The effect of stimulus intensity on the right ear advantage in dichotic listening

The dichotic listening test is non-invasive behavioural technique to study brain lateralization and it has been shown, that its results can be systematically modulated by varying stimulation properties (bottom-up effects) or attentional instructions (top-down effects) of the testing procedure. The goal of the present study was to further investigate the bottom-up modulation, by examining the effect of differences in the right or left ear stimulus intensity on the ear advantage. For this purpose, interaural intensity difference were gradually varied in steps of 3 dB from -21 dB in favour of the left ear to +21 dB in favour of the right ear, also including a no difference baseline condition. Thirty-three right-handed adult participants with normal hearing acuity were tested. The dichotic listening paradigm was based on consonant-vowel stimuli pairs. Only pairs with the same voicing (voice or non-voiced) of the consonant sound were used. The results showed: (a) a significant right ear advantage (REA) for interaural intensity differences from 21 to -3 dB, (b) no ear advantage (NEA) for the -6 dB difference, and (c) a significant left ear advantage (LEA) for differences form -9 to -21 dB. It is concluded that the right ear advantage in dichotic listening to CV syllables withstands an interaural intensity difference of -9 dB before yielding to a significant left ear advantage. This finding could have implications for theories of auditory laterality and hemispheric asymmetry for phonological processing.     

Transcranial magnetic stimulation

We investigated the effects of unilateral cold-water vestibular stimulation on healthy subjects' performance in two cognitive tasks known to be differentially mediated by the two cerebral hemispheres. In a first experiment (right-hemisphere task), subjects memorized object-location associations while being stimulated with cold water in the left ear or right ear or not at all (control group). In the second experiment (left-hemisphere task), subjects memorized a list of sequentially presented function words while being stimulated in the same manner as the subjects in the first experiment. A recall phase followed each encoding phase. In the first experiment, subjects who had been stimulated in the left ear recalled the object locations significantly faster than subjects who had been stimulated in the right ear and those in the control group. The second experiment yielded the reverse pattern: correct word recognition was faster for subjects who had been stimulated in the right ear than for subjects stimulated in the left ear and those of the control group. We suggest that unilateral caloric stimulation leads to a selective activation of contralateral cerebral structures and speeds up cognitive processes mediated by these structures. These results are discussed with respect to findings in neglect patients and functional-imaging studies in healthy subjects.     

Instability of hemispheric asymmetry in dyslexic children.

The study tested the hypothesis of abnormal brain asymmetry in dyslexic children. Two dyslexic groups classified as "phonetic disorder" and "language disorder" and a control group participated in two experiments. In both experiments was employed a dichotic listening procedure consisting in recalling pairs of words presented simultaneously to two ears. In Experiment I the children were to recognize four words presented at a level of loudness typical for natural speech. In Experiment II only two words were used in each trial but they were presented at a low intensity level. The recognition scores for stimuli presented to the left and right ears were compared. In Experiment I all groups of children showed a typical right ear/left hemisphere superiority, i.e. their recognition scores were higher for the right than for the left ear. Dyslexics, however, performed significantly less well. In Experiment II the control children and those from "phonetic disorder" group again performed better when words were presented to their right ears. Unlike those two groups, the children from "language disorder group" showed the right hemisphere superiority, i.e. they performed better in the left ear presentation condition. The results do not support the hypothesis that dyslexic children have abnormal lateralization of verbal functions. They suggest that the pattern of hemispheric asymmetry in dyslexics is less stable and depends both on the kind of dyslexia and on task variation.     

Effects of sex,age, and forced attention on dichotic listening in children: A longitudinal study

Seventy‐six children (38 boys, 38 girls) were tested with dichotic presentations of consonant‐vowel (CV) syllables under three different attentional instructions. Each child was tested twice: at age 8 and 1 year later (at age 9). The study investigated effects of laterally biased attention in dichotic performance in children. In the nonforced (NF) recall condition, children were free to report from both ears; in the forced‐right (FR) condition, they were instructed to attend to and report from only the right ear; in the forced‐left (FL) condition, they were instructed to attend to and report from only the left ear. The results showed a significant right‐ear advantage in both boys and girls and at both test occasions for the NF and FR conditions. During the FL condition, boys still reported more items from the right ear at age 8 but no at age 9, whereas girls reported an equal number of items from both ears at both test occasions. Thus, although hemisphere asymmetry did not change across age, the ability to laterally shift attention did and more so for boys than for girls.     

Dichotic listening,handedness, and reading ability: A meta‐analysis

A meta‐analysis was used to aggregate data regarding the influence of selective attention, handedness, and reading ability on the right‐ear advantage in dichotic listening. Data from 15 independent dichotic studies that employed consonant‐vowel stimuli, free‐recall and directed‐attention conditions, and left‐and right‐handed children (age range 6 to 12 years, IQ > 85) classified as good and poor readers determined the sample. The statistical aggregation of research results based on the use of effect sizes and one‐sample tests indicated that younger (6 years, 0 months to 8 years, 11 months) and older (9 years, 0 months to 12 years, 11 months) good readers and older poor readers shifted attention and thus overcame the right ear advantage bias, whereas younger poor readers shift attention only in the directed‐right condition. Effect size analyses (with individuals combined for age) also indicated that left‐ and right‐handed good readers do not shift attention across directed conditions, but attentional shifting was found for left‐ and right‐handed poor readers. The meta‐analysis clearly demonstrates an interaction among age, reading ability, and handedness on dichotic listening outcome. Differences in patterns of ear report among these groups of children are suggestive of a combination of varied functional lateralization and attentional strategies employed during dichotic consonant‐vowel tasks.     

Lateralization of ERP Components during Verbal Dichotic Information Processing

Two experiments are described in which subjects were requested to recall dichotic digit pairs. The results of both experiments showed a significant right ear advantage for the digits. In the EEG a strongly lateralized sustained positive wave was present in the pre-response period. The asymmetry of this component appeared to be both task dependent, and related to individual differences in ear asymmetry. It is suggested that the sustained wave is a manifestation of differential mobilization of resources of the two hemispheres that compete for limited capacity in a particular stage of the task.     

On the perception of sarcasm in dichotic listening

The purpose of the present study was to investigate the processing of sincere and sarcastic statements by the cerebral hemispheres. Forty right-handed students were asked to localize sincere and sarcastic statements presented dichotically. Participants either indicated the ear that perceived the sarcastic statement or the ear that perceived the sincere statement in counterbalanced blocks of trials. As expected, results revealed a left ear advantage for sarcastic statements and a right ear advantage for sincere statements. In addition, participants showed faster response time when localizing targets (both sarcastic and sincere) to the left ear compared to the right. Finally, a significant negative correlation between laterality effects in the two tasks provided support for causal hemispheric complementarity. Results are discussed with reference to the contribution of the right and left hemispheres to language processing. Their implications for models of sarcasm perception are also discussed.     

Mechanisms underlying the left visual-field advantage in the dual stream RSVP task: evidence from N2pc, P3, and distractor-evoked VEPs

When two letter streams containing two targets (T1 and T2) are presented left and right, T2 is better identified in the left hemifield. This study on 16 healthy participants used evidence from ERPs to decide whether this advantage is due to better processing in the right hemisphere or to overload of the left one. N2pc and P3 components evoked by T1 and T2 were measured, as well as the VEPs evoked by the stream of distractor stimuli. Already at the onset of the stream, these VEPs peaked earlier at the right than at the left hemisphere. N2pc was evoked earlier and P3 amplitudes were larger with left than with right T2. Previously reported side differences in T1-evoked N2pc were no longer obtained after correcting for constant hemispheric differences. The faster VEP latencies at the right hemisphere from the very beginning of the stimulus series may reflect an advantage in structuring fast sequences, which may cause the left visual-field advantage.     

Discourse analysis of logical memory recall in normal aging and in dementia of the Alzheimer type

This study examined the nature of errors in prose recall made in dementia compared with normal aging. Responses by 48 young adults, 47 nondemented older adults, and 70 people with very mild or mild Alzheimer's disease to the Logical Memory subtest of the Wechsler Memory Scale were examined in a propositional analysis. Compared with young adults, healthy older adults showed good immediate recall but deficits in retention over a delay. Demented individuals made errors of omission, not commission, at immediate recall. These errors probably reflect difficulty with attentional control rather than memory per se. In terms of clinical implications, veridical scoring of the Logical Memory subtest provides more sensitive detection of very mild dementia of the Alzheimer type than the current standard criteria for scoring.     

Dichotic listening and corpus callosum magnetic resonance imaging in relapsing-remitting multiple sclerosis with emphasis on sex differences

Twenty-five early-onset relapsing-remitting multiple sclerosis patients (12 women and 13 men) with mild disability were compared with 25 matched controls in a dichotic listening (DL) test under nonforced and forced attentional-shift conditions. Patients showed left ear impairment and no left ear advantage in the forced-left condition. Four corpus callosum (CC) regions were measured in patients on a midsaggital magnetic resonance imaging scan. The right ear score was negatively correlated whereas the left ear score was positively correlated with CC regions (significant only for the nonforced condition). Moreover, in men, the correlations with DL scores were linked mainly to the splenium and posterior isthmus, and in women, they were stronger for anterior isthmus and posterior body. An inverse correlation between months of disease evolution and CC area was found only in women.     

Left hemisphere lateralisation of auditory hallucinations in schizophrenia: A dichotic listening study

Introduction. We propose that auditory hallucinations are internally generated speech misrepresentations that are lateralised to the left temporal lobe. If hallucinations are misrepresentations involving the speech perception area of the left temporal lobe, then hallucinating patients should have problems identifying a simultaneously presented external speech sound, especially when the sound is lateralised to the left hemisphere. Lateralisation of speech perception can be experimentally studied with the dichotic listening task with consonant-vowel syllables. We predicted a negative relation between frequency of auditory hallucinations and performance on the dichotic listening task.

Method. We studied 87 right-handed patients with schizophrenia. Hallucination scores were taken from the BPRS symptom scale. Right and left ear scores in the dichotic listening task were recorded. A right ear advantage is expected in healthy individuals, indicating left temporal lobe processing superiority. The patients were compared with 36 right-handed healthy reference subjects.

Results. A gradual decrease in the ability to process and report the right ear stimulus with increasing frequency of hallucinations was seen in the schizophrenia patients. No such relationship was found for processing and reporting of the left ear stimulus. There were no significant correlations with negative symptoms. Thus, the results were not the consequence of illness severity. There was however a significant correlation with unusual thought content symptom, pointing to a relationship also between delusions and auditory hallucinations.

Conclusion. The results support that auditory hallucinations may be internally generated speech misrepresentations, originating in the left temporal lobe.     

Allocation of attention in dichotic listening: differential effects on the detection and localization of signals.

In 2 dichotic listening experiments, 96 normal right-handed adults attended selectively to the left and right ear and divided their attention equally between both ears. Participants listened for specified targets and reported the ear of entry. The material consisted of pairs of consonant-vowel syllables in Experiment 1 and pairs of rhyming consonant-vowel-consonant words in Experiment 2. Both experiments yielded a right-ear advantage for detection and for localization. Attention instructions had no effect on detection. However, focusing attention on 1 ear increased the number of targets attributed to that ear while decreasing the number of targets attributed to the opposite ear. The dissociation between detection and localization indicates that volitional shifts of attention influence late (response selection) processes rather than early (stimulus identification) processes. Selective-listening effects can be accounted for by a 2-stage model in which a fixed input asymmetry is modulated by a biased selection of responses.     

Associative recognition in Alzheimer's disease: evidence for impaired recall-to-reject

Patients with mild Alzheimer's disease (AD) were compared with age-matched control subjects on an associative recognition task. Subjects studied pairs of unrelated words and were later asked to distinguish between these same studied pairs (intact) and new pairs that contained either rearranged studied words (rearranged) or non-studied words (non-studied). Studied pairs were presented either once or 3 times. Repetition increased hits to intact pairs in both groups, but repetition increased false alarms to rearranged pairs only in patients. This latter pattern indicates that repetition increased familiarity of the rearranged pairs, but only the control subjects were able to counter this familiarity by recalling the originally studied pairs (a recall-to-reject process). AD impaired this recall-to-reject process, leading to more familiarity based false alarms. These data support the idea that recollection-based monitoring processes are impaired in mild AD.     

Neural correlates of dementia: regional brain metabolism (FDG-PET) and the CERAD neuropsychological battery.

The present Investigation examined the biological correlates of the cognitive deficits of Alzheimer's disease and related dementias using the neuropsychological assessment battery of the Consortium to Establish a Registry of Alzheimer's Disease (CERAD) and positron emission tomography (PET). Resting state cerebral glucose metabolism was measured using the labelled radiotracer, [18F] Fluoro-2-deoxyglucose (FDG), in a sample of patients with mild to moderate dementia (n = 66). Specific and predictable relationships were seen between regional brain metabolism (left and right, frontal, temporal, and parietal lobes) and the neuropsychological measures of verbal fluency, constructional praxis, and verbal list learning. On tests of naming and delayed verbal recall only diminished FDG uptake in the left frontal lobe and the left temporal lobe, respectively, approached significance. This study demonstrates the expected relationships between neuropsychological performance and regional cerebral metabolism, thereby providing support for the CERAD battery as a valid measure in the clinical evaluation of dementia and for the use of FDG-PET in brain-behavior studies of dementia.     

Cardiac Response to Speech Sounds in Preterm Infants: Effects of Postnatal Illness at Three Months

The effect of postnatal illness, specifically respiratory distress syndrome (RDS), on preterm infants’ attention to auditory stimuli at 3 months of age was examined in three groups of infants; healthy preterm, preterms who had RDS, and a sample of normal term infants. Infants were presented 2 sessions of 9 trials of synthesized consonant-vowel sounds. The first 7 trials consisted of a binaural presentation in which the CV stimulus /ba/ or /pa/ was presented simultaneously to both left and right ears. The eighth and ninth trials consisted of a dichotic presentation in which the speech stimuli /ba/ and /pa/ were presented, one sound to the left ear and one sound to the right ear. EKG was recorded throughout the presentation. Analyses of covariance with prestimulus heart period as the covariate and heart period during stimulus presentation as the dependent measure were computed to investigate cardiac orienting, habituation and recovery. Results reveal significant cardiac orienting by healthy term and preterm infants on trial 1, habituation across trials 1–7, and recovery of cardiac orienting to stimulus change on the second novel presentation by healthy term and preterm. RDS preterm infants exhibited cardiac orienting to the first stimulus presentation but they did not evidence a change in cardiac level across redundant trials or a response recovery to the novel stimulus. RDS preterms throughout exhibited shorter heart period levels (faster heart rates) than either healthy term or healthy preterm infants. Results are discussed in relation to the effect of RDS on autonomic reactivity.     

STIMULUS REPETITION RATE FACTORS WHICH INFLUENCE THE AUDITORY EVOKED POTENTIAL IN MAN

The slow component of the auditory evoked potential in man was investigated under various conditions which produced an increase in stimulus repetition rate. The left ear received a 1000 Hz tone once each 5 sec. Intervening stimuli, also of 1000 Hz, were presented to (1) the right ear, (2) both ears, and (3) the left ear. Their relative influence on response amplitude to periodic left ear stimulation was evaluated. The results indicated that: (1) intervening right ear stimulation reduced significantly response amplitude to left ear stimulation; (2) presenting the intervening stimuli to both ears simultaneously caused no further decrement in response amplitude; (3) presenting the intervening stimuli to the left ear alone brought about a further and significant decrease in response amplitude to left ear stimulation. These findings imply that habituation of the auditory evoked potential is greatest when the same pattern of neural activity is repeatedly initiated. When the intervening stimuli elicit a different pattern of neural excitation, habituation is less, even though stimulation rate remains unchanged.     

Correspondence between brain ERP and behavioral asymmetries in a dichotic complex tone test

Electrophysiologic correlates of perceptual asymmetry for dichotic pitch discrimination were investigated in 20 normal subjects. Brain event-related potentials (ERPs) elicited by dichotic pairs and binaural probe tones in the Complex Tone Test (Sidtis, 1981) were recorded from homologous scalp locations over left and right hemispheres (F3, F4; C3, C4; P3, P4; O1, O2). Baseline-to-peak amplitudes were measured for N100, P200, and a late positive complex consisting of P350, P550, and slow wave. A left ear advantage (LEA) was evident in 70% of the subjects, and hemispheric asymmetries related to this behavioral asymmetry were found for P350 and P550 amplitudes to probe stimuli. Subjects with a strong LEA had greater amplitudes over the right hemisphere than the left, whereas subjects with little or no LEA showed a nonsignificant trend toward the opposite hemispheric asymmetry. Hemispheric asymmetry of these late ERPs at parietal and occipital sites was highly correlated with behavioral asymmetry. These findings suggest the utility of electrophysiological measures in assessing hemispheric asymmetries for processing complex pitch information.     

Development of the right ear advantage in dichotic listening test: comparison of the findings by cross-sectional and longitudinal studies

The right ear advantage in dichotic listening test for VCV word pairs was investigated using both longitudinal and cross-sectional methods with children whose age ranged from four to six. The degree of a right ear advantage and the population of those who showed a right ear advantage increased with age in the cross-sectional method. In the longitudinal method, however, the above findings could not be confirmed. These results suggest that the application of dichotic listening test may be inappropriate to young children.     

Dual task performance in early Alzheimer's disease, amnestic mild cognitive impairment and depression

BACKGROUND: The dual task paradigm (Baddeley et al. 1986; Della Sala et al. 1995) has been proposed as a sensitive measure of Alzheimer's dementia, early in the disease process. METHOD: We investigated this claim by administering the modified dual task paradigm (utilising a pencil-and-paper version of a tracking task) to 33 patients with amnestic mild cognitive impairment (aMCI) and 10 with very early Alzheimer's disease, as well as 21 healthy elderly subjects and 17 controls with depressive symptoms. All groups were closely matched for age and pre-morbid intellectual ability. RESULTS: There were no group differences in dual task performance, despite poor performance in episodic memory tests of the aMCI and early Alzheimer's disease groups. In contrast, the Alzheimer patients were specifically impaired in the trail-making test B, another commonly used test of divided attention. CONCLUSIONS: The dual task paradigm lacks sensitivity for use in the early differential diagnosis of Alzheimer's disease.     

Volume reduction of the entorhinal cortex in subjective memory impairment

To examine the biological basis of subjective memory impairment (SMI), defined as the feeling of memory worsening with normal memory performance, we measured the volume of the entorhinal cortex (EC) and the hippocampus in SMI subjects, patients with mild cognitive impairment (MCI), patients with Alzheimer's disease (AD) and healthy controls (CO). Compared with controls, the EC was smaller in the SMI group (left: p = 0.060; right: p = 0.045) and in the other two groups in the following order: CO > SMI > MCI > AD. The same sequence was observed with regard to hippocampal volumes, but the volume reduction of the left hippocampus in the SMI group only reached a trend towards significance (p = 0.072) and the right was not significantly smaller compared with controls (p = 0.37). Compared with controls the average (left/right) volume reduction of the EC was 18% (SMI), 26% (MCI) and 44% (AD). The mean volume reduction of the hippocampus was 6% (SMI), 16% (MCI) and 19% (AD). Our results mirror the temporal sequence of neurodegeneration in AD and support the concept of SMI as the first clinical manifestation of dementia.