Dichotic listening
Encyclopedia
In cognitive psychology
Cognitive psychology
Cognitive psychology is a subdiscipline of psychology exploring internal mental processes.It is the study of how people perceive, remember, think, speak, and solve problems.Cognitive psychology differs from previous psychological approaches in two key ways....

, dichotic listening is a procedure commonly used to investigate selective attention in the auditory system
Auditory system
The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...

. In dichotic listening, two different auditory stimuli (usually speech) are presented to the participant simultaneously, one to each ear
Ear
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....

, normally using a set of headphones
Headphones
Headphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...

. 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
Speech shadowing
Speech shadowing is an experimental technique in which subjects repeat speech immediately after hearing it . The reaction time between hearing a word and pronouncing it can be as short as 254 ms or even 150 ms. This is only the delay duration of a speech syllable...

. As Colin Cherry
Colin Cherry
Edward Colin Cherry was a British cognitive scientist whose main contributions were in focused auditory attention, specifically regarding the cocktail party problem. This concerns the problem of following only one conversation while many other conversations are going on around us...

 (1953) found, people recall even the shadowed message poorly, suggesting that most of the processing necessary to shadow the attended message occurs in working memory
Working memory
Working memory has been defined as the system which actively holds information in the mind to do verbal and nonverbal tasks such as reasoning and comprehension, and to make it available for further information processing...

 and is not preserved in the long-term store
Long-term memory
Long-term memory is memory in which associations among items are stored, as part of the theory of a dual-store memory model. According to the theory, long term memory differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 20–30...

. 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
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 to German
German language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

 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 http://www.haskins.yale.edu/publications/pub-r.html demonstrated dichotic perception in the late 1960s and early 1970s at Haskins Laboratories
Haskins Laboratories
Haskins Laboratories is an independent, international, multidisciplinary community of researchers conducting basic research on spoken and written language. Founded in 1935 and located in New Haven, Connecticut since 1970, Haskins Laboratories is a private, non-profit research institute with a...

. 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 at Haskins Laboratories
Haskins Laboratories
Haskins Laboratories is an independent, international, multidisciplinary community of researchers conducting basic research on spoken and written language. Founded in 1935 and located in New Haven, Connecticut since 1970, Haskins Laboratories is a private, non-profit research institute with a...

 that listeners could correctly identify syllables when different components of the syllable were presented to different ears. The formant
Formant
Formants are defined by Gunnar Fant as 'the spectral peaks of the sound spectrum |P|' of the voice. In speech science and phonetics, formant is also used to mean an acoustic resonance of the human vocal tract...

s 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
Lateralization of brain function
A longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. The sides resemble each other and each hemisphere's structure is generally mirrored by the other side. Yet despite the strong anatomical similarities, the functions of each...

 of a cognitive function such as language processing
Language processing
Language processing refers to the way human beings process speech or writing and understand it as language. Most recent theories back the idea that this process is made completely by and inside the brain.- Spoken language :...

. In the early 60s Doreen Kimura reported that verbal stimuli presented dichotically gave a right ear advantage (REA). This was interpreted as showing the result of the structure of the auditory nerves and the left sided dominance for language processing. In the late 1960s and early 1970s Donald Shankweiler http://www.haskins.yale.edu/staff/shankweiler.html and Michael Studdert-Kennedy
Michael Studdert-Kennedy
Michael Studdert-Kennedy is an eminent psychologist and speech scientist. He is well known for his contributions to studies of speech perception, the motor theory of speech perception, and the evolution of language, among other areas. He is a Professor Emeritus of Psychology at the University of...

 http://www.haskins.yale.edu/staff/msk.html of Haskins Laboratories
Haskins Laboratories
Haskins Laboratories is an independent, international, multidisciplinary community of researchers conducting basic research on spoken and written language. Founded in 1935 and located in New Haven, Connecticut since 1970, Haskins Laboratories is a private, non-profit research institute with a...

 used a dichotic listening technique (presenting different nonsense syllables simultaneously to opposite ears) to demonstrate the dissociation of phonetic (speech) and auditory
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 (nonspeech) perception by finding that phonetic structure devoid of meaning is an integral part of language, typically processed in the left cerebral hemisphere
Cerebral hemisphere
A cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...

. A dichotic listening performance advantage for one ear is interpreted as indicating a processing advantage in the contralateral hemisphere
Cerebral hemisphere
A cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...

. In another example, Sidtis (1981) 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.

A study conducted involving the dichotic listening test, with emphasis on subtypes of schizophrenia (particularly paranoid and undifferentiated), demonstrated that paranoid schizophrenics have the largest left hemisphere advantage - with undifferentiated schizophrenics (where psychotic symptoms are present but the criteria for paranoid, disorganized, or catatonic types have not been met) having the smallest. The application of the dichotic listening test helped to further the beliefs that preserved left hemisphere processing is a product of paranoid schizophrenia, and in contrast, that the left hemisphere's lack of activity is a symptom of undifferentiated schizophrenia.

For further details about dichotic listening in neuropsychology
Neuropsychology
Neuropsychology studies the structure and function of the brain related to specific psychological processes and behaviors. The term neuropsychology has been applied to lesion studies in humans and animals. It has also been applied to efforts to record electrical activity from individual cells in...

, see K. Hugdahl (Ed.): Handbook of Dichotic Listening. Chichester, UK: John Wiley & Sons, 1988.
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