Semantic Priming

Semantic priming arises because the brain makes use of relations among similar or related stimuli in addition to using past experiences with the same stimulus. In the basic version of the semantic priming paradigm, subjects are presented with two successive stimuli called the prime and the target. They are usually asked to respond overtly only to the target. When words are the stimuli, the task may be naming or lexical decision. Supposing that the target is the word "nurse," the prime can be a related word (e.g., "doctor"), an unrelated word (e.g., "bread"), or a neutral stimulus (e.g., a row of X's). Under these conditions, the semantic priming or relatedness effect emerges. This effect can be described as a greater speed and accuracy of performance in the response to a target word when it is presented after a semantically related prime word than when it is presented after an unrelated prime word or after a neutral stimulus. This effect has been documented in a variety of situations. The semantic priming effect can occur when people are asked to pay attention to the prime and also when they do not pay attention to the prime, or even when they are unaware of its identity and do not phenomenologically realize that a prime has occurred. That is, semantic priming effects still exist when the target is presented very briefly and masked by visual noise presented immediately following the prime so that people believe they have seen only the visual noise and are not aware of the presence of the prime.

James Neely studied automatic and controlled processes by examining the ability of subjects to switch between semantic categories in an arbitrary fashion. He asked his subjects to think of parts of a building when the prime was the category name "body" (e.g., "body"-"door"). Subjects were able to follow the instructions when the time between the prime and the target (i.e., SOA) was long enough (e.g., 750 msec). That is, they responded faster to "door" following "body" relative to "door" following an unrelated prime (e.g., "tree") or even a neutral prime (e.g., XXXX). However, they were not able to switch from one category to another when the SOA was short (e.g., 250 msec). What was the fate of the rejected category? That is, when the prime was "body" and the subject made an effort to think of the category "building," what happened to parts of the body such as "arm"? "Arm" appearing after "body" was facilitated at short SOAs (<300 msec) and inhibited at long SOAs (between 500 and 2000 msec)—that is, RTs for related trials ("body"-"arm") were longer than RTs for neutral trials and equivalent to those for unrelated trials ("bird"-"arm").

Hence, priming can be achieved both by unintentional automatic activation, as shown by priming from masked words of which subjects are unaware, and from consciously perceptible words at short SOAs even when subjects are trying to think of unrelated words. However, priming can also be achieved by intentionally focusing on a concept and generating possibilities following some rule, even an arbitrary rule as in Neely's "switch condition,'' although such intentional focusing takes more time than automatic activation. Thus, these findings support the existence of two mechanisms of semantic priming. The first is automatic, nonconscious, and can work without attention. By analogy to repetition priming, one might wonder if it is mediated by reduction of neural response in structures that store semantic knowledge. The second is voluntary, conscious, and occupies attention. One might expect that this mechanism would be associated with neural structures involved in the executive control operations of working memory, and that invoking this mechanism would increase neural activation in those structures. To date, however, we do not know of any neuroimaging studies of semantic priming.

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