In this randomised placebo-controlled trial, the investigators will include 30 PD (Parkinson's disease) patients with HY (Hoehn Yahr stage) >2 and L-dopa unresponsive gait characteristics. Each participant will receive taVNS at 25Hz, taVNS at 100Hz and sham VNS (sVNS). During each stimulation, different gait characteristics will be measured with wearable insertion motion sensors.

Participants and study overview:

In this randomised placebo-controlled trial, the investigators will include 30 PD patients with HY>2 and L-dopa unresponsive gait characteristics. Each participant will complete 1 visit, during which the instrumented stand and walk test will be repeated 16 times. The baseline measurement will be followed by three different stimulation conditions sham VNS (sVNS), taVNS at 25Hz (taVNS25) and taVNS a 100Hz (taVNS100). sVNS, taVNS25 and taVNS100 in a randomized order. To enhance gait disturbances, the participants will perform iSAW during each condition twice in silence and twice while simultaneously performing calculations around.

Instrumented stand and walk test (iSAW):

To perform iSAW, participants will first stand still for 30s, after which they will start walking in a straight line for 7m, turn and walk back to the start. The participants will be equipped with 6 inertion Opal ® motion sensors (on both feet, both wrists, on the lumbar back and on their sternum). For the laud counting iSAW condition (iSAW-C), the participants will perform iSAW as described above while also counting backwards by 3 from numbers above 100.

TaVNS:

Noninvasive electrostimulation will be applied to the left ear. taVNS will be applied to the cyma conchae and sVNS will be applied to the earlobe. Both taVNS and sVNS were applied through the Nemos® electrode with the following parameters: square-shaped pseudobiphasic pulse, interpulse duration 80μs, pulse width 300μs, pulse intensity 0,1mA above the perceptual threshold (figure 1). taVNS25 and sVNS will be applied at 25Hz and taVNS100 will be applied at 100Hz. The intensity for each separate stimulation will be adjusted for each participant at the beginning of the experiment. For each condition, the electrode will be placed in the ear by a third person and the ear will be covered by cotton pads and an EEG cap to hide the electrode position from the experimenter. The order of stimulation will not be revealed until the automated data and statistical analysis.

Inclusion Criteria:

• HYstage >2
• L-dopa unresponsive gait characteristics present, preferably with history of freezings

Exclusion Criteria:

• unable to walk for 50m
• unable to follow simple commands due to hearing loss/cognitive impairment

The investigators plan to share the anonymised data with researchers upon request.

Upon request after study publication.

Data will be shared with researchers upon reasonable request.

Noninvasive electrostimulation will be applied to the left ear. taVNS will be applied to the cyma conchae and sVNS will be applied to the earlobe - participants will not be told which is the active and which is the sham condition. Both taVNS and sVNS were applied through the Nemos® electrode. The intensity for each separate stimulation will be adjusted for each participant at the beginning of the experiment. For each condition, the electrode will be placed in the ear by a third person and the ear will be covered by cotton pads and an EEG cap to hide the electrode position from the experimenter. The order of stimulation will not be revealed until the automated data and statistical analysis.