To the Editor: We acknowledge that the design of every study is strongest when groups that are studied are matched for several dependent variables. However, we argue that the data disputed by Drs. Smeets, Jelicic, and Merckelbach are strong and valid. Hippocampal volume was 19.2% smaller and amygdalar volume was 31.6% smaller in the patients with dissociative identity disorder relative to the healthy subjects. Statistical rigor required us to control for the age difference (dissociative identity disorder patients [42.8 years] versus comparison subjects [34.6 years]). In doing so, our findings left only the right amygdalar volume significantly smaller across groups. However, we argue that the comparison data for the hippocampal and amygdalar volumes are valid without correction for this factor. Even though age-related structural alterations in the hippocampus have been identified, there are no previous findings of a relationship between age and hippocampal or amygdalar volume in healthy women in the 20–50-year age group (1, 2), the age group represented by the subjects in our study. A significant negative correlation of age with both left and right hippocampal volumes has been found only in men (a reduction in hippocampal volume of about 1%–1.5% per year). No significant effect of age has been found for amygdalar volume in either men or women. Even when some of these women have entered menopause, no difference in hippocampal volume in pre- versus postmenopausal women has been found (2). The study they cite by Raz and colleagues found differences only for individuals over age 50. There is some evidence in elderly individuals (aged >70) for modest reductions in hippocampal volume with late stages of aging (3, 4). Even if our comparison group had been older, the difference in volumes would still have been present. Accordingly, we argue that the significance levels for the between-group comparisons of both hippocampal and amygdalar volumes are valid without correction for this factor.
1.Pruessner JC, Collins DL, Pruessner M, Evans AC: Age and gender predict volume decline in the anterior and posterior hippocampus in early adulthood. J Neurosci 2001; 21:194–2002.Sullivan EV, Marsh L, Pfefferbaum A: Preservation of hippocampal volume throughout adulthood in healthy men and women: Neurobiol Aging 2005; 26:1093–10983.Lupien SJ, de Leon M, de Santi S, Convit A, Tarshish C, Nair NP, Thakur M, McEwen BS, Hauger RL, Meaney MJ: Cortisol levels during human aging predict hippocampal atrophy and memory deficits. Nat Neurosci 1998; 1:69–734.Sullivan EV, Marsh L, Mathalon DH, Lim KO, Pfefferbaum A: Age-related decline in MRI volumes of temporal gray matter but not hippocampus. Neurobiol Aging 1995; 16:591–606