Testing a linear time invariant model for skin conductance responses by intraneural recording and stimulation.

Samuel Gerster,Barbara Namer,Dominik R Bach,Mikael Elam

doi:10.1111/psyp.12986

Abstract

Skin conductance responses (SCR) are increasingly analyzed with model‐based approaches that assume a linear and time‐invariant (LTI) mapping from sudomotor nerve (SN) activity to observed SCR. These LTI assumptions have previously been validated indirectly, by quantifying how much variance in SCR elicited by sensory stimulation is explained under an LTI model. This approach, however, collapses sources of variability in the nervous and effector organ systems. Here, we directly focus on the SN/SCR mapping by harnessing two invasive methods. In an intraneural recording experiment, we simultaneously track SN activity and SCR. This allows assessing the SN/SCR relationship but possibly suffers from interfering activity of non‐SN sympathetic fibers. In an intraneural stimulation experiment under regional anesthesia, such influences are removed. In this stimulation experiment, about 95% of SCR variance is explained under LTI assumptions when stimulation frequency is below 0.6 Hz. At higher frequencies, nonlinearities occur. In the intraneural recording experiment, explained SCR variance is lower, possibly indicating interference from non‐SN fibers, but higher than in our previous indirect tests. We conclude that LTI systems may not only be a useful approximation but in fact a rather accurate description of biophysical reality in the SN/SCR system, under conditions of low baseline activity and sporadic external stimuli. Intraneural stimulation under regional anesthesia is the most sensitive method to address this question.

Highlights

Model-based Skin conductance responses (SCR) analysis rests on assumptions about the effector organ system that describe how sudomotor action potential bursts generate measured SCR via acetylcholine release from nerve terminals, transmitter diffusion, and processes in the sweat glands
We capitalize on intraneural recordings, and intraneural stimulation under regional anesthesia, to conduct such a formal test of the linear and time-invariant (LTI) model
In the sudomotor nerve (SN) stimulation Experiment 2, we find strong evidence for the LTI model at low rate of SN burst succession

Summary

| INTRODUCTION

(Bach & Friston, 2013). Model-based analysis makes such implicit models transparent and explicit in mathematical form. In the past, the validity of linear models for SN/SCR relationship has sometimes been questioned, including even the informal and loose models used in operational analysis This criticism was mainly on the observation that the relation between SN and SCR amplitude can be variable (Bini, Hagbarth, Hynninen, & Wallin, 1980), and that repeated SN stimulation can lead to SCR with different shapes (Kirno, Kunimoto, Lundin, Elam, & Wallin, 1991; Kunimoto, Kirno, Elam, Karlsson, & Wallin, 1992a, 1992b; Kunimoto, Kirno, Elam, & Wallin, 1991). We have measured the linearity and time invariance of SCR to brief sensory events (Bach, Flandin et al, 2010) This approach cannot distinguish LTI violations in the effector organ system and deviations from the assumptions about the neural system, and provides only an upper bound on effector organ LTI violations. We combined the intraneural recording and intraneural stimulation approaches to quantitatively assess the variance in SCR that is explained in an LTI model of the SN/SCR relationship

| Participants and design

| RESULTS

Findings

| DISCUSSION