Psychology Of Perception Without Awareness Research Paper

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Perception without awareness (or, synonymously, unconscious perception) denotes the fact that a stimulus exerts causal influence on mental processes or behavior without being phenomenally experienced.



Research into unconscious perception began early in the history of experimental psychology (Peirce and Jastrow 1884, Sidis 1898). It is related to other domains working on unconscious processes, such as implicit memory or implicit learning (Reber 1993). From the very beginning, experimenters attempted to prove the mere existence of unconscious perception. Researchers had to set up experimental demonstrations of a stimulus’s causal influence on mental processes without its concomitant phenomenal experience. More recently, theorists began to ask how perception without awareness works.

The proof of perception without awareness usually needs two critical tests: (a) an indirect test that assesses the impact of a stimulus on mental processes or behavior, and (b) a direct test that establishes the absence of phenomenal experience of the stimulus in question. There was rarely doubt about the demonstration of causal influence of a stimulus, but there was—and still is—much skepticism about the proof of absence of phenomenal experience. Therefore, experimenters aimed at enhancing the validity of thresholds in order to ensure that perception was indeed subliminal. Alternatively, some researchers were looking for procedures that circumvented the problem of thresholds. Finally, there is now some evidence for unconscious perception from research in neuroscience.

1. Early Studies

Early studies attempted to show that people were able to guess at above chance level without subjectively perceiving the stimulus (see Adams 1957, for a review). Much of the classical research has been criticized for its lack of proper control conditions and of proper measurements of consciousness (e.g., Eriksen 1960, Holender 1986). Holender, for example, criticized experiments using dichotic listening tasks for their lack of evidence that processing of the stimuli was indeed unconscious. In dichotic listening tasks, participants hear different information in both ears. They are instructed to attend to the information in one ear. It has been shown that information in the unattended ear may exert some influence on ongoing processing without subjective awareness of this information. Of course, if participants turned their attention to the unattended ear, they would be able to identify the content. Holender concluded that one can not be sure that unattended stimuli are in fact unconscious. Therefore, he proposed that manipulations should not limit mental resources by distracting attention, but should limit the available data by rendering perception of stimuli impossible.

2. Perception Below Threshold

At the start of the twenty-first century, several masking techniques are used to render the material—most of them visual stimuli—undetectable or unidentifiable. For example, to measure individual thresholds of detection, masked stimuli are presented on half of the trials. On the other half of the trials, only the mask is shown. Participants have to decide whether a stimulus is present or absent. If accuracy does not exceed chance level (mostly at 50 percent), absence of direct phenomenal experience is assumed.

In a well-known study, Marcel (1983) presented masked words below the subjective threshold of detection. Then he presented two words, one similar and one dissimilar to the masked word, in respect to either graphemic information or semantic content. Participants had to decide which word was similar to the masked word, either graphemically or semantically. They were able to make above-chance guesses for each kind of judgment. As presentation time was decreased, judgments about graphemic similarity dropped to chance level first, followed by judgments about semantic similarity.

The logic underlying this kind of study is to show a significant effect in an indirect perceptual task (e.g., semantic discrimination) when there is a null effect in a direct task (e.g., detection). However, null sensitivity in direct tasks has always been a matter of controversy. If the criteria for null sensitivity in the direct test are too lenient, one may erroneously claim to have demonstrated unconscious perception. Conversely, if these criteria are too strong, it may be virtually impossible to demonstrate perception without awareness.

Therefore, some scholars have tried to circumvent the issue of thresholds and their measurements and taken different routes to demonstrate unconscious perception.

3. Alternative Approaches To Demonstrate Unconscious Perception

Reingold and Merikle (1988) designed an experimental setup that allows measuring unconscious perception without assessment of thresholds. Specifically, they proposed to use identical materials and experimental conditions for direct and indirect tests. The only difference between the two tests lies in the instruction: Conscious perception of stimuli is assessed in a direct test. In contrast, a task without reference to phenomenal experience of the stimulus is given in an indirect test. Normally, one would expect that performance in the direct test would show greater sensitivity than performance in the indirect task because conscious perception optimizes extraction and use of stimulus information. If the indirect measure shows greater sensitivity than the direct measure, however, one may conclude that unconscious processes mediate task performance. This was the case in a study on implicit memory effects by Kunst-Wilson and Zajonc (1980). Participants were exposed to random shapes for 1 ms during a learning session. At test, participants were shown 10 pairs of octagons, one old and one new. They had to decide either which of the two octagons has been shown before (direct test) or which of the octagons they liked better (indirect test). In the direct test, participants scored 48 percent, which was at chance level, but they preferred the old octagon in 60 percent of the cases, indicating greater sensitivity of the indirect than of the direct test. In studies on unconscious perception, however, it proved difficult to find the proposed indirect-greater-than-direct effect pattern.

More recently, Greenwald and his colleagues proposed to use performance in a direct test as independent variable and performance in an indirect test as dependent variable in a linear regression analysis (Greenwald et al. 1995). These authors used a semantic priming paradigm as indirect test to show semantic activation without conscious perceptibility of the prime. It seems plausible that better performance in the direct test results in higher accuracy in the indirect test, yielding a linear regression function with positive slope. X = 0 denotes the point where conscious perception is completely absent. If there existed no effect of unconscious perception, the regression line would cross the zero point: Y = 0 if X = 0. If, however, there were an effect of unconscious perception even under complete absence of conscious identification, the intercept would be positive: Y > 0 if X = 0. In several experiments employing this kind of analysis, Greenwald and his colleagues (Draine and Greenwald 1998, Greenwald et al. 1995, 1996 ) were

able to show reliable indirect effects under absence of direct effects.

4. Evidence From The Neurosciences

Other evidence of perception without awareness comes from patients with damage to the visual cortex. They typically do not see anything in the hemifield of vision that is opposite to the side of the damage (e.g., Poppel et al. 1973, Weiskrantz et al. 1974). However, some of these patients are able to make visual discriminations in the blind hemifield although they reportedly do not have any phenomenal experience of the stimuli. This phenomenon is called ‘blindsight.’ Some authors suggested that blindsight may be caused by small islands of intact tissue in the primary visual cortex (Fendrich et al. 1992). Although correct visual discriminations may be caused by small, intact islands of accurate vision, this hypothesis cannot account for all phenomena observed in both monkeys and human patients (see Weiskrantz 1995).

Brain imaging techniques led to new indirect measures. Using an unconscious priming paradigm, Dehaene et al. (1998) have shown that masked primes influenced subsequent processing, accompanied by measurable modifications of electrical brain activity and cerebral blood flow.

We have discussed studies aimed at demonstrating that unconscious perception exists. We now turn to the question of how unconscious perception works.

5. Differences Between Conscious And Unconscious Perceptual Processes

One may think that unconscious perception is the same as conscious perception, except for the lack of phenomenal experience. In this view, unconscious perception is just a faint shadow of conscious perception. This view has been questioned, and there is good evidence that unconscious perception is driven by a limited-capacity system, unconscious perception by a system that has much more capacity and allows the use of information from multiple sources (see MacLeod 1998). The main evidence comes from demonstrations of qualitative dissociations between conscious and unconscious perception. This means that conscious perception affects a variable in one direction, unconscious perception in the opposite way, suggesting different underlying mechanisms (see Merikle and Daneman 1998). An elegant study showing such a dissociation has been reported by Jacoby and Whitehouse (1989). In a recognition test, a target word was presented and participants had to decide whether or not it had appeared in a list presented before. Shortly before the target, a prime was shown that was either identical to or different from the target. If the primes were shown subliminally, identical primes resulted in more ‘old’ responses than different primes. If the primes were shown supraliminally, however, identical primes resulted in less ‘old’ responses than different primes. This qualitative dissociation suggests that participants who saw supraliminal primes were able to correct their judgments for the supposed influence of the primes. If exposed to subliminal primes, however, participants were unable to avoid the impact of the prime and were biased toward ‘old’ responses when prime and target were identical. Several other studies yielded similar dissociations (e.g., Debner and Jacoby 1994, Marcel 1980, Merikle and Joordens 1997), suggesting that conscious and unconscious perception are governed by different mechanisms.

6. Outlook

There is fair evidence that unconscious perception exists. However, there is no evidence that effects of unconscious perception, caused by exposure to subliminal messages, are of any practical relevance (Greenwald et al. 1991, Vokey and Read 1985).

Beyond the proof of existence, interesting questions about unconscious perception remain open. One question pertains to the durability of unconscious perception (see Merikle and Daneman 1998). It has been shown that implicit memory effects may last for several days. In contrast, most studies on unconscious perception dealt with effects that lasted a few seconds. In implicit memory tasks, encoding is conscious, but retrieval is assumed to be unconscious. In perception without awareness, encoding is unconscious. Do longterm effects exist only if encoding is conscious, or is it possible to find long-term effects of unconscious encoding? Poetzl ([1917] 1960) has shown that briefly presented materials may be manifest in dreams during the following night although people were unable to remember the materials after the initial presentation. The Poetzl phenomenon, together with more recent findings on memory for information encoded during anesthesia (see Merikle and Daneman 1996 for a meta- analysis) suggest that unconscious influences may last for hours.

As discussed above, another major question is how perception without awareness works. This research has only recently begun and will intensify, stimulated by recently developed methods in the neurosciences and cognitive psychology (e.g., the process dissociation procedure, see Debner and Jacoby 1994) that open up opportunities for research into neural correlates and cognitive mechanisms of unconscious perception.

Brain imaging methods may bring new opportunities to look into neural pathways related to unconscious perception. Specifically, these methods may bring new insights into qualitative dissociations between neural substrates connected to conscious perception and neural substrates connected to unconscious perception, as demonstrated in a study by Morris et al. (1998). These authors paired an angry face with (uncomfortable) white noise. Subliminal presentation of the conditioned angry face resulted in increased neural activity in the right, but not left amygdala. In contrast, supraliminal presentation of the same face resulted in increased neural activity in the left, but not right amygdala. Moreover, it has been shown that simple stimulus features may proceed to the amygdala and trigger an emotion without involvement of cortical structures (see LeDoux 1995). Although any conclusion about presence or absence of awareness in perceiving such features is premature, it enables theorists to conceive ways of how information may influence mental processes without reaching consciousness.


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