Another widely used paradigm involving startle response is based on the well-established fact that the intensity of startle reflex depends on the ongoing motivational and affective state of the subject. Both animals and humans studies have shown that this obligatory defensive reflex is facilitated by aversive/defensive motivational states and attenuated by appetitive states (Lang et al., 1993). In humans, startle modulation is typically achieved by pictures with different affective content: pleasant, neutral, and unpleasant. Individual differences in the extent of startle suppression by pleasant pictures and/or potentiation by unpleasant, especially, threatening pictures is viewed as an objective measure of relative sensitivity of the appetitive and aversive motivational systems respectively. The first twin study of emotion-modulated startle (17 MZ and 12 DZ pairs) estimated twin correlations that were suggestive of significant genetic influences on the degree of emotional modulation of startle, but a formal genetic analysis was not performed due to the small sample size (Carlson et al., 1997). However, a subsequent larger study (Anokhin et al., 2007a) failed to show genetic or shared environmental influences on the amount of startle
