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Subliminal Semantic Activation
Subliminal Semantic Activation (SSA) is, “defined as evidence for analysis of a word’s meaning under conditions that prevent awareness of the word’s physical presence”. Another commonly accepted operation definition of SSA is, “a pattern of data showing statistically significant indirect effects for stimuli that produce no evidence of direct effects”. It is an occurrence in which a person develops an association – a definition – for an object (i.e., a word), without consciously realizing that they have been exposed to a connection between two objects. If the person is not consciously aware of what they perceive, the stimulus is said to be below their objective threshold, and therefore the stimulus is considered to be a subliminal stimuli. Researchers use subliminal objects (stimuli) as a means of priming in their tests for SSA.

Beginnings of Subliminal Semantic Activation Research
A.J. Marcel reported findings of the indirect-without-direct-effect data patterns, indicating SSA, in 1980 and 1983. Other investigators produced similar effects, but Daniel Holender soon after called Marcel’s findings into question. In his 1986 article about semantic activation, Holender found Marcel’s procedures did not, “ensure that semantic activation was not accompanied by conscious identification of the stimulus at the time of presentation”. Holender found that Marcel had limitations in the sensitivity of his direct measures.

Cheesman and Merikle supported Holender’s findings by being unable to reproduce Marcel’s indirect-without-direct effect data pattern when using direct measures that were more sensitive than Marcel. Cheesman and Merikle, however, did offer an alternative interpretation that provided more easily obtainable support for SSA. In their 1984 and 1986 articles, they found that indirect effects may be obtained when the perception of the subliminal stimulus was below the subjective threshold, yet while not in the absence of direct effects (i.e., not an objective threshold on the direct measure). They found that only stimuli above the subjective threshold were consciously perceived. Thus, any direct effects witnessed for subliminal stimuli must therefore be unprocessed by unconscious cognition.

Reingold and Merikle subsequently came out with the inclusiveness assumption, which stated that performance on direct and indirect measures can be reflective of both conscious and unconscious cognition. They demonstrated their inclusiveness assumption to provide evidence for unconscious cognition.

Methodology for Testing SSA
Researchers often use priming procedures to test for SSA. A fundamental method of testing has subjects performing a two-choice categorization task. The task is, “supplemented by the presentation of a subliminal prime word shortly before each to-be-judged target stimulus word”. Two widely used methods for testing of subliminal priming using two-choice categorization tasks are having a subject decide if a target letter string forms a word, or if a target word is pleasant or unpleasant. The tasks, often performed on a computer, have subjects use their right and left hands to press a button that corresponds to their response to a prime. Furthermore, researchers make use of response windows (allowed time per response) in experiments involving computer displays, a procedure which, when combined with visual masking (masked priming), “reliably produces statistically strong subliminal priming effects”.

Greenwald et al. (1995) found that using a linear regression analysis analysis method to analyze data obtained in the experiments allowed statistically significant evidence for SSA to be reached.

Experimental Findings
The primary implication of research done on SSA is that humans experience unconscious cognition. Given SSA’s implication, researchers have worked on discovering the capabilities and constraints of unconscious cognition. For example, Greenwald, Drains, & Abrams (1996), have found that, “the central nervous system monitors stimuli outside its current focus of attention.” Furthermore, Greenwald et al., found that activation is very short-lived, with the target word needing to occur within 100 milliseconds of the subliminal prime, but that indeed, visually masked prime words have the ability to influence the meanings of future words. These findings enabled them to conclude that, “the occurrence of semantic activation by consciously unperceived primes indicates that semantic activation does not depend on the prime reaching working memory (awareness)”.

The nature of the two-choice categorization tasks had some researchers concerned that priming is driven by automation of motor responses. That is, word associations made in practice elicit responses that, when the word is later presented as a subliminal prime, activate a word-response mapping that directs the subject to pick the side that correctly corresponds to the prime. Abrams, Klinger, & Greenwald (2002), however, found that in fact, “associations formed in practice that underlie subliminal priming are between words and semantic categories.” Priming is driven by “word-to-category” mapping, and that, “subliminal priming involves unconscious categorization of the prime.” Therefore, practiced subliminal words activate semantic categories, rather than activating automatic word-response mappings, further supporting unconscious cognition.

In 1995, Greenwald, Klinger, & Schuh, performed experiments that analyzed, “regressions of measures of indirect effects of direct effects of marginally perceptible stimuli”. The results produced evidence that supported dissociation of (multiple) network paths. That is, “direct and indirect effects of marginally perceptible stimuli might use separate network paths and, therefore, be independent of (i.e. dissociated from) one another”.