Dissociation of brain areas associated with force production and stabilization during manipulation of unstable objects.

Neuropediatric Research Unit, Department of Women's and Children's Health, Karolinska Institutet, Astrid Lindgren Children's Hospital Q2:O7, 171 76 Stockholm, Sweden.
Experimental Brain Research (Impact Factor: 2.17). 12/2011; 215(3-4):359-67. DOI: 10.1007/s00221-011-2903-9
Source: PubMed

ABSTRACT Multifinger dexterous manipulation of unstable or deformable objects requires control of both direction and magnitude of fingertip force vectors. Our aim was to study the neuroanatomical correlates of these two distinct control functions. Brain activity was measured using functional magnetic resonance imaging while 16 male subjects (age: 26-42, M = 32, SD ± 4 years) compressed four springs representing a 2 × 2 factorial design with two levels of force and instability requirements. Significant activations associated with higher instability were located bilaterally in the precentral gyri, the postcentral gyrus, and the cerebellum. In the main effect for high force, activity was found in areas located in the primary motor regions contralateral to the active hand and bilaterally in the cerebellum. An overlap in activation between the two main effects was found bilaterally in the cerebellum (lobule VI). This study not only confirms a recently described bilateral fronto-parieto-cerebellar network for manipulation of increasingly unstable objects, but critically extends our understanding by describing its differentiated modulation with both force magnitude and instability requirements. Our results, therefore, expose a previously unrecognized and context-sensitive system of brain regions that enable dexterous manipulation for different force magnitude and instability requirements of the task.

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    ABSTRACT: Dexterous manipulation depends on using the fingertips to stabilize unstable objects. The Strength-Dexterity paradigm consists of asking subjects to compress a slender and compliant spring prone to buckling. The maximal level of compression [requiring low fingertip forces <300 grams force (gf)] quantifies the neural control capability to dynamically regulate fingertip force vectors and motions for a dynamic manipulation task. We found that finger dexterity is significantly affected by age (p = 0.017) and gender (p = 0.021) in 147 healthy individuals (66F, 81M, 20-88 years). We then measured finger dexterity in 42 hands of patients following treatment for osteoarthritis of the base of the thumb (CMC OA, 33F, 65.8 ± 9.7 years), and 31 hands from patients being treated for Parkinson's disease (PD, 6F, 10M, 67.68 ± 8.5 years). Importantly, we found no differences in finger compression force among patients or controls. However, we did find stronger age-related declines in performance in the patients with PD (slope -2.7 gf/year, p = 0.002) than in those with CMC OA (slope -1.4 gf/year, p = 0.015), than in controls (slope -0.86 gf/year). In addition, the temporal variability of forces during spring compression shows clearly different dynamics in the clinical populations compared to the controls (p < 0.001). Lastly, we compared dexterity across extremities. We found stronger age (p = 0.005) and gender (p = 0.002) effects of leg compression force in 188 healthy subjects who compressed a larger spring with the foot of an isolated leg (73F, 115M, 14-92 years). In 81 subjects who performed the tests with all four limbs separately, we found finger and leg compression force to be significantly correlated (females ρ = 0.529, p = 0.004; males ρ = 0.403, p = 0.003; 28F, 53M, 20-85 years), but surprisingly found no differences between dominant and non-dominant limbs. These results have important clinical implications, and suggest the existence - and compel the investigation - of systemic versus limb-specific mechanisms for dexterity.
    Frontiers in Neurology 04/2014; 5:53.
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    ABSTRACT: The ability to dynamically control fingertip force vector magnitude and direction is critical for dexterous manipulation. We quantified the dynamic control of fingertip forces to examine how dexterous manipulation declines with age. The strength-dexterity (SD) test measures fingertip forces during compression of a slender spring prone to instability and buckling. The greatest sustained compression (designed to be under 3 N), and force dynamics therein, have been shown to be simple and quick measures of dynamic dexterous manipulation ability. We measured pinch strength and strength-dexterity test in a cross-sectional population of 98 people from 18 to 89 years of age. Dexterous manipulation ability is poorer at older ages, beginning in middle age (p < .001), with greater decline past 65 years of age. Fingertip force dynamics during spring compression and stabilization show a deterioration of neuromuscular control with age. Importantly, this novel detection of decline in dynamic manipulation ability is not correlated with, and thus cannot be entirely explained by, the known decline in pinch strength. We also measured standardized tests of dexterity in participants older than 45, and discuss how the strength-dexterity test uniquely captures features of sensorimotor capabilities for dexterous manipulation in this adult population. Starting in middle age, changes in the functional interactions among sensory, motor, and neural capabilities result in measurably poorer dynamic dexterous manipulation. This deterioration of neuromuscular control motivates and enables future studies to understand the physiological bases for this functional decline so critical to activities of daily living and quality of life.
    The Journals of Gerontology Series A Biological Sciences and Medical Sciences 03/2014; · 4.31 Impact Factor
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    ABSTRACT: Premotor cortex activity is associated with complex motor performance and motor learning and offers a potential target to improve dexterity by transcranial direct current stimulation (tDCS). We explored the effects of tDCS of premotor cortex on performance of a Strength-Dexterity test in healthy subjects. Methods During the test a slender spring held between thumb and index finger should be compressed as much as possible without buckling. Finger forces assessed in the test provided a measure of dexterity. First, task performance was tested in 12 persons during anodal tDCS to the primary motor cortex (M1) contralateral to the performing hand, and sham stimulation. Another 12 persons participated in five sessions of anodal and cathodal tDCS over the left and the right premotor cortex and sham stimulation. Results tDCS over M1 as well as over the left, but not the right premotor cortex resulted in significant improvement of performance. Performance alterations correlated positively between left anodal and right cathodal tDCS and negatively between anodal tDCS of the two sides. Effective polarity for premotor stimulation to improve task performance differed between participants. Individuals who improved with anodal stimulation used lower finger force and experienced the test as more difficult compared to those who improved with cathodal stimulation. Conclusions This study demonstrates that tDCS over the left premotor cortex can improve performance of a dexterity demanding task. The effective polarity of stimulation depends on the task performance strategies. The study moreover shows a functional relevance of interactions between the left and right premotor cortex.
    Brain Research 08/2014; · 2.83 Impact Factor

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