The human brain is a bilaterally organized structure into two hemispheres, where each hemispheres performs different yet similar functions. Even though each hemisphere operates in its own way, coordination is necessary to ensure smooth functioning in activities such as movement, perception and thought.1 The corpus callosum, which is the largest bundle of white matter in the brain, facilitates communication between the two hemispheres.2. Without such connectivity, each hemisphere functions independently, causing chaos and confusion in behavior3 It enables the transfer of sensory, motor and cognitive information from left to right hemispheres and vice versa. It has about 190 to 250 million myelinated axons, which are like tiny wires that help send messages between the two halves of the brain.2 This structure isn’t all the same – it’s made up of different parts that each do specific jobs. These parts are called the rostrum, genu, body, isthmus, and splenium. The genu connects the prefrontal cortices of both hemispheres, supporting executive functions such as planning and decision making, while the body connects motor and premotor areas supporting coordinated movement of both hands. The posterior part, the splenium connects areas responsible for processing touch, vision and spatial awareness 4. This way of being organised helps the brain share information quickly between the two sides, which is important for tasks that require both hemispheres to work together.1The corpus callosum is slow to mature, taking decades to develop from birth into early adulthood. Its development is rapid from 3 to 10, corresponding to the period when there is an improvement in children’s fine motor skills, bilateral hand coordination, and sensory-motor coordination. The interference with the development at this crucial stage may result in difficulties in hand coordination, motor coordination and attention focusing5 One of the most consistent results found in developmental neuroscience is a specific time period from 6 to 12 years that plays a key role in interhemispheric connectivity. Many behavioural studies have confirmed this trend 6 At beginning of this time period, the brain function in this matter is comparable to patients lacking the corpus callosum, while at the end it becomes similar to the performance of adults.7 Chicoine have demonstrated that children aged 6-7 years could not transfer (visuomotor) skills that they learned by one hand to another, a failure that parallels the behaviour of individuals with complete agenesis of Corpus callosum.8&9 However, children aged 11 -12 were able to transfer the skills between the hands effectively; this phenomenon was typical for healthy adults. Similar trends have been observed in bimanual coordination that develops substantially by the end of 1st decade of life 10,11 & 12 and binocular advantage that reaches adult level at the age of 10-11 years 6. Mirror movements-involuntary movement spillover from one hand to the other, an indication of immature callosal inhibition is common in young children but show significant reduction within the same time span 13 14. Functional importance of hemispheric connection is to serve two major purposes; enabling the exchange of information in a way that gets the brain excited and controlling parts of the brain to stop them from working too much.1The connections that help controls the brain, called inhibitory projections that stop one side from doing too much and help the brain divide up task so it can work efficiently whereas the connections that help in sharing information called as excitatory callosal projections which allows brain to share what it sees, hears and moves with the other side 15. This helps the brain work together smoothly and do things in coordinated way15. In school children, connectivity between the left and right sides of the brain an important role in developing abilities which impact their education and overall life.16 These abilities include reading and comprehension-which requires both hemispheres since the left hemisphere is responsible for language (Broca’s and Wernicke’s areas) while the right one works with tone, metaphor and stories17. Bimanual coordination and fine motor skills are required for performing actions like handwriting, drawing, playing musical instruments and sports18Attention and executive functioning refer to abilities including concentration, switching between tasks and impulse control, which are based on the interaction between the brain hemispheres21. Visuospatial processing involves cognitive operations such as mental rotation and navigation which are dependent on interactions between the right and left-brain parts22. Impaired communication between the hemispheres of the brain has been found to be associated with many frequent neurodevelopmental disorders in school aged children such as ADHD, ASD, SLD and Epilepsy 22, 24, 24. Hence, it’s important to understand the connection between the hemispheres of brain both for medical and educational purposes18. A longitudinal study was performed on the development of structural and functional properties of the corpus callosum in children aged 6-8 years26. They used a specific method to look at the shape-based analysis of the mid-sagittal corpus callosum alongside a dichotic consonant-vowel syllable discrimination task as a measure of interhemispheric information transfer 26, 27, these authors found that increases in isthmus thickness were paradoxically associated with decreases in transfer efficiency while decreases in isthmus thickness corresponded to improved transfer26. This result was interrupted by authors as an example of a developmental process of synaptic pruning. Synaptic pruning is responsible for increasing the efficiency and speed of interhemispheric communication of the brain hemispheres.28 29 The human brain exhibits functional hemispheric specialisation, with the left hemisphere being dominant for language in most individuals 30. Handedness represents one of the simplest and the most extensively researched approaches to investigate the differences in the functionality of the two brain hemispheres in terms of motor activity 31. The Edinburgh handedness inventory introduced by Oldfield in 1971 is considered to be the primary instrument for measuring the preference to use either hand while performing different routine actions 32. This test generates the Laterality Quotient (LQ) index, which allows classification of people as right-handed, left-handed and ambidextrous. Handedness is also related to the dominance of the certain hemispheres in performing the language functions 33. While the language processing is located in left brain hemispheres in most cases among the right-handed population, left-handed people may show more variability in this aspect 33. Thus, it is critical to know about the person’s handedness before carrying out any investigation on the inter-hemispheric coordination in order to take into account the individual peculiarities of brain functioning and its lateralisation27 Assessing Interhemispheric communication via behavioural paradigms: In contrast to neuroimaging, behavioural assessment does not require special equipment, cost less involves no harm to the individual and is closer to real life conditions than neuroimaging does35.it allows measuring interhemispheric communication in children35, 36. Many of such tests were successfully used on children. “The Poffenberger test”, introduced in 1912 requires participants to respond with either left or right hand to visual stimuli presented unilaterally to either the left or right hand to visual field. When someone sees something in their left vision, the brain information usually goes to the right side of the brain. If they use right hand to respond, that means the brain had to send the signal across the corpus callosum. In case if they use left hand then it means that no crossing was necessary37 38. The difference in reaction time for two types of reactions is known as crossed-uncrossed difference (CUD) and serves as an indicator of how fast and efficient is visual and motor brain hemispheres interaction 37. For adults, the average CUD is about X milliseconds. For children, these values decrease over time which proves their increasing ability to communicate via brain hemispheres39. The CUD correlates with the anatomy of corpus callosum as shown in DTI studies40 In this literature three complementary behavioural paradigms using non-invasive measure of interhemispheric function have been identified as valid 41 Tactile inter-manual transfer: Tactile inter-manual transfer is the ability to identify or replicate an object that was explored tactually by another hand without visual guidance. This process occurs because touch perception is processed in the contralateral hemisphere of the brain42. For inter-manual transfer to be achieved, tactile information should be transmitted between hemispheres via the corpus callosum43. This activity reveals the efficiency of interhemispheric connection of the corpus callosum in relation to tactile function. The most frequently used test to assess this function is the fingertip cross-lateralisation test during this test, a finger of one hand is stimulated and a child has to indicate the corresponding finger on another hand. The accuracy obtained in the inter-manual transfer condition relative to the intra-manual comparison reveals the efficiency of the information transfer through the corpus callosum44. As the children grow older, the efficiency of performance increases, which corresponds to the maturation of myelin of the posterior portion of the corpus callosum45. The children with various disorders such as corpus callosum agenesis, brain injury, cerebral palsy and down syndrome fail this task.46 Bimanual tapping; this task includes the use of both hands for tapping simultaneously or alternately47. To ensure coordination in the timing of movements produced by the left and right hands, the midbody of the corpus callosum that connects the two major motor areas and the supplementary motor areas is used47,48. The main method for assessing this coordination is through the analysis of inter-tap asynchrony (ITA) that is difference in the duration between the matching taps of both hands47. The lower the ITA, the greater the synchronicity of work performed by both hands. Synchronous tapping is easier because it is performed simultaneously by both hands. But alternating tapping requires additional coordination between the hemispheres of the brain and clearly demonstrates of the brain and clearly demonstrates the immaturity of connections between them. At the age 6 to 7years, children have a high level of ITA and frequent errors in the timing of tapping. At the age of 10- 12 years, their tapping performance approaches that of adults49. Using DTI studies, researchers discovered that high levels of fractional anisotropy within the midbody of the corpus callosum are associated with superior tapping coordination50. Among those who suffer from ADHD, high ITA is evident, just like the results demonstrating underdevelopment of the corpus callosum in children with ADHD51. Bimanual coordination: refers to the process whereby both hands work simultaneously in a well- timed manner52. This function is supported by brain structures such as the corpus callosum including its midbody and splenium, SMA and temporal circuits in the cerebellum52,52. The two functions of the corpus callosum are to transmit time signals from one hemisphere of the brain to another (excitatory) and prevent one hand from working as the other is performing an unrelated activity(inhibitory)53. In clinical practise, clinicians assess bimanual coordination through tests such as the Purdue pegboard bimanual assembly, box and block test and kinematic drawer- opening tasks54. If both hands perform the same actions, such actions appear earlier in development and do not require assistance from corpus callosum. However, when each hand performs different actions, the process requires an active contribution from the corpus callosum and appears later in the school years55. The mirror movements of one hand without intention are common up until age 7 or 8; however, the continuation of these movements indicates the malfunction of the corpus callosum, which prevents unwanted movements56. DTI research in children suffering from CP proves that the condition of splenium of the corpus callosum is correlated with the skills of bimanual coordination. It means that the brain structure affects this skill significantly57. Bimanual coordination disorders are also one of the symptoms of the Developmental Coordination Disorder (DCD); it is checked as a part of the MABC-2 tests58 Despite the increasing number of studies on the role of interhemispheric connection in child development, there are very few comprehensive behavioural assessments using multiple corpus callosum-based tasks and normative data particularly for Indian school-aged children remain limited58. Therefore, this cross-sectional observational study aimed to profile interhemispheric communication in typically developing children aged 6-12years using the tactile inter-manual transfer, bimanual tapping and bimanual coordination tasks and to establish preliminary criteria that could be used for assessing hemispheric interaction related problems by physiotherapists58 GIVE ME REFERNCES PROPERLY DONOT REPEAT THE REFERNCESFAME

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The human brain is a bilaterally organized structure into two hemispheres, where each hemispheres performs different yet similar functions. Even though each hemisphere operates in its own way, coordination is necessary to ensure smooth functioning in activities such as movement, perception and thought.1 The corpus callosum, which is the largest bundle of white matter in the brain, facilitates communication between the two hemispheres.2. Without such connectivity, each hemisphere functions independently, causing chaos and confusion in behavior3 It enables the transfer of sensory, motor and cognitive information from left to right hemispheres and vice versa. It has about 190 to 250 million myelinated axons, which are like tiny wires that help send messages between the two halves of the brain.2 This structure isn’t all the same – it’s made up of different parts that each do specific jobs. These parts are called the rostrum, genu, body, isthmus, and splenium. The genu connects the prefrontal cortices of both hemispheres, supporting executive functions such as planning and decision making, while the body connects motor and premotor areas supporting coordinated movement of both hands. The posterior part, the splenium connects areas responsible for processing touch, vision and spatial awareness 4. This way of being organised helps the brain share information quickly between the two sides, which is important for tasks that require both hemispheres to work together.1The corpus callosum is slow to mature, taking decades to develop from birth into early adulthood. Its development is rapid from 3 to 10, corresponding to the period when there is an improvement in children’s fine motor skills, bilateral hand coordination, and sensory-motor coordination. The interference with the development at this crucial stage may result in difficulties in hand coordination, motor coordination and attention focusing5 FOR THIS I VE ADDED 1. Gazzaniga MS. 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