64. The influence of the reactivity of the electrode-brain interface in therapeutic deep brain stimulation

Nada Yousif1 n.yousif@imperial.ac.uk Richard Bayford2 richard.bayford@gmail.com Xuguang Liu1,3 x.liu@ic.ac.uk

1Department of Clinical Neuroscience, Imperial College London, London, UK
2The Bio-modelling/Bio-informatics Group, Middlesex University, London, UK
3The Movement Disorders and Neurostimulation Unit, Charing Cross Hospital, London, UK

Deep brain stimulation (DBS) is a therapy for treating a number of neurological and psychiatric disorders. The treatment involves the implantation of quadripolar electrodes into condition-specific targets in the human brain and injecting electrical pulses across a depth electrode brain interface (EBI). In our previous work (Yousif et al., 2007; Yousif et al., 2008) using a static finite element method (FEM) model solving the Laplace equation, it was revealed that the resistance of the depth EBI which changes over time post-implantation, has a significant effect on the electric field induced in the surrounding neural tissue. In the present study, we extend our previous EBI model to look at the influence of frequency dependent reactivity of the EBI on the time-varying induced potential distribution, by using anapproach which combines a complex FEM model with Fourier analysis. The main finding of the present study is that, once the electric pulses pass through the peri-electrode space the pulse waveform is distorted, and this distortion is different with changing biophysical properties of the EBI over distinct post-implantation stages. In particular during the acute stage, the interface of lower reactivity behaves similarly to a low-pass filter, and distorts the fast rise and fall of the square pulse; whereas in the chronic stage the stimulating amplitude of the square pulse is reduced >30% by the interface of higher reactivity, and this amplitude slightly falls off during the plateau phase of the waveform (Figure). This indicates that the reactivity of the EBI significantly influences the stimulus waveform in the time domain. In conjunction with our previous studies, we conclude that to make accurate predictions of the stimulation-induced current distribution in the human brain using FEM structural models, both the resistive and reactive components of the EBI need to be taken into account.

Acknowledgements: This work is funded by a grant (id 78512) from the Medical Research Council, UK.

References: Yousif N, Bayford R, Bain PG, Liu X (2007) Brain Research Bulletin 74:361-368.
Yousif N, Bayford R, Wang S, Liu X (2008) Neuroscience 152:683-691.

The voltage waveform in the tissue (solid black line) is distorted from the original waveform (dashed grey line), but in a strikingly different way in acute and chronic post-implant stages.