Dichotic Listening Experiment

Introduction
Method
   -Design
   -Stimuli
   -Equipment Requirements
   -Task
Results
DataFormat
References

Introduction

Some of the most interesting research on right-left brain hemisphere differences has been conducted on clinical populations, people whose brains have been surgically altered or

damaged. But it also possible to study laterality effects in normal populations using one of two laboratory methods. One uses auditory input to the brain and is termed the dichotic listening procedure. The other uses visual input and is termed the divided visual field procedure. This experiment employs dichotic listening. For an experiment using the divided visual field methodology, try the Word Recognition experiment at PsychExperiments. The results of both give credibility to the claim that the left brain hemisphere is specialized for speech perception and verbal functions in general.

Dichotic listening was first introduced by Broadbent (1954) and used by Kimura (1961a, 1961b) as a procedure for studying laterality effects. Kimura's procedure involved presenting digit pairs (1-5, 6-4, etc.) over stereo headsets with one digit going to one ear and the other to the other ear. She and many subsequent investigators found that most people report the right ear digits more often than the left when asked to remember the digits they have heard. A refinement of Kimura's procedure has been to present meaningless syllables rather than actual words (Studdert-Kennedy & Shankweiler, 1970). The PsychExperiments dichotic listening task employs this methodology.
Ť Return to Topť

Method

Design.. The study uses a within subjects design to compare accuracy of sound recognition for the right and left ears. The independent variable, therefore, is ear (right or left) and the dependent variable is the percent of correct identifications.
Ť Return to Topť

Stimuli.. The stimuli for the task are the syllables formed by combining six stop consonants (/b/, /d/, /g/,/k/, /p/, /t/) with the vowel /a/. These consonant-vowel combinations (CVs) were recorded using voice input to AcidWave, a shareware sound editing program from Polyhedric Software http://www.polyhedric.com. The individual CVs were recorded in 16 bit monoaural mode and edited to be approximately 500 msec in duration with maximum amplitude. To form dichotic pairs, individual sounds were opened simultaneously in AcidWave windows. One of these sounds was converted to stereo, then the other sound was pasted on the left channel. To avoid any bias that might be created by subtle inequalities in the sounds, the "swap channel" function of AcidWave was used to mirror each Left-Right pair to an identical Right-Left pair. Using this procedure, all 15 combinations of the six CVs were created and then by mirroring each pair, the full set of 30 sound pairs was obtained.
Ť Return to Topť

Equipment Requirements. Computers used in this research must have sound cards. Research participants must have stereo headsets that they can plug into the "audio out" jack on the sound card.
Ť Return to Topť

Task. The research participant's task is to listen to the 30 CV pairs in sequence, though the order is randomized for each participant. On each pair presentation, one CV is presented to the left ear and the other to the right. The total set of 30 consists of all pairwise permutations of the six CV set. Prior to the task, there is a test to assure that the headset is on properly and that the right and left balance and volume controls are set appropriately
Ť Return to Topť

Results

Data for the analysis consist of two scores: proportion of left ear sounds identified correctly and proportion of right ear sounds identified correctly. The significance of the difference can be computed using a z-statistic or a chi-square statistic.
Ť Return to Topť

Data Format in Downloadable csv file
The data obtained via the Download data link on the PsychExperiments homepage are comma delimited. The first entry (up to the first comma) is the affiliation of the research participant. Typically this is the class to which the participant belongs. Non-affiliated participants use the default entry of Interested Person. Next is the randomly assigned ID code for the research participant. The third entry is the date. The fourth is a measure of "experiment time" in seconds. This is how long it took the participant to complete the experiment, beginning with Trial 1 and ending with the last trial. (Time spent reading instructions etc. are not included). The measure is useful as a screen for "bad" data. The time measure should be appropriate to the task--not too fast and not too slow. Next come entries for the gender of the participant ("M" or :F") and handedness of the participant ("R" for right, "L" for left, and "M" for mixed). Next are the raw data for the experiment organized by trial number. Note that each trial number is repeated twice because there are two sounds presented on each trial and data are recorded for both. Trial numbers are from 1 to 30. After the trial number are text entries to indicate the ear receiving the phoneme, the phoneme, and an integer entry for correctness of the response (1=correct, 0=incorrect).This format is repeated for the second phoneme presented on the trial. Each of these entries is separated by a comma. The table below shows the format trial data in tabular form. Not included in the tabular display are the user information (affiliation, ID, data, and time) which precede these data, as shown in the text line below the table.

Trial Number	Ear	Phoneme	Correct	Trial Number	Ear	Phoneme	Correct	Trial Number	etc.
1,	Left,	ka,	1,	1,	Right,	pa,	1,	2	etc.

An Interested Person,XGZ800,7/6/2002,597,M,R,1,Left,ka,0,1,Right,pa,1,2,Right,ba,0,2,Left,pa,1,3,Left,ga,1,3,. . . 30,Right,ga,1,

References

Broadbent, D.E. (1954). The role of auditory localization in attention and memory span. Journal of Experimental Psychology, 47, 191-196.

Kimura, D. (1961a). Some effects of temporal lobe damage on auditory percpetion. Canadian Journal of Psychology, 15, 156-165.

Kimura, D. (1961b). Cerebral dominance and the perception of verbal stimuli. Canadian Journal of Psychology, 15, 156-165.

Studdert-Kennedy, M. & Shankweiler, D. (1970). Hemispheric specialization for speech perception. Journal of the Acoustical Society of America, 48, 579-594.
Ť Return to Topť