Different values in amplitude, frequency, and various locations on the primary motor cortex all induced SI tics identical to the SO tics. During the NP, stimulation at different locations led to movement of limbs corresponding to the somatotopic location of the stimulation site. Yet, during the TP, stimulation at different locations did not change the location of the SI tic. Tic occurrence in the limb was only constrained to the corresponding location of the bicuculline injection. During TP, manipulations of amplitude above and below threshold elicited similar SI tics. This was also true for variations between high and low frequencies (Israelashvili et al, 2015). SI tics are stereotypic and distinct actions that are induced by cortical activity, but the type of input necessary to induce tics can occur in myriad forms.
However, the timing of tics are affected by the number of stimulation pulses and the amplitude of those pulses. The time between the stimulation burst and tic initiation exponentially decreased as amplitude of each pulse increased. Stimulation significantly above the amplitude threshold led to rapid onset of tics, implying that generation of a tic is additive in terms of cortical input summation. Furthermore, occurrence of tics that occur before stimulation affected the probability of onset of an SI tic and its latent period. A tic that occurred immediately before stimulation inhibited the formation of an SI tic, resulting in an absolute tic refractory (ATR) period, in that no tic could be generated for a certain period of time. A tic that occurred 250-750
ms before the ATR resulted in a relative tic refractory (RTR) period, in that there was a decreased probability of tic generation (Israelashvili et al, 2015). A greater number of stimulation pulses were required to generate another tic. The probability of generating another tic increased sigmoidally as the time difference between the stimulation pulse and the occurrence of the previous tic increased. Factors of cortical input to the striatum and the timing of tics can lead to development of a tic generation model and a greater understanding of the neuronal mechanism of TS in terms of where and when it occurs.