Yes, currently we can lower stimulation frequency only to 3hz with internalized DBS. We used peritriggering technique with 3Hz DBS to couple motor cortical TMS to induce plasticity (Udupa et al, 2016 JNs). As you suggested with externalized electrode we can stimulate at any parameters as required. However, newer electrodes with special programming parameters are coming out which should deliver single pulses. 

Udupa K, Bahl N, Ni Z, Gunraj C, Mazzella F, Moro E, Hodaie M, Lozano AM, Lang AE, Chen R. Cortical Plasticity Induction by Pairing Subthalamic Nucleus Deep-Brain Stimulation and Primary Motor Cortical Transcranial Magnetic Stimulation in Parkinson's Disease. J Neurosci. 2016 Jan 13;36(2):396-404. doi: 10.1523/JNEUROSCI.2499-15.2016. PMID: 26758832

thank you indeed, I am completely aware about your study Dr Udupa, actually the idea started while I was reading some of your articles (the one you have mentioned above and the other one about interaction of afferent and intracortical inhibition). Taking all these into consideration, I was thinking about the possibility of testing thalamic interference with intracortical inhibition in pain patients (sensory thalamus DBS) and the thalamocortical part of cerbellothalamocortical pathway in vim DBS (essential tremor patients). On the top of that, I think that single pulse study using DBS electrode will allow us understand the mechanism of action of DBS, by comparing single pulse to different frequency effect on cortical areas like M1, since DBS stimulation is repetitive and influences the site it is stimulating and remote site connected to the latter by plastic changes.