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Phonemic Restoration Effect is an perceptual phenomenon where under certain conditions, sounds actually missing from a speech signal can be hallucinated by the brain and clearly heard. The effect occurs when missing phonemes in an auditory signal are replaced with white noise, resulting in the brain filling in absent phonemes. The effect can be so strong that listeners do not even know that there are phonemes missing. This effect is commonly observed in a conversation with heavy background noise, making it difficult to properly hear every phoneme being spoken. Different factors can change the strength of the effect, including age and gender.

Background
The Phonemic Restoration Effect was first documented in a 1970 paper by Richard M. Warren entitled "Perceptual Restoration of Missing Speech Sounds". The purpose of the experiment was to give a reason to why in background of extraneous sounds, masked individual phonemes were still comprehensible. In initial experiments, instead of testing the subject's ability to comprehend a sentence, Warren would only replace one phoneme with extraneous noise and ask the subjects to identify what phoneme was missing from the sentence. No subject could accurately point out the location of the missing phoneme, showing an underlying mechanism in our speech perception. This was a phenomenon that was somewhat known at the time, but no one was able to pinpoint why it was occurring or had labeled it. Warren later did much research for next several decades on the subject.

Since Warren, much research has been done to test the various aspects of the effect. These aspects include how many phonemes can be removed, what noise is played in replacement of the phoneme, and how different contexts alter the effect.

Neuroanatomy
Neurologically the signs of interrupted or stopped speech can be suppressed in the thalamus and auditory cortex. It is currently thought to be done at the bottom portion of the speech processing system. It is believed that humans evolved to be able to fill in phonemes that were absent in order to be able to communicate with other humans in noisy situations. It is not fully known at what point in the top-bottom processing the phonemic restoration effect occurs.

Hearing Impairment
People with mild and moderate hearing loss were tested for the effectiveness of phonemic restoration. Those with mild hearing loss performed at the same level of a normal listener. Those with moderate hearing loss had almost no perception and failed to identify the missing phonemes.

Cochlear Implants
For people with cochlear implants, phonemic restoration is only achievable at resolutions higher than 8. Implants that are at 4 or 8 channels do not have the specificity to develop distinct gaps between phonemes, enough to a point that white noise would help fill in the missing phoneme.

Gender
Instead of completely replacing the phonemes, researchers masked them with tones that are informative(helped the listeners pick the correct phoneme), uninformative(neither helped or hurt the listener select the correct phoneme), or misinformative(hurt the listener in picking the correct phoneme). The results showed that women were much more affected by informative and misinformative cues than men. This evidence suggests that Women are more influenced by top-down semantic information more than men.

Age
A large area of study consists of seeing if children are effected by phonemic restoration and if so, at what capacity. Children are able to produce results comparable to adults by about the age of 5, however still not doing as well as adults. This means they are able to use previous knowledge of words to fill in the missing phonemes with much less of their brain developed than adults. In children, there was no difference in gender found in the results.

Setting
The effect reverses in a reverberation room, which echoes real life more so than the typical quiet rooms used for experimentation. This allows for echoes of the spoken phonemes to act as the replacement noise for the missing phonemes. The additional produced white noise that replaces the phoneme adds its own echo and causes listeners to not perform as well. Srinivasana paper.

Rate
Another study by Warren was done to determine the effect of the duration of the replaces phoneme on comprehension. When the gap became approximately the length of the word is when the effect started top breakdown and become ineffective.

Multisensory
Much like the McGurk Effect, when listeners were also able to see the words being spoken, they were much more likely to correctly identify the missing phonemes. This is because vision is the primary sense for humans and provides an advantage during speech processing. Similar to the McGurk effect and lip reading. Will cover the overlap here a bit.

Context
Because languages are structured around distinct structure, the brain has some sense of what word is to come next in a proper sentence. When listeners were listening to sentences with proper structure with missing phonemes, they performed much better than with a nonsensical sentence without a proper structure. This comes from the predictive nature of the pre-frontal cortex in determining what word should be coming next in order for the sentence to make sense.

Volume
Only when the volume of the noise replacing the phonemes is the same or louder as the surrounding words, does the effect properly work.

Dichotic Listening
When a word with the segment 's' is removed and replaced by silence and a comparable noise segment were presented dichotically. When the 's' sound is barely heard by the listener, the effect is dramatically increased.

Language
No research was found on how different languages differ from English in regards to the effect, however it is assumed that this effect is universal for all languages.