The basal ganglia (BG) are part of a basic feedback circuit, regulating cortical function, such as voluntary movement control, via their influence on thalamocortical projections. BG disorders, namely Parkinson's disease (PD), characterized by the loss of neurons in the substantia nigra (SN), involve the progressive loss of motor functions. The process that leads to these neural alterations is still unknown. At the present, PD cannot be cured, but an early diagnosis (ED) could allow to better manage its symptoms and evolution. A branch of neuroscience research is currently investigating the possibility of using motor alterations, e.g. handwriting, caused by the disease as diagnostic signs in the early stage of the disease, expression of small entity of SN lesion. In the present work, we propose a neurocomputational model to investigate the behaviour of the simulated neural system after several degrees of lesion, with the aim of evaluating, if possible, which is the smallest lesion compromising motor learning. The performance of the network in learning a novel motor task has been analyzed, in physiological and pathological conditions. The proposed neural network proves that there may exist abnormalities of motor learning process, due to alterations in the BG, which do not yet involve the presence of symptoms typical of the confirmed diagnosis, since the network shows having some difficulties in motor learning already with 20% DA depletion.