Is There a Causal Link between the Left Lateralization of Language and Other Brain Asymmetries? A Review of Data Gathered in Patients with Focal Brain Lesions

This review evaluated if the hypothesis of a causal link between the left lateralization of language and other brain asymmetries could be supported by a careful review of data gathered in patients with unilateral brain lesions. In a short introduction a distinction was made between brain activities that could: (a) benefit from the shaping influences of language (such as the capacity to solve non-verbal cognitive tasks and the increased levels of consciousness and of intentionality); (b) be incompatible with the properties and the shaping activities of language (e.g., the relations between language and the automatic orienting of visual-spatial attention or between cognition and emotion) and (c) be more represented on the right hemisphere due to competition for cortical space. The correspondence between predictions based on the theoretical impact of language on other brain functions and data obtained in patients with lesions of the right and left hemisphere was then assessed. The reviewed data suggest that different kinds of hemispheric asymmetries observed in patients with unilateral brain lesions could be subsumed by common mechanisms, more or less directly linked to the left lateralization of language.

Investigating Musical Emotions in People with Unilateral Brain Damage

The study under discussion sought to investigate the hemispheric laterality of musical emotions: Is one hemisphere of the brain preferentially involved in recognizing emotions in music? The authors took a neuropsychological approach to answer this question by studying emotional judgments of music in people with brain damage to either hemisphere. Their results indicated that individuals with left hemisphere damage were significantly impaired in recognizing musical emotions as compared to healthy comparison participants. In contrast, individuals with right hemisphere damage were not impaired at identifying emotions in music, but rated the perceived intensity of the emotions lower for sadness and fear (as compared to joy and serenity). Their work suggests that the identification of emotions in music and the perceived intensity of the emotions expressed may rely on different hemispheres of the brain.

Story reading with incidental comprehension and memory: left hemisphere dominance

ABSTRACT Background: There are no studies on adults with unilateral brain lesions regarding story reading with incidental/implicit comprehension and memory, in which memory is only assessed through delayed recall. There is a need for validation of cerebral laterality in this type of verbal recall, which includes spontaneous performance (free or uncued condition (UC)), and induced-through-question performance regarding the forgotten units (cued condition (CC)). Objectives: To explore the effects of unilateral brain lesions, of oral reading with expression (RE) and comprehension (RC) on delayed recall of a story, as either UC or CC; and to validate the ability of UC and CC to discriminate the side of brain injury. Methods: Data were obtained from 200 right-handed volunteers, among whom 42 had left-hemisphere injury (LHI), 49 had right-hemisphere injury (RHI) and 109 were demographically-matched healthy participants (HP). Patients who were unable to read, understand or speak were excluded. Results: LHI individuals presented impairment of both UC and CC, in relation to the other two groups (non-LHI) with sensitivity and specificity above 70%. LHI and RHI individuals were not significantly different in RE and RC, but they were both different from HP in all the assessments except CC, in which RHI individuals resembled HP. Despite this lack of abnormality in RHI individuals during CC, about half of this group showed impairment in UC. Additionally, whereas RE had a significant effect on UC, the moral of the story (RC) had a significant effect on both UC and CC. Conclusions: The left hemisphere was dominant for this memory task involving implicit processing.

A major new dimension in the problem of brain injury

Evidence that neurohormones contribute to the contralateral effects of unilateral brain injury challenges a fundamental assumption of basic neuroscience and clinical neurology.

Simultaneously stimulating both brain hemispheres by rTMS in patients with unilateral brain lesions decreases interhemispheric asymmetry

Background: Interhemispheric asymmetry caused by brain lesions is an adverse factor in the recovery of patients with neurological deficits. Repetitive transcranial magnetic stimulation (rTMS) has been shown to modulate cortical oscillation and proposed as an approach to rebalance the symmetry, which has not been documented well. Objective: In this study, we investigated the influence of repetitive transcranial magnetic stimulation (rTMS) on EEG power in patients with unilateral brain lesions by simultaneously stimulating both brain hemispheres and to elucidate asymmetrical changes in rTMS-induced neurophysiological activity. Methods: Fourteen patients with unilateral brain lesions were treated with one active and one sham session of 10 Hz rTMS over the vertex (Cz position). Resting-state EEGs were recorded before and immediately after rTMS. The brain symmetry index (BSI), calculated from a fast Fourier transform, was employed to quantify the power asymmetry in both hemispheres and paired channels over the entire range and five frequency bands (delta, theta, alpha, beta and gamma bands). Results: Comparison between active and sham sessions demonstrated rTMS-induced EEG after-effects. rTMS in the active session significantly reduced the BSI in patients with unilateral brain lesions over the entire frequency range (t = 2.767, P = 0.016). Among the five frequency bands, rTMS only induced a noticeable decrease in the BSI in the delta band (t = 2.254, P = 0.042). Furthermore, analysis of different brain regions showed that significant changes in the BSI of the alpha band were only demonstrated in the posterior parietal lobe. In addition, EEG topographic mapping showed a decreased power of delta oscillations in the ipsilesional hemisphere, whereas distinct cortical oscillations were observed in the alpha band around the parietal-occipital lobe in the contralesional hemisphere. Conclusions: When both brain hemispheres were simultaneously activated, rTMS decreased interhemispheric asymmetry primarily via reducing the delta band in the lesioned hemisphere.