 Procedure: Acoustic stimuli will be delivered binaurally through headphones Maico,TDH39P. Acoustic startle stimuli will be 40 ms 116 dB white noise bursts with near instantaneous rise time delivered by a computerized electromyographic EMG startle response monitoring system SR-LAB, San Diego Instruments. The eyeblink component of the acoustic startle response will be measured via EMG of the right orbicularis oculi muscle. EMG activity will be amplified and digitized using the SR-LAB San Diego Instruments. According to methods established by Davis and Grillon Grillon et al., 1991; Grillon and Ameli, 1998, aversive stimuli US will be 40-250 ms airblasts with an intensity of 100 psi directed to the larynx, emitted by an compressed air tank attached to a polyethylene tubing and controlled by a solenoid switch. A, B, C, and X will be green, purple, orange, or blue lights counterbalanced color assignment across subjects or pictures. Subjects will be asked to press a button to indicate that they have learned the discrimination between the different colored lights or pictures. The test session will begin with startle probes to reduce initial startle reactivity and rule out nonstartlers. The training phase of the session will consist of trials in which stimuli A and X are paired with the US AX, and trials in which stimuli B and X are not paired with the US BX-. The test phase will include startle probes alone or during presentation of the cues AB and AC the order will be counterbalanced across subjects. Between the presentation of AB and AC, there will be reinforced trials of AX. Total time of test session will be approximately 20 minutes. 