Subliminal stimuli (; sub- literally "below" or "less than") are any sensory stimuli below an individual's threshold or limit for conscious perception, in contrast to supraliminal stimuli (above threshold). Visual stimuli may be quickly flashed before an individual can process them, or flashed and then masked to interrupt processing. Audio stimuli may be played below audible volumes or masked by other stimuli. In 1957, the American cinematographer James Vicary claimed to have increased the sales of Coca-Cola by inserting in his cinema's movies some frames with "Drink Coca-Cola!" written on it. Five years later, however, he admitted to having inflated his results somewhat by including certain data that were labeled scientifically unreliable. However, Vicary's claim increased scientific interest in subliminal messages. Subliminal stimulation is now accepted as a legitimate research field in the scientific literature. A 2012 review of functional magnetic resonance imaging (fMRI) studies showed that subliminal stimuli activate specific regions of the brain despite participants' unawareness, a result corroborated in a meta-analysis from 2023 concerning subliminal stimulation in post-traumatic stress disorder (PTSD).
Effectiveness Applications of subliminal stimuli are often based on the persuasiveness of a message. Research on action priming has shown that subliminal stimuli can only trigger actions a receiver of the message plans to perform anyway. However, consensus of subliminal messaging remains unsubstantiated by other research. Most actions can be triggered subliminally only if the person is already prepared to perform a specific action. The context that the stimulus is presented in affects their effectiveness. For example, if the target is thirsty then a subliminal stimulus for a drink is likely to influence the target to purchase that drink if it is readily available. The stimuli can also influence the target to choose the primed option over other habitually chosen options. If the subliminal stimuli are for a product that is not quickly accessible or if there is no need for it within a specific context then the stimuli will have little to no effect. Subliminal priming can direct people's actions even when they believe they are making free choices. When primed to push a button with their off-hand, people will use that hand even if they are given a free choice between using their off-hand and their dominant hand. However, a meta analysis of many strong articles displaying effectiveness of subliminal messaging revealed its effects on actual consumer purchasing choices between two alternatives are not statistically significant; subliminal messaging is only effective in behaviour in very specific present intentions and contexts, which means they do not have visible results for mischievous use. It is suggested, however, that subliminal stimuli can bias acting decisions, including internally and freely generated ones, but, since that effect remains along with the aforementioned intentions and contexts, any impact on the choice of action are not mischievous but rather appropriate and adaptive.
Method In subliminal stimuli research, the threshold is the level at which the participant is not aware of the stimulus being presented. Researchers determine a threshold for the stimulus that is used as the subliminal stimulus. That 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.
Objective threshold The objective threshold is found using a forced-choice procedure, in which participants must choose which stimulus they saw from options given to them. For example, participants are flashed a stimulus (like 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' results in this task reach the level of (i.e. are no better than) that predicted by chance. The length of presentation that causes chance performance on the forced-choice task is used later in the study for the subliminal stimuli.
Subjective threshold The subjective threshold is determined when the participant reports that their performance on the forced-choice procedure approximates chance. The subjective threshold is 30 to 50 ms slower than the objective threshold, demonstrating that participants' ability to detect the stimuli is earlier than their perceived accuracy ratings would indicate; that is, stimuli presented at a subjective threshold have a longer presentation time than those presented at an objective threshold. When using the objective threshold, priming stimuli neither facilitated nor inhibited the recognition of a color. However, the longer the duration of the priming stimulus, the greater effect it had on subsequent responding. These findings indicate that the results of some studies may be due to their definition of below threshold.
Emotional threshold Some stimuli supposed to elicit a specific emotional reaction (e.g., spider pictures shown to a spider-fearful person) could fail to elicit it even if consciously perceived. This sounds apparently obvious: even if one is arachnophobic, the spider picture could be too brief to elicit a fear reaction. However, this is not obvious at all from the perspective of a phobic person, who is typically afraid even by the mere thought of the phobic stimulus. This lack of emotional response induced by very brief phobic pictures that were nonetheless emerged to awareness has brought to the definition and empirical demonstration of emotionally-subliminal stimuli as stimuli that do not induce the expected emotional reaction even if consciously perceived.
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