Spatial working memory function in twins with schizophrenia and bipolar disorder.
BACKGROUND
Family studies are in conflict as to whether schizophrenia and bipolar disorder have independent genetic etiologies. Given the relatively low prevalence (approximately 1%) of these disorders, the use of quantitative endophenotypic markers of genetic liability might provide a more sensitive strategy for evaluating their genetic overlap. We have previously demonstrated that spatial working memory deficits increase in a dose-dependent fashion with increasing genetic proximity to a proband among the unaffected co-twins of schizophrenic patients. Here, we evaluated whether such deficits might also mark genetic susceptibility to bipolar disorder.


METHODS
The Wechsler Memory Scale-Revised Visual Memory Span and Digit Span subtests were administered to 46 schizophrenic patients, 32 of their unaffected co-twins, 22 bipolar patients, 16 of their unaffected co-twins, and 100 control twins, representing unselectively nationwide twin samples.


RESULTS
Schizophrenic patients and their unaffected co-twins performed significantly worse than control subjects on the spatial working memory task, whereas only the schizophrenic patients performed significantly below the control subjects on the verbal working memory task. Neither bipolar patients nor their unaffected co-twins differed from control subjects on these measures.


CONCLUSIONS
Our findings support the hypothesis that impairment in spatial working memory might effectively reflect an expression of genetic liability to schizophrenia but less clearly to bipolar disorder.
Pirkola
Tiia
T
Department of Mental Health and Alcohol Research, National Public Health Institute, Helsinki, Finland. tiia.pirkola@ktl.fi
Tuulio-Henriksson
Annamari
A
Glahn
David
D
Kieseppä
Tuula
T
Haukka
Jari
J
Kaprio
Jaakko
J
Lönnqvist
Jouko
J
Cannon
Tyrone D
TD
D000368:Aged; D001714:Bipolar Disorder / Q000523:psychology*; D015331:Cohort Studies; D004493:Education; D005260:Female; D005387:Finland; D006801:Humans; D007360:Intelligence; D016014:Linear Models; D008297:Male; D008570:Memory, Short-Term / Q000502:physiology*; D008875:Middle Aged; D011569:Psychiatric Status Rating Scales; D012565:Schizophrenic Psychology; D013028:Space Perception / Q000502:physiology*; D014427:Twins