In cognitive psychology, dichotic listening is a procedure commonly used to investigate selective attention in the auditory system. In dichotic listening, two different auditory stimuli (usually speech) are presented to the participant simultaneously, one to each ear, normally using a set of headphones. Participants are asked to attend to one or (in a divided-attention experiment) both of the messages. They may later be asked about the content of either message.

In a selective attention experiment, the participant may be asked to repeat aloud the content of the attended message, a task known as shadowing. As Cherry (1953)^[1] found, people recall even the shadowed message poorly, suggesting that most of the processing necessary to shadow the attended message occurs in working memory and is not preserved in the long-term store. Performance on the unattended message is, of course, much worse. Participants are generally able to report almost nothing about the content of the unattended message. In fact, a change from English to German in the unattended channel usually goes unnoticed. However, participants are able to report that the unattended message is speech rather than non-verbal content.

Tim Rand[1] demonstrated dichotic perception in the late 1960s and early 1970s at Haskins Laboratories^[2]. This demonstration was originally known as "the Rand effect" but was subsequently renamed as "dichotic release from masking" and then "dichotic perception" or "dichotic listening." Another example of a dichotic listening experiment is Jim Cutting's (1976) demonstration^[3] at Haskins Laboratories that listeners could correctly identify syllables when different components of the syllable were presented to different ears. The formants of vowel sounds and their relation are crucial in differentiating vowel sounds. Yet even though listeners heard two separate signals (no ear received a 'complete' vowel sound), they could identify the syllable sounds.

Dichotic listening can also be used to test the hemispheric asymmetry of a cognitive function such as language processing. In the late 1960s and early 1970s Donald Shankweiler [2] and Michael Studdert-Kennedy [3] of Haskins Laboratories used a dichotic listening technique (presenting different nonsense syllables simultaneously to opposite ears) to demonstrate the dissociation of phonetic (speech) and auditory (nonspeech) perception by finding that phonetic structure devoid of meaning is an integral part of language, typically processed in the left cerebral hemisphere^[4][5][6]. A dichotic listening performance advantage for one ear is interpreted as indicating a processing advantage in the contralateral hemisphere. In another example, Sidtis (1981)^[7] found that healthy adults have a left-ear advantage on a dichotic pitch recognition experiment. He interpreted this result as indicating right-hemisphere dominance for pitch discrimination. For further details about dichotic listening in neuropsychology, see K. Hugdahl (Ed.): Handbook of Dichotic Listening. Chichester, UK: John Wiley & Sons, 1988.

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