Earlier twin studies dealt mostly with evoked potentials elicited in different sensory modalities (for a more detailed review of genetic studies of evoked potentials, see van Beijsterveldt and van Baal (2002)). For example, Lewis et al. studied visual, auditory, and somatosensory evoked potentials in 44 MZ, 44 DZ, and 46 unrelated individuals and found that evoked response waveforms of MZ twins showed a consistently higher degree of similarity than those of the DZ or UR groups across all three sensory modalities and in most of the scalp areas studied. (Maryutina, 1994) used different types of stimuli and noted greater heritability for sensory stimuli relative to semantic stimuli, where shared environmental factors were also significant. Overall, studies of sensory evoked potentials show significant heritability of both amplitude and latency of early ERP components; however, differences in heritability between specific ERP parameters of conditions should be interpreted with caution because most of these earlier studies were based on relatively small samples of twins.
