Delayed auditory feedback (DAF) is a technique in which someone speaks aloud, and hears their voice come back to them at a delay. In all reverberant environments, we hear the echoes of our voice when we speak (as well as the echoes of other sounds) and a lot of the early perceptual processing of sound involves cleaning the acoustic signal up to get rid of these. Around 50ms, however, the delay becomes both noticeable and disruptive to speaking aloud, and at 200ms it becomes maximally disruptive for most people. If you extend the delay over 200mns, it typically stops being so problematic.
People respond to DAF in a variety of ways: some people become very dysfluent and start to ‘stammer’, some people make their voices very flat in pitch and rhythm: some people utterly bellow. Furthermore some people can remain quiet fluent (though their voice will sound affected in some way) while others simply cannot speak. Notable, many people with a developmental dysfluency (e.g. stammering) find it easier to speak under DAF (or other altered auditory environments, such as choral speaking, altered pitch feedback, even a noisy room)
The question of why DAF affects speech is still being unraveled: the maximum disruption of 200ms is about the duration of a normal syllable in speech, and Pete Howell at UCL has argued that this means you are trying to start a new syllable just as you heard the start of last syllable you said: in other words, it makes it hard to speak because it disrupts the timing of normal speech. We have found that the dominant cortical activation associated with DAF is associated with bilateral posterior auditory fields (see below). The brain areas associated with detecting the delays and in compensating for the delays in speech output are in parts of the brain that have been linked to sensory-motor representations and processes of sound and speech (Warren link). Current work in our lab is trying to establish the neural basis of why some people do better (and worse) than others on this task.
DAF has been suggested as a way of deterring unpleasant chanting at football games though this has serious limitations. People were interested in a more sinister application of DAF as a way of jamming the speech of an individual using a directional microphone (e.g. when “some people tend to lengthen their turns or deliberately disrupt other people when it is their turn … rather than achieve more fruitful discussions”). Need we worry about this? It’s clearly at least technically feasible, but remember (1) not everyone is as affected by DAF and everyone else (and some people are dramatically improved by the technique). There are DAF apps which you can use to find out what DAF is like, if you you want to know more about how you'd react (e.g. http://itunes.apple.com/gb/app/daf-assistant/id309496166?mt=8). Also, (2) consider other technology that can come to your aid – wear noise cancelling headphones, and turn them on if you suspect long range DAF might be used on you.
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