2. Demonstration of the existence of subliminal perception

As mentioned earlier, if subliminal means that the individual cannot identify the stimulus, how can it be shown that he or she has indeed perceived it? Many psychological studies of unconscious perception have attempted to prove the existence of unconscious perceptual processes by demonstrating that stimuli are perceived when subjects are not consciously aware of the stimuli. The basic strategy followed in these studies is to establish conditions under which conscious perception does not occur and then to demonstrate that stimuli can nevertheless be perceived under these conditions. The success of these studies depends completely on the acceptability of the method used to establish the absence of conscious perception <12>.

(1) Subjective measures of unconscious perceptual experience 

The concept of subliminal perception is of considerable interest because it suggests that people’s thoughts, feelings, and actions are influenced by stimuli that are perceived without any awareness of perceiving. This interest was reflected in some of the earliest psychological studies conducted during the late 1800s and early 1900s. In these early studies, people were simply asked whether or not they were aware of perceiving. For example, visual stimuli such letters, digits, or geometric figures were presented at such a distance from observers that they claimed either not to see anything at all or to see nothing more than blurred dots<13>.

The consistent result found in these early studies was that the observers’ guesses regarding the stimuli were more correct than would be expected on the basis of chance guessing. In other words, despite the observers’ statements indicating that they were unaware of perceiving the stimuli, their guesses indicated that they did in fact perceive sufficient information to make accurate guesses regarding the stimuli. Over the years, there have been literally hundreds of studies following a similar format. Taken together, these studies show that considerable information capable of informing decisions and guiding actions is perceived even when observers do not experience any awareness of perceiving <13>.

However, the assumption that subjective measures give an accurate indication of a subject’s unconscious perceptual experiences has proved to be problematic <12>.  A major reason for caution is that statements or comments indicating an absence of relevant conscious experiences may simply reflect biases introduced by either the experimental setting or the subjects’ preconceived ideas concerning the value of particular types of perceptual experiences for guiding decisions <9>.

(2) Objective measures of unconscious perceptual experiences

In the 1970s and 1980s, the attention of many psychologists was captured by the results of a number of studies that used objective measures of awareness. The results of these studies suggested that stimuli were perceived under conditions that did not allow discriminations between alternative stimuli. These studies were based on the intuitively appealing assumption that an inability to discriminate between stimuli indicates a complete absence of conscious perception <12>.

 The classic studies were conducted in the 1970s by the British psychologist Anthony Marcel. These experiments were based on previous findings indicating that a decision regarding a stimulus is facilitated or primed when the stimulus follows a related stimulus. Marcel found that words facilitated or primed subsequent word/non-word decisions to letter strings even when the words were presented under conditions that made it difficult if not impossible for the observers to distinguish when the words were presented from when the words were absent. Since the time of Marcel’s original experiments, there have been many other studies that have used similar methods. Not only have these studies confirmed Marcel’s original findings, but they have shown that other stimuli such as pictures, faces and spoken words can also facilitate subsequent decisions when they are presented under conditions that make it difficult to discriminate one stimulus from another stimulus <13>.

Although questions have been raised regarding whether the observers in the studies were completely unable to discriminate one stimulus from another stimulus, the one firm conclusion that can be made on the basis of these studies is that considerable information is perceived even when observers experience little or no awareness of perceiving as indicated by their difficulty in discriminating one stimulus from another stimulus <13>.