Fearful and Sexual Pictures Not Consciously Seen Modulate the Startle Reflex in Human Beings

Background Animal and human studies consistently have demonstrated that the startle reflex elicited by intense auditory stimulation is enhanced by the previous presentation of fear-evoking stimuli. There is also growing and varied evidence of the nonconscious processing of fear stimuli in human beings eliciting brain and autonomic fear responses. Methods We report two studies using the startle probe paradigm and the backward-masking procedure to examine the modulation of the eye-blink component of the startle reflex by consciously and nonconsciously presented emotional pictures. Results Conscious and nonconscious presentation of fearful pictures amplified the magnitude of the startle reflex in both studies. The opposite tendency was observed for conscious and nonconscious presentation of sexually attractive pictures in the second study. Conclusions These findings support the notion that negative (and possibly positive) biologically relevant stimuli can be nonconsciously processed, presumably via amygdala activation, and can affect behavioral responding.