Subliminal stimuli; (literally "below threshold"), contrary to supraliminal stimuli or "above threshold", are any sensory stimuli below an individual's threshold for conscious perception. Some research has found that subliminal messages do not produce strong or lasting changes in behavior. However, a recent review of functional magnetic resonance imaging (fMRI) studies shows that subliminal stimuli activate specific regions of the brain despite participants being unaware. Visual stimuli may be quickly flashed before an individual can process them, or flashed and then masked, thereby interrupting the processing. Audio stimuli may be played below audible volumes, masked by other stimuli, or recorded backwards in a process called backmasking.
The effectiveness of subliminal messaging has been demonstrated to prime individual responses and stimulate mild emotional activity. Applications, however, often base themselves on the persuasiveness of the message. The near-consensus among research psychologists is that subliminal messages do not produce a powerful, enduring effect on behavior; and that laboratory research reveals little effect beyond a subtle, fleeting effect on thinking. Research on those claims of lasting effects - such as weight loss, smoking cessation, how music in popular culture may corrupt their listeners, how it may facilitate unconscious wishes in psychotherapy, and how market practitioners may exploit their customers - conclude that there is no effect beyond a placebo or making the person conscious of their current condition. Importantly, research on action priming has shown that subliminal stimuli can trigger only those actions that one plans to perform anyway: only if a person already has the specific intention to perform a certain action, can this action be subliminally triggered. The following sections have more information on specific studies in which the effectiveness of subliminal stimuli are investigated.
The threshold in subliminal stimuli research is the level at which the participant is not aware of the stimulus being presented. Researchers determine a threshold for the stimulus that will be used as the subliminal stimulus. That subliminal stimulus is then presented during the study at some point and measures are taken to determine the effects of the stimulus. The way in which studies operationally define thresholds depends on the methods of the particular article. The methodology of the research also varies by the type of subliminal stimulus (auditory or visual) and the dependent variables they measure.
The objective threshold is found using a forced choice procedure, in which participants must choose which stimulus they saw from options given to them. Participants are flashed a stimulus (e.g. the word "orange") and then given a few choices and asked which one they saw. Participants must choose an answer in this design. The objective threshold is obtained when participants are at chance level of performance in this task. The length of presentation that causes chance performance on the forced choice task is used later in the study for the subliminal stimuli.
The subjective threshold is determined by when the participant reports that his or her performance on the forced choice procedure is around chance. The subjective threshold is 30 to 50 ms slower than the objective threshold, demonstrating that participants are able to detect the stimuli is present sooner than their perceived accuracy ratings would indicate. In other words, stimuli presented at a subjective threshold have a longer presentation time than those presented at an objective threshold. When using the objective threshold, primes neither facilitated nor inhibited the recognition of a color.
Many studies find a single threshold for a participant and a participant's threshold as defined by the experimenter may change from one moment to the next, depending on many variables. Therefore, the power of the experiment might be compromised. It is possible to estimate the amount of power lost due to variability within the individual.
In order to study the effects of subliminal stimuli, researchers will often prime the participants with specific visual stimuli, often images, and determine if those stimuli elicit different responses. Subliminal stimuli has mostly been studied in the context of emotion, in particular, researchers have focused a lot of attention to the perception of faces and how subliminal presentation to different facial expression affects emotion. Visual subliminal stimuli has also been used to study emotion eliciting stimuli and simple geometric stimuli. A significant amount of research has been produced throughout the years to demonstrate the effects of subliminal visual stimuli.
Attitudes can develop without being aware of its antecedents. Individuals viewed slides of people performing familiar daily activities after being exposed to either an emotionally positive scene, such as a romantic couple or kittens, or an emotionally negative scene, such as a werewolf or a dead body between each slide. After exposure from which the individuals consciously perceived as a flash of light, the participants gave more positive personality traits to those people whose slides were associated with a emotionally positive scene and vice-versa. Despite the statistical difference, the subliminal messages had less of an impact on judgment than the slide's inherent level of physical attractiveness.
Individuals show right amygdala activity in response to subliminal fear, and greater left amygdala response to supraliminal fear. People were exposed to a subliminal image flashed for 16.7 milliseconds that could signal a potential threat and again with a supraliminal image flashed for half a second. Furthermore supraliminal fear showed more sustained cortical activity, suggesting that subliminal fear may not entail conscious surveillance while supraliminal fear entails higher-order processing.
Fear stimuli are processed differently compared to other negative emotional stimuli. Participants rated the affect of target faces in an affect appraisal session and the genuineness of target faces in a genuineness appraisal session. This study found that when participants were primed with fear stimuli compared to happy stimuli, the target was rated as more unpleasant and when primed with fear and disgust subliminal stimuli, the participants rated the target as being less genuine. This study also suggests that our emotional involvement in this case also matters because the subliminal fear stimuli that participants were primed with in the genuineness condition could have invoked feelings of a potential threat; therefore the participant may have been suspicious of the targets and rated them as less genuine.
Suspicion of a potential of threat affects emotional responses in regards to pain that is perceived in others. Participants were shown neutral and semantic pain expressions that were previously primed with their own or others' faces in one experiment. In a second experiment, pictures with pain and neutral scenarios were used in a categorization task. The pictures were primed with their own or others' faces similar to experiment one. Results indicate that an early and late cortical response between pain and no-pain were modulated only when the participants experienced subliminal stimuli of others' faces. These results further support the claim that when negative subliminal stimuli is experienced, the feeling of potential threat arises.
Subliminally primed negative emotions elicit emotional responses that demonstrate negative and defensive effects. Angry and sad faces were used as primes and were compared based on the effects that each emotion would have on agency appraisals. When exposed to subliminal angry faces participants appraised negative events as caused by others, and those exposed to subliminal sad faces appraised the same events as caused by situational factors. These findings supply evidence for subliminal emotion-specific cognitive effects that are not just valence-based effect.
Priming individuals with images flashed for an instant affects experiences of self. Images of a graduate student's faculty adviser's scowling face or an approving face of another person were presented before the graduate students evaluated their own research ideas. Participants who were Catholic were asked to evaluate themselves after being flashed a disapproving face of the Pope or another unfamiliar face. The self-ratings were lower after the presentation of a disapproving face with personal significance; however there was no effect if the disapproving face was unfamiliar.
Emotion eliciting stimuli
A subliminal sexual stimulus has a different effect on men compared to women. Men and women were subliminally exposed to either a sexual or a neutral picture, and their sexual arousal was recorded. Researchers examined the accessibility of sex-related thoughts after following the same procedure with either a pictorial judgment task or lexical decision task. The results revealed that the subliminal sexual stimuli did not have an effect on men, but for women, lower levels of sexual arousal were reported. However, in conditions related to accessibility of sex-related thoughts, the subliminal sexual stimuli led to higher accessibility for both men and women.
Subliminal stimuli have elicit significant emotional changes, but these changes are not valuable for a therapeutic effect. Spider-fearful and non-fearful undergraduates experienced either a positive, negative, or neutral subliminal prime followed immediately by a picture of a spider or a snake. Using visual analogue scales, the participants rated the affective quality of the picture. No evidence was found to support that the unpleasantness of the pictures can be modulated by subliminal priming. In fact, the non-fearful participants rated the spiders as more frightening after being primed with a negative stimuli, however, for the fearful participants, this effect was not found.
Simple geometric stimuli
Laboratory research on unconscious perception often employs simple stimuli (e.g., geometric shapes or colors) whose visibility is controlled by visual masking. Masked stimuli are then used to prime the processing of subsequently presented target stimuli. For instance, in the Response Priming paradigm, participants have to respond to a target stimulus (e.g., by identifying whether it is a diamond or a square) which is immediately preceded by a masked priming stimulus (also a diamond or a square). The prime has large effects on responses to the target; it speeds responses when it is consistent with the target, and slows responses when it is inconsistent. Response priming effects can be dissociated from visual awareness of the prime, such as when prime identification performance is at chance, or when priming effects increase despite decreases in prime visibility.
The presentation of geometric figures as subliminal stimuli can result in below threshold discriminations. The geometric figures were presented on slides of a tachistoscope followed by a supraliminal shock for a given slide every time it appeared. The shock was administered after a five second interval. Electrical skin changes of the participants that occurred before the reinforcement (shock) or non-reinforcement were recorded. The findings indicate that the proportion of electrical skin changes that occurred following subliminal visual stimuli was significantly greater than expected. In contrast, the proportion of electrical skin changes that occurred in response to the stimuli which were not reinforced was significantly less. As a whole, participants were able to make below threshold discriminations.
One method for creating subliminal auditory stimuli is called masking. This method involves hiding the target auditory stimulus in some way. Auditory subliminal stimuli are shown to have some effect on the participant, but not a large one. For example, one study used other speechlike sounds to cover up the target words. The study found evidence of priming in the absence of awareness of the stimuli. But the effects of these subliminal stimuli were only seen in one of the outcome measures of priming, while the effects of conscious stimuli were seen in multiple outcome measures.
Main article: Backmasking
Backmasking is a recording technique in which a sound or message is recorded backward onto a track that is meant to be played forward.
During the 1970s, media reports raised a series of concerns of its impact on listeners, stating that satanic messages were calling its listeners to commit suicide, murder, abuse drugs, or engage in sex, which were all rising at the time. However, both of these studies relating heavy metal music to negative outcomes were correlational in nature. In other words, they did not have random assignment into groups, but rather the groups were pre-selected by participants. The problem with making assumptions based on this type of design is that the two related variables (e.g. suicide and listening to heavy metal music) may not have a causal relationship. So, although the studies show a relationship between certain types of music and suicide, these increases may not be caused by listening to heavy metal music.
Empirical studies have researched the issue of backmasking. As a group, participants report not being able to understand what is said in backward recordings and experimental research has found little evidence that the messages affect participant behavior. While listening to forward messages produced priming effects, listening to backward messages caused no such effect in the participants. Participants were able to discern the gender of the speaker and tell him or her apart from a different speaker. Participants are able to distinguish the language of the message with some degree of accuracy. But the participants were not able to correctly estimate the number of words in the backwards sentences. They were also only slightly better than chance (55% accuracy) at determining whether or not a single word was in the backward sentence. These findings indicate that participants were not able to process much of the meaning of the backward message.
Some self-help audiotapes claim to work through subliminal messaging. Subliminal self-help audio tapes do not work, or at least not through subliminal messages. Subliminal self-help audiotapes produce effects through increasing awareness of the problem or through a person believing they will work.
In a study of self-esteem and memory self-help tapes, performance measures increased for both self-esteem and memory tapes, indicating a placebo effect.However, many participants believed they improved in the measure they thought their self-help tape covered, regardless of whether the tape covered that measure or not.
Similarly, all groups of participants lost weight during a study when only some of the participants were asked to listen to weight loss tapes. The study contained a control group who did not listen to any tape, a placebo group who listened to a tape unrelated to weight-loss, and an experimental group who listened to the weight loss audiotape. The results indicate that the effect of audiotapes is due to awareness of weight rather than a placebo effect or subliminal influence. (A placebo effect would be indicated if the two tape groups lost weight while the non-tape group lost less or no weight.)
Consumption and television
Some studies have looked at the efficacy of subliminal messaging in television. Subliminal messages produce only one-tenth of the effects of detected messages and the findings related the effects of subliminal messaging were relatively ambiguous.
Karremans suggests that subliminal messages have an effect when the messages are goal-relevant. Subliminally priming a brand name of a drink (Lipton Ice) made those who were thirsty want the Lipton Ice. However, those who were not thirsty, were not influenced by the subliminal messages. Karremans did a study assessing whether subliminal priming of a brand name of a drink would affect a person's choice of drink, and whether this effect is caused by the individual's feelings of being thirsty. In another study, participant's ratings of thirst were higher after viewing an episode of "The Simpsons" that contained single frames of the word "thirsty" or of a picture of a Coca-Cola© can. Some studies have shown greater effects of subliminal messaging with as high as 80% of participants showing a preference for a particular rum when subliminally primed by the name placed in an ad backward.
Many authors have continued to argue for the effectiveness of subliminal cues in changing consumption behavior, citing environmental cues as a main culprit of behavior change. Authors who support this line of reasoning cite findings such as the research that showed slow-paced music in a supermarket was associated with more sales and customers moving at a slower pace. Findings such as these support the notion that external cues can affect behavior, although the stimulus may not fit into a strict definition of subliminal stimuli because although the music may not be attended to or consciously affecting the customers, they are certainly able to perceive it.
Long-term effects of subliminal priming can affect academic performance. Participants were subliminally primed with either words related or unrelated to intelligence 1 to 4 days before actual course midterms. The participants who were primed with words related to intelligence had increased performance on their midterms compared to the participants who received neutral primes. These results suggest that subliminal priming may have long-term effects on behavior, especially academic performance.
The psychological processes that are used to assess information are considered to occur below threshold and have an impact on behavior (drug use, smoking). Because the psychological processes that are tapped into are unavailable to conscious introspection, tobacco dependence could be partially explained by cognitive processing biases towards cues that are smoking-related. The processing of smoking-related and affective stimuli that were presented subliminally to nonsmokers, smokers deprived from smoking for twelve hours, and nondeprived smokers. They were presented with a pictorial subliminal prime(smoking-related objects, neutral office supplies, and happy, angry, and neutral faces) followed by a mask, and then the target. The task was to identify the target as quickly as possible. The results indicate that the deprived smokers demonstrated a processing bias for smoking-related stimuli, whereas the nonsmokers and nondeprived smokers did not. The deprivation of tobacco seemed to increase the subliminal processing of smoking-related stimuli for smokers, but did not influence the subliminal processing of affective stimuli.
A high-level origin of visual backmasking deficit exists for schizophrenics. This backward masking deficit in schizophrenia is related to a deficit in the late stages of conscious perception, but subliminal processing of the masked stimuli is preserved. Twenty-eight people with schizophrenia and twenty-eight people who do not have the disease were primed with Arabic digit stimuli, followed by a mask that occurred at different, varying intervals. Patients with schizophrenia had a much longer threshold delay between the digit and conscious perception of the mask, but the subliminal priming of masked numbers was effective and had the same effects for both groups. The researchers conclude that schizophrenics access to conscious report is impaired, whereas fast bottom-up design processing of the same stimuli is preserved.
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