Is basic memory structure invariant across epilepsy patient subgroups?
It is well known that epilepsy patients often exhibit material-specific deficits in memory for verbal versus nonverbal material. However, it is also apparent that such deficits are not always discernible, and that numerous factors may potentially moderate the degree to which fractionation between verbal and nonverbal material is evident. Using confirmatory factor analysis, we compared the relative fit of one-factor (general) and two-factor (material-specific) models of memory in data from 330 patients with intractable seizure disorder. Data from verbal and nonverbal selective reminding tests, as well as Logical Memory and Visual Reproduction (immediate and delayed recall indices), were used in analyses. The one-factor model fit the data poorly in the full sample, whereas the two-factor model fit the data significantly better. We then assessed the invariance of these two models using multiple-group modeling across subsamples of patients with left versus right sided seizure focus, earlier versus later age of seizure onset, lower versus higher Full Scale IQ, fewer versus more years of education, younger versus older patients, and male versus female patients. In all cases, the two-factor model comprised of verbal and nonverbal memory factors fit the data better than a one-factor (general) model. These findings indicate robust consistency of verbal and nonverbal memory constructs in the epilepsy population, which remain viable in this patient group despite considerable heterogeneity in other respects.