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Interference Theory states that interference occurs when the learning of something new causes forgetting of older material on the basis of competition between the two. There are 3 main kinds of Interference Theory: Proactive, Retroactive and Output. The main assumption of Interference Theory is that the stored memory is intact but unable to be retrieve due to competition created by newly accquired information.

The History of Interference Theory
Bergström, a German psychologist, is credited as conducting the first study regarding interference in 1892. His experiment was similar to the Stroop task and consisted of subjects to sort two decks of card with words into two piles. When the location was changed for the second pile sorting was slower showing that the first sorting rules interfered with the learning of the new sorting rules. German psychologists continued in the field with Georg Elias Müller and Pilzeker in 1900 studying Retroactive Interference. To the confusion of Americans at a later date Georg Elias Müller used associative hemming (inhibition) as a blanket term for retroactive and proactive inhibition. The next major progression came from an American psychologist by the name of Benton J. Underwood in 1915. Underwood found that the more lists that were learned, the less the last-learned list was retained after 24 hours. These results were controversial because of the well known effect of the learning theory at the time. In 1924, James J. Jenkins and Dallenback showed that everyday experiences can interfere with memory with an experiment that resulted in retention being better over a period of sleep than over the same amount of time devoted to activity. The United States again made headway in 1932 with John A. McGeoch suggesting that decay theory should be replaced by an Interference Theory. The most recent major paradigm shift came when Underwood proposed that proactive inhibition is more important or meaningful than retroactive inhibition in accounting for forgetting.

Proactive Interference
Proactive Interference is the "forgetting [of information] due to interference from the traces of events or learning that occurred prior to the materials to be remembered". Proactive Interference occurs when in any given context, past memories inhibit an individual’s full potential to retain new memories. It has been hypothesized that forgetting from working memory would be non-existent if not for proactive interference. A real life example of Proactive Interference is if a person had the same credit card number for a number of years and memorized that number over time. Then if the credit card was compromised, and a new card dispensed to the client, the person would then have great difficulty memorizing the new credit card number as the old credit card number is so ingrained in their minds. The competition between the new and old credit card numbers cause Proactive Interference.



Brain Structures
The leading experimental technique for studying Proactive Interference in the brain is the “Recent-Probes” Task, in which participants must commit a given set of items to memory and they are asked to recall a specific item which is indicated by a probe. Using the “Recent-Probes” Task, the brain mechanisms involved in the resolution of Proactive Interference have been identified as the ventrolateral prefrontal cortex and the left anterior prefrontal cortex. These influential areas of the brain have been identified through functional magnetic resonance imaging (fMRI).



Span Performance
Span performance refers to working memory capacity. It is hypothesized that we have a limited span for performance in language comprehension, problem solving and memory. Span performance on later experimental trials is worse than performance of earlier trials. This indicates that Proactive Interference affects susceptibility to span performance.

Retroactive Interference
Retroactive Interference impedes the retrieval and performance of previously learnt information due to newly acquired and practiced information. An example of Retroactive Interference would be if one was to memorize a phone number and then after a few moments memorize another phone number, practicing the second phone number more. When the recall of the first phone number is needed, the recollection will be poor because the last phone number was the item practiced the most. This Retroactive Interference is found because as the second phone number was practiced more, the retention for the first phone number decreases.



Brain Structures
Retroactive Interference has been localized to the left anterior ventral prefrontal cortex by magnetoencephalography (MEG) studies investigating Retroactive Interference and working memory in elderly adults. The study found that adults 55-67 years of age showed less magnetic activity in their prefrontal cortices than the control group. Executive control mechanisms are located in the frontal cortex and deficits in working memory show changes in the functioning of this brain area.

Pitch Perception
Retroactive Interference has also been investigated using pitch perception as the learning medium. The researcher found that the presentation of subsequent stimuli in sucession causes a decrease in recalled accuracy. Massaro found that the presentation of successive auditory tones, confused perceptual short term memory, causing Retroactive Interference as the new tone inhibits the retrieval of previously heard tones.

Motor Movement
Wohldmann, Healey and Bourne found that Retroactive Interference also effects retention of motor movements. Researchers found that retroactive interference effects the performance of old motor movements when newly acquired motor movements are practiced. Physical practice of newly executed motor movements decreased the retention and recall of previously learnt movements. Despite the retroactive interference noted by Wohldmann et al., researchers noted that mental practice decreased the amount of retroactive interference, suggesting that mental practice is more flexible and durable over time. This study of the superiority effect of physical practice is similar to the Word Superiority Effect made famous by Cattell.

Word Tasks
Retroactive Interference increases when the items are similar, therefore increasing association between them as shown by spreading activation. Barnes and Underwood found that when participants in the experimental condition were presented with two similar word lists, the recollection of the first word list decreased with the presentation of the second word list. This finding contrasts the control condition as they had little Retroactive Inference when asked to recall the first word list after a period of unrelated activity.

Output Interference
Output Interference occurs when the initial act of recalling specific information interferes with the retrieval of the original information. Output Interference occurs if one had created a list of items that were to be purchased at a grocery store, which had been forgotten home. The act of remembering a couple items on that list decreases the probability of remembering the other items on that list.

Short-Term Memory
Henry L. Roediger III and Schmidt found that the act of retrieval can serve as the source of the failing to remember, using multiple experiments that tested the recall of categorized and paired associative lists. Three experiments were carried out where subjects were first presented with category lists and then asked to recall the items in the list after being shown the category name as a cue. The further the test position from the category resulted in a decline of the recall of words. A fourth experiment revealed that only recent items were present in output interference in paired associative lists.



Long-Term Memory
Smith found that if categories with corresponding items were successfully recalled, a systematic decline would occur when recalling the items in a category across the output sequence. He conducted multiple experiments to determine the input conditioned necessary to produce Output Interference. In his first experiment word recall per category was greater at 60 sec than 30 sec when taking the last input category out to prevent recency effect. In his second experiment he changed the instructions, words used, and nature of the test for retention, and showed with recognition procedure, there was Output Interference but the effect was limited to the first three output positions. Even if retrieving items is necessary for recall, it is not crucial to performance in a recognition tack. Recall of the organized information from long-term memory had a negative effect on the following item recalled. In long-term memory, Smith suggests that Output Interference has effects on extra-core material, which is represented as contextual information, rather than core material, which is highly available as a result of organization. Both short and long term memories are centralized to the hippocampus and the amygdala.

Effects of Age
In both short-term memory and long-term memory Smith measured output interference in three age groups (aged 20-39, 40-59, 60-80 years). The results of recall performance revealed significant differences due to age where the older group recalled fewer items than the middle group who recalled fewer items than the youngest group. Overall Smith concluded that memory decline appears with increased age with long-term memory forgetting rather than short-term memory forgetting and short-term memory was unaffected by age. However output interference was unable to explain the memory deficit seen in older subject.

Recent research of adult’s free recall and cognitive triage displayed similar findings of recall performance being poorer in older adults compared to younger adults. Although it was also indicated that older adults had an increased susceptibility to output interference compared to younger adults and the difference increased as additional items were recalled.

Decay Theory
Decay theory outlines that memories weaken over time despite consolidation and storing. This is to say that although you remember a specific detail, over time you may have greater difficulty retrieving the detail you encoded. It has been suggested that the time interval between encoding and retrieval determines the accuracy of recall.

A practical example of Decay theory is seen in the financial sector. If you open a bank account and not deposit or withdraw money from the account, after a period of time the bank will render the account dormant. The owner of the account then has to reopen the account for it to remain active. The bank account (the memory) is rendered dormant (the memory weakened) over time if there is not activity on the account (if the memory is not retrieved after a period of time).

Comparing Decay and Interference Theory
Decay theory is similar to Interference Theory in the way that old memories are lost over time. Memories are lost in Decay Theory by the passing of time. In Interference Theory, memories are lost due to newly acquired memories. Both Decay and Interference Theories are involved in psychological theories of forgetting.

Contrasting Decay and Interference Theory
Decay and Interference Theory differ in that Interference Theory has a second stimulus that impedes the retrieval of the first stimulus. Decay Theory is caused by time itself. Decay Theory is a passive method of forgetting as no interference is produced. Interference Theory is an active process because the act of learning new information directly impedes the recollection of previously stored information.

Dual Task Interfence
Dual Task Interference is a kind of interference that occurs when two tasks are attempted simultaneously. Harold Pashler from McMaster University in Hamilton, Ontario Canada wrote a paper summing up the theoretical approaches to Dual Task Interference. The basis of his research looked at, when one attempts two or more tasks at the same time, why in some cases are they successful in completing their task and in other cases not.

Capacity Sharing
Pashler proposed that the brain contains one mental entity to where all tasks must be carried out. A real life example of this could be going to the dentist, the only place to have cavities filled is at a dentist’s office. When the brain is attempting to complete two tasks, both tasks are present in the same mind area and compete for processing ability and speed. This relates to Interference Theory as the tasks compete. Interference Theory says that the learning of new information decreases the retrieval of older information and this is true in Dual Task Interference. The dominant task of the two, inhibits the other task from completion. It is presumed that the dominant task would be a new task as a previously accomplished task would already be stored in memory. The new task would then successfully be completed as more mind effort is required to complete a novel task and the previously completed task would not be completed as the new task dominated the mental capacity. Just as Interference Theory states, the completion of new tasks inhibits the completion of previously completed tasks due to capacity sharing.

Cross Talk Models
Cross talk is the communication between sensory inputs, processing and the thoughts of the individual. The theory is that if two processes are being activated and they are not similar in any way (making cookies and going on vacation), the brain will be confused as separate cognitive areas are being activated and there is conflicting communication between the two. Contrastingly, if the two processes are similar (making cookies and pouring milk), there will be less cross talk and a more productive and uninterrupted cognitive processing.

Cross talk is used by engineers to discuss the degradation of communication channels due to context dependence.

Navon and Miller claim that Dual Task Interference is caused by outcome conflict which is a result of one task producing, “outputs, throughputs, or side effects that are harmful to the processing of the [other task]". This is basically the concept of Interference Theory. The thoughts, outputs and side effects of one task either effect the previous or subsequent recall.

Stroop and Simon Task
The performance of Stroop and Simon tasks were monitored on 10 healthy young adults using magnetic resonance image (MRI) scanning. Functional mages were acquired at specific time intervals during each subjects scan. Brain activation during the Stroop and Simon task was remarkably similar including anterior cingulate, supplementary motor cortex, visual association cortex, inferior temporal cortex, inferior parietal cortex, inferior frontal cortex, dorsolateral prefrontal cortex, and caudate nuclei. Interference effects in the Stroop and Simon tasks activate similar brain regions at similar time distribution.

Competitive Interference and Consumer Memory for Advertising
It has been demonstrated that recall will be lower when consumers have afterwards seen an ad for a competing brand in the same product class. Exposure to later similar advertisements does not cause interference for consumers when brands are rated on purchasing likelihood. This shows that information processing objective can moderate the effects of interference of competitive advertising. Competitive brand advertising not only interferes with consumer recall of advertising in the past but also interferes with learning new distinctive brand information in the future.

Reducing Competitive Ad Interference
Repetition improves brand name recall when presented alone. When competitive advertising was presented it was shown that repetition provided no improvement in brand name recall over a single exposure. The competitive ads interfered with the added learning from repetition. However, when target brand name was showed using varying ad executions interference was reduced. Presenting ads in multi modalities (visual, auditory) will reduce possible interference because there are more associations or paths to cue recall than if only one modality had been used. This is the principle of multimedia learning. Also, interference is increased when competing ads are presented in the same modality. Therefore by presenting ads in multiple modalities the chance that the target brand has unique cues is increased.