Su pervisor: Dr. O t f r i e d Spreen
A B S T R A C T
It is now c o m m o n l y a ck n o w l e d g e d that l e a r n i n g d i s a b l e d
c hi ld r e n are not a h om o g e n e o u s p o pulation, and c u r rent
n e u r o p s y c h o l o g i c a l r es e a r c h in this a re a has f oc u s s e d on
at t e m p t s to i d en t i f y s u b t y p e s of t hese dis o r d e r s . E a r l i e r
s ub t y p i n g studies a do p t e d a subjective, c l i n i c a l - i n f e r e n t i a l
a pp ro ac h to c la ss if i c a t i o n , w h i l e recent r e s e a r c h has
e m p h a s i s e d a s tr ic t l y objective, q u a n t i t a t i v e m o d el w h i c h
involves the use of m u l t i v a r i a t e s ta t i s t i c a l m e t h od s of
classi f i c a t i o n . In the n a t ural sciences, b ot h o b j e c t i v e
q u a n t i f i c a t i o n as well as the d e v e lo pm en ta l, q u a l i t a t i v e
a sp e c t s o f t ax on o m y a r e c on s i d e r e d e q u a l l y e ss e n t i a l for a
g o o d c l a s s i f i c a t i o n (A d a m s ,1985). S ub ty pe s i de n t i f i e d so far
in v a r i ou s studies h a v e had r e l a t i v e l y li t t l e impact on
e i t h e r n e u r o p s y c h o l o g i c a l th e o r y o r c l i n i c a l practice, and
t hi s has b e e n a t t r i b u t e d to a f a i lu re on the part of
r e s e a r c h e r s to i n t e g r a t e the c l i n i c a l - q u a l i t a t i v e a p p r o a c h
w i t h the q u a n t i t a t i v e s ub t y p i n g p r o c e d u r e s (Wilson &
R i s u c c i ,1986).
The p r e sent study a tt e m p t e d to a d d r es s this p r o b l e m by
u s i n g a c o m b i n a t i o n of these two g e n e r a l l y a c c e p t e d methods,
in a n a t te mp t to i de nt i f y r e l i a b l e and m e a n i n g f u l sub t y p e s
w i t h i n a sample o f 275 c l i n i c - r e f e r r e d and 26 n o r m al c o n tr ol
g en e r a t e d : a) u s i n g c l i n i c a l - i n f e r e n t i a l methods, based on
c li n ic a l i n s p e c t i o n of p s y c h o m e t r i c test data, a n d b) using
m u l t i v a r i a t e s t a t i s t i c a l m e t h o ds for the d e r i v a t i o n of
sub t / p e s (cluster analysis). The two s u b t y p i n g s ol u t i o n s were
t h e n compared, a l l o w i n g e a c h to be used to v a l i d a t e the o t her
(Morris & Satz,1984).
The d e r i v e d c lu st er s and cli n i c a l t yp ol og y groups
i d e n t i f i e d all c o m m o n l y f o und s ubtypes as well as most others
r e p o r t e d b y p re v i o u s s u b t y p i n g s t u dies in the literature. The
re su lt s of s e v eral in t e r n a l v a l i d a t i o n p ro c e d u r e s indic a t e d
that the c lu st e r s w e r e r e l a t i v e l y hete r o g e n e o u s , and
t h e r e f o r e s o m e w ha t u n re li ab le , a lt ho u g h the m a j o r i t y of c lu st e r s p r o v e d t o b e m e a n i n g f u l and i nterpretable. C o m p a r i s o n of the two c l a s s i f i c a t i o n s i n d i c at ed a p p r o x i m a t e l y 58% c o r r e s p o n d e n c e in terms of i n d iv id ua l case a s s i g n m e n t s to c o m p a r a b l e su b t y p e s b e t w e e n the typologies. C o m p a r i s o n of T - s c o r e a b i l i t y p r o f i l e s r ev ea le d g e n e r a l l y s a t i s f a c t o r y c o r r e s p o n d e n c e b e t w e e n the p ro fi l e s of c l u s t e r a n a ly s i s d e r i v e d s ub ty p e s a n d t h ose of c o mp ar ab le c li ni ca l subtypes. F u r t h e r a na l y s e s were p e r fo rm ed on s e l e c t e d g r o up s of s u bj ec ts in o r d e r to e xp l o r e s pe ci f i c h yp ot he s e s . Age e ff e c t s
on s u bt yp e p at t e r n s w e r e examined, and the r e su lt s s ug g e s t e d
that s u b t y pe s do p e rs is t o v e r the school age range. However,
a d o l e s c e n t s ub je c t s w e r e m o r e p r o m i ne nt in the severe
l an gu ag e d i s o r d e r subtypes, a n d a large p r o p o r t i o n of the
p e r c e pt u a l problems. R e a d i n g d is a b i l i t y s ub ty p e s w er e a ls o
analysed, i n d i c a t i n g q u a l i f i e d support for D en c k l a ' s (1977)
s u bt y p e s from the c l u s t e r analysis, but c o n s i d e r a b l e
c o n f i r m a t i o n of this t yp o l o g y from the c li ni ca l
c la s si f i c a t i o n . Ro u r k e and F i n l a y s o n ' s (1978) f i n d i n g s in
r e g a r d to s pe ci fi c a r it h m e t i c d i s a b i l i t i e s w e r e not
r e p l i c a t e d in this study. S u b je c t s with s pe ci f i c p r o file
p a t t e r n s were a l s o e x am in ed for e vi de nc e of c h a r a c t e r i s t i c
social, e m o t i o n a l and b e h a v i o u r a l d i f f i c u l t ie s, w it h m ix e d
results. Finally, the o b t a i n e d s ubtypes w er e e x a m i n e d in
o r d e r to d e t e r m i n e p o s s i b l e d i f f e r e n c e s in t e rms of a c a d e m ic
p erf o r m a n c e , in o r d e r to e s t a b l i s h e x t e r n a l v a l i d i t y for the
two c l a s si f i c a t i o n s .
It was c o n c l u d e d that, a l t h o u g h t here are d e f i n a b l e as
w e l l as m e a n i n g f u l s ubtypes of l e a rn in g d i s a b i l it ie s, this
p o p u l a t i o n of c h i l d r e n ca n n o t b e c l a s s i f i e d i n t o d i s c r e t e
s u b t y p e s w i t h c l e a r b o u n d a r i e s and s t r ic t l y d e f i n e d criteria.
In addition, it w a s d e e m e d i m p o r t a n t to r e c o g n i z e that such
d i s o r d e r s range, in d e g r e e of severity, f r o m q u i t e su b t l e to
s e r i o u s l y impaired, so that d i a g n o s t i c "cut off" po i n t s are
i n a p p r o p r i a t e for this p a r t i c u l a r g r o u p o f children.
E x a m i n e r s :
Dr.'O.Spreen
■ n ^ p u e s .Q . jc q S ? S > I 'J T * 3 0 ) su ou iw t / e / *a * JO
T A B L E O F CONTENTS T i t l e P a g e ...i A b s t r a c t ... ii T a ble of C o n t e n t s . . . ... vi List of T a b l e s ... ix List of F i g u r e s ... x A c k n o w l e d g e m e n t s ... xi C h a pter page 1 I n t r o d u c t i o n ... 1 R e vi ew of the L i t e r a t u r e ...6 D e f i n i t i o n and S a m p l i n g I s s u e s ... 6 R e s e a r c h S t ra t e g i e s in the S t u d y of L e a r n i n g D i s a b i l i t i e s ... 15 The S e a r c h for S u b t y p e s ... 19 C l in i c a l I n fe r e n t i a l C l a s s i f i c a t i o n s ...21 M u l t i v a r i a t e S t a t i s t i c a l C l a s s i f i c a t i o n ... 36 C l a s s i f i c a t i o n Issues in S u bt yp e R e s e a r c h ... 46 2 O b j e c t i v e s of the S t u d y ... 54 De s i g n of the S t u d y and T h e o r y F o r m u l a t i o n ... 55 H y p o t h e s e s ... 60 3 M e t h o d ... 65 S u b j e c t s ... 65 Test M e a s u r e s ... 74 P r o c e d u r e s ... 76
4 A n a l y s e s and R e s u l t s ... 81
P h a s e I (Clinical c l a s s i f i c a t i o n ) . . . . ... 81
P h a se II (Statistical c l a s s i f i c a t i o n ) ... 112
R e su lt s and Int e r n a l V a l i d a t i o n (Sample A)....116
R e su lt s and In t e r n a l V a l i d a t i o n (Sample B),...118 I nt er pr e t a t i o n of C l u s t e r s (Sample A ) ... 119 I n t e r p r e t a t i o n of C l u st e r s (Sample B )... 131 5 H y p o t h e s i s T e s t i n g A na ly se s and R e s u l t s ... ...142 H y p o t h e s i s 1 ... 142 H y p o t h e s i s 2 ... 142 H y p o t h e s i s 3 ... ... 144 H y p o t h e s e s 4 and 5 ... 157 H y p o t h e s i s 6 ... ... 164 H y p o t h e s e s 7 and 8 ... 172 H y p o t h e s i s 9 ... 181 6 D i s c u s s i o n ... 202 H y p o t h e s e s 1 and 2 ... 202 H y p o t h e s i s 3 ... 203 H y p o t h e s e s 4 and 5 ... 206 H y p o t h e s i s 6 ... ... 207 H y p o t h e s e s 7 and 8 . . . ... ...211 H y p o t h e s i s 9 . . ... 213 G e n e r a l D i s c u s s i o n . . ... 216 C o n c l u s i o n s . . ... 223 L i m i t a t a t i o n s of S t u d y , ... 226 F u t u r e R e s e a r c h ... 227 R e f e r e n c e s ... 229
A P P E N D I C E S page
A Clinical I nf e r e n t i a l S u b t y p i n g of L e a r n i n g
D i s a b i l i t i e s ... 2 45
B Decision Rules for C la s s i f y i n g T y p ol o g y
S u b t y p e s . . . „ ... 252
C Sta n d a r d i z e d Parent I n t e r v i e w ... 269
D Parent I n f o r m at i o n F o r m ... 272
E Letters to School P ri n c i p a l s and P a r ents
L I S T OF TABLES
T a b l e page
1. D y sl ex ia S y n d r o m e s (Mattis, 1 9 7 5 ) ...
2. D y s l e x i a Su b t y p e s (Denckla, 1 9 7 7 ) ...
3. D y s l e x i a S u b t y p e s (Denckla, 1 9 7 9 ) ...
4. S u m m a r y S t a ti st i c s for Two C l i n i c - r e f e r r e d Groups and C o n t r o l g r o u p (V I Q , P I Q , F S I Q , S e x and S E S ) ___ .68 5. F r e q u e n c y D i s t r i b u t i o n of SES r a t i ng in S a m pl e A s u b j e c t s ... ... 6. F r e q u e n c y D i s t r i b u t i o n of SES ra t i n g in S a m p l e B s u b j e c t s ... 7. F r e q u e n c y D i s t r i b u t i o n of SES r a t i n g in N o r m a l C o nt r o l s u b j e c t s ... ... 8. A b b r e v i a t i o n codes for S ub t y p e s in C li ni c a l T y p o l o g y ... 9. M e a n s and S t an d a r d D e v i at io ns of V ar ia b l e s : C l i n i c a l T y p o l o g y Gr o u p s (Sample A ) . . . ... (C o n t d . ) ... ... . 10. M e a n s and S t a n d a r d D e v i a t i o n s of V ar ia b l e s : C l i n i c a l T y p o l o g y G r o u p s (Sample B ) . ... . 1 05 ( C o n t d , ) ... . 1 06
1 1 . A b b r e v i a t e d Cod.-.s for C li n i c a l T y p o l o g y Subtypes:
S a m p l e B ... ... 1 07 12. M e a n s and S t a n d a r d D e v i a t i on s of C l u s t e r i n g V a r i a b l e s (Sample A ) ... . 1 20 (C o n t d . ) ... . 121 13. M e a n s and S t an d a r d D e v i at io ns of C l u s t e r i n g V a r i a b l e s (Sample B ) ... 14. S u m m a r y of C l u s t e r A na ly se s on T h r e e Age Groups.. .167 15. Ag e E f fe ct s - Cli n i c a l T y p o l o g y ... . . 1 71
L I S T O F F I G URES
Fi g u r e page
1 H i e r a r c hi ca l C l a s s i f i c a t i o n of L e a r n i n g
D i s a b i l i t i e s ... 86
2 Profiles of C l i n i c a l T y p o l o g y Groups (Sample A ) . . . . 98
(Contd. )... 99 to 104
3 P rofiles of C l i n i c a l T y p o l o g y Groups (Sample B ) ...107
(Contd, )... 1 08 to 111 4 C l us t e r P r of il es - S a m p le A ...122 (Contd. )... 1 23 to 1 27 5 C l u s t e r P r of il es - S a m p le B ... 132 (Contd. )... 133 to 1 35 6 A r i t h m e t i c D i sa b i l i t y P r o f i l e s V a r i a b l e L i s t ... 160 A r it h m e t i c D i s a b i l i t y P r o fi le s (Group 3 ) ... 161 7 A r i t h m e t i c D i s a b i l i t y P ro fi l e s (Group 2 ) ... 163 8. C l i n i c a l / C l u s t e r P r o f i l e C o m p a r i s o n s (Sample A ) . . . 183 (Contd. )... 1 84 to 1 89 9. C l i n i c a l / C l u s t e r P r o f i l e C o m p a r i s o n s (Sample B). . . 1 9 6 (Contd. ) . ... 1 97 to 1 99
A C K NO WL E D G E M E N T S
I w o ul d like to thank a number of people whose help
d u r i n g the c o m p l e t i o n of this d i s s e r t a t i o n was much
app r e c i a t e d . I am mo s t g r a t e f u l to Penny Hobson U n d e r w o o d
a n d C a t he ri ne M a h o n e y for cheir generous h e l p and a d v ic e in
r e g a r d to s t a t i s t i c a l ana l y s i s procedures. I would a ls o
l i ke to t h a nk Leif B lu c k a n d Rich a r d Chadwick, w i t hout
w h o s e p at i e n t and c he e r f u l tea c h i n g in the s t a t istics l a b . ,
I c ou l d not h av e c o m p l e t e d this research. My sincere thanks
go to M ar y A n n e M a h o n e y for c o d i ng the v a l i d a t i o n da t a and
to O d e tt e G o u l d for her c om p e t e n t ha n d l i n g of the graphics.
Finally, I a m e s p e c i a l l y g r a t e f u l to Dr. Otfried Spreen,
fo r w hose u n f a i l i n g s u p port and e n c o u r a g e m e n t throughout my
CH A PTER I
I N T R O D U C T I O N
It is now g e n er al ly a cc e p t e d that c h i l d r e n wi t h
l ea rn i n g d is ab i l i t i e s do not c o n s t i t u t e a homoge n e o u s
p o pulation. Over the last 20 years, t h e r e has been
i n c r e a s i n g recogn i t i o n of the f a l l a c y of the "unitary
d e f i c it " hyp o t h e s i s as the u n d e rl yi ng b a sis of such
d i s orders, and the r es ea r c h st r a t e g y in r el a t i o n to this
t o pi c has c ha n g e d radically. I n s tead of the tradit i o n a l
c o n t r a s t i n g g roup de s i g n model, v/hich a t t e m p t e d to identify
a single u n de rl yi ng v ar ia bl e that w o u l d d is c r i m i n a t e
b e t w e e n groups of learning d i s a b l e d a n d no r m a l children,
c u r r e n t r es ea r c h in the n e u r o p s y c h o l o g y of l ea rn i n g
d i s a b i l i t i e s has f o cussed almost e x c l u s i v e l y on a tt em p t s to
i d en ti fy r e l a ti ve ly h o m og en eo us s ub t y p e s of these disorders.
M et h o d s of c la ss if i c a t i o n h a v e i n v o l v e d two ma i n
appro a c h e s : a) the d iv i s i o n into s u b ty pe s on the basis of
s u b j e ct iv e clinical i n t e r p r e t a t i o n of p s y c h o m e t r i c test
pr o f i l e s (Mattis, French & Rapin, 1975; Denckla, 1979) and
b) the use of e m p i ri ca l and o bj e c t i v e m u l t i v a r i a t e
s ta t i s t i c a l c l as si fi ca ti on p rocedures, such as Q-type
fa c to r analysis or c lu s t e r a na ly si s of psy h om e t r i c data
(Doehring & Hoschko, 1977; Fisk & R ou rk e, 19 79 ; Satz &
M o r r i s ,1981). M a n y of the e ar l i e r s tu d i e s a do p t e d a
vi s u a l i n s p e c t i on of the test data and c l inical
o b s e r v a t i o n s of lea r n i n g d i s a b l e d children. However, almost
a ll recent n e u r o p s y c h o l o g i c a l r esearch in this T e a has
i n v o l v e d a quanti t a t i v e , m u l t i v a r i a t e approach.
A n o t h e r d i m e n s i o n r e l a t i n g to c l a s s i f i c a t io n research
in this area con c e r n s the q u e s t i o n of w h e t h e r subjects are
c l a s s i f i e d on the b asis of a c a d e m i c p e r f o r m a n c e measures
(e.g. Boder , 1 9 7 3 ) or p r o c e s s i n g d e f i c i e n c i e s based on
c o g n i t i v e and n e u r o p s y c h o l o g i c a l test performance.
T h o s e studies in w h i c h sub t y p e s have b e e n det e r m i n e d
o n the b a sis of a n u m b e r of c o g n i t i v e and n e u r o p s y c ho lo gi ca l
v a r i a b l e s h a v e b e e n c r i t i c i z e d as gener a l l y f a i ling to
e x a m i n e the na t u r e of the r e a d i n g (or learning) dis o r d e r in
detail. Similarly, s t u d i e s in w h i c h subtypes have been
d e t e r m i n e d on the b asis of r e a d i n g tests of various types
h a v e f a i l e d to i n t e g r a t e the r e s ults wi t h either
n e u r o p s y c h o l o g i c a l , m e d i c a l or d e v e l o p m e n t a l information in
o r d e r to v a l i d a t e the s u b ty pe s that have been identified.
A c o m m o n p r o b le m is that, in studies u s i n g clinical
data, t h er e has been li t t l e c o n s i s te nc y in the selection
c r i t e r i a for subject s a m p l e s or in the ch o i c e of variables
u s e d for c l a s s i f i c a t i o n, w h i c h tends to limit comparisons
o f s u b t y p e solut i o n s b e t w e e n studies.
In a d i s c u s s i o n of t h e o r e t i c a l issues in subtype
sciences, t a x o n o m i c r esearch has been c o n d ucted by the use
of both "phyletic" and "phenetic" approaches. The p hy le t i c
compo n e n t e m p h a s i z e s the theoretical, phylogenetic,
d ev el op me n ta l and q ua li ta t i v e aspects of taxonomy; in
contrast, the phenetic c o m p on en t e m p hasizes the o b j ec ti ve
q ua n t i f i c a t i o n in c la ss if i c a t i o n through the use of
e m p i r i c a l or d e r i v e d m a t h e m a t i c a l models, based on a
f r a m e w o r k of measurement. T h ese two ways of u n d e r s t a n d i n g
the ta x o n o m y are c o n s i d e r e d c o m p l e m e n t ar y and bo t h are
essen t i a l for a g o o d c l a s s i f i c a t i o n sy s t e m (A d a m s ,1985).
Such a concept is e q u a l l y important in c l a s s i f i c a t i o n
r e s e a r c h p e r t a i n i n g to o t h e r d i s c iplines, and A d ams
e m p h a s i s e s the i m po r t a n c e of a n ad e q u a t e t h e o r e t i c a l
(phyletic) f r a m e w o r k to g u i d e the use of the m a t h e m a t i c a l
procedures, w h e n e m b a r k i n g u p o n n eu ro ps y c h o l o g i c a l
s u b t y p i n g research.
The process of f o r ming a v a lid c l a s s i f i c a t i o n is
d i r e c t l y inv o l v e d m d e v e l o p i n g r e l ia b l e d i a g n o s t i c
c r i t e r i a from w h i c h theories a n d t h e r a p e u t i c plans can be
g e n e r a t e d (K e n d e l l ,1975). Thus a good, w e l l - v a l i d a t e d
c l a s s i f i c a t i o n of lea r n i n g d i s a b l e d c h i l d r e n s h o u l d p r o v i d e
not on l y a g r e a t e r u n d e r s t a n d i n g of l e a r n i n g d i s a b i l i t i e s
as a whole, but a l s o s t i m u l a t e f u r ther r e s e a r c h into the
e t iologies, p r o g n o s e s and t r e atments of the v a r i o u s
s u b t y p e s .
s u b t y p in g literature could be c r i t i c i z e d for the failure to
u n d e r s t a n d the underlying c o n c eptual f ra m e w o r k and purpose
of classification. In particular, they criticize the failure
of most s u b type research to recog n i z e the value and
im p or ta nc e of relevant and r eliable s u btypes to the whole
p ro c e s s of clinical d i a g n o s i s and treatment of learning
d i s a b i l i t i e s in children. A l t h o u g h some d e f i n a b l e subtypes
h av e been i d e ntified in v a r ious s t u dies using clinical or
s ta t i s t i c a l approaches, many of them h a v e had little impact
on ei t h e r n e u r o ps y c ho lo gi ca l t h e or y or c li ni c a l practice.
W i l s o n and Risucci (1986) a t t r i b u t e this to a failure on
the part o f researchers to i n t e g r a t e clinical-inferent.ia.1
a n d q u a n t i ta t i ve m e t hods into the a s se ss me nt and val i d a t i o n
p r o c e s s e s .
The p r e sent study a t t e m p te d to a dd r e s s this p r o b l e m by
u s i n g a c o m b i n a t i o n of the two g e n e r a l l y ac c e p t e d m e t h o d s
e m p l o y e d in subtyping research, namely: a ) c l i n i c a l
i n s p e c t i o n of test data and s o rt in g techniques, and b)
m u l t i v a r i a t e statistical methods. Roth m et h o d s are o r i en t e d
t o w ards the same goal, and m a n y p s y c h i a t r i c c l a ss if i c a t i o n
s ys t e m s h a v e used stati s t i c a l techi q u e s in c o n j u n c t i o n with
c l i n i c a l l y d e r i v e d methods, thus a l l o w i n g ea c h to be used
to v a l i d a t e the c l a s s i f i c a t i o n s d e r i v e d by the other
(Morris a n d Satz, 1984). A recent c l a s s i f i c a t i o n s t udy on
l a n g u a g e d i s o r d e r e d c h i l d r e n has al s o a d o p t e d this model
The subject sa m p l e in the present study includes
clinic referred c h i l d r e n w i t h learning, behavioural or
social d i f f i c u l t ie s as well as a small g r o u p of normal
chi l d r e n without any such problems. As the v a l u e of any
typology to be used for e x p l a n a t o r y p u r po se s d ep e n d s upon
its validity, e v a l u a ti on s of the rel i a b i l i t y , h o m o g e n e i t y
and coverage of the s u b t y p i n g s o l ut io n w e r e made
susbsequent to the s t a t i s t i c a l c l a s s i f i c a t io n. Ex t e r n a l
v a l i d i t y of the t y p o l o g y w i l l be e v a l u a t e d b y c o m p a r i s o n
with a number of e x t e r n a l v a r ia b l e s , i n c l u d i n g a c a de mi c
performance, beha v i o u r a l , social, m e d i c a l a n d d e v e l o p m e n t a l
factors. This p r o c e d u r e is a l r e a d y in p r o g r e s s and will
R E V I E W OF THE L I T E R A T U R E
Th i s r e v i e w of the lit e r a t u r e w i l l b e g i n by dis c u s s i n g
the v a r io us issues r e l a t i n g to the d e f i n i t i o n and sampling
of a p o p u l a t i o n of l ea rn i n g d i s a b l e d children. The
l i t e r a t u r e w h i c h d e s c r i b e s the s e a r ch for sub t y p e s in
l e a r n i n g a n d r e a d i n g d i s a b i l i t i e s w i l l form the m a i n focus
of the review, and w i l l i nc l u d e s tu d i e s w h i c h involve
v a r i o u s d i f f e r e n t p o p u l a t i o n s am p l e s of l ea rn i n g disabled child r e n , as w e l l as a v a r i e t y of m e t h o d s u s e d in the c l a s s i f i c a t i o n . L i t e r a t u r e i n v o l v i n g the t heoretical a s p e c t s a n d p r ob l e m s of c l a s s i f i c a t i o n will also be r e v i e w e d . D e f i n i t i o n a n d S a m p l i n g Issues
R e s e a r c h into r e a d i n g and g e n e r a l lea r n i n g
d i s a b i l i t i e s in c h i l d r e n has b e e n c h a r a c t e r i z e d by
c o n f l i c t i n g a n d c o n f u s i n g results. It is g e n e r a l l y
c o n c l u d e d that the r e a s o n for this is b a s ed upon the lack
of a g r e e m e n t a b out t e r m i n o l o g y as w e l l as d e f i n i t i o n s in
r e g a r d to t h e s e dis o r d e r s . This, in turn, has r esulted in
w i d e v a r i a t i o n a m on g s t s u b ject s a m p l e s b e t w e e n studies, w h i c h can o b s c u r e r e s e a r c h f i n d i n g s a n d i n v a l i d a t e c o m p a r i s o n s b e t w e e n studies. A c e r t a i n p r o p o r t i o n of c h i l d r e n w i t h i n the regular sch o o l s y s t e m h a v e t r o u bl e a c q u i r i n g a c a d e m i c skills, for r e a s o n s a s s o c i a t e d w i t h m e n t al r e t a r d a t io n, gross
n e u r o l o g i c a l impairment, severe emotional disturbance,
c ultural d i s a d v a n t a g e or i n a dequate teaching. However,
since the e a r l y fifties, it has been r e c o g n i z e d that many
chi l d r e n e x p e r i e n c e c o n s i d e r a b l e d i f f i c u l t y in l e a r n i n g
basic a c a d e m i c skills, a l t h o u g h none of the above factors
seem to a c c o u n t for their pro b l e m s in learning.
S i nce the first case studies of c h i l d r e n with such
u n e x p l a i n e d le a r n i n g d i f f i c u l t i e s , some form of c o n g e n i t a l
d efect wi t h a n e u r o l o g i c a l em p h a s i s has b e e n assumed.
M o r g a n (1896) r e p o r t e d the ear l i e s t ca s e of what ca m e to be
k n own as " c o n g e n i t a l word blindness", and H i n s h e l w o o d
s u b s e q u e n t l y p u b l i s h e d two m o n o g r a ph s on this topic
(1900,1917). He d e f i n e d the c o n d i t i o n as a " c o n g e n i t a l
defect o c c u r r i n g in c h i l d r e n w i t h o t h e r w i s e normal
u n d a m a g e d brains, c h a r a c t e r i z e d by a d i s a b i l i t y in l e a r n i ng
to read so great that it is m a n i f e s t l y due to a
p a t h o l o g i c a l condition, a n d w h e r e a ttempts to teach the
c h ild by o r d i n a r y m e t h o d s ha v e failed". H i n s h e l w o o d
e m p h a s i s e d "the g r a v i t y of the d e f ect" and the "purity" of
the symptoms, which s h o u l d be ident i c a l to those c a s e s of
a c q u i r e d w o r d - b l i n d n e s s w i t h p r e s u m e d lesions in the
a n g u l a r g y r u s area of the brain. He al s o d i s t i n g u i s h e d
b e t w e e n this pure form and cases of c h i l d r e n v/ith m i l d e r
d i s o r d e r s e.g. some s l i g h t l y d e f e c t i v e d e v e l o p m e n t of
v i s u a l memory, and s u g g e s t e d the term "conge n i t a l d y s l e x i a "
a n d b e c a m e u s e d for all forms of reading disability,
w h e t h e r m i l d o r m o re severe.
T h e W o r l d F e d e r a t i o n of N e u r o l o g y agr e e d o n a
d e f i n i t i o n of d e v e l o p m e n t a l dyslexia, which reads as
follows: "A d i s o r d e r m a n i f e s t e d by a d i f f i c u l t y in learning
to read, d e s p i t e c o nv e n t i o n a l instruction, ade q u a t e
i n t e l l i g e nc e, and s o ci o- c u l t u r a l opportunity. It is
d e p e n d e n t up o n f u n d a m e n t a l c o g n i t i v e d if f i c u l t i e s w h i c h are
f r e q u e n t l y of a c o n s t i t u t i o n a l cha r a c t e r " (C r i t c h l e y ,1970).
The wo r k of Stra u s s and W e r n e r (1938), S t r a u s s and
L e h t i n e n (1948), S t r auss and K ep h a r t (1955), and
C r u i c k s h a n k (1966) was i nf lu en t i a l in e x p a n d i n g the c o n cept
of d y s l e x i a to e n c o m p a s s all forms of a c a d e m i c diffi c u l t y ,
i n c l u d i n g r e a d i n g dis o r d e r s . A n d K i r k (1963) is c r e d i t e d
w i t h c o i n i n g the term " s p e c i f i c l earning d i s a b i l i t y " to
d e s c r i b e this group.
The first w i d e l y a c c e p t e d d e f i n i t i o n of l e a r n i n g
d i s a b i l t i e s w a s the one put forth in 1975 by the 94th U.S.
C o n g r e s s , and reads as follows:
S p e c i f i c l e a r n i n g d i s a b i l i t y is a d i s o r d e r in o n e or m o r e of the b a s i c p s y c h o l o g i c a l processes i nv ol v e d in u n d e r s t a n d i n g or in u s i n g language, spoken or written, w h i c h m a y m a n i f e s t itself in an imperfect a b i l i t y to
listen, think, speak, read, write, spell or to d o m a t h e m a t i c a l c a l c u l a t i o n s . The term in c l u d e s such c o n d i t i o n s as p e r c e p t u a l handicaps, brain injury,
m i n i m a l b r a i n d y s f u n c t i o n , dyslexia, and d e v e l o p m e n t a l aphasia. T h e t e r m d o e s not include c h i l d r e n who have l e a r n i n g p r o b l e m s w h i c h are p r i m a r i l y the re s u l t of visual, h e a r i n g or m o t o r handicaps, of m e n t a l
environmental, or e c o n o m i c disadv a n t a g e .
Such d efinitions ha v e b e c o m e the source of much
c o n t r o v e r s y and criti c i s m over the years, both for the
a mb i g u i t y in t e rminology as well as for the fact that
d i a gn os is of dyslexia n e c e s s a r i l y b e co me s one of exclusion.
For example, Ru t t e r (1978) s ug g e s t e d that the a mb i g u o u s
w o r d i ng of the d e f i ni ti on c o uld i mply that a d ia g n o s i s of
" le a r n i n g disabled" was i n va li d if a c hi l d had visual or
h ea r i n g problems, a lower than a v e r a g e IQ score, came from
a poor family or had an u n c o n v e n t i o n a l background.
The res e a r c h group a s s o c i a t e d w i t h the Florida
L o ng i t u d i n a l Project (Satz, M o r r i s and colleagues) w er e
a mo n g s t the critics of the W o r l d F e d e r a t i o n definition, and
w e r e p a r t i cu la rl y c on c e r n e d a b o ut the a s s u m p t i o n in the
an d can be d if fe re n t i a t e d fr o m o t h e r d i s a b l e d readers.
U si ng this definition, t he y s e l e c t e d a g r o up of "dyslexic"
d is ab l e d readers, who m e t the e x c l u s i o n a r y criteria, and
c o m p a re d them to a m a t c h e d g r o u p of " no n - d y s l e x i c " d is a b l e d
readers, w h o had failed to meet o n e or m o r e of the
e x c l u s i o n a r y criteria. B o t h g r o u p s as w el l as a normal
c on t r o l g r o up re c e i v e d a n u mb er of n e u r o p s yc ho lo gi ca l,
e du c a t i o n a l and p e r s o n a l i t y tests, a nd the results showed
that both groups d i f f e r e d from the c o nt ro l group, but there
w e r e no d if f e r e n c e s b e t w e e n the " dy sl e x i c " and the non-
d y s l e x i c p o o r readers. T hi s s t u dy (Taylor, Satz & Friel,
w o r th le s sn es s of the e x c lu si on ar y criteria and the c l a ssic
d e f i n i t i o n of dyslexia, p a r t i c u l ar ly for the selec t i o n of
subjects from a g en e r a l school population. However,
a l t h o ug h the v ig o r o u s c r i t i c i s m l evelled against the W o r l d
F ed e r a t i o n d e f i n i t i o n of dys l e x i a is, no doubt, justified,
it should be m e n t i o n e d that both Be n t o n (1975) a n d Mat t i s
(1978) have since p ro p o s e d a l ternative definitions, w hi c h
have p os it i v e d e f i n i n g c riteria and a void e xc l u s i o n a r y
c l a u s e s .
The m e d i ca l c o n cept of dyslexia as a s y ndrome of
c o ns ti tu t i o n a l origin, w it h the ass u m p t i o n of a
n e ur ol og ic al basis, has a l s o been the source of mu c h
c o n t r o v e r s y o v e r the years. Although a neurological basis
for sp e c i f i c l e a r n in g d is ab il it ie s has never been
established, it r em a i n s a wi d e l y held premise, wi t h broad
a cc e p t a n c e by c l i n i c i a n s (G a d d e s ,1985; Hooper & B o y d , 1986;
R o u r k e ,1975). However, a lt ho u g h it is a likely p r es um pt io n
that d ys le xi a i n v o l ve s a d e v el op me nt al failure in neural
integration, for w h i c h t he r e is m uc h c i r c u m stantial support,
B e n to n (1975) has e m p h a s i s e d that it is still only a
presumption, l a c king any c lear sci e n t i f i c evi d e n c e to this
effect. N ev er th e l e s s , b as e d on this premise, it has been
a ss u m e d by p hy si c i a n s and many c li ni c i a n s that c h i l d r e n
with c o n s t i t u t i o n a l l y b a s ed reading or other learning
d i sa b i l i t i e s can and s h o u ld be c o n c e pt ua ll y se p a r a t e from
E d u c a t o r s and o t h e r p r o f e s s i o n a l s o b j e c t to this
a ssumption, and prefer to use an o p e r a t i o n a l d e f i n i t i o n of
p o o r r e a d i n g a b i l i t y in children, namely: a " r e t a r d e d
reader" is one whose reading a ch ie ve m e n t is b e l o w w ha t is
n o r m al ly e xp e c t e d for the c h i ld's age, a b i l i t y and g rade
level; a "serious" r e a d i ng d ef i c i t is d e f i n e d as two or
m o re years b e l o w grade level, and is termed a "disability",
f r e q u e n t l y a t t r i b u t ed to c u l tu r a l or e m o t i o n a l d e p r i v a t i o n
or o t h er e nv i r o n m e n t a l factors.
R u t t e r (1978) a ls o a r g ue d that the W o r l d F e d e r a t i o n of
N e u r o l o g y ' s d e f i n i t i o n was i m p r a c ti ca bl e for g e n e r a l use.
He p r e s e n t e d ev i d e n c e from e pi d e m i o l o g i c a l s t ud ie s in
L o n d o n a n d the Isle of Wight to show that, on s t a t i stical,
m e d i c a l a n d e d u c a t i o n a l grounds, the d i s t i n c t i o n b e t w e e n
two g r ou ps of u nd e r a c h i e v i n g readers (the " b a ck wa rd
r ea de r s " and those wi t h "spec i f i c r e a d i ng r et a r d a t i o n " ) was
valid. T h o s e w ho s e r e a di ng level was w e l l b e l o w that
e x p e c t e d for t he i r c hr o n o l o g i c a l age, but c o n s o n a n t w it h
t he i r IQ level were t e r m ed " ba c k w a r d reade r s " ; c h i l d r e n
w h o s e r e a d i n g a c h i e v em en t was low, a ft e r t a k i n g b o t h age
and IQ level into account, b e l o n g e d to the " s p e c i fi c
r e a di ng r e t a r d at i o n" g r ou p (Rutter, T iz a r d & W h i t m o r e , 1970;
R u t t e r & Y u l e , 1973,1975; Yule, Rutter, Berger, & Thompson,
1974). The f indings of such s t u dies are s u m m a r i z e d as
follows: a) The s y n d r o m e of " s p e c i f i c r e a d i n g r e t a r d a t i o n "
no r ma l a ch i e v e m e n t d i s t r i bu ti on , f o r ming a "hump" on the
l ow e r e n d of the d i s t r i b u t i o n curve; b) T here was a higher
r a t i o of boys to g irls in the S RR g ro u p ( 3.3 to 1),
w h e r e a s sex d i s t r i b u t i o n in the "general r e a ding
b a c k w a r d n e s s " (GRB) g r o u p w a s al m o s t e q ual (1.3 to 1); c)
O v e r t n e u r o l o g i c a l d i s o r d e r w as m o r e frequent in the GRB
g r o u p (11.4%) .is w er e w i d e r a n g e of "dubious" neurological
d ef ic it s, i nc l u d i n g motor, p r a x i c and speech a b n o r m a l i t i es
( 25.3%). In contrast, t h er e was no a s s o c i a t i o n with overt
o r "hard" n e u r o l o g i c a l s ig n s in the SRR group, a lt ho u g h
" du bi ou s" (or "soft") n e u r o l o g i c a l signs were evident to a
l es se r d e g r e e t h a n in the G R B g r o u p (18.6%). ("Soft" signs
c an r e f le ct e i th e r s t r u c t u r a l d e f e c t s in the b rain or
d e v e l o p m e n t a l delay), d) H ow ev er , the SRR was st r o n g l y
a s s o c i a t e d w i t h s p e ec h a n d l a n g u a g e impairments, e) A
g r e a t e r p r o p o r t i o n of the G R B g r o u p came from s o c i a ll y
d i s a d v a n t a g e d homes, and the f a m i l i a l i n c i d e n c e of reading
d i f f i c u l t i e s a n d d e l a y e d s p e e c h a c q u i s i t i o n in both
r e t a r d e d r e a d i n g gr o u p s w a s a b o u t three times that of the
co nt ro l group, f) O n f o l l o w - u p at a g e 14, d e s p i t e their
g e n e r a l l y h i g h e r IQ level, the S R R c h i l d r e n ma d e
s i g n i f i c a n t l y less p r o g r e s s t ha n the GRB g ro u p in read i n g
a n d spelling, but s i g i f i c a n t l y m o r e p r o gr e s s in arithmetic,
a l t h o u g h both g r o u p s w e r e s t i l l i m p a i r e d in all three
subjects.
w as based e n t i r e l y u p o n a s t a t i s t i c a l definition, the
concept of SRR is c le a r l y m e a ni ng fu l both c l i n i c a l l y as
w el l as in terms of prognosis. The high male and fa m i l i a l
in c i d en ce and the a b n o r m a l i t i e s in language d e v e l o p m e n t are
c h a r a c t e r i s t i c of g r o up s that c ould be diagn o s e d as
"dy s l e x i c " a c c o r d i n g to the W o r ld F ed e r a t i o n of N e u r o l o g y ' s
de f i n i t i o n . However, h a v i n g been very critical of the term,
"dyslexia", for not r e f e r r i n g to any w e l l - d ef in ed o r e a s i l y
d i a g n o s e d disorder, R u t t e r (1978) suggested the c o n c e p t of
S R R as an alte r n a t i v e , w h i c h c o u l d be ea s i l y d e f i n e d by
u s i n g the a pp r o p r i a t e r e g r es si on e quation for the
p r e d i c t i o n of a c h i evement, based upon the o bs er ve d
c or r e l a t i o n s b e tw ee n e du ca ti o n a l level, age and I Q in the
ge ne ra l p o pulation. In so doing, it w ou l d al s o be p os s i b l e
to avoid a n y i mp l i c a t i o n s of an u n d e r l y in g
n e u r o p a t h o l o g i c a l condition. R u t t e r (1978) al s o t o o k c a r e
to e m p h as is e that SRR was m u l t i f a c t o r i a l l y d e termined,
i n v o l vi ng such f a c tors as f a m i ly size. SES, l o c a t i o n of
habitat, type of school and t em pe ra m e n t a l c h a r a c t e ri st ic s,
p r es u m a b l y to a v o i d a n y s u g g e s t i o n of a u n i t a ry cause, such
as dyslexia.
In d i s c u s s i n g the r e l a t i o n s h i p b e t w e e n n e u r o l o g i c a l
i m p a ir me nt and l e a r n i n g d is ab i l i t i e s , S p r e e n (1989) cites
the above s e r ie s of e p i d e m i o l o g i c a l studies, p o i n t i n g out
the clear p ar al le l to a s tu d y by D i n g m an and T a r j a n (1960).
a t t e n t i o n to the e x pected f r e q u e n c i e s of v a r y i n g degrees of
i n te l l i g e n c e u nder the G a u ss ia n d i s t r i b u t i o n curve, and
c o n c l u d e d that there was an excess of cases at the lower
e n d of the d i s t r i b u t i o n (a result a l s o f ou n d by Roberts,
1952). The a ut h o r s p o s tulated that the ex c e s s was due to
p a t h o l o g i c a l factors (e.g. b r a in d a m a g e of pre-or p o s t
n a t a l origin) and that this p o p u l a t i o n c o uld be viewed as
h a v i n g a sep a r a t e d is t r i b u t i o n c u r ve (with a me a n of 32 and
a s t a n d a r d d e v i a t i o n of 16), w hi c h e xt e n d s well into the
n o r m a l IQ range. Spreen su g g e s t s that the excess in the
r a t e of the S R R g ro u p in R u tt er and Yu l e (1974,1975)
s t u d i e s m i ght also reflect a " pa t h o l o g i c a l excess" of an
o t h e r w i s e n or m a l l y d i s t r i bu te d ability, i.e."that we can
e x p e c t a c e r t ai n p r op or ti on of p e op le s p e c i f i c a l l y dis a b l e d
in intell i g e n c e , reading ability, a n d i n d ee d m us i c a l or
m a t h e m a t i c a l o r any other a b i l it ie s in any large pop u l a t i o n
sample. However, the p r o po rt io n at the low end of the
d i s t r i b u t i o n is hi g h e r than expected; this "excess" may be
c o n s i d e r e d as r e s u l t in g from p at h o l o g i c a l causes" (p. 393).
S p r e e n t e n t a t i v e l y concludes that a c e r t a i n p ro po r t i o n of
d y s l e x i c s sh o u l d be e xpected on the b asis of normal
d i s t r i b u t i o n o f abilities alone, but the m os t likely
e x p l a n a t i o n f or the "excess" of cases is a g e n e t i c
p r e d i s p o s i t i o n for n e u r o l og ic al d y s f u n ct io n, ci t i n g Regehr
R e s e a r c h Str a t e g i e s in the S t udy of L e a r n i n g Disa b i l i t i e s
Traditio n a l l y , d y s l e x i a was c o n c e p t u a l i z e d as a
u ni t a r y phenomenon, w i t h ma n y e a r l i e r resear c h e r s p r o p o s i n g
theories c o n ce rn i ng the b a s i c u n d e r l y i n g "cause" of the
problem. For example, O r t o n (1937) p r o po s e d inadequate
d e v e l o p me nt of h e m i s p h e r i c d o m i n a n c e as the underlying
problem; Keph a r t (1960) and C r u i c k s ha nk (1968) both
s ug ge st ed p e r c e p t u a l - m o t o r deficits; Smith and Carrigan
(1959) p ro po se d a t h e o r y of immature s y naptic t ra n s m i s s i o n
to a cc o u n t for this con d i t i o n , and V e l l u t i n o (1979)
p os t u l a t e d that a v e r b a l p r oc es si ng deficit was the single
u n d e r l y i n g fa c t o r in dyslexia. Similarly, some writers
c o n c e p t u a l i z e d a c o m m o n b a s i c d e f i c i t as u n d e r l y i n g the
b ro a d e r c a t e g o r y of s p e c i f i c l ea rn i n g d i s a b i l i ti es (for
example, Smith, Coleman, D o k ecki and D a v i s , 1971).
A s u b s t a nt ia l b o d y of \ e s ea r c h was p r o d u c e d b a s e d u p o n
this " un it ar y d e f i c i t " h y p o t h e s i s , w i t h p r e d i ct ab le
i n c o n s i s t e n c y in the findings. As D oe hr in g (1978) pointed
out, a l t h o u g h such s t u d i e s can o f t e n be c r i t ic iz ed in terms
of m e t h o d o l o g y as w e l l as i n te rp re ta ti on of results, the
basic p r o b l e m is the r e s e a r c h p a r a d i g m employed.
This "single s y n d r o m e parad i g m " in w h i c h groups of
poor r e a ders o r poor l e a r n e r s w e r e c o m p a r e d to groups of
normal learners in r e l a t i o n to a single a b i l i t y (e.g.
v i s u a l p e r c e ption), a s s u m e s that poor r e a ders r e p r e s e n t a
is e ss e n t i a l for learning to re a d (A p p l e b e e , 1 9 7 1 ; W ie n e r &
Cromer, 1967). Such a model, as Do e h r i n g (1978) points out,
has e n j o y e d a th r i v i n g e x i s t e n c e in l e a r n i n g d i s a b i l i t i e s
a nd d y s l e x i a r es e a r c h o ve r the years, and the n u merous
s t u d i e s u s i ng this p ar ad i g m have, in effect, p r o vi de d a m ple
e v i d e n c e to s h o w that a v e r y w i d e range of f a c tors are
a s s o c i a t e d w it h r e a ding a n d learning disabi l i t i e s . These
i n cl ud e the following: p r e -and p e r in at al factors (Kawi &
P a s a m a n i c k , 1959); finger l o c a l ization and r i g ht -l ef t
d i s c r i m i n a t i o n problems (Kinsbourne & W a rrington, 1963);
t em p o r a l o rd e r recall and s e q uencing as well as serial
p o s i t i o n i n g d e f i c i t s { Bakker, 1967, 1972; Corking, 1974);
b i s e n s o r y m e m o r y (Senf,1969; Senf & F r e u n d l ,1971);
p e r c e p t u a l - m o t o r m at ch i n g (Kephart, 1967; def i c i t s in
c e r e b r a l d o m i n a n c e (Orton, 1 928,1 93 7; Satz, R a r di n &
R o s s , 1971; Yeni-Kom s h i a n , I s e nb er g & G o l d s t e i n , 1975; Zurif
& C a r s o n , 1970); cro s s m o d a l integration (Birch & Belmont,
1 964,1965); and p sy ch ol i n g u i s t i c defi c i e n c i e s (D e n c k l a ,1972;
J o h n s o n & M y k l e b u s t ,1967; Wiig, Semel & C r o u s e , 1973, a m o n g s t
m a n y others). R u t t e r (1978) also men t i o n s t em p e r a m e n t a l
a t t r i b ut es , such as hyperactivity, poor c o n c e n t r a t i o n and
i m p u l s i v i t y as a s s oc ia te d factors (De H ir s h et al.,1966;
K a g a n , 1965).
The e v i d e n c e that a w i d e v ar i e t y of d ef ic it s we r e
a s s o c i a t e d w i t h r ea d i n g a n d learning d i s a b i l i t i e s p ro mp te d
a n e x p a nd e d version of the c o n t r a s t i n g g r ou p model. It
involved comparisons b e t w e e n groups of ret a r d e d r e a ders and
normal r e a d e r s based on their scores from a large n u m b e r of
c o g n i t i v e and n e u r o p s y c h o l o g i c a l m ea su re s (D o e h r i n g , 1968).
However, such a research d e s i g n a l s o had predic t a b l e
pro b l e m s r e l a t e d to the i n c o n s i s t e n c y of results b e t w e e n
studies, l a r g e l y due to the h e t e r o g e n e i t y of the d y s le x i c
groups. S u c h res e a r c h a p p r o a c h e s e v e n t u a l l y p r o vi d e d the impetus f o r c o n c e p t ua li z i ng l ea rn i n g d i s ab il it ie s in a m u l t i d i m e n s i o n a l fashion, w h i c h r e s u l t e d in the a t t e m p t to i d e n t i f y s p e c i f i c h o m o g e n e o u s sub t y p e s w i t h i n this p o p u l a t i o n of children, b as e d e i t he r o n p atterns of a c a d e m i c o r r e a ding skill d e f i c i t s or n e u r o p s y c h o l o g i c a l
v a r i a b le s (the "multiple s y n d rome" r e s e a r c h paradigm,
a c c o r d i n g to Doehring, 1 978). Such c l a s s i f i c a t i o n attempts,
however, we r e not en t i r e l y n e w ( p a r t i c u l a r l y in r e l a t i o n to
d y s l e x i a ) , as is shown in the f o l l o w i n g sect i o n of this
review, i n w h i c h classi f i c a t i o n s b a s e d upon e t i o l o g y a r e
outlined. Where the c u r rent a p p r oa ch es b r e a k ground,
however, is in a t t e m pt i n g to find p a r t i c u l a r subtypes of
the r e a d i n g d i s a b i l i t y itself, w h i c h c o n st it u t e s a
m u l t i p l e - s y n d r o m e model, w i t h i nf e r r e d m ul ti pl e etiologies.
One f u r t h e r aspect of r e s e a r c h s t r a t e g y involves the
s e l e c t i o n of subject samples, w h i c h t y p i c a l l y varies
Thus, in e a r l i e r studies, s u b ject selec t i o n was o ft e n b a sed
u p o n the e x c l u s i o n a r y c r i t e r i a of the c l a s s i c d e f i ni ti on of
dyslexia, c o m b i n e d wi t h a c l i n i c a l e x a m i n a t i o n w hich included
a m e a s u r e of r ea d i n g skills. However, w h e n d i s c u s s i n g the
s tu d y of dysle x i a , B e n t o n (1978) c o m m e n t e d upon the da n g e r
of a do pt in g t o o r e s t r i c t i v e a d e f i n i t i o n , w i t h c riteria
w h i c h m a y w e l l e xc l u d e m a n y c a ses w hi c h are relevant
e xa mp le s of a s pe ci f i c r e a d i n g (or learning) disability. He
e m p h a s i s e d the d a n g e r of e x c l u d i n g s o c ia ll y d is a d v a n t a g e d
and e m o t i o n a l l y d i s t u r b e d c h i l d r e n w i t h severe r e a ding (or
learning) p r o b l e m s from c o n si de ra ti on , b e ca us e this may
limit the d i a g n o s t i c c a t e g o r y of in t e r e s t to a very select
a n d "at y p i c a l " subgroup. B e n t o n s u g g e s t e d that it may,
therefore, be i m p o r t a n t to "cast a wi d e net" and s tudy
c h i l d r e n w i t h a w i d e v a r i e t y of p r o b l e m s and deficits,
i n c l u d i n g t h o s e w i t h n e u ro lo gi ca l, intellectual, social and
e m o t i o n a l h a n d i c a p s as w e l l as t h o s e w i t h o u t any r e a d i ng
(or learning) p r o b l e m s at all.
The e m e r g e n c e of m u l t i v a r i a t e s t a t i s t i c a l
c l a s s i f i c a t i o n t e c h n i q u e s has, in fact, made such an
a p p r o a c h to s u b j e c t s e l e c t i o n q u i t e feasible, and is the
m e t h o d e m p l o y e d in such c l a s s i f i c a t i o n s t u dies as Satz and
M o r r i s (1981) a n d Morris, B l a s h f i e l d and Satz (1986), to be
d i s c u s s e d l a t e r in this review. I m p o r t an t a s p ects of these
studies are: a) the i n c l u s i o n of no r m a l read e r s in the
t he s ubtypes on a large number of e x t e r n a l variables,
i n c l u d i n g birth data, n e u r o logical ratings, SES and
p a r e n t a l education, t e a cher ratings of b e h a v i o u r as w e l l as
p e r s o n a l i t y test data. In this way, e t i o l o g i c a l factors
h a v e a l s o been i ncluded into a c l a s s i f i c a t i o n of r e a d i n g -
d i s a b l e d children.
T he Se a r c h for Sub t y p e s
A t t e m p t s to c l a s s i f y s p ecific l e a r n i n g d i s a b i l i t i e s
d a t e from the 1960's and have i n v o l v e d s e v e r a l d i f f e r e n t
a p p r o a c h e s . A c l a s s i c a p p r o a c h follows the s i n g l e syndrome,
m u l t i p l e e t i o l o g y model, in w h i c h d y s l e x i a was still
t r e ated as an e n t i t y w h i l e m a n y e t i o l o g i c a l c a u s e s w e r e
s u g g e s t e d ( B a n n a t y n e , 1971; K e e n e y & K e e n e y , 1968;
R a b i n o w i t c h , 1968). Most of t hese e t i o l o g i c a l l y b a s e d
c l a s s i f i c a t i o n s d i s c r i m i n a t e d b e t w e e n a " s p e c i f i c (or
primary) d e v e l o p m e n t a l dyslexia", a n d a ty p e (or types)
w h i c h w e r e l a b e l l e d as " secondary" or " s y m p t o m a t i c " , i.e. r e a d i n g r e t a r d a t i o n w h i c h was s e c o n d a r y to o r g a n i c p a t h o l o g y of any sort, or r e s u l t i n g f r o m e m o t i o n a l or m o t i v a t i o n a l factors, c u l t u r a l d e p r i v a t i o n or o t h e r e n v i r o n m e n t a l c i r c u m s t a n c e s , i n c l u d i n g o v e r t b r a i n damage. Q u a d f a s e l a n d G o o d g l a s s (1968) s e p a r a t e d out the c a t e g o r y of e a r l y b r a i n d a m a g e into a d i s t i n c t ty p e of d y s l e x i a
(labelled sympto m a t i c ) , in w h i c h the r e a d i n g e r r o r s w e r e
s i m i l a r to those of the p r i m a r y d y s l e x i c categ o r y .
in c lude not o n l y r e a d i n g d i s a b i l i t i e s but all language
d i s o r d e r s in c h i l d r e n as well, u s i n g a hier a r c h i c a l model.
T h e groups in this c l a s s i f i c a t i o n included: intellec t u a l l y
retarded; e m o t i o n a l l y dis t u r b e d ; minimal n e u r o logical
d y s f u n c t i o n ; aphasic; dyslexic; autistic; as well as
l a n g u a g e d e p r i v a t i o n ; c u l t u r a l or e d ucational deprivation;
a n d g e n e t i c dyslexia.
A r i s i n g out of the v o l u m i n o u s re s e a r c h of the sixties
a n d s e v e n t i e s came the r e c o g n i t i o n of the h e t e r o g e n e i t y of
l e a r n i n g and r e a d i n g disorders, leading to n umerous studies
w h i c h a d o p t e d the " m u l t i p l e syndrome" m e t h o d of
c l a s s i f i c a t i o n . The two m a i n a p p r o a c h e s were: a) clinical-
inf e r e n t i a l , in w h i c h the d i v i s i o n into subtypes was based
u p o n c l i n i c a l i n t e r p r e t a t i o n of p s y c h o m e t r i c ass e s s m e n t
p r o f i l e s (Boder,1979; D e n c k l a ,1977; J o h n s o n & Myklebust,
1967; K i n s b o u r n e & W a r r i n g t o n , 1963; and Mattis, Fr e n c h &
Rapin, 1975), a n d 2) the use of empir i c a l and o b j e c t i v e
m u l t i v a r i a t e s t a t i s t i c a l c l a s s i f i c a t i o n p r o c e d u r e s such as
Q - T y p e f a c t o r a n a l y s i s or c l u s t e r a n alysis of p s y c h o m e t r i c
d a t a (Doehring & Hoshko, 1977; F i s k & Rourke,1979; Lyon,
1982; P e t r a u s k a s & R o u r k e , 1 9 7 9 ; Satz & M o r r i s ,1981). A
f u r t h e r d i m e n s i o n of c l a s s i f i c a t i o n i n volved the actual
p e r f o r m a n c e v a r i a b l e s used, such as w h e t h e r s u bjects we r e
c l a s s i f i e d a) o n the b a s i s of a c a d e m i c p e r f o r m a n c e (Boder,
1973; D o e h r i n g & H o s h k o , 1977), o r b) p r o c e s s i n g
test p e r f o r m a n c e (Petrauskas & Rourke 1979).
A l m o s t all the e a rly c l a s s i f i c a t i o n s w e r e c o n c e r n e d
w i t h r e a d i n g d i s a b i l i t i e s (dyslexia). This r e v i e w will,
therefore, b e g i n by d i s c u s s i n g the r elevant r e s e a r c h in
r e l a t i o n to r e a d i n g disabilities, and then g o on to r e v i e w
the s u b t y p i n g studies c o n c e r n i n g o t h e r l earning
d i s a b i l i t i e s .
C l i n i c a l I n f e r e n t i a l C l a s s i f i c a t i o n s
As e a r l y as 1896, Charcot p r o p o s e d the e x i s t e n c e of
two types of learners - vi s i l e and audile, a c c o r d i n g to
F r eud (1953). A l s o t e a c h e r s through the years ha v e a s s u m e d
f r o m o b s e r v a t i o n that c h i l d r e n with r e a d i n g d i f f i c u l t i e s
c ould be d i v i d e d into t h o s e with "audi t o r y c h a n n e l d e f i c i t s "
a n d those w i t h "visual c h a n n e l defic i t s " (Mann & Suiter,
1974). K i n s b o u r n e and W a r r i n g t o n (1963,1966) d i v i d e d
c h i l d r e n (and o n e adult) o n the basis of d i s c r e p a n c y scores
on the W I S C (VI Q / P I Q d i f f e r e n c e s of 20 points or more).
J u d g i n g f r o m t h e i r p e r f o r m a n c e on c e r t a i n tasks, t h ese a u t h o r s s u g g e s t e d two d i f f e r e n t types of r e a d i n g a n d w r i t i n g r e t a r d a t i o n , n a m e l y a) a language d e f i c i t group, a n d b) a G e r s t m a n n s y n d r o m e group, f r o m w h i c h they d r e w a n a l o g i e s to two s y n d r o m e s of c erebral c o r t i c a l d i s o r d e r in a d u l t s .
S m i t h (1970) a l s o e x a m i n e d WISC profiles, from w h i c h
w e r e i d e n t i f i e d t hree s ubtypes of r e t a r d e d readers: (1)
s p a t i a l a b i l i t y was intact, (2) Spatial-p e r c e p t u a l skills
we r e weak, w h i l s t a u d i t o r y s e q u e n c i n g and symbol
m a n i p u l a t i o n we r e intact, a n d (3) Both spatial a b i lity and
a u d i t o r y s e q u e n c i n g / s y m b o l m a n i p u l a t i o n were deficient,
s u g g e s t i n g a "mixed" subtype. B a t e m a n (1968) also
i d e n t i f i e d three groups of p o o r readers, based on
e x a m i n a t i o n of IT P A (Illinois Test of Psych o l i n g u i s t i c
Abi l i t i e s ) profiles: G r o u p 1 had go o d visual m e m o r y but
p o o r a u d i t o r y memory; G r o u p 2 had go o d a u d i t o r y m e m o r y but
p o o r v i s u a l memory; and G r o u p 3 had both poor visual and
a u d i t o r y memory. Similarly, Ingram, M a s o n and B l a c k b u r n
(1970) e x a m i n e d 82 h i g h l y p r e - s e l e c t e d children, with
r e a d i n g and spe l l i n g d i f f i c u l t i e s , e m p l o y i n g an
e x c l u s i o n a r y d e f i n i t i o n for "dyslexia". Th e y i d e n t i f i e d
th ree s u b g r o u p s of dyslexics: (1) an a u d i o p h o n i c type, with
p r o b l e m s in sound d i s c r i m i n a t i o n and sound blending, as
we l l as d i f f i c u l t i e s in p h o n i c analysis, (2) a visuo-
s p a t i a l type, wi t h d i f f i c u l t i e s in visual d i s c r i m i n a t i o n
a n d o r i e n t a t i o n (e.g. p,b, a n d d confusion) as w e l l as in
w o r d r e c o g n i t i o n , and (3) a m i x e d type, who had both
v a r i e t i e s of problems.
A l t h o u g h some of t h e s e s t u d i e s lacked a c o n t r o l g r o u p
of n o r m a l readers, and the s e l e c t i o n of subjects was not
c l e a r l y d e s c r i b e d in others, t h e r e is a great d e a l of
a g r e e m e n t in results, w h i c h a l l suggest a f undamental
subtypes of dyslexia, w i t h a t h ird s u b t y p e w h i c h is a
m i x t u r e of these two problems. J o h n s o n a n d M y k l e b u s t (1967)
a l s o a r g u e for two broad s u btypes of dyslexia: the a u d i t o r y
a n d the visual subtypes, a l t h o u g h these c o n c l u s i o n s are
b a s e d so l e l y on o b s e r v a t i o n and e x t e n s i v e c l i n i c a l
experience, a n d are not d e r i v e d from any s p e c i f i c studies.
However, they d e s c r i b e t h ese p a r t i c u l a r t y pes of r e a d i n g
pro b l e m s in such detail, w i t h s p e c i f i c e x a m p l e s of
p a r t i c u l a r d e f i c i t s and the a s s o c i a t e d a c a d e m i c
diffic u l t i e s , that their c o n t r i b u t i o n to the f i e l d of
l e a r n i n g d i s a b l i t i e s has be e n mo s t influential.
The s t u dies d e s c r i b e d a b o v e w e r e a m o n g s t t h e f i rst
s u g g e s t i n g that there m i g h t be two or m o r e s u b t y p e s of
r e a d i n g problems, and b y the 1970's, t h e r e was i n c r e a s i n g
a c c e p t a n c e of the fact that d y s l e x i a was not a h o m o g e n e o u s
d i a g n o s t i c entity. A m o n g the n o t a b l e r e s e a r c h e r s w h o a t t e m p t e d to d e m o n s t r a t e this h e t e r o g e n e i t y are B o d e r (1973), Mattis, French a n d R a p i n (1975), D e n c k l a (1977, 1979), M y k l e b u s t (1978) a n d B a k k e r (1982), all of w h o m ha v e a d o p t e d a c l i n i c a l - i n f e r e n t i a l , t h e o r e t i c a l or o t h e r w i s e n o n - s t a t i s t i c a l a p p r o a c h to typology. T h e i r c o n t r i b u t i o n s w i l l be d i s c u s s e d s e p a r a t e l y below. W h i l e a c k n o w l e d g i n g
t h a t there are a number of o t h e r s w h o h a v e c o n t r i b u t e d to
s u b t y p e research, i n c l u d i n g P i r o z z o l o (1979) a n d A a r o n
(1978,1982), the present r e v i e w must n e c e s s a r i l y be
r e p o r t e d two b a s i c s u b t y p e s of r e a d i n g d i s abilities, which
i n v o l v e d the d i f f e r i n g p r o c e s s i n g skills of the two
he m i s p h e r e s , b ased u p o n p r e c i s e m e a s u r e m e n t s of eye-
m o v e m e n t s (Pirozzolo) or d i f f f e r i n g i n f o r m a t i o n pro c e s s i n g
s t r a t e g i e s (Aaron).
Ingram, M a s o n and B l a c k b u r n (1970) had i d e ntified
t h r e e s u b t y p e s of d y s l e x i c c h i l d r e n on the basis of their
p e r f o r m a n c e o n r e a d i n g a n d s p e l l i n g tasks. B o d e r (1971,
1973), how e v e r , was the f irst to d e f i n e s p e c i f i c d i a g n o s t i c
c r i t e r i a for i n c l u s i o n in ea c h g r o u p of d y s l e x i c subtypes,
o n the b a s i s of their r e a d i n g - s p e l l i n g errors. Her research
is n o t e w o r t h y b e c a u s e of the c a r e f u l q u a l i t a t i v e as well as
q u a n t i t a t i v e a n a l y s i s of su c h errors, by w h i c h she was able
to c l a s s i f y a p r e - s e l e c t e d g r o u p of d y s l e x i c c h i l d r e n (who
f i t t e d the s t a n d a r d d e f i n i t i o n ) into three subtypes: (1) a
" d y s p h o n e t i c " group, w h e r e the r e a d i n g - s p e l l i n g p a t t e r n
r e f l e c t e d a p r i m a r y d e f i c i t in p h o n e t i c w o r d - a n a l y s i s and
s y n t h e s i s skills, so that w o r d s w e r e re a d as wholes, with
s u b s t i t u t i o n o f s e m a n t i c a l l y s i m i l a r r a t h e r than
p h o n e t i c a l l y s i m i l a r words. T h e largest p r o p o r t i o n (67%) of
t h e s a m p l e b e l o n g e d to this group; (2) 10% o f the subjects
w e r e c l a s s i f i e d into a " d y s e i d e t i c " group, w h e r e the prim a r y
d e f i c i t i n v o l v e d the v i s u a l p e r c e p t i o n of w h o l e w o r d s or
the r e v i s u a l i z a t i o n of w o r d s in spelling; (3) a m i x e d
" d y s p h o n e t i c - d y s e i d e t i c " g r o u p w h e r e b o t h t ypes of reading/
