The sending and receipt of non-verbal messages are expressed and perceived in a subliminal way; some behavior are so little, like eyebrow flash or tongue-showing, that we made them without awareness.
Sometimes, nonverbal movements are coarse and showy, line changing of posture, crossing arms o legs, but even we should see them, we usually don’t remark them or don’t think they have a meaning; in this situations, they are not imperceptible, but they are virtually subliminal.
In classical scientific researches or in advertising subliminal visual stimuli are words or pictures that are presented so as to be unidentifiable to the viewer’s conscious perception.
For example, images may be flashed before the eye too quickly for the conscious mind to apprehend. Such stimuli can nevertheless exert an effect on judgment and behavior.
In recent years, scientists begun to use as subliminal stimuli facial expressions shown for a brief moment and functional magnetic resonance imaging to see how the brain reacts; in this way, they found out that the amygdala, a primitive structure of the cetral nervous system, is a key structure involved in the rapid and unconscious processing of facial emotions.
Other researchers observed that amygdala process even other kind of nonverbal behaviors.
In conclusion, we can state that amygdala process nonverbal messages without our awareness.
Below you will find a few articles on this astonishing topic.

.  Author: Timothy E. Moore
Review:Gale Encyclopedia of Psychology, 2nd ed. Gale Group, 2001
Title: Subliminal influence

.  Authors: Piotr Winkielman, Kent C. Berridge, and Julia L. Wilbarger
Source:Glendon College, York University
Title: Unconscious Affective Reactions to Masked Happy Versus Angry Faces Influence Consumption Behavior and Judgments of Value PDF

.  Author: Randy Dotinga
Source: Health Day News
Title: Subliminal Smiles Can Sway You

.  Authors: Mathis D,  Mozer M
Source: Web Page – Abstract (one can donwload the full text article)
Title: Conscious and unconscious perception: A computational theory

.  Authors: Merikle P, Daneman M
Review: Journal of Consciousness Studies, 1998, 5
Title: Psychological investigations of unconscious perception – PDF

.  Authors: Merikle, P. M., & Joordens, S.
Source: Scientific Approaches to Consciousness
Title: Measuring unconscious influences – PDF

.  Authors: Merikle PM, Reingold EM
Source: Journal of Experimental Psychology: General, 127, 304-310
Title: On demonstrating unconsc. perception: Comment on Draine and Greenwald – PDF

.  Authors: Philip M. Merikle and Eyal M. Reingold
Source: Perception without awareness , 1992 (pages  55-80). New York: Guilford Press.
Title: Measuring unconscious perceptual processes

.  Authors: Visser, T. A. W., & Merikle, P. M.
Review: Consciousness and Cognition, 8, 94-113.
Title: Conscious and unconscious processes: The effects of motivation – PDF

.  Authors: Merikle, P. M., Smilek, D., & Eastwood, J. D.
Source: Cognition, 79, 115-134.
Title: Perception without awareness: Perspectives from Cognitive Psychology – PDF

.  Authors: Reingold EM, Merikle PM
Source: Perception & Psychophysics, 1998, 44, 563-575
Title: Using direct and indirect measures to study perception without awareness

.  Authors: Reingold EM, Merikle PM
Source: Mind & Language, 1990, 5, 9-28
Title: On the inter-relatedness of theory and measur. in the study of unconscious processes

.  Authors: Eyal Reingold and others
Source: different reviews
Title: Eyal Reingold’s collection of articles on Subliminal Percep. and related Topics

.  Authors: Hakwan C. Lau1, and Richard E. Passingham1,
Source/Review: The Journal of Neuroscience, May 23, 2007 • 27(21):5805–5811
Title: Unconscious Activation of the Cognitive Control System in the Human Prefrontal Cortex PDF

.  Author: Hellew L
Source: Web Page
Title: Unconscious Processing: Subliminal Perception, Neuropsychology, and the I-Function

.  Author: Timothy E. Moore
Source: Skeptical Inquirer
Title: Scientific Consensus and Expert Testimony: Lessons from the Judas Priest Trial

.  Authors: Draine S, Greenwald T
Souce: Different Review and Web Documents
Title: Sean Draine & Tony Greenwald’s Abstracts on Subliminal Perception

.  Author: Stark Todd
Source: Web Article
Title: Is Subliminal Influence Used in Advertising?

.  Authors: Morris JS, Öhman A, Dolan R
R eview: Nature, 1998, 393, pages 467 – 470
Title: Conscious and unconscious emotional learning in the human amygdala

.  Author: Stark Todd
Source: Web Article
Title: How and Why Does Subliminal Influence Work?

.  Author: Stark Todd
Source: Web Article
Title: Is Subliminal Influence a Threat?

.  Author: Stark Todd
Source: Web Article
Title: Do Subliminal Audio Tapes Work?

.  Authors: Takeo Watanabe, José E. Nanez & Yuka Sasaki
Review: Nature, 413, 844-848, 25 October 2001
Title: Perceptual learning without perception

.  Author: Phil Merikle
Review: Different Journals
Title: P. Merikle’s Articles and Papers

.  Author: Nick Epley
Source: Cognitive Studies/Computer Science/Psychology/201/Cognitive Science in Context Laboratory
Title: Science or Science Fiction?:Investigating the Possibility (and Plausibility) of Subliminal Perpception

.  Author: Adam Chen
Source: undefined
Title: Expert discusses the effects of subliminal advertising