This point mass is located at the center of mass. COG—the resultant force of all of small attractive forces of the mass particles of which the body is composed is the body's weight, and the location at which the resultant force is assumed to act. Ground reaction force vector GRF —the resultant of a pressure distribution under the foot or feet. Base of support BoS , is defined as the possible range of the COP, which is loosely equal to the area below and between the feet in two-feet standing Winter et al.
The most simplified biomechanical model assumes the human as one rigid body, where the COM is located at the waist, a pivot axis at the ankle, and a COP where the GRF vector acts. The assumptions used in the presented model are those of the inverted pendulum model of human standing balance Winter and Eng, : 1 The balance problem can be completely described by the movement of the whole-body COM, 2 the distance l from the axis of rotation to the COM remains constant, and 3 the excursions of the COM are small with respect to l.
Computational Neuroscience Models of the Basal Ganglia
Where small body movements cause acceleration of the COM, a radial acceleration exists leading to priority of equilibrium control during almost all motor tasks including quiet standing aimed at reposition the COG over the COP Gatev et al. The muscles around the ankle and hip joints work continuously as the human body struggles to maintain balance. One can see that as long as the COP is kept beyond the COM position, with respect to the rotation center at the ankle, the body is accelerated back to the upright position.
A major problem for human standing posture is the high center of gravity COG maintained over a relatively small base of support. In attempting to understand motor mechanisms involved in the development of balance, research on postural control has focused mainly on two types of study: a balance with respect to external conditions, b postural adjustments to anticipated internal disturbances of balance. Unexpected external disturbances reveal centrally programmed patterns of postural responses.
Afferent feedback also influences posture when the initial setting is disturbed. The second type of disturbance reveals feed-forward postural adjustments for review, Dietz, By feed-forward, we mean that the controller predicts an external input or behaves using higher-order processing rather than simple negative feedback of a variable Gatev et al. Studies of the postural responses to unexpected small and slow external disturbances in the antero-posterior direction found that most people reposition the COG by swaying as a flexible inverted pendulum primarily about the ankles with little hip or knee motion.
The choice of a postural strategy to disturbance was found to depend on the available appropriate sensory information Nashner et al.
Locomotion is fundamental for an optimal child development. The ability to smoothly and adequately navigate through the environment enables the child to interact with the environment.
ADHD and autistic spectrum individuals have reported significant motor difficulties, both fine and gross Melillo and Leisman, According to Patla et al. The bipedal walking pattern that humans have adopted over time constitutes an elegant way to meet these requirements in an efficient and economic way. Several findings with respect to motor control in children with DCD and ADHD, however, indicate that they could have problems to meet some of these constraints related to neuromuscular control.
Raynor observed decreased muscular strength and power in children with DCD, accompanied by increased levels of co-activation in a unilateral knee flexion hand extension task.
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Similar neuromuscular problems, indicating difficulties with the selective muscle control necessary for rhythmic coordination, were found in a unilateral tapping task by Lundy-Ekman et al. Likewise Volman and Geuze showed that these rhythmic coordination difficulties of children with DCD are not restricted to the control of unilateral tapping. By means of a bimanual flexion-extension paradigm they found that relative phase stability of children with DCD was less stable than in controls.
From studies where upright stance was perturbed by means of a sudden displacement of a moveable platform it was concluded that the balance recovery strategy of children with DCD was different Williams, Their strategy was characterized by a top-down muscular activation pattern compared to the distal-proximal pattern displayed by children without DCD, which was argued to be more efficient.
In stance, the projection of the center of mass has to be kept within the borders of the base of support, in order to maintain balance. For locomotor balance however, one must achieve a compromise between the forward propulsion of the body, which involves a highly destabilizing force, and the need to maintain the overall stability Winter and Eng, Taking into account this complexity with respect to the control of posture during locomotion it can be hypothesized that the balance problems experienced by children with DCD might be a limiting factor for their locomotor activity.
So far, descriptions of the gait pattern of children with DCD are limited to some qualitative observations.
Basal ganglia models and theory
Larkin and Hoare have notified for example poor head control, bent arms in a guard position, jerky limb to limb transitions, excessive hip flexion, pronounced asymmetry, wide base of support, short steps, foot strike with flat foot and toe-walking. This index is based on a comparison of four spatio-temporal gait parameters time of opposite toe-off, single stance time, total stance time, and step length with reference parameters of the San Diego database Sutherland, From their calculations Woodruff et al.
This one-dimensional measure of walking performance is useful for classifying and evaluation of gait performance in clinical practice; however, it does not explain the nature or source of atypical gait see Figure 5. In addition, comparison of gait variables with a reference population without controlling for stature or leg length and body weight might obscure deviations and lead to imprudent conclusions, since the walking pattern is highly dependent on anthropometrical characteristics Therefore, in order to gain insight into the gait pattern of children Hof, ; Stansfield et al.
Figure 5. Stick-figures of the body configuration at initial FS left and TO right. Feet with broken lines are the contralateral feet. Figure 6. The horizontal broken line indicates the cut-off value 2. Different studies report balance testing included a disruption of sensory signals.
Contributions of the Basal Ganglia to Temporal Processing: Evidence from Parkinson’s Disease
During dynamic posturography ADHD-participants showed mild balance problems, which correlated with findings in cerebellar children. ADHD children showed abnormalities in a backward walking task and minor abnormalities in the paced stepping test. They did not differ in treadmill walking from the controls. These findings support the notion that cerebellar dysfunction may contribute to the postural deficits seen in ADHD children. However, the observed abnormalities were minor. It needs to be examined whether balance problems become more pronounced in ADHD children exhibiting more prominent signs of clumsiness.
In the past, motor clumsiness had been viewed as a neurological rather than as a psychiatric disorder. Motor control problems were first noted in what were then called minimal brain dysfunction syndromes or MBD. Several studies by Denckla and others Denckla and Rudel, ; Denckla et al. Minor developmental deviations were reported to consist of dyscoordination, fine motor deviations, choreiform movements, and abnormalities of muscle tone. Researches that have dealt with these minor neural developmental deviations tend to look at motor dysfunction as a sign of neurological disorder that may be associated with other problems such as language and perception dysfunction.
In Asperger's syndrome, it has been noted that individual's have significant degrees of motor incoordination. Wing and Attwood noted that posture, gait, and gesture incoordination were most often seen in Asperger's syndrome and that children with classic autism seem not to have the same degree of balancing and gross motor skill deficits.
However, it was also noted that the agility and gross motor skills in children with autism seem to decrease as they get older and may eventually present in similar or at the same level as Asperger's syndrome. Gillberg and Gillberg also reported clumsiness to be almost universal among children that they had examined for Asperger's syndrome. The other associated symptoms noted consisted of severe impairment and social interaction difficulties, preoccupation with a topic, reliance on routines, pedantic language, comprehension, and dysfunction of nonverbal communication. In subsequent work, Gillberg included clumsiness as an essential diagnostic feature of Asperger's syndrome.
Klin et al. They further noted that all 21 Asperger's cases showed gross motor skill deficits, but 19 of these also had impairment in manual dexterity, which seems to suggest that poor coordination was a general characteristic of Asperger's. With studies like these, many researches have noted dysfunction of fine motor coordinative skills as a feature of autistic spectrum disorders. Walker et al. Vilensky et al. Hallett et al. Using a computer assisted video kinematic technique; they found that gait was atypical in these individuals.
The authors noted that the overall clinical findings were consistent with a cerebellar rather than a basal ganglionic dysfunction. Kohen-Raz et al. These objective measures were obtained using a computerized posturographic technique. It has been also noted that the pattern of atypical postures in children with autism is more consistent with a mesocortical or cerebellar rather than vestibular pathology. Numerous investigators Howard et al. Makris et al. They noted that ADHD is hypothesized to be due, in part, to structural defects in brain networks influencing cognitive, affective, and motor behaviors Leisman et al.
Although the literature on fiber tracts is limited in ADHD, Makris and colleagues note that gray matter abnormalities suggest that white matter connections may be altered selectively in neural systems. A prior study, Ashtari et al. In this study of adults the authors hypothesized that fiber pathways subserving attention and executive functions would be altered. Relative to controls, the fractional anisotropy FA values were significantly smaller in both regions of interest in the right hemisphere, in contrast to a control region the fornix , indicating an alteration of anatomical connections within the attention and EF cerebral systems in adults with childhood ADHD.
Researchers at Stanford University have observed that in children with ADHD, also known as childhood hyperkinetic disorder Wing and Attwood, frontal-subcortical connections are disrupted by subcortical dysfunction showing decreased glucose consumption in frontal cortex, and decrease nigrostriatal D2 receptor uptake ratios The Stanford study used functional MRI to image the brains of boys between the ages of 8 and 13 while playing a mental game.
Ten of the boys were diagnosed with ADHD and six were considered normal. When the boys were tested there appeared to be a clear difference in the activity of the basal ganglia with the boys with ADHD having less activity in that area than the control subjects. After administering methylphenidate, the participants were scanned again and it was found that boys with ADHD had increased activity in the basal ganglia whereas the normal boys had decreased activity in the basal ganglia.
Interestingly, the drug improved the performance of both groups to the same extent. This may be a similar finding as the PET scans on patients with hyperactivity disorder, where normal appearing frontal metabolism existed with decreased caudate and putamen metabolism Gillberg and Gillberg, Methylphenidate, a dopamine reuptake inhibitor, may increase function in a previously dysfunctional basal ganglia whereas raising dopamine levels in normal individuals would most likely result in decreased activity of the basal ganglia to prevent overproduction of dopamine.
The previously dysfunctional basal ganglia would have most likely resulted in decreased frontal metabolism with increased thalamo-cortical firing; this would result in decreased cognitive function with increased hyperkinetic hyperactive behavior.
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Increasing dopamine levels may increase frontal metabolism due to increased activity of the striatum with decreased firing of the globus pallidus thereby inhibiting thalamo-cortical firing decreases which in turn decreases hyperkinetic behavior. This would make sense based on the findings of fMRI before and after, and the fact that both groups showed equal improvement in performance. Etiological theories suggest a deficit in cortico-striatal circuits, particularly those components modulated by dopamine and therefore discussed in comparison with the other basal ganglia related disorders in the paper.
Teicher et al. Daily treatment with methylphenidate significantly changed the T2 relaxation times in the putamen of children with ADHD. There was a similar but non-significant trend in the right caudate. Converging evidence implies the involvement of dopaminergic fronto-striatal circuitry in ADHD.
Anatomical imaging studies using MRI have demonstrated subtle reductions in volume in regions of the basal ganglia and prefrontal cortex Castellanos et al. Cognitive functioning is mildly impaired in this disorder Seymour et al.
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In particular, cognitive control, the ability to inhibit inappropriate thoughts and actions, is also affected and therefore we are again dealing with a disorder of inhibition. Several studies have shown that this impairment is related to the reduction in volume in fronto-striatal regions Sergeant et al. Durston et al. Volumetric abnormalities have also been associated with the basal ganglia and in turn with ADHD.
Qiu et al. The basal ganglia caudate, putamen, globus pallidus were manually delineated on magnetic resonance imaging from typically developing children and children with ADHD. LDDMM mappings from 35 typically developing children were used to generate basal ganglia templates. These investigators found that boys with ADHD showed significantly smaller basal ganglia volumes compared with typically developing boys, and LDDMM revealed the groups remarkably differed in basal ganglia shapes. Volume compression was seen bilaterally in the caudate head and body and anterior putamen as well as in the left anterior globus pallidus and right ventral putamen.
Volume expansion was most pronounced in the posterior putamen.