User:Poornima Venkataraman/sandbox

[[File:Pens.JPG|thumb|Pens.

Initially, the person picks out the red pen and therefore it is the prime target while the remaining pens in the holder are considered to be prime distractors. When the person wants to use the blue pen (probe target) instead, negative priming effects are observed as the blue pen was previously ignored as the prime distractor.]]

[[File:Pens.JPG|thumb|Pens.

Initially, the person picks out the red pen and therefore it is the prime target while the remaining pens in the holder are considered to be prime distractors. When the person wants to use the blue pen (probe target) instead, negative priming effects are observed as the blue pen was previously ignored as the prime distractor.]]

Negative Priming is the implicit memory effect in which prior exposure to a stimulus unfavorably influences the response to the same stimulus. It falls under the category of priming, which is a memory effect in which exposure to a stimulus influences response to a later stimulus without any conscious awareness. Negative priming is the effect where a reaction to a stimulus that is previously ignored is slow and error-prone. For example, when you are trying to pick out the red pen from the pen holder, the red pen is the target of your attention and the remaining pens are blocked as distractors. If you want to switch to the blue pen next, there is a momentary delay in this action. This slow reaction is caused by the negative priming effect as the blue pen was once a distractor.

Negative priming is believed to play a crucial role in selective attention and memory retrieval processes. Both attention and memory are similar in that many internal representation are activated simultaneously and only certain of them are selected to be filed in the short term memory. Negative priming is assumed to reflect both the excitation and inhibition associated with the competing distractors during the selection process.

Negative priming is also known as the mechanism using which we apply inhibitory control over cognition. This refers only to the inhibition stimuli that can interfere with the current short term goal of creating a response. The effectiveness of inhibiting the interferences depends on the cognitive control mechanism as a higher number of distracters yields higher load on working memory. Increased load on working memory can in turn result in slower perceptual processing leading to delayed reaction. Therefore, negative priming effect depends on the amount of distractors, effectiveness of the cognitive control mechanism and the availability of the cognitive control resources.

Distractor Inhibition model
Distractor Inhibition model explains the negative priming effect as the result prior inhibition of the stimulus as a distractor in order to facilitate the selection of the target. When the distractor stimulus becomes the target, the response is subsequently hampered due to residual inhibition. This selection of a target refers to selective attention or the ability to respond to a specific object when other distractors also compete for a response. While excitation is known to facilitate attention, distractor inhibition model considers a dual mechanism involving excitation to boost target signal and inhibition of distractors to be more efficient in selective attention. This inhibition is triggered when there is a mismatch between the internal representations of the target and a distractor stimulus and this disparity suppresses the activity. Inhibition of the distractor’s internal representation functions to facilitate selective attention and decays gradually when the stimulus is no longer present. When a distractor stimulus is re-encountered as the target, the internal representation of this stimulus may continue to be suppressed due to residual inhibition. This inhibition results in the impairment of processing an appropriate response to the new target stimulus, which is observed by the greater reaction time.

There are a few problems associated with this inhibition model. This model accounts for negative priming only when the stimuli are ignored repetitively as distractors under conditions of goal directed behavior of selecting the target stimulus. Another issue is that negative priming effects have been found to be long-term compared to transient residual inhibition as proposed by this model. Long-term negative priming effects have been confirmed by various research and is one of the main concern in this model.

Following the development of these issues, Tipper and Houghton modified the distractor inhibition model to account for long term negative priming effects. Initially, inhibition process was thought to happen only once, when the distracter stimulus is encountered. This was modified in the Houghton-Tipper Model, in which the inhibition of the stimulus as distractor is cued again when the same stimulus is encountered as the new target. This model further suggests that inhibition happens twice with initial occurrence during selective attention and the second occurrence of inhibition when the distracter becomes the target.

Episodic Retrieval Model
The Episodic Retrieval model theorizes that each encounter with a stimulus is encoded and stored separately as an individual episode. Each episode includes perceptual details of both the stimuli and the response developed for that stimulus. When a stimuli is encountered the second time, the first episode of the stimuli is retrieved automatically. When the repetitively ignored distractor stimulus becomes the target, the tag associated with the response to the stimulus is also retrieved. This tag associated with response to a distractor will likely be "do-not-respond" tag as opposed to "respond". Retrieval of such a tag presents a conflict of whether to respond or not. Resolving this conflict is time-consuming and produces negative priming effect.

This model has gained more popularity over the last decade compared to the Distractor Inhibition model due to its account of long term negative priming effects. The main distinguishing factor between the two models is that the inhibition model occurs during the encoding of the distractor stimuli, whereas, episode retrieval claims that the negative priming occurs only when the memory of the stimuli is retrieved. Essentially, these models vary by their forward view of negative priming as a process inn selective attention and backward view of episode retrieval. Empirical evidence supporting the Episode Retrieval model comes from studies that determined the decay of negative priming effects as a function of RSI. Episode Retrieval model explains these results in terms of the ability to retrieve an episode based on the ratio of 2 consecutive trial intervals of prime and probe. When the most recent interval is short and the initial interval is long, there is maximum decay in the negative priming effect. However, when the intervals of both trials are the same, no decay is found. These show that the negative priming effect is produced by the retrieval of that episode. Recent findings lean towards this model but the model itself is not entirely complete. It lacks in explaning how the retrieval of the initial episode creates the longer reaction time. The “do-not-respond” tag is also vague and needs more concrete evidence to support this model.

Feature Mismatch hypothesis
This theory proposes that negative priming effect is the result of interference due to the target being located where the distractor was once located. This theory originated from the findings that selection of a target is not enough to cause negative priming effect, which were observed from experiments where the subjects arbitrarily chose the target out of two possibilities. Feature Mismatch hypothesis is based on the findings of neural facilitation when the target and location remain same and inhibition when there is a mismatch in the target and its location. This theory is also related to Simon effect, which refers to the innate tendency to respond faster and more accurate when stimuli occur in the same location. This theory may explain the effects of location specific negative priming but lacks in its explanation of negative priming when location is not involved.

Rather than a complete theory that explains negative priming, it only offers more loop holes in the Distractor Inhibition model. This theory principally differs from the distractor inhibition model in its stand that selection of a target stimulus where the distractor is ignored is not necessary for negative priming. This theory also provides evidence that negative priming is not restricted to cases of selective attention.

Temporal Discrimination model
Temporal Discrimination Model attempts to blend in both the selective attention and memory retrieval aspects of negative priming in a less complex model. It is based on the assumption that negative priming is caused only at the moment of response to a stimulus that was previously considered distractor. This model explains negative priming as the delayed response due to confusion in classifying a stimulus as old or new. A new stimulus is immediately classified as new and undergoes perceptual processing. A repeated old stimulus is familiar and cues the automatic retrieval of the prior episode. A stimulus that has been repetitively ignored prior to becoming the target is neither entirely new nor old and falls under the gray area. This "gray area" ambiguity slows down the processing of the stimuli. The temporal discrimination model points to this ambiguity as the cause of slowed categorization of the stimulus leading to negative priming effect. Like feature mismatch hypothesis, this model also claims that negative priming is not due to selective attention to the prime target and inhibition of the distractor. This model argues that "negative priming is an emergent consequence of a discrimination process that is inherent to memory retrieval". Temporal discrimination model explains negative priming without reference to inhibition of distractors or the "do not respond" tag and by simple discrimination of "old", "new" and "in between" categories.

Neuroanatomy and Imaging
Neurological evidence of negative priming effects is being researched to help understand the physiological aspects and to develop more accurate models. The most common method to find such neurological evidence is by neuroimaging the brain using fMRI while subjects go through experiments of tasks that prompt negative priming effects. The two primary bases for neurological evidence are the internal representations of stimuli and memory retrieval. Most significantly activated regions of the brain are the left temporal lobe, inferior parietal lobe, and the prefrontal cortex of the frontal lobe. Evidence for internal representations are found in the left anterior temporal cortex, which has been associated with abstract semantic knowledge representations. Left anterolateral temporal cortex was found to be directly related to the magnitude of negative priming effect. The inferior parietal lobe is implicated with the overt shifts of attention that occurs when attending to the distractors and the target. The inferior parietal cortex activated whenever there was a shift in attention from the distractor to target stimulus or vice versa. Another significant area of activation was found in the prefrontal cortex. The superior, inferior, and medial frontal gyri, and the medial prefrontal cortex exhibited activation during the negative priming tasks. Activations in the frontal lobe has been associated with inhibitory network and selective attention. Similarly, evidences for semantic representations and temporal lobe activations are used to support the episode retrieval model. Additional investigations of the neurophysiological data of negative priming are necessary to further clarity the dynamic relationship between selective attention and memory in negative priming.

Stimulus Modality
The primary two stimulus modalities used for negative priming research are visual and auditory stimulus materials. The stimulus presented varied from objects or symbols in visual field to human voices or artificial sounds. Stronger negative priming effects are found for auditory stimulus but the standardized effect sizes between the modalities did not vary. Evidence for negative priming has also been found across various modes of response including vocal naming, manual key press, and reaching. Negative Priming was observed for various types of judgment such as identification, categorization, matching, counting and localization. The tasks used to find evidence for negative priming includes stroop color-word task, lexical decision task, identification, matching, and localization tasks. The stroop color-word task utilizes the stroop effect to observe the distracter suppression and negative priming. Identification tasks present a set of images, sounds, words, symbols, or letters and require the subject to select the prime target based a particular feature that differentiates the target from the distracter. Lexical decision utilizes semantic knowledge of the subject and tests the subject ability to remember the multiple meanings and uses of one word. For example, the word "bank" has multiple meanings and can be referred in different contexts such as "bank is a place where money is deposited" or "banks of a river". Matching tasks require subjects to respond “same” or “different” by matching the target letters or shapes with the explicitly specified goal while ignoring the distractor. Localization tasks require some form of movement of subjects to respond to the location of the target stimulus. This type of localization task is especially used to test the Feature Mismatch hypothesis as it provides evidence for negative priming during the mismatch of the location and target stimuli.

Prime, Probe and Interference
Prime trial is the initial presentation of stimuli that may be repeatedly displayed and/or ignored before the actual testing for the stimulus. Both the target and the distractor are presented during the prime trial. The subject is instructed to identify the target based on the color or other form of identification. Probe trial is the actual testing during which the response is used to measure the negative priming effect. Some experiments may use different forms of interference such as changes in the position of the stimuli or the presentation of completely irrelevant stimuli. Probe interference is negative priming effect caused by the presence of distractor stimuli present in the probe itself. Probes that did not require the selection of the target over the distractor had a lower negative priming effect. This is being used as counter evidence for the Distractor Inhibition theory as it would need a distractor in its probe to test the inhibition of the distractor.

RSI Effects
Response-Stimulus interval (RSI) is the time difference between the response to prime target and the onset of probe trial stimulus. Negative priming effects are observed for delays of 20 ms to 8000 ms between the prime and probe by various experiments, during which negative priming decays rapidly. However, a fixed interval or rate of this decay has not been established due to contradicting results from various experiments. This decay has been taken into account by the two both the Distractor Inhibition model and the Episode Retrieval model and use varying results to explain the delay as a part of the model of negative priming. Further research is needed to determine concrete RSI data and establish short-term and long-term negative priming limits.

Conclusion
Among the four theories, the Feature Mismatch hypothesis and the Temporal Discrimination model lack adequate evidence to support them and only incorporate the other two models in some fashion. For example, the Feature Mismatch hypothesis and the Temporal Discrimination model can be easily incorporated into Distractor Inhibition model and Episode Retrieval model respectively. The Distractor Inhibition model was most popular and dominant model until recent contradicting findings pointing to a retrieval mechanism in negative priming. Episode Retrieval model is gaining more support for the memory based negative priming but lacks in its concrete explanation of the association tags. Perhaps, further research exploring both these models might pave a way to better understand the role of negative priming in both selective attention and memory.