Perceptual and Motor Skills, 1992, 7 5 , 691-695. O Perceptual and Motor Skills 1992
STIMULUS-DEPENDENT EAR ASYMMETRY IN A DICHOTIC MONITORING TASK ' LUTZ JANCKE Department of Psychobiology and Pzychocybernetics Heinrich-Heine Uniuersify Summary.-Auditory lateralization was investigated in 26 right-handed and 26 lefthanded, normal subjects using two dichotic monitoring tasks in each proband [dichotic consonant-vowel (CV) syllable monitoring once with the syllable /ta/ and once with the syllable Ida/ as target]. Subjects were instructed to monitor for the presence of a target CV which could occur in either ear. They responded by depressing a response button; reaction time (RT) and hit rates were recorded. In right-handers the syllable /ta/ presented to the right ear was detected more frequently, on the average, than presented to the left ear. Also, RT was shorter for detection of /ta/ in the right ear than for detection in the left ear For both right- and left-handers. The detection of Ida/ showed no ear advantage in hit rate and RT either for right-handers or for left-handers. These results demonstrate the existence of a right-ear advantage in dichotic monitoring of the target syllable /ta/ but not for the target syllable Ida/. This difference in evoking a right-ear advantage is attributed to a difference in the difficulty of detection of both targets. It is argued that the detection of Ida/ is too difficult to evoke phonetic processing, leading to a right-ear advantage.
Since Kimura's original dichotic digit-list paradigm (Kimura, 1961a, 1961b), several paradigms using other verbal stimuli have been proposed to increase the reliabihty and predictive validity of the dichotic method with respect to language lateralization (for overviews see Bryden, 1982, 1988). Among these tasks, monitoring of dichotically presented verbal stimuli has consistently produced the most robust right-ear advantages in right-handed individuals, an effect thought to be related to the superior temporal discrirnination of acoustic signals by the dominant hemisphere (Springer, 1971; Geffen, Traub, & Stierman, 1978; Geffen & Caudrey, 1981). During such monitoring paradigms subjects were required to press a button whenever they heard the target stimulus. To date, it is not known whether the target stimuli should comprise special phonetic properties to evoke a strong right-ear advantage in such paradigms. The present study addressed this problem. The question at issue is whether targets embedded in identical dichotic stimuli induce different ear advantages in a group of right- and left-handers. Therefore, we report hit rates and reaction times for two dichotic consonant-vowel (CV) monitoring tasks which are comprised of the same test stimuli but differ with respect to target syllables (once performed with the syllable /ta/ and once with the syllable Ida/ as target). 'Address correspondence to Lutz Jancke, Ph.D., Department of Psychobiology and Psychocybernetics, Heinrich-Heine University, Universitatsstrak 1, D-4000 Diisseldorf 1, Federal Republic of Germany.
METHOD Subjects We examined 52 healthy, paid volunteers without hearing deficits in the speech frequency range (less than 1OdB hearing loss on either ear tested by standard audiometry). Of the 52 subjects 26 were right-handers (mean age: 26.4 yc; age range: 21 to 35 years) and 26 were left-handers (mean age: 24.9 yr.; age range: 2 1 to 34 years). Hand preference was assessed using the fiveitem questionnaire of Bryden (1977, 1982). This scoring procedure yields a score ranging from - 1.OO (extreme left-handed) through + 1.00 (extreme righthanded). According to this score subjects were classified either as left-handers (score < 0) or right-handers (score > 0). Most of the tested subjects were university students. There were 13 men and 13 women in each handedness group. Dichotic Monitoring Test The CV monitoring test comprised pairs of natural speech CV syllables ( / b a t , /gal, Ida/, /pa/, /ka/, and /ta/) spoken by a trained phonetician (microphone AKG 16OEl) which had been band-pass filtered (Rockland 432, upper corner frequency 10 kHz 24 dB/octave and lower corner frequency 80 Hz 24 dB/octave). Each syllable was digitized and stored in computer memory (12-bit A/D converter, digitizing rate 20.3 Hz, ATARI MEGA ST4). The onset, duration, intensity, and fundamental frequency of the stimuli were then edited and synchronized by means of a speech editor. The criterion for temporal alignment of the competing syllables was the onset of articulatory release. Each syllable started with zero intensity of 100 msec. duration before start of the articulation and stopped with zero intensity of 50 msec. duration after vowel end. This manipulation served to remove clicks at the beginning and end upon D/A conversion. The duration of syllables ranged from 310 to 360 msec., depending on voice onset times, with a vowel duration of 300 msec. Voice onset times (in msec.) for the stops were approximately b = 10, g = 20, d = 15, p = 40, k = 60, t = 50. The duration of the entire signal (syllables, leading and end interval with zero intensity) ranged from 460 to 510 msec. These edited signals were stored on hard disk for further use. For dichotic presentation, the stored signals were D/A converted (12-bit, four-channel D/A converter, digitizing rate 20.3 kHz) by converting alternatively a value of the first syllable and then the second syllable. Each syllable was converted using a different channel of the D/A converter. With this technique, competing syllables were aligned with an accuracy of 49.4 psec. After conversion the output of each D/A channel was amplified (70 dB SPL as measured by a Bruel & Kjaer sound level meter) and presented via stereo headphones (Sennheiser HD450). During the experiments, the subjects were seated in a soundattenuated chamber and received instructions through a two-way intercom
AUDITORY LATERALIZATION
693
set. The stimuli were randomized and arranged in eight blocks, each containing 30 syllable pairs, resulting in a total of 240 trials. Each of the 30 possible pairwise permutations of the six CV syllables occurred equally often. Because all possible CV pairs were equiprobable, one-third of the trials consisted of pairs containing the target. Fifty percent of the targets were delivered to the right and 50% to the left ear. Thus, the a pviori probability of target occurrence for each ear was 0.167. The interval between syllable pairs was 1.9 sec. Subjects were instructed to pay attention to both ears and press a button with the forefinger when the target syllable was heard. Reactions within 100 to 1000 msec. after stimulus onset were accepted. Subjects had to accomplish this procedure two times, once with the target /ta/, and once with the target Ida/. Half of the subjects started with the target syllable /ta/ and the other half started with target syllable Ida/. Headphone orientation and responding hand were counterbalanced for the whole experiment. Thus, 26 subjects started with headphone orientation A, 13 of whom started to respond with the right hand (R) and another 13 of whom started to respond with the left hand (L). The other 26 subjects started with reversed headphone orientation (B) using the same order of responding hands. The responding hand and headphone orientations were changed after each block (ABABABAB with RLRLRLRL or LRLRLRLR; BABABABAB with RLRLRLRL or LRLRLRLR). I n both groups, the numbers of right-handers, left-handers, men, and women were counterbalanced. The frequency of target detection (hit rate) and the reaction times (RT) for each ear were stored and used for further analysis. All subjects were given 30 practice trials for each target syllable prior to the experiment. For the purpose of statistical analysis a two-sided test problem and a significance level of p = 0.05 was chosen. Because a series of statistical tests were performed on the same sample the alpha-adjusting procedure of Holm (1979) was used. RESULTSAND DISCUSSION Right-handed subjects generated larger hit rates and RTs for detection of the target syllable /ta/ presented to the right ear as for detection of the target syllable /ta/ presented to the left ear (hit rate: t,, = 3.99, p
STIMULUS-DEPENDENT EAR ASYMMETRY IN A DICHOTIC MONITORING TASK ' LUTZ JANCKE Department of Psychobiology and Pzychocybernetics Heinrich-Heine Uniuersify Summary.-Auditory lateralization was investigated in 26 right-handed and 26 lefthanded, normal subjects using two dichotic monitoring tasks in each proband [dichotic consonant-vowel (CV) syllable monitoring once with the syllable /ta/ and once with the syllable Ida/ as target]. Subjects were instructed to monitor for the presence of a target CV which could occur in either ear. They responded by depressing a response button; reaction time (RT) and hit rates were recorded. In right-handers the syllable /ta/ presented to the right ear was detected more frequently, on the average, than presented to the left ear. Also, RT was shorter for detection of /ta/ in the right ear than for detection in the left ear For both right- and left-handers. The detection of Ida/ showed no ear advantage in hit rate and RT either for right-handers or for left-handers. These results demonstrate the existence of a right-ear advantage in dichotic monitoring of the target syllable /ta/ but not for the target syllable Ida/. This difference in evoking a right-ear advantage is attributed to a difference in the difficulty of detection of both targets. It is argued that the detection of Ida/ is too difficult to evoke phonetic processing, leading to a right-ear advantage.
Since Kimura's original dichotic digit-list paradigm (Kimura, 1961a, 1961b), several paradigms using other verbal stimuli have been proposed to increase the reliabihty and predictive validity of the dichotic method with respect to language lateralization (for overviews see Bryden, 1982, 1988). Among these tasks, monitoring of dichotically presented verbal stimuli has consistently produced the most robust right-ear advantages in right-handed individuals, an effect thought to be related to the superior temporal discrirnination of acoustic signals by the dominant hemisphere (Springer, 1971; Geffen, Traub, & Stierman, 1978; Geffen & Caudrey, 1981). During such monitoring paradigms subjects were required to press a button whenever they heard the target stimulus. To date, it is not known whether the target stimuli should comprise special phonetic properties to evoke a strong right-ear advantage in such paradigms. The present study addressed this problem. The question at issue is whether targets embedded in identical dichotic stimuli induce different ear advantages in a group of right- and left-handers. Therefore, we report hit rates and reaction times for two dichotic consonant-vowel (CV) monitoring tasks which are comprised of the same test stimuli but differ with respect to target syllables (once performed with the syllable /ta/ and once with the syllable Ida/ as target). 'Address correspondence to Lutz Jancke, Ph.D., Department of Psychobiology and Psychocybernetics, Heinrich-Heine University, Universitatsstrak 1, D-4000 Diisseldorf 1, Federal Republic of Germany.
METHOD Subjects We examined 52 healthy, paid volunteers without hearing deficits in the speech frequency range (less than 1OdB hearing loss on either ear tested by standard audiometry). Of the 52 subjects 26 were right-handers (mean age: 26.4 yc; age range: 21 to 35 years) and 26 were left-handers (mean age: 24.9 yr.; age range: 2 1 to 34 years). Hand preference was assessed using the fiveitem questionnaire of Bryden (1977, 1982). This scoring procedure yields a score ranging from - 1.OO (extreme left-handed) through + 1.00 (extreme righthanded). According to this score subjects were classified either as left-handers (score < 0) or right-handers (score > 0). Most of the tested subjects were university students. There were 13 men and 13 women in each handedness group. Dichotic Monitoring Test The CV monitoring test comprised pairs of natural speech CV syllables ( / b a t , /gal, Ida/, /pa/, /ka/, and /ta/) spoken by a trained phonetician (microphone AKG 16OEl) which had been band-pass filtered (Rockland 432, upper corner frequency 10 kHz 24 dB/octave and lower corner frequency 80 Hz 24 dB/octave). Each syllable was digitized and stored in computer memory (12-bit A/D converter, digitizing rate 20.3 Hz, ATARI MEGA ST4). The onset, duration, intensity, and fundamental frequency of the stimuli were then edited and synchronized by means of a speech editor. The criterion for temporal alignment of the competing syllables was the onset of articulatory release. Each syllable started with zero intensity of 100 msec. duration before start of the articulation and stopped with zero intensity of 50 msec. duration after vowel end. This manipulation served to remove clicks at the beginning and end upon D/A conversion. The duration of syllables ranged from 310 to 360 msec., depending on voice onset times, with a vowel duration of 300 msec. Voice onset times (in msec.) for the stops were approximately b = 10, g = 20, d = 15, p = 40, k = 60, t = 50. The duration of the entire signal (syllables, leading and end interval with zero intensity) ranged from 460 to 510 msec. These edited signals were stored on hard disk for further use. For dichotic presentation, the stored signals were D/A converted (12-bit, four-channel D/A converter, digitizing rate 20.3 kHz) by converting alternatively a value of the first syllable and then the second syllable. Each syllable was converted using a different channel of the D/A converter. With this technique, competing syllables were aligned with an accuracy of 49.4 psec. After conversion the output of each D/A channel was amplified (70 dB SPL as measured by a Bruel & Kjaer sound level meter) and presented via stereo headphones (Sennheiser HD450). During the experiments, the subjects were seated in a soundattenuated chamber and received instructions through a two-way intercom
AUDITORY LATERALIZATION
693
set. The stimuli were randomized and arranged in eight blocks, each containing 30 syllable pairs, resulting in a total of 240 trials. Each of the 30 possible pairwise permutations of the six CV syllables occurred equally often. Because all possible CV pairs were equiprobable, one-third of the trials consisted of pairs containing the target. Fifty percent of the targets were delivered to the right and 50% to the left ear. Thus, the a pviori probability of target occurrence for each ear was 0.167. The interval between syllable pairs was 1.9 sec. Subjects were instructed to pay attention to both ears and press a button with the forefinger when the target syllable was heard. Reactions within 100 to 1000 msec. after stimulus onset were accepted. Subjects had to accomplish this procedure two times, once with the target /ta/, and once with the target Ida/. Half of the subjects started with the target syllable /ta/ and the other half started with target syllable Ida/. Headphone orientation and responding hand were counterbalanced for the whole experiment. Thus, 26 subjects started with headphone orientation A, 13 of whom started to respond with the right hand (R) and another 13 of whom started to respond with the left hand (L). The other 26 subjects started with reversed headphone orientation (B) using the same order of responding hands. The responding hand and headphone orientations were changed after each block (ABABABAB with RLRLRLRL or LRLRLRLR; BABABABAB with RLRLRLRL or LRLRLRLR). I n both groups, the numbers of right-handers, left-handers, men, and women were counterbalanced. The frequency of target detection (hit rate) and the reaction times (RT) for each ear were stored and used for further analysis. All subjects were given 30 practice trials for each target syllable prior to the experiment. For the purpose of statistical analysis a two-sided test problem and a significance level of p = 0.05 was chosen. Because a series of statistical tests were performed on the same sample the alpha-adjusting procedure of Holm (1979) was used. RESULTSAND DISCUSSION Right-handed subjects generated larger hit rates and RTs for detection of the target syllable /ta/ presented to the right ear as for detection of the target syllable /ta/ presented to the left ear (hit rate: t,, = 3.99, p
