Dichotic listening

Dichotic listening is the auditory process that involves listening with both ears. Dichotic listening can be broken into two different processes: binaural integration and binaural separation ¹. Binaural integration is the ability to perceive different acoustic messages presented to the left and right ears at the same time ¹. Binaural separation is the ability to perceive an acoustic message in one ear while ignoring a different acoustic message in the other ear ¹. In order to perceive the acoustic messages in both ears, the outer, middle, and inner ears must be working properly, but more importantly, the auditory brainstem nuclei, auditory cortical neurons, as well as neurons in the corpus callosum must be functioning properly. Individuals with dichotic listening deficits often have difficulty hearing in the presence of background noise ¹.

Dichotic listening has been used for many purposes to understand the nature and capacities of selective attention. For example, it has been used with a concurrent visual task to investigate the extent to which visual and auditory attention compete with one another—an important issue for understanding when and how humans are able to multitask.

During a typical dichotic listening assessment, the subject is presented with pairs of consonant–vowel syllables (such as /ba/ and /da/) via headphones; one syllable is played to the right ear, and the other is played simultaneously to the left ear ². The common dichotic listening paradigm consists of three different instructions. In the nonforced condition, the subject is asked to report freely what he or she heard the best ². In the forced‐right condition, the subject is instructed to report the stimuli played to the right ear, and in the forced‐left condition, the subject is instructed to report the stimuli played to the left ear. The right ear advantage (right ear advantage) is a benchmark finding, showing a preference to report stimuli presented to the right ear ³. Since the typical pathway for language processing is contralateral, a right ear advantage in the nonforced condition reflects the superior processing capacity for the right ear stimulus in the left hemisphere. This has also been validated through studies using brain imaging ⁴. On the other hand, a left ear advantage reflects the superior processing capacity for the left ear stimulus in the right hemisphere, and a no ear advantage reflects no superior processing of any ear in either hemisphere.

Whereas a right ear advantage is seen in the majority of older children and adults, a left ear advantage is seen in about 10% of the population ⁵. Younger children are known to have a less pronounced right ear advantage, but parallel to language development, there is a gradual shift towards the left hemisphere ⁶. The scores of correct responses, to both left and right ear stimuli, indicate the degree of lateralisation. The laterality index is a measure of the relative difference between the ear scores independent of correct responses. The forced‐right and forced‐left conditions are seen as tests of attention. These two instructions reflect different cognitive processes. In the forced‐right condition, the instruction to report from the right ear follows the “bottom‐up” bias towards the right ear stimulus, resulting in an increase of the right ear advantage. In the forced‐left condition, on the other hand, the instruction is opposite to the stimuli‐driven bottom‐up perception, and demands strategies for executive cognitive control ⁷. The bottom‐up processes are innate, favouring the right ear, while response to the top‐down forced‐left condition is seen first at around the age of 10 ⁸. Changes in the bottom‐up and top‐down capacities may also interact with the functions of the reading network in the brain ⁹.

The analyses of dichotic listening can be complex, as some studies indicate that handedness, age, and gender may interact with the dichotic listening scores ¹⁰. However, in a large scale study of subjects ranging from age 10 to older adults, it was concluded that handedness did not affect any of the findings, and no sex differences were seen in children and older adults ¹¹.

The dichotic listening technique has been used to explore laterality effects in subjects with agenesis of the corpus callosum, partial and complete commissurotomy ¹², congenital hemiplegia ¹³, acquired and congenital brain injury ¹⁴ and cortical hemispherectomy ¹⁵. The results of these studies suggest that the ear advantage in dichotic listening is a valid measure of hemispheric specialization ¹⁶ and that a reduced overall ear score can indicate abnormal hemispheric specialization. Reversed speech lateralization as measured by a left ear advantage was documented in many of these studies, although care must be taken to distinguish a reduced ear score due to a contralateral lesion effect from a true shift of language dominance. The timing of a lesion and its underlying pathology, as well as presence of seizures, was shown to influence the degree of lateralization as measured by the ear advantage. Congenital lesions seem to reduce the magnitude of abnormal laterality, possibly due to increased cortical reorganization ¹⁷.

Some data gathered from dichotic listening test experiments suggests that there is possibly a small-population sex difference in perceptual and auditory asymmetries and language laterality. According to Voyer ¹⁸, “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” ¹⁹, 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 ¹⁸.

Dichotic listening experiments have shown that different sorts of input show different ear preferences. Subjects with left-hemispheric language lateralization, which possibly constitute more than 95 percent of the population ²⁰, are more accurate in reporting items arriving at the right ear than items arriving at the left ear when the input is verbal. This tendency is commonly referred to as the right-ear advantage for verbal stimulus. Conversely, the majority of people have a left-ear advantage for tasks involving the recognition of music or environmental sounds ²¹.

There are two explanations of these ear advantages in dichotic listening:

1. The structural theory ascribes the advantages to anatomic properties of the auditory system. Kimura ²² explains the right-ear advantage for verbal stimulus by the fact that the right ear is connected to the language dominant left hemisphere of the brain and the left ear to the right hemisphere through the contralateral pathways. She claims that the contralateral pathways are more preponderant than the ipsilateral pathways which constitute the link between the ear and the hemisphere on the same side.
2. The attentional theory also maintains that the basis of the laterality effect is lateralized cortical functions, but emphasizes the influence of attention in priming a particular hemisphere. An expectation of verbal stimuli, for instance, serves to prime the language dominant hemisphere and make it extra sensitive to stimuli ²³.

Current evidence indicates that neither of these two views, in their most extreme forms, is entirely correct. If attentional mechanisms were all that was relevant, then one would predict that it would not be possible to activate both the left and the right hemisphere simultaneously. According to Bryden ²¹, however, one can obtain right-ear advantages for verbal material and left-ear advantages for non-verbal material at the same time. On the other hand, it is proven that attentional factors do make some difference: the same dichotic stimuli that produce an left-ear advantage when one expects non-verbal stimuli, can produce a right-ear advantage when one expects verbal stimuli ²⁴. It therefore seems that both structural and attentional components are relevant to the production of dichotic laterality effects.

Dichotic listening test

Dichotic listening test is a psychological test commonly used to investigate selective attention and the lateralization of brain function within the auditory system ²⁵. Dichotic listening test is used within the fields of cognitive psychology and neuroscience. In a standard dichotic listening test, a participant is presented with two different auditory stimuli simultaneously such as words, sentences, or musical stimuli, into different ears over headphones ²⁶. Dichotic stimulation creates a perceptual conflict between the two ears. In one type of test, participants are asked to pay attention to one or both of the stimuli; later, they are asked about the content of either the stimulus they were asked to attend to or the stimulus that they were not told to ignore ²⁷. In normal subjects dichotic listening test can reveal the dominant hemisphere. Unilateral lesions of the auditory radiations or cortex typically produce poor performance for the contralateral ear with dichotic testing. However, lesions of the dominant hemisphere sometimes can disconnect the auditory areas of the two hemispheres and thereby produce poor performance for the ipsilateral ear, so-called paradoxical ear extinction.

Dichotic listening is a non-invasive method for measuring cerebral hemispheric specialization of auditory processing ²⁸. A general right-ear advantage for verbal material and a left ear advantage for non-linguistic stimuli have been demonstrated in healthy individuals ²⁹. The ear advantage is attributed to the dominance of the contralateral cerebral hemisphere for processing the stimuli. Kimura’s anatomical model ³⁰ posits that although both ears are represented in both hemispheres, the ipsilateral connections are weaker and suppressed during simultaneous presentations of similar auditory stimuli. As a result, each ear primarily projects to the contralateral cerebral hemisphere, and more accurate responses from one ear are interpreted to reflect the specialization of the contralateral hemisphere for processing the stimuli ³¹. However, different dichotic listening tests yield different degrees of hemispheric specialization and create different degrees of ipsilateral suppression. Dichotic listening tests are therefore differentially susceptible to attention and memory load. Kinsbourne ³² proposed an alternative, attentional, model of the ear advantage. In their view, hemispheric specialization is associated with attention shifts to the contralateral half of space. Indeed, attentional focus can modulate incomplete suppression ¹², and there appear to exist separate auditory channels for attended and unattended information. However, the attentional model was not supported by the only reported case of dichotic listening during forced attention in a patient with left hemispherectomy ³³. Thats study revealed a complete right-ear extinction irrespective of attentional instruction.

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 ³⁴. 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. In 1994, M.F. Green and colleagues tried to relate “the functional integration of the left hemisphere in hallucinating and nonhallucinating psychotic patients” using a dichotic listening study. The study showed that auditory hallucinations are connected to a malfunction in the left hemisphere of the brain ³⁵.

Dichotic listening paradigm

It is a well-known fact that the human ability to process incoming stimuli is limited. Nonetheless, the world is complicated, and there are always many things going on at once. Selective attention is the mechanism that allows humans and other animals to control which stimuli get processed and which become ignored. Think of a cocktail party: a person couldn’t possibly attend to all of the conversations taking place at once. However, everyone has the ability to selectively listen to one conversation, leading all the rest to become unattended to and nothing more than background noise. In order to study how people do this, researchers simulate a more controlled cocktail party environment by playing sounds to participants dichotically, i.e., by playing different sounds simultaneously to each ear. This is called a dichotic listening paradigm.

Dichotic listening paradigm was developed and described in the early 1960s and 1970s ³⁶. It has been used as a non‐invasive method to study brain lateralization of speech perception, has been applied to many clinical disorders to assess impairments within attention, working memory and executive functions, and has been validated through numerous studies using different methods ³⁷.

One of the most influential applications of the dichotic listening paradigm has been in the study of the human brain’s lateralization for processing language. The human brain is divided into two hemispheres. Generally speaking, the right hemisphere is wired to the left side of the body, and the left hemisphere is wired to the right. This means that auditory stimuli played exclusively to one ear will be routed, first, to the opposite brain hemisphere. It is also known that the left hemisphere is generally specialized for language processing. Therefore, the prediction is that stimuli played to the right ear should be processed more effectively than stimuli played to the left ear. This has been confirmed in many dichotic listening studies, and it makes dichotic listening a useful paradigm for investigating language deficits thought to be associated with left hemisphere brain damage, as often occurs after a stroke, but without using a brain scan.

Dichotic listening task

Dichotic listening task is a very useful way to study selective attention ³⁸. Dichotic listening task asked an observer to pay attention to one of two different messages, each delivered to one ear over stereo earphones. For example, the left earphone may present Lincoln’s Gettysburg Address, and the right earphone may give a list of random words. The observer is asked to pay attention to the Gettysburg Address and ignore the random words in the other ear, as shown in Figure 1.

To make sure that attention is strictly focused on one message, the observer repeats aloud that message. This task is called shadowing and is summarized in Figure 2. It is difficult to do, and it effectively fixes attention to that one ear. Observers repeat the shadowed message in a stiff voice, showing that they are putting out quite a bit of effort to do the shadowing well.

When questioned about the message in the unattended ear, observers usually cannot report anything about it. They may be able to say that someone was speaking and identify the sex of the speaker. But they consciously recall nothing about the contents of the message except the last few words before they report and the number of words observers do recall has been used as a measure of how long echoic sensory memory lasts. There is one exception: about 1/3 of the time observers notice strongly emotional words in the unattended ear — their name ³⁹ or sexually explicit “taboo” words, especially if the observer had a higher anxiety level ⁴⁰.

Figure 1. Dichotic listening task

Figure 2. Dichotic listening task

Dichotic listening experiment

The simplest way to carry out a dichotic listening experiment is to use a prerecorded tape and stereophonic headphones. The subject will be presented with one stimulus sound in the right channel of the headphones and another in the left. The channels in the recording should be switched regularly to make up for possible differences in hearing between the left and the right ear.

In some cases the subjects are told that they will receive a different stimulus item in each ear and are instructed to report both these items. The number of correct reports from the right and the left ear are compared. In other experiments the informants are not informed that they will receive dichotic input. The stimulus material should then be short and carefully selected so that the two sounds “fuse into one perceptual unit” ²¹. Listening to this type of input material, subjects often claim to hear only one single sound, localized in the centre of the head. One then compares the number of reports of the material presented to the right ear and the number of reports of the stimulus presented to the left ear. Perhaps the former technique could be called an informed dichotic experiment (as the subject would be informed of the dichotic nature of the input) while the latter approach could be called non-informed .

Dichotic listening experiment procedure ⁴¹

1. Apparatus and stimuli

1. Use two sets of headphones and two pieces of hardware for playing auditory stimuli. Ultimately, the experiment requires the ability to play two separate auditory signals through separate headphones. This can be done with a sound mixer or with a computer. But it can be done easily and without technical expertise by using two separate pieces of hardware.
2. Select three different recordings with informational content that can be tested for comprehension. These recordings are the stimuli for dichotic listening. Comprehension questions are also necessary. A good source for this kind of material is publicly available reading comprehension texts and questions from exams like the Scholastic Aptitude Test. For this demonstration, two pieces of text and their associated questions selected can be found here (https://www.jove.com/files/ftp_upload/10101/Jon_Flombaum_Dichotic_Listening_Appendix_1.pdf).
3. Record one person reading each of the selected pieces, and create three individual audio files.
4. Print out the questions for each passage for the participant to complete after hearing the relevant recordings.

2. Procedure

1. Keep in mind that the goal of this experiment is to compare the ability to retain information for selectively attended stimuli compared to unattended stimuli. Set up the experiment to include two listening and test sessions.
   1. The first session is a baseline, with only a single audio passage intended to measure baseline listening comprehension without a second stimulus present.
   2. The second session involves two different passages played simultaneously, one to each ear, with instructions to attend to one of them.
2. Explain the instructions for the baseline condition to the participant, as follows:
   “In a moment, I will ask you to put one headphone in each of your ears. I will press play, and a short passage will be read aloud to you through the headphone in your right ear. Nothing will come from the other. Please pay attention and listen carefully while the passage is read. Afterwards, you will be asked to answer some questions about the passage.”
3. Play one of the passages through the headphone attached to the right ear. For control purposes, the participant should have a headphone in the left ear as well, but nothing should play through it.
4. Once the passage has finished playing, give the participant the set of questions associated with it, and allow them 15 min to answer as many as possible.
5. For the dichotic condition, explain the procedure to the participant, as follows:
   • “In a moment, I will ask you to place a headphone in each of your ears again. And again, a passage will be read aloud through the headphone played to your right ear. Please play close attention to that passage because you will be asked questions about it after. But this time, a different passage will be read aloud through the headphone in your left ear. You should do your best to ignore that passage, and direct your attention to your right ear.”
6. Once the participant has the headphones placed in their ears, press play simultaneously on each.
7. When the passages are done, give the participant the questions for the two passages. They should be in the same document and randomly intermixed. Explain this to the participant:
   • “Now, I’d like you to answer as many questions as you can about the passages you just heard. About half of the questions are from the passage I asked you to pay attention to. But the other half will be from the other passage. Please do your best. And if you feel like you don’t know an answer, just guess.”
8. Allow 30 min for the participant to answer the questions. Once the participant has left, score the answers.
9. Once the questions are scored, calculate the proportion correct associated with each individual passage. Graph the scores.

Dichotic listening training

Dichotic listening training can involve dichotic listening training (bottom-up approach), compensatory strategies training (a top-down approach) and a combination of both treatments (combined therapies) at improving the listening abilities (listening without background noise, listening ability in background noise and sound localisation ability) of children with auditory processing disorder (APD).  Auditory Processing Disorder (APD) formerly referred to as central auditory processing disorder is an abnormality in the brain’s ability to filter and process sounds and words. Individuals with Auditory Processing Disorder have a neurological defect in the pathways from the auditory (hearing) nerve through the higher auditory pathways in the brain. This causes distortion and/or delay in auditory signal transmission, which results in inaccurate or incomplete coding of sound. Since these individuals struggle to process (or interpret) what they hear, it causes listening problems that often mimic a hearing loss.

Most people with Auditory Processing Disorder will usually pass a hearing test and often have normal intelligence. However, since the brain receives sounds incorrectly, they may not recognize subtle differences between sounds in words (duh and guh for example), and they may have difficulty using those sounds for speech and language.

Children with Auditory Processing Disorder have trouble screening out background noise, so surrounding sounds from air conditioners, hallways, and noisy environments such as gymnasiums make it very difficult to understand speech. It’s like listening to a radio station with static or other stations interfering with the reception. In addition, these children typically try so hard to understand that they often forget parts of what they hear.

There are many types of Auditory Processing Disorder, and no two individuals are exactly alike in their symptoms. Auditory processing evaluations are most often performed on school-age children, although many adults are also tested.

Some of the more common problems in individuals with auditory processing disorder include poor ability to:

• Direct, sustain, or divide attention
• Discriminate subtle differences in sounds and words
• Hear in noisy situations
• Recognize and integrate a sequence of sounds into words or other meaningful combinations
• Comprehend rapidly spoken speech
• Remember and/or comprehend spoken information
• Follow multi-task directions

Formal auditory training has been recommended to improve auditory processing skills in order to capitalize on the neural plasticity of the central nervous system ⁴². When the listener receives appropriately designed sensory stimulation, neural changes should occur, including enhanced myelin development and transmissions across synapses ⁴³.

This study ⁴⁴ describes one treatment designed for children with deficits in dichotic listening skills. Dichotic listening tasks require listeners to attend to different words, phrases, or short sentences presented simultaneously to each ear. For example, a listener under headphones may be presented the double-pair digits “6–2” in the left ear and, at the same time, “4-1” in the right. The ability to process both inputs improves with age, reaching 90%-96% accuracy by age 12 ⁴⁵.

Musiek, Gollegly, and Baran hypothesized that when children demonstrate difficulties in this area (particularly with input presented to the left ear), the poor transfer of auditory input across hemispheres may be due to delays in the myelination of the corpus callosum ⁴⁶ Hence, appropriate auditory training would be expected to stimulate this myelin development and improve interhemispheric transfer.

The results of that study showed that for most subjects, providing auditory stimulation to the left ear only improved left-ear dichotic deficits as measured by the Dichotic Digits Test. However, it is not known why some subjects responded to dichotic listening training faster than others or why some participant did not achieve normal test scores after 12 sessions. And it remains to be determined if improvements in dichotic listening skills affect other auditory processing skills or academic success.

Future studies should include randomized assignments to treatment/different-treatment/no-treatment groups. In addition, a multiple baseline design would allow each subject to serve as its own control against the possibility of maturation effects. Other auditory training procedures and outcomes also need to be reported for our field to advance in these areas.

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