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Dichotic stimulation is a form of auditory stimulation in which different sounds are presented to each ear simultaneously.

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."[1] During a standard dichotic listening test, a participant is simultaneously presented with two different auditory stimuli (usually speech) separately to each ear over headphones.[2] Participants are asked to distinguish/identify one or (in a divided-attention experiment) both of the stimuli. Later, they may 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)[3] 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. In addition to this, if the content of the unattended message contains certain information, such as the listener's name, then the unattended message is likely to be noticed and remembered.[4] Also if the message contains sexual words then people usually notice them immediately.[5] This suggests that the unattended information is also undergoing analysis and keywords can divert out attention to it.

During the early 1970s, Tim Rand [1] demonstrated dichotic perception at Haskins Laboratories.[6] 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."[6] 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),[7] 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.


Uses

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 dichotically presented verbal stimuli (specifically spoken numerals) presented to a participant produced a right ear advantage (REA).[8] 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." [9] According to her study, this phenomenon was related to the structure of the auditory nerves and the left-sided dominance for language processing.[10] It is important to note that REA doesn't apply to non-speech sounds. What is more is in "Hemispheric Specialization for Speech Perception," Studdert-Kennedy and Shankweiler (1970)[11] examine dichotic listening of consonant-vowel-consonant (CVC) syllable pairs. The six stop consonants (b, d, g, p, t, k) are paired with the six vowels and a variation in the initial and terminal consonants are analyzed. REA is the strongest when the sound differs for the initial consonant and it is the weakest when the vowel experiences the variation. Asbjornsen and Bryden (1996) state that "many researchers have chosen to use consonant-vowel (CV) syllable pairs, usually consisting of the six stop consonants paired with the vowel \a\. Over the years, a large amount of data has been generated using such material." [12]

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 and is typically processed in the left cerebral hemisphere.[13][14][15] A dichotic listening performance advantage for one ear is interpreted as indicating a processing advantage in the contralateral hemisphere. In another example, Sidtis (1981)[16] 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[17] 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 (consonant-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." [17] Similarly, Westerhausen and Hugdahl (2008) [18] 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." [18]


Studies in different populations

Sex differences

Additionally, dichotic listening has revealed a possible small-population sex difference in perceptual and auditory asymmetries and language laterity. According to Voyer (2011),[19] "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."[20] 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,"[21] 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.[19]

In schizophrenia

A study conducted involving the dichotic listening test,[22] with emphasis on subtypes of schizophrenia (particularly paranoid and undifferentiated), demonstrated that people with Paranoid Schizophrenia 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.

Versions

The "Dichotic Fused Words Test" (DFWT) is a modified version of the dichotic listening test. It was originally explored by Johnson et al. (1977)[23] but in the early 80's Wexler and Hawles (1983)[24] modified the test to ascertain more accurate data for hemispheric specialization of language function. In the DFWT, a participant is presented with pairs of monosyllabic rhymed CVC words and each word varies in the initial consonant. The significant difference in this test is "the stimuli are constructed and aligned in such a way that partial interaural fusion occurs: subjects generally experience and report only one stimulus per trial." [25] According to Zatorre (1989), some major advantages of this method include but are not limited to "minimizing attentional factors, since the percept is unitary and localized to the midline" and "stimulus dominance effects may be explicitly calculated, and their influence on ear asymmetries assessed and eliminated."[25] Thus, it is no surprise that Wexler and Hawles obtained a high test-retest reliability (r=0.85) in their study.[24]

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 have evidence of Broca's area and Wernicke's area, both located in the left hemisphere. In contrast, the left ear (and therefore the right hemisphere) is often better at processing nonlinguistic material.[26] 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.[27] 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.

Variations in procedure

The manipulation of voice onset time (VOT) during dichotic listening has provided novel insights regarding brain function.[28] To date, the most common design is the utilisation of four VOT conditions: short-long pairs (SL), where a CV syllable with a short VOT is presented to the left ear and a CV syllable with a long VOT is presented to the right ear, as well as long-short (LS), short-short (SS) and long-long (LL) pairs. In 2006, Rimol, Eichele, and Hugdahl [29] first reported that in healthy adults SL pairs elicit the largest REA while, in fact, LS pairs elicit a significant left ear advantage (LEA). A study of children aged 5–8 years of age has shown a developmental trajectory whereby long VOTs gradually start to dominate over short VOTs when LS pairs are being presented under dichotic conditions.[30] Converging evidence from studies of attentional modulation of the VOT effect shows that at around 9 years of age children lack the adult-like cognitive flexibility required to exert top-down control over stimulus-driven bottom-up processes.[31][32] Arciuli et al.(2010) further demonstrated that this kind of cognitive flexibility is a predictor of proficiency with complex tasks such as reading.

For further details about dichotic listening in neuropsychology, see K. Hugdahl (Ed.): Handbook of Dichotic Listening. Chichester, UK: John Wiley & Sons, 1988.


See also

References

  1. Ingram, John C. L. (2007). Neurolinguistics : an introduction to spoken language processing and its disorders, 1. publ., 3. print., 381, Cambridge: Cambridge University Press.
  2. Ingram, John C. L. (2007). Neurolinguistics : an introduction to spoken language processing and its disorders, 1. publ., 3. print., Cambridge: Cambridge University Press.
  3. Cherry, E. C. (1953). Some experiments on the recognition of speech, with one and two ears. Journal of the Acoustical Society of America 25, pp. 975–979.
  4. Moray, N. (1959), Attention in dichotic listening: Affective cues and the influence of instructions. Quarterly Journal of Experimental Psychology, 11, 56-60.
  5. Nielson, L. L., and Sarason, I. G. (1981). Emotion, personality, and selective attention. Journal of Personality and Social Psychology, 41, 945-960.
  6. 6.0 6.1 Rand, T. C. (1974). Dichotic release from masking for speech. Journal of the Acoustical Society of America, 55, 678-680.
  7. Cutting, J. E. (1976). Auditory and linguistic processes in speech perception: inferences from six fusions in dichotic listening. Psychological Review 83, pp. 114–140.
  8. Kimura, D (1961). Cerebral dominance and the perception of verbal stimuli. Canadian Journal of Psychology, 15, 166-171
  9. Ingram, John C. L. (2007). Neurolinguistics : an introduction to spoken language processing and its disorders, 1. publ., 3. print., 115, Cambridge: Cambridge University Press.
  10. Kimura, D. (1967). Functional asymmetry of the brain in dichotic listening. Cortex, 3, 163-178
  11. Studdert-Kennedy, Michael, Donald Shankweiler (August 1970 1970). Hemispheric Specialization for Speech Perception. The Journal of the Acoustical Society of America 48 (2): 579–594.
  12. Asbjornsen, Arve, M.P. Bryden (1996). Biased attention and the fused dichotic words test. Neuropsychologia 34 (5): 407.
  13. Studdert-Kennedy, M., & Shankweiler, D. P. (1970). Hemispheric specialization for speech perception. Journal of the Acoustical Society of America, 48, 579-594.
  14. Studdert-Kennedy, M., Shankweiler, D., & Schulman, S. (1970). Opposed effects of a delayed channel on perception of dichotically and monotically presented CV syllables. Journal of the Acoustical Society of America, 48, 599-602.
  15. Studdert-Kennedy, M., Shankweiler, D., & Pisoni, D. (1972). Auditory and phonetic processes in speech perception: Evidence from a dichotic study. Journal of Cognitive Psychology, 2, 455-466.
  16. Sidtis, J. J. (1981). The complex tone test: Implications for the assessment of auditory laterality effects. Neuropsychologia 19, pp. 103–112.
  17. 17.0 17.1 Hugdahl, Kenneth (2003). Dichotic Listening Performance and Frontal Lobe Function. Thomas Bodner, Elisabeth Weiss, Thomas Benke 16: 58–65.
  18. 18.0 18.1 Westerhausen, Rene, Kenneth Hugdahl (2008). The corpus callosum in dichotic listening studies of hemispheric asymmetry: A review of clinical and experimental evidence. Neuroscience and Biobehavioral Reviews 32: 1044–1054.
  19. 19.0 19.1 Voyer, Daniel (2011). Sex differences in dichotic listening. Brain and Cognition 76: 245–255.
  20. Voyer, Daniel (2011). Sex differences in dichotic listening. Brain and Cognition 76: 245–246.
  21. Voyer, Daniel, Jennifer Ingram (2005). Attention, reliability, and validity of perceptual asymmetries in the fused dichotic word test. Laterality: Asymmetries of Body, Brain, and Cognition 6: 545–561.
  22. Michelle S. Friedman, M.A.(2001). Perceptual Asymmetries in Schizophrenia: Subtype Differences in Left Hemisphere Dominance for Dichotic Fused Words. <http://psychophysiology.cpmc.columbia.edu/pdf/friedman2001a.pdf
  23. Johnson et al (1977). Dichotic ear preference in aphasia. J. Speech Hear Res 20: 116–129.
  24. 24.0 24.1 Wexler, Bruce, Terry Hawles (1983). Increasing the power of dichotic methods: the fused rhymed words test. Neuropsychologia 21.
  25. 25.0 25.1 Zatorre, Robert (1989). Perceptual asymmetry on the dichotic fused words test and cerebral speech lateralization determined by the caroid sodium amytal test. Neuropsychologia 27 (10).
  26. Grimshaw et al (2003). {{{title}}}. Neuropsychologia 41.
  27. Hahn, Constanze Smoking reduces language lateralization: A dichotic listening study with control participants and schizophrenia patients. Journal.
  28. Arciuli J (July 2011). Manipulation of voice onset time during dichotic listening. Brain Cogn 76 (2): 233–8.
  29. Rimol, L.M., Eichele, T., Hugdahl, K. (2006). The effect of voice-onset-time on dichotic listening with consonant-vowel syllables. Neuropsychologia. 44(2): 191–196.
  30. Westerhausen, R., Helland, T., Ofte, S., Hugdahl, K. (2010). A longitudinal study of the effect of voicing on the dichotic listening ear advantage in boys and girls at age 5 to 8.. Dev Neuropsychol. 35(6): ):752–761.
  31. Andersson, M., Llera, J.E., Rimol, L.M., Hugdahl, K. (2008). Using dichotic listening to study bottom-up and top-down processing in children and adults.. Child Neuropsychol. 14(5): 470–479.
  32. Arciuli, J., Rankine, T., Monaghan, P. (2010). Auditory discrimination of voice-onset time and its relationship with reading ability.. Laterality. 15(3): 343–360.

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