We have an intuitive view of perception that is simple and compelling. We assume that energy from our environment tickles our sensory organs, which then just deliver it as images to the brain. The objects exist out in reality, so we see them. Then we think and feel and act based on what we perceive. 

If nature had the same view, there would be no room for subliminal perception, we would either perceive something or we wouldn't. 

And in fact, conscious noticing works very much like that. Either we see it or we don't.  But it turns out that perception is not the same thing as noticing. We can perceive things and respond to them, without noticing their effect, or even their existence. Why should this be possible ?

The brain didn't evolve to serve strictly as a faithful recorder of images, but as a guidance system for our survival. As a result, it records things as faithfully as is useful, and distorts them when it is useful. Our sensory systems evolved to be very flexible in interpreting what we perceive, so that we can respond quickly and adaptively to changes around us.  

This doesn't doesn't  mean that we create our own awareness out of whole cloth.  It means that we take an active role, through our learned experience, expectations, values, and needs, in interpreting what we perceive.  The remarkable and counter-intuitive thing about this is that much of it can happen unconsciously.  

One of the most fascinating tricks used by nature to aid our survival is to design our brain as a prediction engine.  We constantly predict what we will perceive.  This happens largely without our awareness.  We are continually guessing, beneath awareness, about what to expect.

These guesses are called by various names, depending on which theory is being considered.  Expectancy, perceptual hypotheses, and schemata are all different ways of describing how we organize our experience and response partly through prediction.

Prediction serves a number of useful purposes.  It makes us ready to act more quickly with a prepared response, and it provides a way for us to quickly and efficiently recognize when we're off course and need to make a correction.   Things that violate our expectations get our attention, setting off an alarm bell in our brain.  When things are going along according to expectations, familiar patterns tend to fade out of awareness, a process known as habituation.  So we largely respond to change rather than to stable stimuli themselves.

Readiness is so fundamental that researchers sometimes find that even in a spontananeous task, readiness potentials in the brain occur  before we realize we have decided to respond.  (Libet et al, 1982)   That is, the decision process can be unconscious. 

Most importantly of all, our state of readiness even helps determine what we notice consciously.  We don't just passively receive sensory impressions and bring them to awareness.  Our awareness and understanding are based in a fundamental way on the prior experience of our body interacting with the environment, and the perceptual assumptions we make. 

There simply is not enough information available to our senses to solve some of the perceptual problems we manage to solve routinely, unless we were already prepared with assumptions.  That's why psychologists are so obsessed with illusions, they reveal the assumptions we make in perception.  

Meaning derives from personal and physical experience.  Awareness is intimately dependent upon meaning, meaning derived from prior experience.  The manner in which prior experience structures our perception and action is guided by the unconscious processing of subtle cues and the imagery and emotions triggered by those subtle cues.

Sensory perception begins as unconscious detection of features and then becomes aware at some point if sufficient higher processing area neurons are activated.  This can be shown in the brain as a distinction between changes corresponding to noticing, vs. changes in response to stimuli. (Lumer et al, 1998; Schall, 1999)

There are five basic phenomena of subliminal stimulation:

1.  Mere Exposure Effect   --  Exposure to an image without awareness predisposes us to prefer that image over others.

2.  Poetzl Effect  --  Words or images perceived without awareness appear in altered form in imagery and dreams some short time later.

3.  Affective Priming  --  Exposure to an emotionally compelling image without awareness causes us to respond emotionally without knowing why.

4.  Semantic Priming  --  Exposure to a word without awareness tends to bias our perception of subsequent words for a fraction of a second. 

5.  Psychodynamic Activation  --  Exposure to certain kinds of  fantasy images or suggestion without awareness  can influence mental state or psychosocial adaptation in a meaningful and persistent way.