Evoked Potentials

Overview:

An evoked potential is a change in membrane potential that is directly induced by an electrical stimulus or the presence of a drug and they are distinct from spontaneous potentials. Evoked potentials are described by their tendency to depolarize (excite) or hyperpolarize (inhibit) the membrane potential and are referred to as excitatory (EPSP) or inhibitory post-synaptic potentials (IPSP).

Evoked potentials refer to responses obtained following stimulation of central nervous system structures such as the cerebral cortex, brain stem, spinal cord and peripheral nerves.



Evoked potentials include:

• Steady-state evoked potentials – when the stimuli are in a rapid succession, the evoked potentials overlap in time until it forms a steady wave
• Motor evoked potentials (MEP) - recorded from muscles following direct stimulation of exposed motor cortex, or magnetic/electrical transcranial stimulation of motor cortex
• Sensory evoked potentials (SEP) - recorded from the central nervous system following stimulation of sense organs
  • Visual evoked potentials - generated by a flashing light or changing pattern on a monitor
  • Auditory evoked potentials – generated by click or tone stimulus presented through earphones
  • Somatosensory evoked potentials (SSEP) - generated by tactile or electrical stimulation of a sensory or mixed nerve in the periphery.

Method:

Evoked potential amplitudes tend to be low, ranging from less than a microvolt to several microvolts and therefore usually require signal averaging to resolve these low-amplitude potentials against the background of other biological signals and noise.

Evoked potentials may be recorded using PowerLab and suitable amplifiers that include intracellular amplifiers manufactured from Dagan Corporation and Warner Instruments. For further information, see the:

• Intracellular Recordings application page
• Patch & Voltage Clamp application page
• Extracellular Recordings application page
• EEG application page

Software:

LabChart
LabChart software (for Windows and Macintosh) combines the familiar simplicity of a traditional strip chart recorder with the powerful analysis features of a digital acquisition system. LabChart software and a PowerLab data acquisition unit provide data integrity, easy selection of hardware settings, powerful online and offline analysis, procedure automation, seamless extraction of experimental data and flexible display options. Acquisition and analysis capabilities can be further increased with LabChart Extensions and LabChart Modules. LabChart Modules are available as part of LabChart Pro.

LabChart software is also suitable for evoked potential recordings that do NOT require waveform averaging or if more than two recording channels are required. The triggering option within LabChart is extremely useful to synchronize recordings with a stimulus, while the analog or digital outputs on the PowerLab may be used to control a third-party stimulator. The LabChart software can be used to identify the specific waveform components such as amplitude and latency.

LabChart Extensions are free for download from the website for existing LabChart users and they include:

• Event Manager (Win Only): Allows the user to monitor user defined events online using different criteria, and to perform a variety of user defined actions.
• Export Axon (Win Only): Allows the user to save LabChart files in the ABF (Axon binary format) which can be read by pClamp.
• Evoked Response (Mac Only): This extension analyzes physiological responses to a stimulus (evoked response experiments). A number of response parameters can be measured and logged to the Data Pad. This process can be automated to analyze a series of stimuli/response cycles.
• Peak Parameters (Win or Mac): Allows the user to determine a number of parameters for an individual peak. Parameters calculated include peak height, width, slope and various time parameters. It is useful for determining parameters of action potentials such as cardiac potentials, EPSP and IPSP.
• Read Scope (Mac Only): Read Scope is a LabChart extension which allows LabChart for Mac to read Scope for Mac files directly.
• Export Matlab(Win only): Allows LabChart for Windows files to be saved and exported as MATLAB compatible files. MATLAB is a flexible data analysis program available from The Mathworks Inc. (www.mathworks.com).
• Translate Binary (Win only): Translate Binary is a LabChart extension which allows LabChart for Windows files to be saved and exported in a simple binary format. Translate Binary can import documents that have either been exported from LabChart, or generated by another application.
• Translate EDF (Win only): Allows LabChart to save data as an EDF file, and to read EDF files. Translate EDF does not support the EDF+ format.
• Telegraph (Win only): Makes use of the gain-telegraph output from an electrophysiological amplifier to continue to display data at the correct scale after a gain change. The Telegraph Extension uses the gain telegraph voltage from the amplifier to automate the display of electrophysiological data in LabChart, so that the correct units and scale are used.

Scope
Scope software, supplied with PowerLab systems, provides powerful display, recording and analysis features to transform your computer into a two-channel storage oscilloscope, XY plotter or Power Spectrum (FFT) analyser. Scope is used commonly to measure any high-frequency signal that is time-locked to a stimulus such as action potentials and evoked responses. For analyzing Evoked Potentials, the Scope Software is recommended because it provides signal averaging functions that are necessary to extract the evoked response from background noise.

Therefore, Scope is an ideal software program for recording any type of evoked potential because each successive stimulus and evoked potential may be recorded in a single sweep and the results of multiple sweeps may be superimposed or averaged to minimize signal noise and isolate the evoked potential. Scope is useful in signal averaging techniques to improve the signal-to-noise ratio and minimize artifacts induced by related events such as the blink-response. Features such as filtering and spectral analysis may be used to identify oscillating potentials. The analog output of the PowerLab data acquisition system may be used to control a stroboscopic flash unit (with suitable inputs) for presentation of the light stimuli.

ScopeView
By providing digital oscilloscope features, Scope View facilitates recording and viewing consecutive sweeps of data. These sweeps, or pages, can be overlaid and averaged.  There are several powerful features in Scope View that enhance analysis and display.

• Provides the ability to record, display and analyze any high frequency signal that is time-locked to a stimulus
• Synchronizes sweeps with recorded or built-in stimulation patterns
• Provides a range of real-time and offline analysis features
• Generates stimuli of differing intensities and waveform structures (i.e. single-pulse, multiple pulse, simple ramps) and control an external stimulator using the analog output on the front of the PowerLab
• Create pages or sweeps from Comments in Chart View in addition to block mode and Event Mode.
• Events in Blood Pressure and ECG Module can be used to generate pages in Scope View.
• Use the new 3D Depth and Saturation Overlay feature in the Scope Overlay Options to help distinguish overlaid traces.
• Alter the active trace and overlay trace colors.
• Change the display background to black for better contrast.

Scope View is recommended because it provides signal averaging functions that are necessary to extract the evoked response from background noise.

GLP and 21 CFR Part 11
For those researchers working within a laboratory requiring GLP and 21 CFR Part 11 compliance the GLP Client and GLP Server are available for use with LabChart (Windows only) and PowerLab data acquisition systems. For more information, visit the For more information, visit the Good Laboratory Practice application page or contact your nearest ADInstruments representative.

Hardware:

PowerLab Data Acquisition Systems

• PL3504 PowerLab 4/35 - 4 Channels
• PL3508 PowerLab 8/35 – 8 Channels
• PL3516 PowerLab 16/35 – 16 Channels

Instruments

• STG4004 4 Channel Stimulus Generator
• STG4008 8 Channel Stimulus Generator
• ML1001 Electronic Stimulator and ML1101 Stimulus Isolator

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