In this study by J. Brinkman and H.G.J.M. Kuypers, the neuronal connections from the cerebral cortex to the spinal chord were examined in order to determine how the descending contralateral and ipsilateral pathways supply the upper extremity muscle fibres. This experiment suggests that in rhesus monkeys, the descending contralateral brain neurons control the arm, hand and finger movements independently, while the ipsilateral brain neurons supply only arm movements. To test this hypothesis, seven rhesus monkeys had a cerebral commissurotomy procedure performed while preserving their intercollicular commissure.
Their brain was cut along the sagittal plane in order to separate the two hemispheres and prevent any connections from passing between. Motor performance of the monkeys were studied with their vision restricted to one eye, so that ocular input was secluded to a single hemisphere. The monkey’s task was to pick up a piece of food from a table with either their contralateral or ipsilateral hand to the blindfolded eye. The independent variable studied was the arm being tested (ipsilateral or contralateral), and the dependent variable was the fluidity of the arm, hand, and finger movements. At the initial assessment, the contralateral arm, hand and fingers were able to smoothly coordinate action to pick up the food effectively. The ipsilateral side had more difficulties: the arm could move towards the food easily, but the hand and fingers were not able to recruit motor neurons to independently pick up the piece until hand contact was made with the food. The tactile sensation of the food activated full motor recruitment between both cortexes, instead of the only single hemisphere, allowing the hands and fingers to pick up the food. To minimize this effect, a more precise test was derived to focus specifically on hand movements. The food pieces were slotted into a board so that they could not be felt at the surface; tactile guidance would not lead to inaccurate readings of hand and finger movement. The monkeys still had partial vision but this time required their fingers to dislodge the piece involving more accuracy and control from the distal extremities.
The results on the contralateral side were consistent with the previous test; however, the ipsilateral side was only able to move the arm to the general location but could not retrieve the particle. The experimenters introduced larger pieces of food into the grooves of the board, which could be felt at the surface. The ipsilateral hand and fingers were able to pick up the food, but demonstrated behaviors comparable to an individual who was blind (feeling around for the item). This concluded that when vision was partially blocked, the contralateral side was able to coordinate the movements of the arm, hand and fingers to grasp the object, whereas the ipsilateral side could move the arm towards the area but could not exhibit the intricate movements from the hand and fingers.
These findings were the same as those found in humans; however, other research papers that involved monkeys have varying conclusions. The limitations are that additional research is necessary to find one conclusive result among all papers, and that error in the treatment methods may be the reason for several conclusions. Brinkman and Kuypers findings pertain to the ipsilateral control only for the independent distal movements of extremities, which could be masked through tangible interaction of the object. Further, these findings may provide a viable tool in diagnosing brain damage in individuals who exhibit difficulty with fine movements in upper extremity control.