Stimulation-based state monitoring – new developments

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Stimulation-based state monitoring – new developmentsS. Kalitzin, D.Velis, F. Lopes da Silva

Epilepsy Institute of The Netherlands (SEIN), Medical Physics Department Epilepsy Institute of The Netherlands (SEIN), Department of Clinical Neurophysiology Swammerdam Institute of Life Sciences, University of Amsterdam, The Netherlands

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Phase clustering index (PCI)

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S. Kalitzin, J. Parra, D. Velis, F. Lopes da Silva, Enhancement of phase clustering in the EEG/MEG gamma frequency band anticipates transition to paroxysmal epileptiform activity in epileptic patients with known visual sensitivity, IEEE-TBME, v.49, 11 p 1279-1286, 2002

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Question 1: how to stimulate and measure the response ?

Decision on stimulation sequence

Biphasic Monophasic

BiCAPCyclic Alternating Pattern (CAP)

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Non-linear response to direct electric stimulation

What is a linear response ?

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How can we test this ?

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Averaged triggered response analysis

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Linear Response Model

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For linear response model: total PCI=0

CAP – polarity alternation

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BICAP: Separating PCI of the non-linear response

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Stimulation in conducting fluid (NaCl)

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Stimulation frequency

Example from patient case –MONOCAPStimulation frequency invariant response

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Application to interictal localization of the SOSCase: left TLE

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Robustness to stimulation frequency

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Question 2: where to stimulate and where to measure the response ?

(1)Averaged rPCI(2) Collective PCI

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Processing features of the collective response to stimulation in one location

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Triggered response and correlation

Collective phase clusteringand bi-local phase coherence

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Application to interictal localization of the SOSCase: (predominantly) right TLE

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Collective Impact estimations

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Speculation: going beyond locality (bilateral case)

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What about Seizure prediction ?

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Summary & Conclusions:

1. rPCI feature successfully detected the SOS in six new TLE cases. 2 left, 2 right 2 bilateral.

2. BICAP stimulation solves the problem of high PCI due to linear conductance.

3. For interictal localization tasks, the best spatial coding so far is at the stimulation site and the best response measure is the collective response.

4. Bi-local rPCI spread studies deserve attention.

5. Seizure prediction in time remains a statistical observation at best. Electrode size, recording sites and stimulation protocol might be critical. Robustness is therefore crucial.