User:Vsoccer36

In cognitive psychology and neuroscience, dichotic listening is a procedure commonly used to investigate selective attention in the auditory system. More specifically, it is "used as a behavioral test for hemispheric lateralization of speech sound perception." During a standard dichotic listening test, a participant is simultaneously presented with two different auditory stimuli (usually speech) separately to each ear over headphones. Participants are asked to distinguish/identify one or (in a divided-attention experiment) both of the stimuli. Later, they are 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 Colin Cherry (1953) found, people recall 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.

During the early 1970s, Tim Rand demonstrated dichotic perception at Haskins Laboratories. In his study, the first formant (F1) was presented to one ear while the second (F2) and third (F3) formants were presented to the other ear. F2 and F3 varied in low and high intensity. Ultimately, in comparison to the binaural condition, "peripheral masking is avoided when speech is heard dichotically." This demonstration was originally known as "the Rand effect" but was subsequently renamed as "dichotic release from masking" to "dichotic perception" or "dichotic listening." Similarly, around the same time, another investigator at Haskins Laboratories, Jim Cutting (1976), investigated how 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. That being said, even though the listeners heard two separate signals (no ear received a 'complete' vowel sound), they could still 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 early 60s, Doreen Kimura reported that verbal stimuli (specifically spoken numerals) presented dichotically gave a right ear advantage (REA). She attributed the right-ear advantage "to the localization of speech and language processing in the so-called dominant left hemisphere of the cerebral cortex." According to her study, this phenomena was related to the structure of the auditory nerves and the left sided dominance for language processing. In the late 1960s and early 1970s Donald Shankweiler and Michael Studdert-Kennedy  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 and is typically processed in the left cerebral hemisphere. A dichotic listening performance advantage for one ear is interpreted as indicating a processing advantage in the contralateral hemisphere. 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.

Since dichotic listening can be used as a lateralized speech assessment task, neuropsychologists have utilized the technique to explore the role of singular neuroanatomical structures in speech perception and language asymmetry. For example, Hugdahl et al (2003), investigated dichotic listening performance and frontal lobe function in left and right lesioned frontal lobe nonaphasiac patients compared to healthy controls. In the study, all groups were exposed to 36 dichotic trials with pairs of CV (constant-vowel) syllables and each patient was asked to state which syllable he or she heard best. As expected, the right lesioned patients showed a right ear advantage like the healthy control group but the left hemisphere patients displayed impairment when compared to both the right lesioned patients and control group. From this study, researchers concluded "dichotic listening as into a neuronal circuitry which also invoves the frontal lobes, and that this may be a critical aspect of speech perception." Similarly, Westerhausen and Hugdahl (2008) analyzed the role of the corpus callosum in dichotic listening and speech perception. After reviewing many studies, it was concluded that "...dichotic listening should be considered a test of functional inter-hemispheric interaction and connectivity, besides being a test of lateralized temporal lobe language function" and "the corpus callosum is critically involved in the top-down attentional control of dichotic listening performance, thus having a critical role in auditory laterality."

Additionally, dichotic listening has revealed a possible small-population sex difference in perceptual and auditory asymmetries and language laterity. According to Voyer (2011), "Dichotic listening tasks produced homogenous effect sizes regardless of task type (verbal, non-verbal), reflecting a significant sex difference in the magnitude of laterality effects, with men obtaining larger laterality effects than women." However, the authors discuss numerous limiting factors ranging from publication bias to small effect size. Furthermore, as discussed in "Attention, reliability, and validity of perceptual asymmetries in the fused dichotic words test," women reported more "intrusions" or words presented to the uncued ear than men when presented with exogenous cues in the Fused Dichotic Word Task which suggests two possibilities: 1) Women experience more difficulty paying attention to the cued word than men and/or 2) regardless of the cue, women spread their attention evenly as opposed to men who may possibly focus in more intently on exogenous cues.

There is an emotional version of the dichotic listening task in which individuals listen to the same word in each ear but they hear it in either a surprised, happy, sad, angry, or neutral tone. They are then asked to press a button indicating what tone they heard. Normally dichotic listening tests show a right-ear advantage for speech sounds, specifically for those with stop constants. This right-ear/left-hemisphere advantage is expected, as we are aware of Broca’s and Wernicke’s areas, both located in the left hemisphere. In contrast, the left ear (and therefore the right hemisphere) is often better at processing nonlinguistic material. The emotional dichotic listening task is consistent with these other studies, as individuals who perform this task tend to have more correct responses to their left ear than to the right. It is important to note that the emotional dichotic listening task is seemingly harder than the phonemic dichotic listening task, eliciting more incorrect responses overall.

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