Long latency evoked potentials in a case of corpus callosum agenesia

"The Dichotic Digit Test" as an Index Indicator for Hearing Problem in Dementia: Systematic Review and Meta-Analysis

BACKGROUND

Patients with dementia commonly have problems processing speech in the presence of competing background speech or noise. This difficulty can be present from the very early stages of dementia, and may be a preclinical feature of Alzheimer's disease.

PURPOSE

This study investigates whether people with dementia perform worse on the dichotic digit test (DDT), an experimental probe of speech processing in the presence of competing speech, and whether test performance may predict dementia onset.

RESEARCH DESIGN

Systematic review and meta-analysis.

DATA COLLECTION AND ANALYSIS

A literature search was conducted in Medline, Embase, Scopus, and Psycinfo. We included (1) studies that included people with a diagnosis of dementia and a healthy control group with no cognitive impairment; (2) studies that reported results from a DDT in a free-recall response task; and (3) studies that had the dichotic digit mean correct percentage score or right-ear advantage, as outcome measurements.

RESULTS

People with dementia had a lower DDT total score, with a pooled mean difference of 18.6% (95% confidence interval [CI]: 21.2-15.9). Patients with dementia had an increased right-ear advantage relative to controls with a pooled difference of 24.4% (95% CI: 21.8-27.0).

CONCLUSION

The DDT total scores are lower and the right-ear advantage increased in cognitively impaired versus normal control participants. The findings also suggest that the reduction of dichotic digit total score and increase of right-ear advantage progress as cognitive impairment increases. Whether abnormalities in dichotic digit scores could predict subsequent dementia onset should be examined in further longitudinal studies.

Lateralisation of auditory processing in Down syndrome: a study of T-complex peaks Ta and Tb

It has long been argued that abnormal cerebral lateralisation might underlie the language problems that characterise Down syndrome, but to date only behavioural evidence has been provided. We used the auditory event-related potentials Ta and Tb of the T-complex to investigate lateralised processing of speech (vowels) and non-speech (simple and complex tones) sounds in children with Down syndrome and age-matched typically developing children. We also explored associations with speech and language abilities. Although changes in the Ta and Tb in response to increases in stimulus complexity and ‘speechness’ were similar across group, the Tb peak was delayed in children with Down syndrome across conditions. In addition, marked differences in the patterns of lateralisation of Ta latency and Tb amplitude were observed in children with Down syndrome, in response to both speech and non-speech sounds. No associations were found between Ta and Tb characteristics and speech and language abilities in children with DS.

Hemispheric differences in processing dichotic meaningful and non-meaningful words

Classic dichotic-listening paradigms reveal a right-ear advantage (REA) for speech sounds as compared to non-speech sounds. This REA is assumed to be associated with a left-hemisphere dominance for meaningful speech processing. This study objectively probed the relationship between ear advantage and hemispheric dominance in a dichotic-listening situation, using event-related potentials (ERPs). The mismatch negativity (MMN) and a late negativity (LN) were measured for bisyllabic meaningful words and non-meaningful pseudowords, which differed in their second syllable. Eighteen normal-hearing listeners were presented with a repeating diotic standard ([beI-gi:] or [leI-gi:]) and an occasional dichotic deviant (a standard presented to one ear and a deviant [beI-bi:], [beI-di:], [leI-bi:] or [leI-di:] presented to the opposite ear). As predicted there was a REA for meaningful words compared to non-meaningful words. Also, dipole source analysis suggested that dipole strength was stronger in the left than the right cortical region for meaningful words. However, there were differences in response within meaningful words as well as between meaningful and non-meaningful words which may be explained by the characteristics of embedded words and the position-specific probability of phoneme occurrence in words.

Effects of aging and gender on interhemispheric function

The ability of the two hemispheres of the brain to communicate with one another via the corpus callosum is important for a wide variety of sensory, motor, and cognitive functions, many of them communication related. Anatomical evidence suggests that aging results in structural changes in the corpus callosum and that the course over time of age-related changes in corpus callosum structure may depend on the gender of the individual. Further, it has been hypothesized that age- and gender-related changes in corpus callosum structure may result in concomitant decreased performance on tasks that are reliant on interhemispheric integrity. The purpose of this study was to investigate the effects of age and gender on auditory behavioral and visuomotor temporal indices of interhemispheric function across the life span of the normal adult. Results from 120 consistently right-handed adults from age 20 to 75 years revealed that interhemispheric integrity, as measured by dichotic listening, auditory temporal patterning, and visuomotor interhemispheric transfer time tasks, decreases relatively early in the adult life span (i.e., between the ages of 40 and 55 years) and shows no further decrease thereafter. In addition, the course over time of interhemispheric decline is different for men compared to women for some tasks. These findings suggest that decreased interhemispheric function may be a possible factor contributing to auditory and communication difficulties experienced by aging adults. In addition, results of this study hold implications for the clinical assessment of interhemispheric function in aging adults and for future research into the functional ramifications of decreased multimodality interhemispheric transfer.

Speech Lateralisation in Callosal Agenesis Assessed by the Dichotic Fused Words Test

Child patients with left-hemisphere damage (n = 2), with total callosal agenesis (n = 2), and with partial callosal agenesis or hypotrophy (n = 3) were submitted to dichotic verbal stimulation by the Fused Words Test. The controls were nine normal children, right-, left-, and mixed-handed. As expected, the left-injured patients presented a massive advantage of the ear contralateral to the intact hemisphere. Among the controls the right-handers showed a significant right-ear prevalence, whereas left-and mixed-handers exhibited rather inconsistent earasymmetries. The major finding of this study is the striking difference in perceptual asymmetry between partial and total acallosals. Based on the notion that the dichotic verbal asymmetry is an effect of the underlying functional asymmetry for language, these findings are interpreted as an indication that development of the unilateral speech control is influenced by the presence, during ontogenesis, of callosal connections.