## 4 Discussion  
For the first time, accurate recording of both c and oVEMP responses during simultaneous monopolar DBS was possible due to a combination of the \(\upmu\)VEMP device parameters and a customised signal filtering algorithm. Over 160,000 people have undergone DBS surgery worldwide for both neurological and non-neurological health conditions [17]. As well as being an accepted adjunct therapy for PD, DBS is also approved by the US FDA for the neuro-modulation treatment of essential tremor, dystonia, epilepsy and obsessive-compulsive disorder. In addition, DBS is being investigated as a potential treatment for multiple other conditions, including addiction, major depression, chronic pain, Tourette's syndrome, and cluster headache [18]. As dizziness and imbalance are common presentations in the general population, it is therefore likely that there are significant numbers of people with monopolar DBS attending neuro-otological clinics for vestibular assessment who would not be able to undergo dynamic otolithic functional assessment with VEMPs due to the artifacts produced by their implanted device. The current study demonstrates that the portable \(\upmu\)VEMP device, which connects to a laptop, tablet, or smartphone and which has a large dynamic input range and software for DBS noise filtering, can accurately record the presence of both oVEMPs and cVEMPs during continuous monopolar DBS. This allows VEMP assessments to be performed during the participants' usual care in the community and without the need for a supervising neurologist to alter the parameters of the DBS device. The bilaterally absent ACS cVEMP responses in two subjects and bilaterally absent ACS oVEMP responses in three subjects were not unexpected. Figure 5: Validation part 2. oVEMP responses from one participant (P5) with DBS ON with filtering vs. DBS OFF. The graphs show an averaged 100 ms epoch starting 20 ms prior to stimulus at 0 to 80 ms post-stimulus. \(X\) axis is in ms. \(Y\) axis is the reflex amplitude in \(\upmu\)V. The orange line shows a trace with DBS ON post filtering. The blue line shows a trace with DBS OFF. Coloured arrows indicate first positive and negative vestibular reflex peaks. Note the similarities between the two curves. DBS—deep brain stimulation, oVEMP—ocular vestibular evoked myogenic potential. The mean age of the four male participants was 67.5 \(\pm\) 4.93 years, and there are known age-related effects on both oVEMP and cVEMPs with ACS in the healthy population. These include increased oVEMP and cVEMP thresholds [19]; peak-to-peak amplitude reduction; and response rate decline. cVEMP and oVEMP ACS response abnormalities have also been reported in PD [3, 20, 21, 22]. In the healthy population, tap-evoked responses at Fz have been reported to be less affected by age both for cVEMPs [19, 23] and oVEMPs. This is also reflected in these results, with 7 out of the 10 ears stimulated having a present hammer oVEMP response and 9 out of 10 ears having present hammer cVEMP responses. Sensorineural hearing loss does not have an impact on VEMPs' presence [4], though a limitation of this study was that conductive hearing loss was not specifically screened; however, the use of tap-evoked VEMPs mitigates this potential impact on our findings. During part one of the validation process, we introduced DBS noise to a non-DBS VEMP response and applied filtering. A smoothing effect of the curve can be shown in the graph due to low-frequency filtering (100 Hz). This filtering is used to remove the artifacts from the DBS, in our case at around 190 Hz. A smooth curve might lose details in the waveform, but the main peaks and troughs used for the metrics are intact, and so the diagnosis remains unaffected. During part two of the validation process, the absent responses to left ear ACS oVEMP and cVEMP testing in P5 were expected and secondary to the patient's previous left ear surgery; where such procedures are known to impair ACS VEMP response [24].