Dissociating unconscious processes through differential physiological responses

Recent studies show physiological responses to images suppressed from awareness, indicating unconscious perception. Such findings demonstrate similarities between conscious and unconscious perception, but the subjective difference between these modes raises the question of how they differ at the physiological level. Consciously viewed emotional images evoke a coherent array of physiological responses across multiple systems, but to date, physiological responses produced without stimulus awareness have largely been assessed using single measures. Would emotional images presented outside awareness also evoke a coherent set of physiological responses? Here, we measured four responses simultaneously – electrodermal response, phasic heart rate, startle eyeblink reflex, and post-auricular reflex. Participants viewed arousing emotional images (both positive and negative), which were occasionally accompanied by sudden bursts of white noise to elicit startle reflexes. For half of the participants, the images were masked from awareness using continuous flash suppression. The aware group showed the typical coherent pattern of physiological responses to emotional (compared to neutral) images: larger electrodermal responses, increased heart rate deceleration, potentiated eyeblink reflexes during negative images and potentiated post-auricular reflexes during positive images. In contrast, we found a striking dissociation between measures for the unaware group: electrodermal responses and post-auricular reflexes were sensitive to emotion, but heart rate deceleration and eyeblink reflexes were not. Our findings suggest that although a specific subset of physiological systems can respond to emotional stimuli presented outside of awareness, indicating unconscious processing, consciousness may be characterised by coherent emotional responses across physiological systems.