Left turning (swivel) in manic patients

Early uneven ear input induces long-lasting differences in left-right motor function

How asymmetries in motor behavior become established normally or atypically in mammals remains unclear. An established model for motor asymmetry that is conserved across mammals can be obtained by experimentally inducing asymmetric striatal dopamine activity. However, the factors that can cause motor asymmetries in the absence of experimental manipulations to the brain remain unknown. Here, we show that mice with inner ear dysfunction display a robust left or right rotational preference, and this motor preference reflects an atypical asymmetry in cortico-striatal neurotransmission. By unilaterally targeting striatal activity with an antagonist of extracellular signal-regulated kinase (ERK), a downstream integrator of striatal neurotransmitter signaling, we can reverse or exaggerate rotational preference in these mice. By surgically biasing vestibular failure to one ear, we can dictate the direction of motor preference, illustrating the influence of uneven vestibular failure in establishing the outward asymmetries in motor preference. The inner ear–induced striatal asymmetries identified here intersect with non–ear-induced asymmetries previously linked to lateralized motor behavior across species and suggest that aspects of left–right brain function in mammals can be ontogenetically influenced by inner ear input. Consistent with inner ear input contributing to motor asymmetry, we also show that, in humans with normal ear function, the motor-dominant hemisphere, measured as handedness, is ipsilateral to the ear with weaker vestibular input.

Despite a long-standing fascination with asymmetries in left–right brain function, very little is known about the causes of functional brain asymmetry in mammals, which appear independent of the mechanisms that create anatomical asymmetries during development. Asymmetries in motor function are a common example and include preferred turning direction, handedness, and footedness. In this study, using mouse models, we establish a causal link between transient imbalances in degenerating inner ear function and the establishment of stable asymmetries in neural pathways that regulate motor activity and in motor behavior. Our study also suggests that shared mechanisms may underlie lateralized motor behaviors across mammalian species. For example, we show that in humans with normal ear function, the strength of the vestibular response from each ear in the forebrain correlates with asymmetric motor behavior, measured as handedness. In a broader sense, our study reveals a conceptually novel role for sensory input in shaping the asymmetric distribution of brain function, a process for which there is otherwise no clear mechanism.

Instability in functional motor laterality of children and adolescents with endogenous psychosis and predominantly motor disturbances

A group of eight unmediated right-handed children and adolescents with endogenous psychosis and predominantly motor disturbances and two right-handed control groups (6 healthy subjects and 10 patients with different psychiatric diseases) were investigated with the help of a tapping-test series. The most important finding was related to differences in the stability of functional motor laterality between controls and psychotics. Stability of instability in functional motor laterality was identified by referring to the standard deviation (SD) of percentile right-left tapping differences calculated for each subject for the various parts of the tapping-test series. The high SDs in psychotics, in contrast to the low SDs in both control groups, point to increased variations or instabilities in the functional superiority of the preferred hand. Instability in functional motor laterality in this test is considered characteristic of this subgroup of patients, and may be due to a partial relapse to a lower hierarchical stage of handedness.