3. Qualitative differences between conscious and unconscious perception

In this section, five studies that demonstrate qualitative differences for consciously and unconsciously perceived stimuli will be described. Each study provides a demonstration of a different characteristic that distinguishes conscious from unconscious perception. Together, the results of these studies provide rather compelling evidence for the importance of unconscious perceptual processes in influencing reactions to stimuli<12>.

(1) Affective reactions

The affective primacy hypothesis by Zajonc holds that affective reactions can be elicited with minimal stimulus input. This hypothesis challenges the cognitive appraisal viewpoint, which maintains that affect cannot emerge without prior cognitive mediation <18>. 

The affective primacy hypothesis was first suggested by a mere exposure experiment by Kunst-Wilson and Zajonc (1980), in which subjects, by virtue of repeated exposures, developed affective preferences for previously novel Chinese ideographs<18>. Recently, Murphy and Zajonc (1993) obtained more convincing evidence for the importance of unconscious perception in determining affective reactions by showing that affective reactions are more likely to be influenced by unconsciously perceived stimulus than by consciously perceived stimuli <12>.

In the experiments conducted by Murphy and Zajonc, subjects were shown a clearly visible, Chinese ideograph on each of a series of trials. The subjects were asked to indicate on a five-point scale whether they thought each ideograph represented a “good” or a “bad” concept. The critical aspect of the experiment concerned what happened immediately before each ideograph was presented. For one group of subjects, the presentation of each ideograph was preceded by a picture of a human face that expressed either happiness or anger. For this group of subjects, each face was presented for such a brief duration (i.e., 4 msec) that no subject reported awareness of the faces. For second group of subjects, the same ideographs and faces were presented, but the duration of each face (i.e., 1000 msec) was sufficiently long so that all subjects reported awareness of the faces. The important result found by Murphy and Zajonc is that only the briefly presented, unconsciously perceived faces influenced the subjects’ ratings of the ideographs <12>.

(2) Predominant codes

An interesting qualitative difference first demonstrated by Groeger<5> is that unconsciously perceived words are coded differently than are consciously perceived words. In an experiment using visual stimuli, Groeger presented a single target word on each experimental trial and required subjects to select the target word from a matrix of 24 words that was presented immediately following the target word. The critical aspect of the experiment was that the matrix never contained the actual target word presented on the trial. Rather, the matrix included some words that were semantically similar to the target word and some words that were structurally similar to the target word <12>

The result of this experiment showed that in a situation in which the target words were presented for such a brief duration that the subjects did not report any awareness of the target words, the subjects tended to select the semantically similar words. However, in a situation in which the target words were presented for a duration that was sufficiently long for the subjects to report awareness of the target words, the subjects tended to select the visually similar words <12>.

(3) Automatic reactions

A common belief regarding consciousness is that conscious perception enables one to use the perceived information to act on the world and to produce effects on the world. In contrast, unconsciously perceived information leads to more automatic reactions that cannot be controlled by the perceiver.

This distinction between the active and passive consequences of perception has been captured in a number of experiments demonstrating that consciously perceived stimuli allow subjects to follow instructions, whereas unconsciously perceived stimuli lead to much more automatic reactions <12>.

(4) Predictive strategies 

Another example of how unconscious perception leads to automatic reactions and conscious perception allows individuals to modify their reactions comes from a series of experiments showing that prediction based on stimulus redundancy only occurs when the predictive stimuli are consciously perceived <12>.

On each experimental trial, the subjects were first shown either the word RED or the word GREEN and these words were presented so that they were either consciously or unconsciously perceived. The subjects were then shown a patch of color that was either red or green. The task for the subjects was simply to name each color patch as fast as possible. The standard result found with this task is that it takes more time to name a color patch when it follows an incongruent color word than when it follows a congruent color word. This standard interference effect in color naming was found independent of whether the preceding words were consciously or unconsciously perceived.

However, when the experimental conditions were changed so that incongruent word/color-patch pairings occurred much more often than did congruent pairings, the results depended on whether the words were consciously or unconsciously perceived. When the words were consciously perceived, it actually took less time to name a color patch when it followed an incongruent color word than when it followed a congruent color word. What seems to have happened is that the subjects capitalized on the predictive information provided by the words: they learned to expect that the color patch on each trial would be the color not named by the preceding word. In contrast, when the words were unconsciously perceived, the standard result was found independent of the predictive relation between the words and color patches.

(5) Influence of context

Marcel<18> hypothesized that the influence of context on perception in limited to information that is consciously perceived. To investigate this issue, he conducted an experiment involving the perception of individual words. In his experiment, Marcel presented sequences of three words: the first word was a clearly visible context word (e.g., hand or tree), the second word was always a polysemous word with two possible meanings (e.g., palm), and the third word was a clearly visible target word related to one of the possible meanings of the polysemous word (e.g., wrist). The purpose of the context words was to bias imterpretation of the polysemous words, which were presented under conditions so that they were either consciously or unconsciously perceived <12>.

It is known that time to respond to a target word is facilitated when the immediately preceding word is semantically related. Therefore, if the context biases interpretation of the polysemous word, then time to respond to the target word should be faster if the context word and the target word are related to the same meaning of the polysemous word than if the context word and the target word are related to the different meanings of the polysemous word.

When the polysemous word was consciously perceived, time to respond to the target word was faster if it was preceded by a meaning-related context word; when the polysemous word was not consciously perceived, time to respond to target word was facilitated if it was preceded by a polysemous word with a related meaning, independent of the bias introduced by the context word. The results of this experiment are consistent with the idea that conscious perception of a stimulus is constrained by context but that unconscious perception leads to automatic reactions that are relatively unconstrained by context.