Journal

of

Psychonomic

Science

VoL

28,

No.

1

Stimulated

visual area

ssta

wage

Figure

4. The results of fMRI measurernent. The figure shows the retinotopic area in 5 slices of the

sagittal _plane with a 10-mm width for three _{participants.} The retinotopic areas are indicated

bv

patterned patches on

the

brain

images

(see

_quadrant sectors on the right

for

the relationship

between

patterns and visual _quadrants),

The

sLatistical threshold was set atp<.05, with a correction based on

the falsediscovery rate.

illlifiilii

ig#ifim

l I

tttt

t

'

･''l't.I'

.

,d

'

'

'

''

' . . . e' .

-.

'

'

-.

'

. e

.

- op

ee

Numberofparticipants:

o

1

3

s7

9M

Figure 5. Histograrn of the t.otal number of significantly activated channels summed across 20

participants. The maps represent the results

from

stimulation of the upper Ieft,upper righL lower

left,

and lower right visual field_quadrant

(frorn

leftto righO. Smail dots indicaLe the location of each

measurernent channel,

Gray

and

b]ack

circles show the result

for

oxy-Hb and

deoxy-Hb,

respectively.

The

radius of each circlc

indicates

the number of the _participants who showed a significant

Hb

changes

(p<.05)

at the corresponding channel.

S.TAyA et aL: Hemodynamic changes inresponse tothe stirnu]ated visual quadrants 41

difference

between the two indices suggests thatthe

measurements of oxy-Hb and

deoxy-Hb

concentra-tionsreflect

different

aspects of neuro-vascular

acti-vation.

OuT

results are

in

accordance with the results

of Suh, Bahar, Mehta, & Schwartz, 2006, who

re-ported that decreases in the deoxy-Hb

concentra-tionswere

less

localized

than the

increase

in

thetotai

hemoglobin concentration.

fMRI

data.

We

found

that

NIRS

can

detect

the

Hb

changes in the occipital area corresponding with stjmulation of the lower visual

field.

On

the other

hand, no channels detected significant Hb changes

with stimulatjon of the upper visua]

fie]d,

One

plau-sible explanation of theseresults

is

thatonly thearea representing the lower visual fieldlocated around

the $urface of occipital cortex

is

included

in

the

range of NIRS measurement, while the area

repre-senting the upper visual

field

is

located

far

deeper

than the measurable range of NIRS. The aim of the

fMRI

measurement was toexamine this_possibility.

Itcan

be

seen

from

Figure

4

thatstimulation of the

lower visual field activated the cortical region

around theoccipita] surface, whi]e stirnulation of

the

upper visual fie]dactivated the deeper regions from

the occipital surface

(more

than

30rmm).

In

sum-mary, theresults ef

fMRI

scans suggest thatcortical regions corresponding to the upper visual fieldare

indeed

located

too

deeply

to

enable

NIRS-measurements of Hb changes.

Individual

differences.

Figurc

5

shows

the

fre-quency distributtenof significant Hb changes across

20 _{participants.} The ehannels that detected

sig-nificant

H.

b

changes are

located

around a certaip area

of the visual cortex _presumably represcnting each

visual _quadrant. However, itshould be noticed tbat several channels

located

far

from

the center of the

population still

detected

statistically significant

Hb

changes. 1'hiswide-spread

distribution

of activated channels shows that large individual differencesin

size and location of thehurnan visual cortex exist.

Discussion

We

found

significant

Hb

changes whdn stirnuli

were presented tothelower visuaL

field,

Incontrast,

significant

Hb

changes were not

detected

in

aliof the

channels when stimuli were presented tothe upper visual

field.

These results sugge$t that NIRS can

detcct

activation of the cortical area corresponding

tothe

lower

visual

field,

but

can

hardly

detect

activa-tion of the cortical area corresponding to theupper visual field.

The

absence of significant

Hb

change

during

stimulation of the upper visual fie]dmight bedue to

the

fact

that the area representing the upper visual

fieldislocatediardeeper than the measurabLe depth

with standard

NIRS

instruments,

The

upper and

tower visual field,respectiveLy, are represented by

the cortical area located above and below the cal-carine sulcus,

Vjewed

from

the side,thesulcu$ slants upward from the _posteriortothe anterior _partof the

occipita]

lobe.

Thus,

viewed

from

the

back,

the

area

representing the lower visua] fie]d

is

exposed to-'

wards the su]cus, while covering thearea

represent-ingthe upper visual

field

that

is

located

in

the

deeper

partof theoccipkal lobe.As mentioned earlier, NIRS measures

hemodynamic

changes approxtmately

20-30 mm under the scalp, TheTeiore, the visual area representing

the

upper visual

field

might

be

located

fardeepeT than the measurable depth, We confirmed

this

possibility using fMRI. The imaging data

showed that the stronger activation was obtained at

the deepcr _partof the occipital

lobe

in

the conditions

of

tine

upper visual-field stimulation than

in

the

con-dition

ofthc

lewer

visual-field stimulation

_(Figure

4).

A recent study has shown thata new optical

jmaging

tcchnique, called diffuseoptical tomography

<DOT),

enables us tomonitor Hb changes inthe deeper

part

of the cortjcal region

because

of a

longer

optodc

distance of up to48 mm

(Zeff

etaL 2007). ZeffetaL

showed that DOT can monitor Hb changes

corre-sponding tothe upper visual

field

$timulation. How-ever,

increase

of signal nolse

is

inevitable

when

NR

lights

penetrate

far

deeper from the scalp

(Sase,

Eda,

Seiyama, Tanabe, Takatsuki, & Yanagida. 2001).

Thus, our results suggest that researchers still

should

be

careful about monitoring the acti,vation of visual cortex representing upper visual

field,

even

when using rhe

DOT

technique.

We found individua] differenccsjntlhelocationof

NII-Electronic Library Service

42 The

_Japanese

_Journal

of Psychonomic Science VoL 28,No. 1

Hb changes

{Figure

5}.This finding isinaccordance with the

individua]

differences

in

size and

]ocation

of

the visuai cortex observed in our fMRI scanning

(Figure

4),

a$ well as

in

previous studies

(e.g.

Dougherty et al,,2003;Takahashi et al.,2001; Zeffet

al.,

2007).

The

differences

suggest that pre-measurements

for

deciding the region-of-interest are advisable formonitoring the activation of the visual cortex with

NIRS;

i.e.

a prellminal expertment using

NIRS to find the channels which can be activated with a `localizer'

(e,g,

a

flickering

checker-pattern as used inthisstudy) could be made beforc examining

the main experimental stimulus. Another _possible

factor

for

the

individuai

differences

is

the way of

djrecting attention. fMRI studies have shown that

the cortical activation of _primary visual cortex can

be modulated by spatial attention

(e.g.

Brefezynski &

DeYoe,

1999).

Although

we asked our participants to

fixtheir_gaze on thefixationsquare, we did not _give

any jnstruction about attention, thus itis_possibLe

that the

difference

in

attended parts caused the

indi-vidual differences of the NIRS activation map.

In

summary, we

demonstraLed

that

NIRS

can

de-tect tl]ehemodynarnic changes caused by

stirnula-tion

of

the

lowcT

visual fic]d,

but

hardly

dcteets

_the activation in cortical regions corresponding to the

upper visual field.Our finding suggests that in

fu-tureexperiments one should

bear

this

in

inind when monitoring the activation of human vjsual cortex,

On

the other

hand,

our results also show that

NIRS

is

quite sensitive tothe cortical activation ifa target

area

i$

included inthe measurable rangc. Furthcr,

the

di

fference

ofoxy-Hb and

deoxy-Hb

de]n

onstrated

here imply that thcse two indices could reflect the

differentaspects of neLrro-vascular functioning, By

comparing oxy-Hb and deoxy-Hb, we could obtajn a

more cornplete _picture of the connection between

neurat and vascular responses.

Taken

together,

NIRS can provide us with a greatcr chance toexplore

thc brain functioning that cannot

be

assessed with

othcr

1rnaging

techniques. NIRS would be a

desir-able technique formonitoring the cortical activity of

infantsand neonates

<e.g,

Otsuka et al. 2007;Taga, et

al. 2003). Monitoring thc brain activity associated

with tasks that cannot be _perforrned in an fMRI

scanner isalso a _promising directionof NIRS usage

{e.g.

Hatakenaka,

Miyai,

Mihara,

Sakoda,

&

Kubota,

2007).

Acknowledgements

We

are _grateful to Dr.

Gerard

Remijn

for

he]pful

comrnents on this papcr. We also thank to Dr.

Yasuto Tanaka and Dr.Yusuke Morito for_their

co-operation tothe

[MRI

analy$is,

Thi$

research was

supported by the

COE

_program of Kanazawa

Univer-sity on

Innovative

Brain

Science

of the

Japanese

)v･Tinistr},of Education, Culture, Sports,Science and

Tcchnology.

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