Berridge, K. C., & Winkielman, P. (2003). What is an unconscious emotion: The case for unconscious 'liking'. Cognition and Emotion, 17, 181-211.

Ever since William James, psychologists of emotion have tended to view affective states as intrinsically conscious.  We argue that nonconscious affect also exists, and focus specifically on the possibility of unconscious 'liking'.  We present evidence that positive and negative affective reactions can be elicited subliminally, while a person is completely unaware of any affective reaction at all (in addition to being unaware of the causal stimulus).  Despite the absence of any detectable subjective component of emotion, subliminally-induced unconscious ‘liking’ can influence later consumption behavior.  We suggest that unconscious ‘liking’ is mediated by specific subcortical brain systems, such as the nucleus accumbens and its connections.  Ordinarily, conscious liking (feelings of pleasure) results from the interaction of separate brain systems of conscious awareness with those core processes of unconscious affect.  But under some conditions, activity in brain systems mediating unconscious core ’liking’ may become decoupled from conscious awareness.  The result is a genuinely unconscious emotion.