The startle (eye blink) response is a defensive reflex, which occurs during the presentation of an unexpected auditory stimulus (probe). Its strength is measured by the electromyographic (EMG) changes in the orbicularis oculi region of the eye and is highly sensitive to ambient emotional cues. It is well established that the magnitude of the blink (EMG) response to the startling acoustic probe is enhanced when subjects view unpleasant or negative emotional pictures in comparison to pleasant and neutral pictures (e.g., Cook, Davis, Hawk, Spence, & Gautier, 1992; Vrana, Spence, & Lang, 1988), whereas the response is reduced during viewing of positive as opposed to negative or neutral emotional cues (Cuthbert, Bradley, & Lang, 1996). Startle potentiation by unpleasant stimuli is thought to reflect activation of the aversive/defensive motivational system, while startle inhibition by positive emotional cues may reflect activation of appetitive or approach motivational processes (Lang, Bradley, & Cuthbert, 1990; Lang, Bradley, & Cuthbert, 1992). The neural circuitry underlying the startle response may involve several structures also associated with nicotine administration and withdrawal, such as the prefrontal cortex, hippocampus, amygdala,
