Invariance of symptoms of mood disorder in people with temporal lobe epilepsy TLE Stephen Bowden, Ra

1
Invariance of symptoms of mood disorder in
people with temporal lobe epilepsy (TLE)Stephen
Bowden, Rachel Reilly,Fiona Bardenhagen and Mark
Cook.University of Melbourne St. Vincents
Hospital, Melbourne.
INTRODUCTION To fully represent a theoretical or
latent-trait structure of behaviour, it is
necessary to model observed variable intercepts
and factor means in addition to the familiar
factor loadings, factor variances and
covariances, and residual variances (Meredith,
1993 Widaman Reise, 1997). Represented in this
way, it is possible to fully examine measurement
model invariance across groups. The centrality of
assumptions of invariance for fairness in
testing, and theoretical equivalence of validity
patterns are widely acknowledged (Byrne et al.,
1988 Meredith, 1993). Since identification of
clinical disorders usually involves
interpretations of patterns of means or patterns
of validity correlations, it is important to
examine the assumptions of invariance. Widaman
and Reise (1997) have defined several levels of
invariance. Configural invariance involves the
finding of a similar pattern of factors and
indicator-variable assignments. Configural
invariance is seen in the type of factor-analysis
replication commonly reported in the clinical
literature and, when reported from different
samples, indicates a similar, but not necessarily
identical pattern of latent variables structure.
In contrast metric invariance involves
successively more restrictive assumptions about
the measurement model, (i) weak metric invariance
assumes identical factor loadings, (ii) strong
metric invariance assumes identical observed
variable intercepts, and (iii) strict metric
invariance assumes identical reliabilities,
across groups. If strong metric invariance
holds, group differences in both means and
variances on the latent variables, which
represent the constructs in psychological
theories, are reflected in group differences in
means and variances on the measured variables
(Widaman Reise, 1997, p. 295). In clinical
evaluation of psychological adjustment amongst
people diagnosed with TLE there is much
speculation about the meaning of psychological
symptoms. Some have suggested that symptoms of
distress and poor adjustment should be
interpreted as features of personality and
psychopathology specific to people with TLE
(Blumer, 1999). In contrast, others have
suggested that exotic formulations of
psychopathology distract from recognition and
treatment of familiar psychological disorders,
commonly seen in many people with significant
disability (Devinsky, Souhel, 1999). Central to
the debate are symptoms of depression. If
symptoms of mood disorder in people with TLE
reflect behaviours specific to this clinical
population, then it is reasonable to expect that
elements of a measurement model of depression
should differ from the model of depressive
symptoms seen in other samples. In contrast, if
we were to observe invariance of depressive
symptoms across samples, then this observation
would provide compelling evidence for the
assumption that symptoms of mood disorder in
people with TLE should be interpreted and treated
as depression.
RESULTS Data analysis Scores on the BDI in both
samples were subjected to maximum-likelihood
confirmatory analysis using Lisrel 8.12
replicating, in both sample, the oblique
three-factor solution reported by Byrne (1998),
which measure the latent variables of (i)
Negative Affect, (ii) Work/Performance
Inhibition, and (iii) Somatic Elements. When this
three-factor baseline model was analysed
simultaneously in both groups, using the same
method of model identification to that reported
by Widaman Reise (1997, p. 305) configural
invariance was observed and the resulting fit
statistics are reported in Table 1 (Model 1).
Next, invariance constraints were placed on the
factor-loading (?) matrix, the variable intercept
(t) vector, and the error variance matrix (T), to
test the weak, strong, and strict invariance
models (Models 2-4), respectively. The increments
in ?2 and values of other goodness of fit
statistics reported in Table 1, suggest that
strong invariance obtains, and strict invariance
is a plausible assumption.

__________________________________________________
_____________________________________________ Tabl
e 1 Invariance Model ?² df ??²
?dF RMSEA SRMR CAIC TLI CFI _____________________
__________________________________________________
________________________ 1 Baseline 619.41
372 .045 .066 1519 .86 .87 (configural
invariance) 2 Weak 642.43 390 23.02 18
.044 .075 1419 .86 .87 (Model 1 and ?
invariant) 3 Strong 669.62 408 27 19
18 .044 .075 1324 .86 .87 (Model 2 and t
invariant) 4 Strict 708.78 429 39.16 21
.044 .079 1220 .86 .86 (Model 3 and T
invariant) ______________________________________
__________________________________________________
________

DISCUSSION The results of the present study show
that a well validated model of depressive
symptoms is invariant across samples of patients
with TLE and heterogeneous neurological
disorders. Initial modeling (Model 1), showed
configural invariance across samples, which
implies that the same latent variable structure
is measured by an identical pattern of items from
the BDI. In other words, it can be assumed that
the BDI measures a set of depressive symptoms in
people with TLE, which is the same as that
observed in our neurological reference group, and
has been observed in numerous other samples
(Byrne, 1998). In addition, the measurement
model displayed precise evidence of strong metric
invariance (Model 3), which implies that the same
set of latent variables is measured across
groups, with the same metric. The assumption of
strong metric invariance is necessary for the
uncomplicated (scale equivalent) interpretation
of patterns of elevated scores, and external
validity patterns (Widaman Reise, 1997).
Overall, this pattern of strong factorial
invariance provides a precise test of the
assumption that the behaviours underlying scores
on the BDI are the same in people with TLE, as
they are in people with other neurological
conditions, and numerous other community samples
(Byrne, 1998). The results suggest that exotic
formulations of personality structure and
depressive symptoms in people with TLE should be
avoided, and the symptoms of mood disturbance
should be treated in the same way as in other
clinical populations, namely, as potential
markers of depression. Strong validity data, of
the kind reported in this study, should sensitise
clinicians to the presence of mood disturbance in
patients with organic conditions.
METHOD Samples Two samples were employed for
this study, the first comprising 187 consecutive
patients undergoing video-telemetry at St.
Vincents Hospital Melbourne, for the
investigation of TLE. The second sample comprised
150 consecutive patients admitted to the Clinical
Neurosciences Department for investigation and
treatment of heterogeneous neurological
disorders, but not seizure disorders. The sample
of patients with TLE comprised 94 females and 93
males, with a mean age of 35.6. The neurological
sample comprised 59 females and 91 males, with a
mean age of 46.0. Measure Depressive symptoms
were measured with the Beck Depression Inventory
(BDI), an instrument which is based on a
well-validated clinical model of depression
(Byrne, 1998 Tanaka Huba, 1984). Mean BDI
score for the TLE sample was 10.6, and for the
neurological sample 12.8.

Address for correspondence s.bowden_at_psych.unimelb
.edu.au
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