Basal ganglia are sub-cortical ganglia, which are involved not only in the control of the voluntary movement, but also in cognitive processes such as decision making. They have projections with thalamus and cerebral cortex. In recent years, deep brain stimulation -which is being used to reduce the effects of some psychiatric disorders, especially Parkinson disease- is regulating the damage related to cortex, basal ganglia, thalamus loop. In order to improve the regulation, mathematical models are used. In this study, this loop has been modelled as a mass model, in a way that it is convenient for neuropsychiatrical test applications. Bifurcation analyses for different parameters and for input current have been done. These analyses explain the dynamical behavior of the cortex-basal ganglia-thalamus loop.