(Editor: Pamela Marek)
Some of the most interesting research on the right-left hemisphere differences
has been conducted on people whose brains have been surgically altered or
damaged. Perhaps the most well known studies were conducted in the 1960's when
epilepsy was treated by severing the corpora callosa of patients, thus allowing
for the study of the split brain or laterality (Harrington, 1998). Today,
psychologists are more likely to study this phenomenon using brain imaging
devices. Nevertheless, the intrigue of brain laterality continues to be a rich
source of research by all types of psychologists.
In addition to the procedures described above, it is also possible to study
laterality in normal populations using a dichotic listening experiment.
Laterality can be studied by presenting two stimuli, one to each ear, and to
investigate the ability to accurately identify the sounds. Auditory information
is processed as illustrated in Figure 1. A second way to study
laterality is through visual processing and a divided visual field procedure
(this study is also located in this laboratory). Study of auditory laterality
offers insight into two areas of psychology, perception and cognition (Hugdahl,
1998). This technique is still used by researchers to study how information is
processed with respect to hemisphere specific tasks.
Study of laterality using the dichotic listening procedure was first introduced
by Broadbent (1954) and later refined by Kimura (1961a, 1961b). Kimura's
procedure involved presenting pairs of digits through stereo headsets different
digits set to each ear. She and others (see Hugdahl, 1998) found that most
people correctly recalled digits presented to their right ear more often than
those presented to the left ear. Studdert-Kennedy and Shankweiler (1970) used a
similar procedure substituting meaningless syllables rather than actual words.
This study used the classic procedure for testing dichotic listening - the
Each participant should be provided with stereo handsets that can be plugged
into the computer audio jack.
This study is designed as a within subjects study. In other words, each person
responds to a series of trials and the analysis is conducted to compare how
well an individual can distinguish sounds based on whether they received the
information to the left or right ear. The independent variable is sound
presentation (right or left ear) and the dependent variable is ability to
correctly distinguish the sound.
The stimuli developed for this study are nonsense syllables consisting of one
of a series of consonants (b, d, g, k, p, t) paired with the vowel "a". This
pairing of sounds results in 36 possible combinations (i.e., ba-ga, da-ga).
Although it is possible to derive 36 combinations, only 15 of these
combinations were used in this experiment. These sounds were recorded in 16 bit
mono-aural mode and edited to 500 milliseconds in duration. These sounds were
carefully recorded so that they are equally balanced for both ears.
Each person must listen to 30 presentations of the stimuli. Each presentation
or trial involves two different consonant-vowel pairings presented to each ear.
For example, the sound "pa" and "ka" are presented at the same time, but each
to a different ear. This presentation of sounds or pairs are reversed, sounds
are presented to the opposite ears, for a total of 30 trials. For example,
instead of presenting the pa and ka sound to the right and left ear
respectively, the sounds are presented to the left and right ear. This dichotic
listening procedure helps to illustrate the right ear advantage because the
accuracy of correctly identified sounds should be higher with the right, rather
than the left ear. A total of 15 different pairings were created for this
In this study we are interested in determining if the right ear advantage to
listening can be replicated, thus providing evidence that language is
dominantly processed by the left hemisphere. To determine if these results are
repeated, calculation of proportion of correct responses for each ear would
constitute the dependent variable measure. One possible analysis would entail
use of a chi-square analysis. The independent variable would be the left and
right ear, and a simple comparison of proportion of correctly identified sounds
would be used to test the hypothesis.
Dichotic listening tasks have been used extensively to develop and test different models of attention. Broadbent's early work (e.g., Broadbent, 1954), a basis for his filter model of attention, suggested that people attended to information from one ear at a time. However, subsequent research using shadowing tasks suggested otherwise. In a shadowing task, people are instructed to attend to only one ear and to ignore the input to the other ear. Despite these instructions, Moray found that people still recognized their name when it was presented in the unattended ear (the cocktail party effect). In another shadowing investigation, Treisman (1964) noticed that people grasped phrases from the unattended ear if the phrases were connected with the information to which they were attending, leading to the development of an attenuation model of attention.
Shadowing tasks have been used in more recent research as well. For example, in the clinical realm, Ingram, Berner, and McLaughlin (1994) induced a negative mood by asking people to think about a negative event while listening to sad music. After the mood induction, people who experienced symptoms of depression prior to the experiment were more apt than others to let emotionally-laden words intrude into a message they were shadowing, suggesting differential information processing. In social psychology, Baumeister, DeWall, Ciarocco, and Twenge (2005) used a shadowing task as an indicator of self-regulation to determine how people reacted to the possibility of social exclusion. People who were led to believe that they would be alone later in life performed more poorly on the shadowing task than did people who were told they would experience rewarding relationships or would become accident-prone, implying that contemplating exclusion reduced self-regulation.
Insofar as the right ear advantage, researchers have extended its application to Alzheimer's disease, sleep deprivation, and dyslexia. Using Broadbent's (1954) digit task, Duchek and Balota (2005) reported that people with mild Alzheimer's disease showed a much greater right ear advantage than did those with very mild Alzheimer's disease or normal controls, thereby demonstrating that the disease influences attentional processes. Sleep deprivation also impacts attentional control. Using consonant vowel (CV) syllables similar to those used in the present experiment, Johnson, Laberg, Eid, and Hugdahl (2002) found that, compared to a control group, sleep-deprived Navy cadets were less successful at shifting attention to the left ear when asked to do so. Finally, Helland and Asbornsen (2001), also using CV syllables, reported that dyslexic children displayed less laterality of language than did normal controls. In summary, the dichotic listening task is a promising diagnostic tool.
Definitional information for each of the labeled columns appears in Figure 2.
Data is downloadable in three formats (XML, Excel spreadsheet format, and comma
delimited for statistical software packages like SPSS). The columns of data are
labeled. We are providing definitional information for each of the labeled
Baumeister, R. F., DeWall, C. N., Ciarocco, N. J., & Twenge, J. M. (2005).
Social exclusion impairs self-regulation. Journal of Personality and
Social Psychology, 88, 589-604.
Broadbent, D. E. (1954). The role of auditory localization in attention and memory span.
Journal of Experimental Psychology, 44, 51-55.
Duchek, J. M., & Balota, D. A. (2005). Failure to control prepotent pathways in
early stage dementia of the Alzheimer's type: Evidence from dichotic
listening. Neuropsychology, 19, 687-695.
Helland, T., & Asbjornsen, A. (2001). Brain asymmetry for language in dyslexic
children. Laterality, 6, 289-301.
Ingram, R. E., Bernet, C. Z., & McLaughlin, S. C. (1994). Attentional allocation
processes in individuals at risk for depression. Cognitive therapy and
Research, 4, 317-332.
Johnson, B. H., Laberg, J. C., Eid, J., & Hugdahl, K. (2002). Dichotic
listening and sleep deprivation. Scandinavian Journal of Psychology,
Moray, N. (1959). Attention in dichotic listening: Affective cues and the
influence of instructions. Quarterly Journal of Experimental Psychology,
Treisman, A. (1964). Verbal cues, language and meaning in selective attention.
American Journal of Psychology, 77, 206-209.