Spreading activation

Priming is an implicit memory effect in which exposure to one stimulus influences the response to another stimulus. Studies show that people are faster in deciding that a string of letters is a word when it follows an associatively or semantically related word. For example, ‘nurse’ is recognized more quickly following ‘doctor’ than ‘bread.’ As another example, if the original concept is ‘red’ and the word ‘vehicles’ is primed, people are much more likely to say ‘fire engine’ instead of something unrelated to vehicles, such as ‘cherries.’ If instead ‘fruits’ was primed, they would likely name ‘cherries.’

Priming can also be visual, rather than semantic; if people see an incomplete sketch they are unable to identify and they are shown more of the sketch until they recognize the picture, later they will identify the sketch at an earlier stage than was possible for them the first time. The effects of priming can be very salient and long lasting, even more so than simple recognition memory. Unconscious priming can affect word choice long after the primes have been consciously forgotten. Priming works best when the two stimuli are in the same modality. For example visual priming works best with visual cues and verbal priming works best with verbal cues. But priming also occurs between modalities.

Explicit memory is the conscious, intentional recollection of previous experiences and information (e.g. remembering the time of an appointment or recollecting an event from years ago). Implicit memory is an unconscious, unintentional form of memory. Remembering a specific driving lesson is an example of explicit memory, while improved driving skill as a result of the lesson is an example of implicit memory. In addition to priming, implicit memory also leads to the ‘illusory truth effect’ (the tendency to believe information to be correct when exposed to it repeatedly). In daily life, people rely on implicit memory in the form of procedural memories, information for the performance of particular types of action that can be recalled with little to no conscious thought (e.g. how to tie shoelaces or ride a bicycle). The multiple memory system theory ascribes the differences in implicit and explicit memory to the differences in the underlying structures. The theory says that explicit memories are associated with a declarative memory system responsible for the formation of new representations or data structures. In contrast, implicit memories are associated with a procedural memory system where memories are just modifications of existing procedures or processing operations.

The terms positive and negative priming refer to when priming affects the speed of processing. A positive prime speeds up processing, while a negative prime lowers the speed to slower than un-primed levels. Positive priming is caused by simply experiencing the stimulus, while negative priming is caused by experiencing the stimulus, and then ignoring it. Positive priming effects happen even if the prime is not consciously seen. Positive priming is thought to be caused by spreading activation, a theory of how the brain moves through an entire network of ideas to retrieve specific information. The spreading activation theory presents the array of concepts within our memory as cognitive units, each consisting of a node and its associated elements or characteristics, all connected together by edges. This means that the first stimulus activates parts of a particular representation or association in memory just before carrying out an action or task. The representation is already partially activated when the second stimulus is encountered, so less additional activation is needed for one to become consciously aware of it.

Negative priming is less understood. Many models have been hypothesized, but currently the most widely accepted is the episodic retrieval theory, which hypothesizes that ignored items are flagged ‘do-not-respond’ by the brain. Later, when the brain acts to retrieve this information, the tag causes a conflict. The time taken to resolve this conflict causes negative priming. A less favored theory, the distractor inhibition model, posits that the activation of ignored stimuli is inhibited by the brain.

The difference between perceptual and conceptual primes is whether items with a similar form or items with a similar meaning are primed, respectively. Perceptual priming is based on the form of the stimulus and is enhanced by the match between the early and later stimuli. Perceptual priming is sensitive to the modality and exact format of the stimulus. An example of perceptual priming is the identification of an incomplete word in a word-stem completion test. The presentation of the visual prime does not have to be perfectly consistent with later testing presentations in order to work. Studies have shown that, for example, the absolute size of the stimuli can vary and still provide significant evidence of priming.

Amnesic patients are described as those who have suffered damage to their medial temporal lobe, resulting in the impairment of explicit recollection of everyday facts and events. Amnesic patients do as well on perceptual priming tasks as healthy patients, however they show some difficulties completing conceptual priming tasks, depending on the specific test. Patients with Alzheimer’s disease (AD), the most common form of dementia, show decreased priming effects on word-stem completion and free association tasks, while retaining normal performance on lexical decision tasks. These results suggest that AD patients are impaired in any sort of priming task that requires semantic processing of the stimuli, while priming tasks that require visuoperceptual interpretation of stimuli are unaffected. Current research indicates that perceptual priming is controlled by the extrastriate cortex and conceptual priming is controlled by the left prefrontal cortex, but this is undoubtedly an oversimplified view of the process, and current work is focused on elucidating the brain regions involved in priming in more detail.

Priming is thought to play a large part in the systems of stereotyping. This is because attention to a response increases the frequency of that response, even if the attended response is undesired. The attention given to these response or behaviours primes them for later activation. Another way to explain this process is automaticity. If trait descriptions, for instance ‘stupid’ or ‘friendly,’ have been frequently or recently used, these descriptions can be automatically used to interpret someone’s behavior. An individual is unaware of this, and this may lead to behavior that may not agree with their personal beliefs.

This can occur even if the subject is not conscious of the priming stimulus. In a 1996 study, subjects were implicitly primed with words related to the stereotype of elderly people (example: ‘Florida,’ ‘forgetful,’ ‘wrinkle’). While the words did not explicitly mention speed or slowness, those who were primed with these words walked more slowly upon exiting the testing booth than those who were primed with neutral stimuli. Similar effects were found with rude and polite stimuli: those primed with rude words were more likely to interrupt an investigator than those primed with neutral words, and those primed with polite words were the least likely to interrupt.

These findings have been extended to therapeutic interventions. For example, a therapist presented with a depressed patient who ‘self-stereotypes herself as incompetent,’ ‘can find ways to prime her with specific situations in which she had been competent in the past… Making memories of her competence more salient should reduce her self-stereotype of incompetence.’

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