Cortical control of posture
Postural control refers to the maintenance of body posture in space. Central nervous system interprets the sensory inputs to produce motor output to maintain upright posture. Definition. Postural control is defined as achievement, maintenance or regulation of balance during any static posture or dynamic activity for the regulation of stability and orientation. The interaction of the individual with the task and the environment develops postural control. Stability refers to maintenance of the centre of mass within the base of support while orientation refers to maintenance of relationship within the body segments and between body and the  environment for the task. These stability and orientation challenges necessitate change in the task and environment, thereby making postural control the most essential pre requisite for most of the tasks. There are two types of postural control strategies: predictive and reactive, which utilize the feed forward and feedback postural control respectively in order to maintain stability during various circumstances. Feed forward postural control refers to the postural adjustments made in response to the anticipation of a voluntary or a self-generated movement that may be destabilizing in while feedback postural control refer to the postural adjustments made in reaction to  sensory stimuli from the externally generated perturbation. Furthermore, these strategies may involve either a fixed-support or a change-in-support response depending on the intensity of the perturbation. Systems involved in posture control. Postural control involves a complex interaction of multiple systems in order to maintain stability and orientation. Multi-components of the conceptual model of postural control include: The functional task and the environment define the precise organization of the postural systems. Background. Traditionally postural control was regarded an automatic response to sensory stimuli generated by subcortical structures such as the brainstem and spinal circuits. Since postural responses  are generated quickly, without voluntary intent and with less variability than cued, voluntary movements, cerebral cortex was not considered to be involved in postural control. Hence cortical contribution to postural regulation is controversial and highly debatable. Furthermore, it was assumed that since postural control is automatic in nature, it utilized only limited attentional resources. However, current evolving evidence from numerous neurophysiological, behavioural and neuroimaging studies (as given below) suggest cortical involvement in postural control and maintenance of balance. Studies also indicate that postural control involved significant attentional resources and does require cognitive and sensory processes depending on other influencing factors . Supportive evidence. Behavioral studies. Based on the central capacity theory, behavioral studies using dual-task paradigm are instrumental in evaluating the demands of attention for various postural tasks. According to the central capacity sharing theory, there are limited attentional resources and performance of any task demands utilization of some proportion of these resources. In this model, there is simultaneous processing of multiple tasks and sharing of resources or processing capacity which results in interference leading to slower processing of either of the tasks. Dual task which involves performance of primary postural task (maintenance of a specific static or dynamic posture ) and a secondary task (focal task involving use of the working memory) concurrently. This can lead to two possible outcomes: allocation of attentional resources to focal task during stance leading to impaired postural stability or allocation of attentional resources to maintain postural stability leading to poor performance on the focal task. Thus, such studies suggested the attentional demands of postural tasks. Kerr et al. demonstrated recall on a spatial memory task was affected negatively (increased errors) during concurrent standing balance task due to sharing of common neural resources for postural stability and cognitive processing. Furthermore, it was also found that  attentional cost increased as the challenge to maintain balance on equilibrium tasks increased. This was demonstrated by increased reaction time to auditory stimulus when subject performed standing and walking rather than sitting. It has been found that postural sway increased on performance of a cognitive task concurrently with a postural task. Dual task training improved dual task performance. Neurophysiological studies. An initial postural reaction on exposure to an external perturbations was shown to be generated by the brainstem and spinal cord in animal and human studies (short latency mono or polysynaptic spinal loop 40-65ms) followed by the later part  of the reaction which is modified by direct transcortical loops (long latency loops, ~132ms). Cerebral cortex via cerebellum which helps in adapting by using prior experience or via basal ganglia which helps generating a response based on the current context, modifies the postural response. Neuroimaging studies. Various functional neuroimaging techniques such as Functional near-infrared spectroscopy, Functional magnetic resonance imaging, Positron emission tomography have been used to elucidate cortical control in static and dynamic postures. Using PET, Ouchi Y et al. 1999 evaluated mechanisms involved in bipedal standing and confirmed the pivotal contribution of cerebellar vermis in maintenance of standing  posture and further suggested involvement of the visual association cortex in controlling postural equilibrium while standing. Mauloin et al. 2003 using PET studied motor imagery of locomotion under four conditions and confirmed supraspinal control in locomotion by demonstrating activation in the dorsal premotor cortex and precuneus bilaterally, the left dorsolateral prefrontal cortex, the left inferior parietal lobule, and the right posterior cingulate cortex. There was increased engagement of higher cortical structures noted with increase in demands of locomotor tasks. Using FMRI, Jahn et al. 2004 studied the activation pattern with three imagined conditions and found that standing was associated with  activation of the thalamus, basal ganglia, and cerebellar vermis. Using FNIRS, Mihara M et al. 2008 studied activation related to external perturbation and suggested prefrontal cortex to be involved in adequate allocation of visuospatial attention. Zwergal A et al. 2012 studied role of aging on activation pattern in standing and found more activation in bilateral insula, superior and middle temporal gyrus, inferior frontal gyrus, middle occipital gyrus and postcentral gyrus suggesting decreased reciprocal inhibition of these areas. Factors affecting cortical control. The attentional demand to postural control is modulated by injury, pathology, aging, complexity of postural task and availability of  sensory information. Previous studies by Fearing studied postural sway with a simultaneous focal task in a quiet upright stance and found that reduced stability with age may increase attentional resources to balance which were shared during focal task performance leading to reduced postural stability. With aging there is reduced multisensory input required for postural control leading to increased fall risk and injuries. Slower processing ability and reduced attention capacity contribute to disturbance in postural control and further more, in presence of a focal task during maintaining an erect posture, there is reduced performance on the focal task due to structural  interference. Shumway Cook et al. studied the performance of two tasks (on a sentence completion task and a judgement-of-line-orientation task) during stance in young and elderly subjects with and without a fall history. It was found that there was a greater increase in center of pressure (COP) excursion for the elderly adults with a history of falls as compared to elderly adults without a history of falls.