How to analyse evoked responses, where the peak is negative?

LabChart's  Peak Analysis Module has different presets to analyse your data:
  • General - Unstimulated
  • Evoked Response
  • Population Spike
  • Synaptic
  • Action Potential
  • Cardiac Action Potential
The Detection settings for Evoked Responses, Population Spikes, Action Potential and Cardiac Action Potentials expect a positive response or positive peak. On occasion, especially in evoked responses you might see a negative response.
To overcome this limitation there are different ways to approach the problem. The most suitable solution depends on the actual signal waveform and the parameters you like to extract.

A) Invert the signal
Use the Channel Calculation Arithmetic -1*Ch1, where Ch1 is the recording of the evoked response. This can be done in the same channel or in a separate channel. This calculation inverts the signal so the response is now positive.

B) Remove the stimulus artifact and use the "General - Unstimulated" preset
Stimuls artifacts have a short duration, so using the Channel Calculation Smoothing (Median Filter; window width 5 Samples) will eliminate the artifact (NB the recorded data are not manipulated; the calculation can be undone by selecting No Calculation from the Channel Calculation menu). In Peak Analysis Settings choose the Analysis Type General - Unstimulated. For the Peak Detection Settings use the preset General - Sine shape or General - Spikey shape. Go to Customize... and choose under Detection Parameters Trigger: Minimum. You might have to tweak the detection further by changing the Normalization settings.
Within the Welcome Center under the Getting Started Tab > Peak Analysis you'll find an example file for unstimulated negative peaks.

C) Use a Macro
If you are interested in certain peak parameters, which can be extracted from the Data Pad, it might be easier to use a Macro to find events, make appropriate selections and add to the Data Pad. A simple pseudo code could look like this:

Go to Start of File
Find all
Find Stimulus Artifact
Select x ms before the artifact (baseline) and add to Data Pad
Move past the artifact and select to local Minimum (peak height, time to peak) and add to data Pad

  Please feel free to contact your local ADInstruments support, if you need any help.