Eyelid conditioning and extinction procedures were embedded in a masking probability learning task. On non-CS trials interspersed with extinction trials, a 100% masking (100% M) group received the US along with the masking stimuli, whereas a masking + free-shock (M + FS) group received US-alone presentations. A control (C) group received the same stimuli as did Group 100% M but with neutral instructions. Group 100% M revealed more, but Group M + FS revealed less, resistance to extinction than did Group C. Only the M + FS group extinguished completely.
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