Evidence of Dysfunction of a Prefrontal-Limbic Network in Schizophrenia: A Magnetic Resonance Imaging and Regional Cerebral Blood Flow Study of Discordant Monozygotic Twins Daniel R. Weinberger, Richard Suddath,

M.D., Karen Faith Berman, M.D., M.D., and E. Fuller Torrey, M.D.

Objective: The authors phrenia the affected twin

previously reported that in monozygotic almost invariably had a smaller anterior

with

imaging

magnetic

resonance

(MRI),

and

invariably

had

twins discordant pes hippocampus,

less regional

for schizomeasured

cerebral

blood

flow

(rCBF) in the dorsolateralprefrontal cortex duringperformance ofthe Wisconsin Card Sorting Test. The present study was an investigation ofthe relationship between hippocampalpathology and prefrontal hypofunction in the same twin pairs. Method: Nine pairs of monozygotic twins discordant for schizophrenia underwent MRI scanning for determination of anterior hippocampalvolume andxenon-inhalation rCBF testingfor determination of prefrontal physiological activation associated with the Wisconsin Card Sorting Test. Results: The differences within twin pairs on the MRI and rCBF measures were strongly and selectively correlated. Specifically, the more an affected twin differed from the unaffected twin in left hippocampal volume, the more they differed in prefrontal physiological activation during the Wisconsin Card Sorting Test. In the affected twins as a group, prefrontal activation was strongly related to both left and right hippocampal volume. These relationships were not found in the group ofunaffected twins. Conclusions: This finding is consistent with the notion that schizophrenia involves pathology ofand dysfunction within a widely distributed neocortical-limbic neural network that has been implicated in, among other activities, the performance ofcognitive tasks requiring working memory. (Am

J

Psychiatry

1992;

149:890-897)

I

is increasingly appreciated that schizophrenia is associated with deficits in various cognitive functions, especially attention, memory, and “executive” functions (1 ). Although the mechanisms underlying these deficits are uncertain, recent speculation has focused on dysfunction of temporal-limbic and prefrontal brain regions as explanations for at least the deficits in memory and executive functions, respectively. Poor perfonmance on prefrontal-type executive tasks, such as the Wisconsin Card Sorting Test, has been directly linked to physiological hypofunction of the prefrontal cortex in patients with schizophrenia (2-7), and subtle anatomical deviations in anteniomedial temporal lobe

Received July 10, 1 991; revision received Nov. 27, 1991; accepted Dec. 31, 1991. From the Clinical Brain Disorders Branch, intramural Research Program, NIMH. Address reprint requests to Dr. Weinberger, NIMH Neuroscience Center, St. Ehizabeths Hospital, 2700 Martin Luther King Jr. Ave., S.E., Washington, DC 20032. Supported

The authors cedures.

890

in part

thank

by NIMH

grant

Mr. Alex lerrazas

MH-41176.

for help with statistical

pro-

structures

implicated

in memory

processing

have

been

interpreted as a pathological correlate of the impairments on memory tests also manifested by these patients (8-1 1). Furthermore, our group has reported data suggesting that these neurobiological findings might be related (12), and we have speculated that the heart of the matter is dysfunctional neocortical-limbic connectivity (12-15). In a recent investigation of sets of monozygotic twins discordant for schizophrenia, we found that the affected twin almost invariably performed worse on memory tests (8) and on the Wisconsin Card Sorting Test (7, 8) than did the unaffected sibling. Moreover, we also found that the affected twin almost invariably had a smaller anterior pes hippocampus ( 1 6) and invariably had diminished relative prefrontal activity as reflected in a measure of regional cerebral blood flow (nCBF) during performance of the Wisconsin Card Sorting Test (7). In this investigation we examined the nelationship between these putative abnormalities in hippocampal anatomy and prefrontal cortical physiology

in this group

of monozygotic

Am

J

twins

Psychiatry

in an effort

149:7,

July

to test

1992

WEINBERGER,

directly

the

limbic

network.

hypothesis

of a dysfunctional

pus junction. The volumes of the pes hippocampus on both the right and left sides were determined. To indude a hypothetically “neutral” MRI measure (i.e., a

neocortical-

measure

that

did

not

discriminate

affected

from

Subjects The study group consisted of nine pains of monozygotic twins discordant for schizophrenia who were recruited as part of a multidimensional study of schizophrenia

in monozygotic

twins.

These

nine

pairs

are

all

of the discordant twin pairs who had undergone both magnetic resonance imaging (MM) and rCBF measurement. Characteristics of these twin pairs were described in detail previously (7, 8, 16). In brief, monozygosity was confirmed by analysis of 19 RBC antigens. Psychiatric diagnosis was established by structured diagnostic interview (SCID parts I and II [17]). The affected memben of each twin pair fulfilled the DSM-III-R criteria for chronic schizophrenia, and the unaffected member had no evidence of major psychopathology and did not fulfill criteria for an axis I psychiatric disorder. Three pains were women, and six pairs were men; their mean age was 32 years (range, 25-44 years). Each twin pair had been discordant for at least 4 years when the study began and had remained discordant for an additional 3year follow-up period. The affected twins were all taking antipsychotic medications at the time of the study. They were in excellent general medical health and had no sign of dehydration or malnutrition. They had been screened for major medical problems and for history of noteworthy

drug

abuse,

head

trauma,

and

other

neuno-

logical problems. The results of rCBF measurements of this group have been reported in detail elsewhere (7). The MM scans of these subjects were part of a study of a larger group of monozygotic twins discordant for schizophrenia,

the MM

and

scans

have

the

anatomical

also

been

measurements

reported

previously

from

measures

have

Regional

Cerebral

Resonance

been

given

Blood

elsewhere

(16).

Flow

The method for determining rCBF by xenon inhalation has also been described in detail elsewhere (2, 4, 7). In brief, the procedure involved a 60-second inhalation of a mixture of xenon-133 gas and air followed by 14 minutes

of “washout,”

during

which

only

room

first

measurement

was

made

during

an eyes-closed

unitless

Initial lapsed

measure

of cortical

slope values from into five regions,

7). The

raw

rCBF

data

gray

matter

32 cortical as previously have

been

perfusion

given

elsewhere

Analysis

Imaging The purpose of this study tionship between anatomical

was to examine the reladifferences in the volume

of the

physiological

unaffected

In the

prior

anatomical

study

(16), the volume of the anterior pes hippocampus was the most robust discriminator of affected from unaffected twin and was selected a priori as the anatomical measure of interest for this investigation. This measure was determined by summing the areas of the pes hipin the

including

J

Psychiatry

first

four

a slice through

149:7,

(7).

(16).

image

pocampus

(18).

regions were coldescribed (2, 4,

a relationship. 1 For each twin pair an anatomical difference was computed by subtracting the pes hippocampal ume in the affected twin from the same measure

system.

rest-

ing condition that served to acclimate the subjects to the testing procedure. The second and third conditions were an automated version of the Wisconsin Card Sorting Test and a simple sensonimotor control task. They were presented in counterbalanced sequence to control for the possibility of an order effect. The control task was designed to serve as a nonspecific sensonimotor baseline activation condition so that rCBF during the Wisconsin Card Sorting Test could be compared with a control value (2). We calculated rCBF as the initial slope, a

The method for making anatomical measurements from MRI scans has been described in detail elsewhere (16). In brief, we used spin-echo coronal sections weighted for T1 relaxation time (repetition time=800 msec, echo time=20 msec); the sections were S-mmthick contiguous slices parallel to the floor of the fourth ventricle and were acquired with the same 1 .S-tesla magnet (General Electric Signa) for each of the subjects. We used an interactive method for outlining structures of interest on digitized images displayed on a computer analysis

air was

inhaled. Regional cerebral blood flow was calculated (by using a biexponential model for gray and white matter compartments) from the radioactivity desatunation curves generated by 32 extracranial gamma ray detectors. Each subject underwent three consecutive rCBF procedures separated by approximately 30 minutes and carried out under three different testing conditions. The

Statistical Magnetic

Am

unaf-

fected twin), we derived cerebral volume by adding the total cerebral areas in the four slices through the antenor hippocampus. The raw data concerning these

METHOD

dala,

El AL.

SUDDATH,

BERMAN,

July

slices

posterior

to the

the amygdala-hippocam-

1992

amyg-

pes hippocampus

and

differences

in prefrontal Sorting Test schizophrenia.

activation during the Wisconsin Card within monozygotic pains discordant for To achieve this goal, we selected two meas-

ures

as those

a priori

most

likely

to test

the presence

.

twin.

If we

is an ideal subject twin in each pair, can

be taken

putative phrenia activation

as a measure

anatomical in the

2. For each

assume

associated

the

unaffected

value volin the twin

with the affected coanatomical difference

of the

pathology

affected

pair

that

for comparison the within-pair

of

relative

extent

associated

with

of the

schizo-

twin.

we computed with

the

the difference Wisconsin

Card

in rCBF Sorting

891

PREFRONTAL-UMBIC

DYSFUNCTION

TABLE 1. Within-Pair Monozygot

Ic

IN SCHIZOPHRENIA

Differences in MRI AnatomiCal for Schizophrenia Within-Pair

Difference

Volume

Twin

(cm3)a

Cerebral

Twin

Hippo-

Hippocampus

Volume Measure

Pair

-0.6 -5.2 -2.2 -8.5 -4.3 11.0 -0.7 4.9 -1.9 in unaffected

campus

1 2

38 38

F F

3

33

M

4 S 6 7 8

31 25 28 44 28

F M M M M

0.35 0.06 0.14 0.16 0.24 0.35 0.36 0.24

M

0.03

0.14 -0.02 0.16 0.20 0.01 0.76 0.29 0.08 0.13

twin was subtracted

from volume

9

27

in affected

First,

each

performance

individual’s

of the

determined.

This

rCBF

Wisconsin

was

activation

Card

accomplished

rCBF data during the control task during the Wisconsin Card Sorting

Sorting

related Test

by subtracting from Test.

during

rCBF

studies

(2, 19).

this

the

differ-

ence in rCBF “activation” for the affected twin was subtracted from the analogous value for the unaffected twin. This within-pair physiological difference can be taken as a measure of the relative extent of the cognition-linked dorsolateral prefrontal cortical hypofunction in the affected twin. The correlation of these within-pair differences in left and night pes hippocampal volumes and within-pair differences in rCBF activation was examined by several procedures, both parametric and nonparametnic. Simplc nonparametnic correlations were performed with the Spearman rho test. Because cerebral perfusion is highly intercorrelated across brain regions and the majority of the variance in regional perfusion is accounted for by the level of global brain perfusion (20), correlations, if found, may not reflect an anatomical nelationship with cortical neuronal activity that is uniquely regional. Alternatively, such correlations may be obscured by the “noise” introduced by global perfusion variance. To account for these possibilities, we performed

a partial

correlation

procedure

to address

the

principal theoretical question of this study, namely, whether and to what extent the MRI data relate to rCBF data (and, by inference, neuronal activity) that are uniquely regional (i.e., not accounted for by whole brain CBF). This approach is analogous to other methods, such as analyses of covaniance (20), that attempt to highlight local brain activity by adjusting regional data for the level of global flow, but the partial correlation approach addresses the strength of the relationship between

procedure the effect

892

variables

rather

involved using of the differences

than

differences

linear regression in whole brain

in means.

Temporal

Parietal 4.0 -4.3 -0.5 -7.6

1 2 3 4 S

23.4 -7.2 1.7 -10.2 4.3

10.7 -5.7 0.1 -7.8 4.0

8.6 -4.8 -0.1 -2.3

6

12.9

10.1

-2.3

7 8 9

19.6 -8.8 4.0

aRgionah

-6.2

8.8 -9.2 -11.5

cerebral

blood

Parieto-

Whole

Occipital

Brain

5.8 -1.8 -9.3 -4.7

8.7

7.7

10.5 -8.8 -9.5

during

10.5 -4.5 -0.7 -8.2 1.9

2.1

2.9

7.9 -4.9 -9.4

flow

Activationa

10.3

11.2 -6.9 -11.4

11.5 -8.2 -8.6 task was subtracted

a control

to

the rCBF data This “subtnac-

Second,

in rCBF

was

tion” method has become widely accepted as the “gold standard” for isolating behavior-specffic regional brain activity

Central

Difference

from rCBF during the Wisconsin Card Sorting Test. The difference value for the affected twin was subtracted from the difference for the unaffected twin. The values shown here are (unitless) initial slope values from 32 cortical regions collapsed into five regions the raw data have been given elsewhere (7).

nwn.

Test.

Prefrontal

Twins Discorda nt for

Test for Monozygotic

Within-Pair

Right

Sex

aVohume

Wisconsin Card Sorting Schizophrenia

in

Left Age (years)

Pair

Measures for

Twins Discordant

The

to remove CBF on the

regional the MRI

differences

and

difference

data.

as a “semipartial

correlating

This

the

residuals

procedure

correlation”

(21).

with

is referred

Because

to

of the nela-

tively small number of subjects, the residual rCBF diffenences and the MM differences were also subjected to a nonparametric Spearman correlational analysis. The statistical procedures were performed on a mainframe computer with Statistical Analysis System programs (SAS Institute, Cary, N.C.).

RESULTS

The twin pains’ age, sex, and in the anatomical measures are within-pair differences in rCBF Wisconsin Card Sorting Test are hippocampal

ences

anatomical

correlated

p0.7,

(rho=0.82,

pO.lS).

pected, rCBF differences were highly correlated between all regions and with mean whole brain CBF (in all cases, nho>0.90, p0.20

0.13

>0.70

-0.02 -0.63 -0.30

>0.90 0.43

0.04 -0.71 -0.29

>0.90 0.40

0.33 -0.60 0.10

>0.30 0.70

-0.19 0.44 -0.35

>0.60 >0.20 >0.30

aff7

tions were found for both left and right hippocampus and residual panietal flow. This correlation suggests that more hippocampal pathology is associated with greater panietal activation. The results of a test for a relationship between within-pair differences in the cerebral volume measure and residual prefrontal rCBF activation were not significant (Spearman nho=0.40, p=O.29; Pearson r=0.26, p=O.SO). Post hoc correlations were performed to address whether the results depended on the presence of illness per se. In the unaffected twins as a group, there were no significant correlations between pes hippocampal volume and rCBF activation during the Wisconsin Card Sorting Test, either without or with partial correlations (p>O.2 in each case). In the affected group, after the partial correlation procedure was performed prefrontal activation correlated strongly with both the left (rho= 0.75, p=O.O2) and right (rho=0.82, p