The right ear advantage revisited: Speech lateralisation in dichotic listening using consonant–vowel and vowel–consonant syllables

The dichotic listening task is typically administered by presenting a consonant–vowel (CV) syllable to each ear and asking the participant to report the syllable heard most clearly. The results tend to show more reports of the right ear syllable than of the left ear syllable, an effect called the right ear advantage (REA). The REA is assumed to be due to the crossing over of auditory fibres and the processing of language stimuli being lateralised to left temporal areas. However, the tendency for most dichotic listening experiments to use only CV syllable stimuli limits the extent to which the conclusions can be generalised to also apply to other speech phonemes. The current study re-examines the REA in dichotic listening by using both CV and vowel–consonant (VC) syllables and combinations thereof. Results showed a replication of the REA response pattern for both CV and VC syllables, thus indicating that the general assumption of left-side localisation of processing can be applied for both types of stimuli. Further, on trials where a CV is presented in one ear and a VC is presented in the other ear, the CV is selected more often than the VC, indicating that these phonemes have an acoustic or processing advantage.     

Verbal and nonverbal auditory processing among left- and right-handed good readers and reading-disabled children

Cerebral lateralization of left- and right-handed good readers and left- and right-handed reading disabled was examined with a sample of 60 children who ranged in age from 7-13 years via a dichotic selective attention task (free recall, directed left, directed right) using consonant-vowel (CV) and tonal stimuli. Several ANOVAs were conducted to evaluate gender, reader group, handedness, and stimuli effects of left- and right-ear reports across dichotic conditions. Results indicated males outperformed females across stimuli and conditions regardless of handedness and all subjects recalled more tonal stimuli than CV stimuli. More importantly, the expected REA (left hemisphere processing) was found for CV stimuli only by right-handed good readers across all three dichotic conditions. The left-handed good readers and left-handed reading-disabled children were left ear (LE) dominant in free recall and in the directed left condition, but were right ear (RE) dominant in the directed right condition. Conversely, right-handed reading-disabled children produced a REA during free recall and directed right conditions, but were LE dominant in the directed left condition. In contrast, a significant LEA (right hemisphere processing) was found for tonal stimuli across all dichotic conditions for all four groups. These findings lend support to the hypothesis that attentional factors have a greater influence on auditory processing of verbal than nonverbal stimuli for various groups of children and also suggest reversed or bilateralized processing abilities for language in strongly left-handed children with sinistral relatives.     

Priming inhibits the right ear advantage in dichotic listening: implications for auditory laterality

The typical finding in dichotic listening with verbal stimuli is the right ear advantage (REA), indicating a left hemisphere processing superiority, thus making this an effective tool in studying hemispheric asymmetry. It has been shown that the amplitude of the REA can be modulated by instructions to direct attention to left or right side. The current study attempted to modulate the REA by changing the dichotic listening stimulus situation. In Experiment 1, a consonant vowel (CV) syllable prime was presented binaurally briefly before the dichotic stimuli (consisting of two CVs). The prime could be the same as either the left or right ear dichotic stimulus, or it could be a different stimulus. Participants were instructed to report the CV they heard best from the dichotic syllable pair. The traditional REA was found when the prime was different from both dichotic stimuli. When the prime matched the CV in the left half of the subsequent dichotic pair, the REA was increased, while if the prime matched the right half, the REA was reduced. In order to see at which perceptual stage the modulation takes place, in Experiment 2 the prime was visual, presented on a PC screen. The same effect was seen, although the modulation of the REA was weaker. We propose that the memory trace of the prime is a source of interference, and causes cognitive control of attention to inhibit recognition of stimuli similar to recent distractors. Based on previous studies we propose that this inhibition of attention is performed by prefrontal cortical areas. Similarities to the mechanisms involved in negative priming and implications for auditory laterality studies are pointed out.     

The effects of hemispatial and asymmetrically focused attention on dichotic listening with normal and learning-disabled children

The effects of hemispatial and focused attention were examined with 50 normal and learning-disabled children to determine the extent of these two attentional strategies influenced perceptual laterality as reflected by the dichotic listening right-ear advantage (REA). Twenty-five normal children (8 females, 17 males, mean age 9.10 yr) matched with 25 learning-disabled children (8 females, 17 males, mean age 10.1 yr) were administered a dichotic consonant-vowel (CV) and consonant-vowel-consonant (CVC) syllable task. The two types of stimuli were compared across focused attention (free report, focused left, focused right) and hemispatial (central, left hemispace, right hemispace) conditions implemented independently and in systematic combinations. A four-factorial analysis of variance (groups x stimuli x conditions x ears) resulted in a significant REA for normal children across all attentional conditions whereas learning-disabled did not produce a consistent REA across all attentional conditions, and in several instances, produced equivalent left and right hemisphere processing. Right hemispatial orientation increased the magnitude of the REA (i.e., left hemisphere processing) for both groups, whereas left hemispatial orientation increased the magnitude of the left ear report (i.e., right hemisphere processing) only in learning-disabled subjects. Focused attention to the right ear also increased left hemisphere efficiency for both groups of children; however, focused attention to the left ear produced symmetrical functioning by learning-disabled subjects. Congruent combinations of focused attention and hemispatial orientation were not found to enhance the REA beyond its magnitude when each strategy was assessed independently. When focused attention and hemispatial conditions were employed in opposing directions, normal children were more susceptible to the "rightward" direction regardless of the strategy whereas learning-disabled subjects were more susceptible to the "verbal" nature of the strategy. Higher overall processing performance was exhibited for CVC stimuli when compared to CV stimuli. These findings lend support to the hypothesis that hemispatial and asymmetrically focused attention strategies interact with structural mechanisms in producing the observed REA in dichotic listening and do so differentially for normal and learning-disabled children.     

Verbal dichotic listening in developmental stuttering: subgroups with atypical auditory processing.

OBJECTIVE: The major aim of this study was to determine whether adults with persistent developmental stuttering have atypical auditory processing. BACKGROUND: Stuttering has been attributed to aberrant hemispheric dominance, and auditory processing deficits have been found in some adults who stutter. Dichotic listening, an indirect measure of auditory processing, has not been used to study auditory laterality in right- and left-handers who stutter. Because left-handers and people who stutter may have aberrant hemispheric dominance, it is important to examine auditory neural systems in right- and left-handed people who do and do not stutter. METHODS: Adults with persistent developmental stuttering (n = 18) and matched controls (n = 28) were studied by simultaneous binaural (dichotic) presentation of consonant-vowel stimuli in three attention conditions: nondirected attention, attention directed right, and attention directed left. Sex-handedness groups (stutter and control) included right-handed men and women and left-handed men, but not left-handed women because this stutter subgroup could not be recruited. To study ear advantage and auditory laterality, two dependent measures were examined: percent left and right ear responses and lateralization shift magnitude. Potential relationships between degree of handedness and dichotic listening measures were also examined. RESULTS: Matched controls and right-handed men who stutter had the expected right-ear advantage (REA) in the nondirected attention condition. In contrast, left-handed men who stutter had a left-ear advantage (LEA), and right-handed women who stutter did not have a lateral ear bias in the nondirected attention condition. Right-handed women who stutter had the greatest tendency to hear a sound that was not presented to either ear, and were relatively unable to selectively direct attention left or right. In contrast, left-handed men who stutter were able to shift attention to the left and right ear better than any other group. For the fluent control group, there were no significant relationships among degree of handedness and dichotic-listening variables. For the stutter group, degree of handedness was significantly related to percentage left and right ear response and to the lateralization shift magnitude. CONCLUSIONS: Left-handed men who stutter and right-handed women who stutter have atypical auditory processing but differ in important ways. The left-ear bias found in left-handed men who stutter in the nondirected attention condition suggests that their right temporal lobe may be important in perceiving speech, and, therefore, they have mixed dominance. These subjects were also better at shifting attention in both directions in comparison to all other groups; thus, the right hemisphere, which is dominant for shifting attention to both right and left space, may be activated. In contrast, the right-handed women who stutter had no ear bias in the nondirected attention condition, made more perceptual errors, and had difficulty shifting attention to the left and right. Although these results suggest that right-handed women who stutter have attentional deficits, the relationship between attentional disorders and stuttering remains to be elucidated. Because right-handed men who stutter were not different from controls, aberrant hemispheric dominance cannot fully account for stuttering. Unfortunately, left-handed women were not examined in this study. Therefore, these interesting sex-handedness effects found in left-handed men and right-handed women who stutter must be interpreted with caution.     

Speech lateralization and attention/executive functions in a Finnish family with specific language impairment (SLI)

Hemisphere lateralization for speech perception was investigated in a Finnish family with specific language impairment. We used dichotic presentations of consonant-vowel (CV) syllables, consisting of the six stop-consonants paired with the vowel /a/, under three different attentional instructions. The dichotic listening technique means that two different speech stimuli are presented simultaneously, one in each ear. Left hemisphere dominance for speech perception is assessed from a preference for the right ear stimulus. Response accuracy was compared in five members, over three generations, of a family with SLI with 5 healthy control subjects. The dichotic listening task was performed under three different instructions, to attend only to the right ear stimulus, to attend only to the left ear stimulus, or with no instruction about attention. The subjects indicated orally which of the six CV-syllables they heard on each trial. There were 36 dichotic presentations of the CV-syllables for each instruction. The CV-syllables were played from a CD with digitized stimuli. The results showed no difference between the groups during the no instruction condition, indicating normal speech lateralization in the SLI group. However, the SLI group was deficient in modifying the ear advantage through focused attention to the left ear, thus indicating an attentional/executive deficit in addition to a deficit in left hemisphere lateralization. It is concluded that individuals with specific language impairment may have a cognitive deficit related to attention in addition to a language processing deficit related to left hemisphere function.     

Normative Data for Equivalent,Parallel Forms of the Judgment of Line Orientation test

Hemisphere lateralization for speech perception was investigated in a Finnish family with specific language impairment. We used dichotic presentations of consonant–vowel (CV) syllables, consisting of the six stop-consonants paired with the vowel /a/, under three different attentional instructions. The dichotic listening technique means that two different speech stimuli are presented simultaneously, one in each ear. Left hemisphere dominance for speech perception is assessed from a preference for the right ear stimulus. Response accuracy was compared in five members, over three generations, of a family with SLI with 5 healthy control subjects. The dichotic listening task was performed under three different instructions, to attend only to the right ear stimulus, to attend only to the left ear stimulus, or with no instruction about attention. The subjects indicated orally which of the six CV-syllables they heard on each trial. There were 36 dichotic presentations of the CV-syllables for each instruction. The CV-syllables were played from a CD with digitized stimuli. The results showed no difference between the groups during the no instruction condition, indicating normal speech lateralization in the SLI group. However, the SLI group was deficient in modifying the ear advantage through focused attention to the left ear, thus indicating an attentional/executive deficit in addition to a deficit in left hemisphere lateralization. It is concluded that individuals with specific language impairment may have a cognitive deficit related to attention in addition to a language processing deficit related to left hemisphere function.     

The corpus callosum in dichotic listening studies of hemispheric asymmetry: a review of clinical and experimental evidence

The dichotic listening paradigm using verbal stimulus material typically yields a right ear advantage (REA) which indicates the left-hemisphere dominance for speech processing. Although this interpretation is widely accepted, the cerebral hemispheres also interact through the corpus callosum. Moreover, the two most influential theoretical models of dichotic listening, the structural and the attentional model, both refer to the functional integrity of the corpus callosum, when explaining the REA. However, the current review of the available data reveals several aspects that can not be explained by the dichotic listening models. For example, an individual's ability to direct attention to either ear is mediated by callosal fibers. Consequently, the corpus callosum not only has to be considered as a channel for the automatic exchange of information between the cerebral hemispheres, it rather allows for a dynamic and flexible interaction in supporting both bottom-up and top-down stimulus processing. The review has also revealed how inter-individual variability in callosal fiber structure affects both bottom-up and top-down performance on the dichotic listening task.     

Overcoming the right-ear advantage: A study of focused attention in children

Abstract

Several studies indicate that normal right-handed children of various ages show a right-ear advantage (REA) for dichotic verbal stimuli even when instructed to attend to the left ear. Other evidence, however, suggests that selective listening ability begins to develop in early childhood and that children reliably can overcome the REA by the age of 8 or 9 years. We used a signal detection procedure to address this apparent contradiction. In the first of two experiments, 58 children in two age groups (M=7 and 10 years) were able to overcome the REA for dichotic consonant-vowel (CV) stimuli when instructed to focus attention on the left ear. Success in detecting and localizing signals from the left ear, as reflected in hit rates, was independent of age and reading level. A second experiment, in which strings of dichotic monosyllabic words were presented to 56 children at the same two age levels, yielded similar results. These findings challenge the claim that ability to overcome the REA when attending to the left ear reflects an abnormality of cerebral functioning.     

The influence of musical experience on lateralisation of auditory processing

The influence of musical experience on free-recall dichotic listening to environmental sounds, two-tone sequences, and consonant-vowel (CV) syllables was investigated. A total of 60 healthy right-handed participants were divided into two groups according to their active musical competence ("musicians" and "non-musicians"). In both groups, we found a left ear advantage (LEA) for nonverbal stimuli (environmental sounds and two-tone sequences) and a right ear advantage (REA) for CV syllables. Dichotic listening to environmental sounds was uninfluenced by musical experience. The total accuracy of recall for two-tone sequences was higher in musicians than in non-musicians but the lateralisation was similar in both groups. For CV syllables a lower REA was found in male but not female musicians in comparison to non-musicians. The results indicate a specific sex-dependent effect of musical experience on lateralisation of phonological auditory processing.     

Dichotic listening performance suggests right hemisphere involvement in PTSD

The present study focuses on language laterality as measured with dichotic listening (DL) to consonant-vowel syllables (CV syllables) in refugees with post-traumatic stress disorder (PTSD). PTSD is associated with impaired callosal transfer and with increased right hemisphere activation and impaired executive skills that could influence the processing of dichotic stimuli. A total of 22 participants with PTSD were compared to 23 participants without a diagnosis of PTSD. All participants had similar experiences of acts of war and political violence. They were tested with dichotic listening to CV syllables with free recall and directed attention following the forced attention paradigm. The PTSD group showed increased right ear advantage due to impaired left ear reporting and also smaller attention modulation compared to the control group, and the performance shared variance with self-report measures of arousal and intrusive memories. The results are discussed towards a model of impaired functionality of the frontal lobe and right hemisphere versus impaired callosal transfer, both yielding predictions for the processing of the left ear input and the ability to attention modulation of the performance.     

Dichotic listening performance suggests right hemisphere involvement in PTSD

The present study focuses on language laterality as measured with dichotic listening (DL) to consonant-vowel syllables (CV syllables) in refugees with post-traumatic stress disorder (PTSD). PTSD is associated with impaired callosal transfer and with increased right hemisphere activation and impaired executive skills that could influence the processing of dichotic stimuli. A total of 22 participants with PTSD were compared to 23 participants without a diagnosis of PTSD. All participants had similar experiences of acts of war and political violence. They were tested with dichotic listening to CV syllables with free recall and directed attention following the forced attention paradigm. The PTSD group showed increased right ear advantage due to impaired left ear reporting and also smaller attention modulation compared to the control group, and the performance shared variance with self-report measures of arousal and intrusive memories. The results are discussed towards a model of impaired functionality of the frontal lobe and right hemisphere versus impaired callosal transfer, both yielding predictions for the processing of the left ear input and the ability to attention modulation of the performance.     

Dichotic listening asymmetry: sex differences and menstrual cycle effects

The impact of menstrual cyclicity and sex differences on dichotic listening was studied in 25 women and 20 men (aged 20-25 years). Dichotic listening was administered using consonant-vowel (CV) stimuli and tested across three attention conditions. Women were tested at two points in the menstrual cycle (Day 2-5: low oestrogen and progesterone/Low-EP; Day 18-25: high oestrogen and progesterone/High-EP). Men were tested once. Performance averaged across attention conditions was analysed for menstrual cycle and sex effects. Significant menstrual cycle phase effects were observed in women. At the High-EP phase women showed a greater right ear advantage (REA) compared to the Low-EP phase. Sex differences were found when dichotic listening asymmetry in men was compared to women at the Low-EP, but not the High-EP phase. In contrast to laterality effects, baseline perceptual performance (total right plus left ear response) was similar in men and women at both phases of the menstrual cycle. Results support a role for ovarian hormones in shaping laterality of speech perception in women. This study also emphasises the importance of considering menstrual cycle effects when evaluating sex differences in dichotic listening.     

Interaction of attention and acoustic factors in dichotic listening for fused words

Two dichotic listening experiments examined the degree to which the right-ear advantage (REA) for linguistic stimuli is altered by a “top-down” variable (i.e., directed attention) in conjunction with selected “bottom-up” (acoustic) variables. Halwes fused dichotic words were administered to 99 right-handed adults with instructions to attend to the left or right ear, or to divide attention equally. Stimuli in Experiment 1 were presented without noise or mixed with noise that was high-pass or low-pass filtered, or unfiltered. The stimuli themselves in Experiment 2 were high-pass or low-pass filtered, or unfiltered. The initial consonants of each dichotic pair were categorized according to voice onset time (VOT) and place of articulation (PoA). White noise extinguished both the REA and selective attention, and filtered noise nullified selective attention without extinguishing the REA. Frequency filtering of the words themselves did not alter performance. VOT effects were inconsistent across experiments but PoA analyses indicated that paired velar consonants (/k/ and /g/) yield a left-ear advantage and paradoxical selective-attention results. The findings show that ear asymmetry and the effectiveness of directed attention can be altered by bottom-up variables.     

Segmental and metrical complexity during non-word repetition in adults who stutter

Non-word repetition is weaker for adults who stutter (AWS) compared to adults who do not stutter (AWNS) as phonological demands increase. However, non-word stimuli used in previous studies varied by length, but did not vary with regard to segmental or metrical complexity. The purpose of the present study was to examine the unique influence of these two distinct types of complexity on non-word repetition in AWS and AWNS via administration of the Test of Phonological Structure (TOPhS). Twenty-four adults (12 AWNS, 12 AWS) repeated 96 non-words within a soundproof booth immediately after auditory presentation. All 96 non-word targets included on the TOPhS were one to four syllables in length and ranked based on segmental complexity (simple, moderate and complex) and metrical complexity (simple, moderate and complex). No main effect of metrical complexity was detected between groups, and no differences in accuracy were observed for non-words with simple or moderate segmental complexity. However, AWS were significantly more likely to produce a phonemic error when repeating words with complex segmental structure than AWNS, irrespective of metrical complexity. Segmental complexity may contribute to the differences in phonological working memory in AWS when controlling for metrical complexity and length.     

Blind individuals show enhanced perceptual and attentional sensitivity for identification of speech sounds

We report on enhanced processing of speech sounds in congenitally and early blind individuals compared with normally seeing individuals. Two different consonant-vowel (CV) syllables were presented via headphones on each presentation. We used a dichotic listening (DL) procedure with pairwise presentations of CV syllables. The typical finding in this paradigm is a right ear advantage, indicating better processing of the CV-syllable stimuli in the left hemisphere. The dichotic listening procedure involved three different conditions, with instructions to pay attention to the right ear stimulus, the left ear stimulus or no specific instruction. The participants were 14 congenitally or early blind Finnish-speaking individuals that were compared with 129 normal seeing Finnish-speaking individuals. The blind participants reported overall significantly more correct syllables than seeing control subjects. When instructed to pay attention to the left ear stimulus and only report from the attended channel, they were again significantly better than the seeing control subjects. These findings indicate effects of hemispheric reorganization in blind individuals at both the sensory and cognitive levels of information processing in the auditory sensory modality.     

Constant and variable aspects of the dichotic listening right-ear advantage: A comparison of standard and signal detection tasks

The study was designed to disentangle perceptual and attentional components of the dichotic listening right-ear advantage (REA). A total of 120 normal adults (108 right-handers and 12 left-handers) were administered the Halwes Fused Dichotic Words Test (FDWT) under standard conditions, which entail a forced-choice response format and no attention instructions. Two weeks later, the same stimuli were administered in a signal detection format with varied attention instructions (left, right, and equally divided). The standard task yielded the expected REA. The signal detection task yielded an REA for detection of targets that remained invariant across attention conditions. In contrast, ear asymmetry for localisation of targets varied with attention instructions. The REA on the standard task was correlated with the asymmetry of target detection (r = .57) on the signal detection task but not with the asymmetry of localisation or response bias. Ear asymmetry on the standard test seems to reflect an early stage of processing that is not altered by shifts of attention. Volitional shifts of attention influence the localisation of targets, but this component of signal processing is unrelated to the REA obtained on the standard test.     

The development of the right ear advantage in dichotic listening with focused attention

The influence of selective attention on cerebral asymmetry in 6, 8 and 10 year old children was studied using pre-cued dichotic recall of 1 to 4 pairs of digits. A constant right ear advantage for correct recall from the attended ear and intrusions from the unattended ear was found in all three age groups. The ear difference increased with set size and across serial position with a set size of 4 digit pairs. Thus the number of items in acoustic memory determined the degree of lateral asymmetry. The ability to focus attention on the left ear improved between 6 and 8 years. It was concluded that control of directed attention, which improves with development, and left hemisphere specialization for speech processing, which remains constant, both determine performance in dichotic verbal listening.     

Constant and variable aspects of the dichotic listening right-ear advantage: a comparison of standard and signal detection tasks

The study was designed to disentangle perceptual and attentional components of the dichotic listening right-ear advantage (REA). A total of 120 normal adults (108 right-handers and 12 left-handers) were administered the Halwes Fused Dichotic Words Test (FDWT) under standard conditions, which entail a forced-choice response format and no attention instructions. Two weeks later, the same stimuli were administered in a signal detection format with varied attention instructions (left, right, and equally divided). The standard task yielded the expected REA. The signal detection task yielded an REA for detection of targets that remained invariant across attention conditions. In contrast, ear asymmetry for localisation of targets varied with attention instructions. The REA on the standard task was correlated with the asymmetry of target detection (r = .57) on the signal detection task but not with the asymmetry of localisation or response bias. Ear asymmetry on the standard test seems to reflect an early stage of processing that is not altered by shifts of attention. Volitional shifts of attention influence the localisation of targets, but this component of signal processing is unrelated to the REA obtained on the standard test.     

Material type and attention control in dichotic listening

The present study investigated the effects of semantic and emotional content on laterality effects found in focused attention dichotic listening tasks. A total of 80 right-handed participants completed one of four conditions. They were presented either with dichotic pairs of consonant-vowel (CV) syllables (ba, da, ga, ka, pa, or ta) or words (bower, dower, power, or tower), pronounced in either a neutral tone or one of four emotional tones (happy, sad, angry, neutral). In all conditions, participants were instructed to circle the stimulus presented to the attended ear on each trial. The attended ear was randomised between blocks of trials for each participant. A significant condition by ear attended by ear of presentation interaction emerged. The typical right ear advantage (REA) was found in the neutral conditions, regardless of ear attended. In contrast, the emotional conditions revealed an REA when participants focused on the right ear, whereas a left ear advantage was found when they focused on the left ear. These findings suggest that the emotional component resulted in a shift in ear advantage coinciding with instructions possibly because of the influence of this component on attention and task difficulty. The role of semantic processing in words versus syllables is also discussed.