Tonic skin conductance levels can never be negative. Possible reasons for observing negative values (negative conductance) for an EDA (GSR/skin conductance) recording and methods for resolving the issue:
Calibration is incorrect
If using the EDA100C or GSR100C, make sure to follow the calibration procedures outlined in Application Note 187. Note that any version of AcqKnowledge later than 4.0 allows calibration to be performed automatically. If the template file or graph window had channels set up using the “Add New Module” button, then the software walked through steps that should have correctly calibrated the module. For instructions on the proper setup of channels, please see Setting Up Channels in AcqKnowledge.
The recording is in AC mode
While tonic skin conductance can never be less than zero, phasic skin conductance can. This is normal behavior and it is a function of the filter response. A negative phasic skin conductance means that the tonic skin conductance is decreasing. Most of the automated analysis routines and much of the science of skin conductance requires a measure of tonic skin conductance. To measure this signal, both of the HP filters at the bottom of the front face of the amplifier should have their switches set to the “DC” position for GSR100C or EDA100C. Similarly, if using a PPGED-R (BioNomadix PPG and EDA amplifiers), dip switch 6 on the receiver should be in the down position (as set at the factory). See the MP Systems Hardware Guide for information about BioNomadix receiver dip switches.
Interference from another channel
Make sure no other amplifier or external input is set on the same channel. In the case of an EDA100C or GSR100C, the channel is specified by the position of the white switch on top of the module. Verify that the position of this switch is unique (no other modules with that switch in that position are connected to the MP system), that nothing is connected into the corresponding channel on the UIM100C or HLT100C (if there is one), and similarly that no other module (e.g., OXY100C) that can use multiple channels is trying to use the channel that the EDA amplifier is using. To see how to set up channels and ensure that no two modules are configured to use the same channel, see Setting Up Channels in AcqKnowledge.
Interaction with bioimpedance measurements
If recording with NICO100C or EBI100C, some considerations must be made with respect to proper connections. See “Multiple amplifiers per subject or multiple subjects per system” for additional information.