Large individual differences exist in the absolute magnitude (μV) of the human startle response to an acoustic probe as measured by EMG activity in the orbicularis oculi muscle (Blumenthal et al., 2005). One way to minimize the variance due to individual differences and improve the probability of detecting the variance due to experimental conditions is to use standardized scores (Blumenthal et al., 2005). Prior to analyses, valence-related startle responses were standardized using each participant's own startle response elicited during the ITI, when no background cues were presented, as the reference distribution (Bonnet, Bradley, Lang, & Requin, 1995). First, we calculated the overall means and standard deviations of all ITI-related startle responses for each participant across all four sessions. We then applied the equation T score = 50 + 10 * [(startle response to valence condition – overall mean of ITI-related startle response)/overall standard deviation of ITI-related startle response] to all valence-related responses to convert the absolute startle amplitudes to T scores. Outlying responses, defined as observations three standard deviations above the mean T score of 50, were identified and excluded
