Evoked Potentials
Overview:
The evoked potential technique is a simple and non-invasive means of measuring nerve transmission from the senses to the brain.
Evoked potentials are nervous system responses to an artificial stimulus applied to a particular sensory system (see below). Signal latency and waveform components are measured from the cerebral cortex using EEG.
The evoked potential technique is amenable to any sensory modality:
- Vision: ‘visual evoked potentials’ (VEP)
- Hearing: ‘brainstem auditory evoked potentials’ (BAEP)
- Touch: ‘somatosensory evoked potentials’ (SEP)
- Taste: ‘gustatory evoked potential’ (GEP)
- Smell: ‘olfactory evoked potential’ (OEP)
- Balance: ‘vestibular evoked myogenic potential’ (VsEP)
Evoked responses from the CNS to effector organs are also used clinically and in research. Where the effector organ involves skeletal muscle, the technique is called the motor-evoked potential (MEP). This involves measuring muscle responses following stimulation of the motor cortex, either invasively or with transcranial magnetic stimulation.
Method:
Electrodes
Bipolar recording electrodes can be Ag/AgCl or gold-plated surface electrodes (or clips for earlobes). For invasive recordings tungsten or stainless steel wire electrodes can be used. Stimulating electrodes should also be bipolar with an interelectrode distance of <35 mm.
Recording electrode placement
Recommended scalp recording locations (10-20) for somatosensory (SEP), auditory (BAEP) and visual (VEP) evoked potentials are as follows:
- SEP: Midway C3-P3-C4-P4 (active), Fz (reference)
- BAEP: Cz (active), left/right earlobe (reference)
- VEP: Oz (active), Fz (reference)
For gustatory and olfactory evoked potentials T3-T5 or T4-T6 with Fz reference may be suitable. For vestibular evoked potentials Cz with left/right earlobe references should be used (as per BAEP).
For montage configurations, please refer to the American Neurophysiology Society Guidelines for detailed instructions.
10-20 International Standard for EEG Electrode Placement
Triggering
- Stimulator: run preset stimulator protocol on start of sampling
- Sampling control: automated fixed duration repeated sampling with fixed-sweep size averaging in Scope View. Alternatively, sampling may be trigger-controlled (external TTL, or analogue input) e.g., for manual stimulus presentation control, or automated protocols controlled by other software.
Recording
Evoked cortical response amplitudes may range from 0.25-5 µV depending on the type of potential being measured, which gives an SNR of 1:10 to 1:200 against background EEG noise of 50 µV. Thus evoked potentials cannot be seen in raw data and must be averaged. As a rule of thumb, the number of averaging sweeps required depends on the square of background noise divided by signal amplitude (up to 4000 samples required for BAEPs).
- Bandpass filtering: VEP 1-100 Hz, BAEP 100-3000Hz, SSEP 30-3000 Hz.
- Required analysis times (for Scope View): 250ms for VEP; 40 ms for median nerve SEP; 60 ms for posterior tibial nerve SEP; 10-15 ms for BAEP.
Stimulation
- SEP the stimulus should be a square wave pulse 2-200 ms in duration and 10-50 mA in current intensity. For upper extremity testing, stimulation is applied to the median or ulnar nerve. For lower extremity testing stimulation is applied to the posterior tibial nerve.
- BAEP, a 10Hz 100-200 µs square wave sound pulse delivered to a single ear at a volume of 65 dB while white noise is played to the contralateral ear. At least 1000 samples per ear is required for averaging.
- VEP, unpatterned (flash) or patterned (e.g., alternating checkerbox) presented on a computer screen at a rate of 0.5-2 Hz.
Analysis
Use Scope View to measure event onset times, interpeak latencies, peak heights and number of event components.
Software:
The LabChart Advantage:
(may require additional Modules and Extensions)
- Suitable for evoked potentials that do not require averaging (e.g., VEPs)
- Record multichannel montages for SEPs to track ascending signal latencies
- Improve SNR with digital filtering
- Customizable stimulator settings to control visual, tone, or electrical stimuli
- Synchronize recordings with stimulus
- Control sampling using external trigger
- Automated extraction of data from recordings using online Timed Add to Data Pad or offline using Multiple Add to Data Pad
- Extracted parameters in the Data Pad or ECG Table View can be easily exported for further analysis, e.g in Excel or Matlab
LabChart
LabChart software (for Windows and Macintosh) together with a PowerLab data acquisition system offers up to 32 channels of real-time data acquisition, data integrity, easy selection of hardware settings, powerful online and offline analysis, procedure automation, seamless extraction of experimental data and flexible display options. Additional acquisition and analysis functionality is provided with the use of specialized LabChart Extensions and LabChart Modules. LabChart Modules are available as part of LabChart Pro and LabChart Extensions are free for download from the website for existing LabChart users.
Scope View
Scope View, supplied with PowerLab systems, provides powerful display, recording and analysis features to transform your computer into a two-channel storage oscilloscope. Scope is used commonly to measure any high-frequency signal that is time-locked to a stimulus such as 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.
Features and benefits:
- Provides the ability to record, display and analyze stimulus-locked waveforms
- Average over a fixed number of samples (page-based) or fixed recording intervals (time-based)
- Synchronize sweeps with automated comments or event markers (stimulus)
- Analyze evoked potential latency and peak amplitude using Marker tool
- Customizable 3-D overlay options
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 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
Signal Conditioners
Biopotential Amplifiers
Human EEG
- A differential amplifier that consists of eight electrically isolated differential input AC amplifiers
- A shared ground connection across all eight inputs.
- Supplied with two packets of MLA0310 Lead Wires (1.8 m, 10 snap on)
- A single channel, differential amplifier that is suitable for recording evoked potentials at one location
- Supplied with a MLA2340 3 Lead Shielded Bio Amp Cable
- Supplied with MLA2503 Shielded Lead Wires (3 snap-on)
- A dual channel differential amplifier that is suitable for recording evoked potentials at two locations
- Supplied with an MLA2540 5 Lead Shielded Bio Amp Cable
- Supplied with MLA2505 Shielded Lead Wires (5 snap-on)
Animal EEG
- An isolated, high performance and software-controlled differential amplifier
- Has three 2mm input sockets allow the direct connection of electrodes to this amplifier
- Supplied with MLA1215 Animal Bio Lead Wires
- Also suitable for use with
Stimulators
Electrical
- ML1001 Electronic Stimulator and ML1101 Stimulus Isolator
- MLADDF30 Stimulating Bar Electrode
Visual & Auditory
- MLE1320 SuperLab and Stim Tracker
- Recommended stroboscopic flash unit: Grass Telefactor: The PS33 may be controlled via digital TTL outputs on the PowerLab using the Event Manager software (Windows) or Timed Events Extension (Macintosh)
Accessories
Bio Amp Cables:
- MLA1340 3 Unshielded Lead Bio Amp Cable
- MLA1540 5 Unshielded Lead Bio Amp Cable
- MLA2340 3 Shielded Lead Bio Amp Cable
- MLA2540 5 Shielded Lead Bio Amp Cable
Leads:
Leads compatible with both shielded (MLA2340 & MLA2540) and unshielded (MLA1340 & MLA1540) cables:
- MLA1505 Unshielded Lead Wires (5 alligator clip)
- MLA0311 Unshielded Lead Wires (1 m, 10 Snap on)
- MLA0310 Unshielded Lead Wires (1.8 m, 10 snap on)
- MLA1203 Needle Electrodes (29 gauge, 5)
Leads compatible with shielded cables (MLA2340 & MLA2540) only:
- MLA1605 Shielded Lead Wires (5 alligator clip, 25 cm)
- MLA1610 Shielded Lead Wires (5 Micro-Hooks)
- MLA1615 Shielded Lead Wires (5 Alligator Clip, 100 cm)
Leads that directly connect to the Dual Bio Amp
Leads that directly connect to the Animal Bio Amp
Electrodes:
Checktrode®:
Model 1089 MK III Checktrode®:
- Designed to rapidly evaluate the integrity of electrode-skin contact in electrophysiological recordings such as EEG.
- Aids in reduction of electrode contact impedance with the skin.
- May be used to investigate the integrity of electrodes used in these applications.
Leads and electrodes that are compatible with the Model 1089 MK III Checktrode®
- MLAWBT9 Flat EEG Electrodes
- MLA0310 Lead Wires (1.8 m, 10 snap on)
- MLA0311 Lead Wires (1 m, 10 snap on)
- MLA0315 Lead Wires (5 snap on)
- MLA1505 Lead Wires (5 alligator clip)
- MLA1203 Needle Electrodes (5) 29 gauge
- MLADDB30 Recording Bar Electrode
- MLA1010 Disposable ECG Electrodes (100)
- MLA1010B Disposable ECG Electrodes (1000)
Consumables:
EEG Caps (Not suitable for use with the ML136 Animal Bio Amp):
Histamine facilitates in vivo thalamocortical long-term
potentiation in the mature visual cortex of anesthetized rats
M. C. Kuo and H. C. Dringenberg., European Journal of Neuroscience, 1731 - 1738, 2008
Anatomical evidence for the projections from the basal nucleus of the amygdala to the primary visual cortex in the cat
Chen Y, Zhu B, Shou T, Neuroscience Letters, 126–130, 2009
Prostaglandin E Receptor Subtype EP4 Agonist Protects Cochleae Against Noise-Induced Trauma
R. Hori, T. Nakagawa, Y. Sugimoto, T. Sakamoto, N. Yamamoto, K. Hamaguchi and J. Ito
, Neuroscience, 813-819, 2009
The material on this page is provided in good faith and believed accurate at the time of writing. No responsibility will be taken, or liability accepted, for damages arising from the use of information herein. Readers are urged to check with respective manufacturers the accuracy of all product related information.
