Auditory feedback
Auditory feedback is an aid used by humans to control speech production and singing by helping the individual verify whether the current production of speech or singing is in accordance with his acoustic-auditory intention. This process is possible through what is known as the auditory feedback loop, a three-part cycle that allows individuals to first speak, then listen to what they have said, and lastly, correct it when necessary. From the viewpoint of movement sciences and neurosciences, the acoustic-auditory speech signal can be interpreted as the result of movements of speech articulators. Auditory feedback can hence be inferred as a feedback mechanism controlling skilled actions in the same way that visual feedback controls limb movements.
Speech
Auditory feedback allows one to monitor their speech and rectify production errors quickly when they identify one, making it an important component of fluent speech productions. The role of auditory feedback on speech motor control is often investigated by exposing participants to frequency-altered feedback. Inducing brief and unpredictable changes in the frequency of their auditory feedback has consistently been shown to induce a "pitch-shift reflex", which suggests that this reflex aids in stabilizing voice frequency around the desired target.However, due to the fact that auditory feedback needs more than 100 milliseconds before a correction occurs at the production level, it is a slow correction mechanism in comparison with the duration of speech sounds. Thus, auditory feedback is too slow to correct the production of a speech sound in real-time. Nonetheless, it has been shown that auditory feedback is capable of changing speech-sound production over a series of trials. 10 minutes is typically sufficient for a nearly-full adaptation. Research has also shown that auditory linguistic prompts resulted in greater correction to acoustic perturbations than non-linguistic prompts, reflecting the decrease in accepted variance for intended speech when external linguistic templates are available to the speaker.
Speech Acquisition and Development
Auditory feedback is an important aid during speech acquisition by toddlers, by providing the child with information about speech outcomes that are used to pick-up and eventually hone speech motor planning processes. Auditory inputs are typically produced by a communication partner and heard by the toddler, who subsequently tries to imitate them. Children as young as the age of four have demonstrated the ability to adapt speech motor patterns to perceived changes in vowel auditory feedback, which enables them to maintain the accuracy of their speech output. However, children's speech motor adaption abilities are not fully optimised due to their limited auditory perceptual skills. Thus, improvements in children's ability to perceive relevant acoustic property will usually be followed by an improvement in their speech adaption performance.Individuals who are born deaf often fail to acquire fluent speech, further reinforcing how auditory feedback plays a crucial role in speech acquisition and development.
Delayed auditory feedback experiments indicate that auditory feedback is important during speech production, even in adults. It has been shown that severe disfluencies in speech occur when the timing of voice feedback is delayed for a normal speaker. Individuals who become deaf post-lingually and are unable to receive vocal feedback anymore also typically experience a deterioration in speech quality, highlighting the importance of auditory feedback in speech formation throughout one's lifetime.
Impacts on speech disorders
Stuttering
Stuttering is said to be due to ineffective monitoring of auditory feedback, mainly caused by a deficit in the cortical auditory system modulation during speech planning. When fluent speakers detect a sudden irregularity in a specific acoustic parameter of their auditory feedback, they are able to instantly correct the error in their speech production. Individuals who stutter, on the other hand, are found to have weaker-than-normal abilities to correct such errors. Individuals that stutter hence demonstrate ineffective auditory comparisons of desired speech movements, as compared to fluent speakers.Delayed auditory feedback has been found to be an effective treatment for some individuals who stutter, since extending the time between speech and auditory perception allows for more time to process and correct errors.
Apraxia of speech
It is posited that individuals with apraxia of speech have weak feedforward programs, which results in the disfluencies of their speech. These individuals hence develop a heavy reliance on auditory feedback to minimize and repair speech errors even in later stages of their lives, while fluent speakers easily transitions from feedback dependent to feedforward-dominant. This is not ideal since heavy reliance on mostly auditory feedback is said to be inefficient for the production of rapid and accurate speech.Auditory masking has been found to decrease disfluency duration and increase vocal intensity as well as syllable rate in some individuals with apraxia of speech. Since apraxia of speech is said to be due to weak feedforward programs and high dependence on auditory feedback, auditory masking can be reasoned to increase fluency by decreasing the frequency of a speaker attending auditorily to speech errors, and hence reducing the likelihood of disfluency-generating corrections.
Impacts on Visually Impaired individuals
Enhanced auditory processing can be observed in individuals with visual impairment, who partially compensate for their lack of vision with greater sensitivity in their other sensories. Their increased sensitivity to auditory feedback allows them to demonstrate impressive spatial awareness despite their lack of sight.Desktop Assistance
Studies have shown that when vision is no longer the primary source for obtaining information, focus shifts from vision to hearing in the desktop environment. Currently, there are assistive technologies such as screen readers, which aids visually impaired individuals in obtaining information on their desktop screens via auditory feedback. The assistance can come in the form of either speech based auditory feedback or non-speech based auditory feedback. Speech based interfaces are based on human speech, whilst non-speech based interfaces are based on environmental sounds such as music or artificial sound effects.For the visually impaired, sole reliance on speech based auditory feedback imposes a heavier cognitive load which is irritating for users. In contrast, non-speech auditory feedback is pleasant and conveys information more quickly, but lacks detailed information in their conveyance and training is required to understand the cues. Hence, the most ideal interface currently is adaptive auditory feedback, which automatically transitions between speech and non-speech cues based on the user state. Such an interface has been found to be more comfortable and generates higher satisfaction among visually impaired users.
Impacts on other disorders
Graphomotor learning in writing disorders
A trial was conducted to explore whether auditory feedback had an influence on learning how to write. It was found that in adults, auditory feedback enabled the writer to better discern their writing motions. This resulted in an increase in flow and quickness of writing when using sounds to learn the writing of new characters. Subsequent studies then tested the use of auditory feedback as an aid for children with dysgraphia to learn how to write. It was found that after multiple sittings of using auditory feedback while writing, children could write more smoothly, rapidly and clearly.Products based on auditory feedback principles have been invented to aid individuals with such writing disorders. Children with speech disorders can also benefit from such products. For example, a headphone called Forbrain uses a bone conductor and a series of dynamic filters to correct the perception of one's own voice. This improves concentration, attention, speech, coordination, and other sensory functions. It was awarded by the BETT Show in 2015 in the category "ICT Special Educational Needs Solutions".