Figure 5.

Frequency-dependent responses (short-term plasticity) of the average, first and last fEP during stimulation trains at SNr recording sites. (A) Sample raw traces from the same patient showing the first three and last three post-stimulus fEPs induced during a 30 Hz stimulation train and the first and last five fEPs induced during a 50 Hz stimulation train. The dashed line represents the ‘baseline’ fEP amplitude during the 1 Hz stimulation train, which remained constant. (B) Frequency-dependent responses in first, last, and average fEP amplitudes during stimulation trains. As the frequency of stimulation was increased, there was a rapid attenuation of the amplitude of the average fEP. However, the first-fEP within each train became potentiated as the stimulation frequency increased. The inhibitory potentiation (facilitation) effect was induced by the stimulus delivered by the previous stimulation train. As the facilitation increased, the synaptic depression effect correspondingly increased as well, demonstrated conjointly by the attenuation of average and last-fEP amplitudes. These phenomena are believed to be, in part, modulated by depletion of releasable stores of GABA within presynaptic terminals. *P < 0.05, **P < 0.01, ^†P < 0.001.