Electrodes©

Dennis L. MolfeseUniversity of Nebraska - Lincoln

• Website: http://psych.unl.edu/erp/

• User: PsychERP

• P-d: April2011

Electrode PlacementElectrode selection, placement, and number depend on test goals, subject age, task factors.No universal or “correct” system.50+ different systems used.Need to document placements, references.Speed of application important with infants, children (< 10 min. optimal)

Electrode Terms

Monopolar vs. bipolar electrode pairs“monopolar” suggests 1 electrode not active or at neutral site Irrelevant issue todayEach electrode ALWAYS compared to something when input to amplifierDifferential amplifiers compare electrode (+ input) to another electrode (- input)

Electrodes & Babies

Electrode Related Issues

The closer an electrode is to reference, the smaller the amplitude of the signalElectrodes detect current changes at scalp generated by structures at different sites within the brain plus muscle activity plus other artifacts

Electrode Related Issues

Electrodes may not indicate signal because:Electrode too close to reference (flat line)Dipole generating response not perpendicular to brain/scalp surfaceFilters screen out signal of interest

Common Electrode References

NoseCz (vertex)Linked mastoids or one mastoidLinked Ears or one earOff the head (e.g., back, leg)Average (preferred if many electrodes)

Reference Points

Choose your reference wisely

Choice of Reference affects dataNo “zero” activity point on a bodyAverage reference is the best approximation of zero for large number of electrodes

LinkedMastoid

Reference Effects” Choice of Reference “Changes” the Data !

RightMastoid

LeftMastoid

AverageReference

Reference Effects” Choice of Reference “Changes” the Data !

Surface Laplacian Issue1. Scalp potential shape depends on reference location.

2. Average reference partially mitigates distortion.

a. Incomplete coverage of scalp

b. Finite number of electrodes

3. Scalp distribution blurred relative to cortex by low scalp conductivity

4. Surface scalp Laplacian better approximates cortical-dura surface potentials

Current moves up until hits scalp-air boundary where it moves tangentially

Scalp surface Laplacian is proportional to the local surface divergence of the scalp current density.

Brain

Tangential current flow

BEWARE: Currents move through skull at many locations and will SUM when one current meets another.

Brain

Tangential current flow

(signalscancel)

Common Electrode Application System (Jasper, 1958)

Ten-Twenty [10-20] System: Common procedure using nasion, inion, left & right pre-auricular points; employs % distances.

Ten-Twenty [10-20] System

Top Viewnose

Ten-Twenty [10-20] System

Lateral View

nose

Proportional placement of electrodes saves time during pediatric applications

nose

Remember: Electrode placement systems arbitrary

Sub_Num Gender Gest. Age (wks) Head Circ. (cm) Nas-Ini (cm) Mea-Mea (cm)1 f 28 28 19 162 m 30 33.5 21 203 f 30 30.5 19.5 174 f 33 33.5 20.5 19.55 f 33 30 19.5 17.56 m 33 32.5 20 20.57 m 33 33.5 19.5 208 f 35 30.5 20 189 f 35 29.5 21 18

10 m 36 34.5 19.5 2011 m 37 33 20 20.512 m 38 35 22 20.513 m 38 35 22.5 2114 f 38 35 22 2115 m 39 34.5 21 20

Variations in head sizes in neonates

Head Shape Influenced by Type of Delivery

NEONATE VARIATIONS IN HEAD SHAPE: Vaginal Delivery

NEONATE VARIATIONS IN HEAD SHAPE: C-section

NEONATE VARIATIONS IN HEAD SHAPE

NEONATE VARIATIONS IN HEAD SHAPE

NEONATE VARIATIONS IN HEAD SHAPE

NEONATE VARIATIONS IN HEAD SHAPE

NEONATE VARIATIONS IN HEAD SHAPE

General strategies

1:1 ratio electrode pair to amplifierIf not enough amplifiers ->montage

Montage Strategies

Montage strategies! ! In cases where the number of

electrodes is more than the number of amplifiers available for testing ! ! Recordings can not be made

from all the electrodes all of the time.

Montage Strategies

1) Montage - different patterns of connections between electrodes and amplifiers.

2) No montage system inscribed in STONE.

! a) record between pairs of symmetrically placed electrodes;

! b) connections between anterioposterior or transverse lines;

Application - classic

1) comb hair, measure & mark positions;2) rub area with abrasive (pumice);3) clean away application material;4) apply electrolytic paste;5) apply electrode, tape;6) needle electrodes: sterilize (autoclave,

soak), then insert needle 5 mm, 1 mm depth; tape electrode to scalp.

Application - classic

Application - classic

Electrode Impedances

Low input impedance amplifiers require impedances 1-5 k OhmsHigh input impedance amplifiers tolerate well impedances 40-60 kOhms

Electrode Application

Electrode Application

Electrode Application

More recent speedy alternatives:Electrode capsElectrode nets

Electrode Application

Electrode CapsMost require abrading skin with blunt needle, applying electrode paste, monitoring impedance, re-abrading skin.Pressure applied to head (tight cap, straps around chest, chin strap).Infection risk.Attrition rates with infants & young children ~ 50%; rates up to 75-80% in children with disabilities18 - 24 month olds difficult (just say, “No!”)

Electrode Application

EGI HYDROCEL ELECTRODE NET

Advantages: Not Abraiding Scalp So No Pain Infection Risk is Minimal Rapid Application (10 Sec) Electrodes distributed Symetrically Tolerate Variations in Hair Styles

Cautionary notes: Do NOT mix different electrode net styles in same study. Some find Salt in saline itchy (dilute solution)

NEONATE ELECTRODE NET

NEONATE ELECTRODE NET

NEONATE ELECTRODE NET

PRETERM INFANT ELECTRODE NET

EGI Electrode Nets

NEWBORNPRESCHOOL

ELEMENTARY SCHOOLADULT

SPECIAL POPULATIONS

Individual Electrode Application

Electrode Caps & Electrode Nets Vary in Size

Geodesic Sensor Net64 Channel V2.0

For questions or additional assistance please refer to the EGI System 200 Technical Manual or contact us at:Electrical Geodesics, Inc1600 Millrace Drive, Suite 307Eugene, Oregon USA 97403Phone: (541) 687-7962 Fax: (541) 687-7963Email: info@egi.com or support@egi.com

For questions or additional assistance please refer to the EGI System 200 Technical Manual or contact us at:

Electrical Geodesics, Inc1600 Millrace Drive, Suite 307Eugene, Oregon USA 97403Phone: (541) 687-7962 Fax: (541) 687-7963Email: info@egi.com or support@egi.com

Geodesic Sensor Net256 Channel V1.0

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HydroCel Geodesic Sensor Net

256 Channel MapVersion 1.0

For questions or additional assistance please refer to the EGI Sensor Net Technical Manual or contact us at:

Electrical Geodesics, Inc.

1600 Millrace Drive, Suite 307

Eugene, Oregon 97403

Phone: (541) 687-7962 Fax:(541) 687-7963

Email: support@egi.com or info@egi.com

APPLICATION OF ELECTRODE NET TO NEWBORN

APPLICATION OF ELECTRODE NET TO WILLIAMS SYNDROME CHILD

APPLICATION OF ELECTRODE NET TO 3-YEAR OLD

APPLICATION OF ELECTRODE NET TO 6-YEAR OLD

APPLICATION OF ELECTRODE NET TO 8-YEAR OLD

APPLICATION OF ELECTRODE NET TO ADULT

Questions ???

Electrode placement - sites & strategies used

! 1) Jasper, H.H. (1958). EEG, 10, 370.

! 2) Gibbs, F.A. & Gibbs , E.L. (1950). Atlas of

! Electroencephalography, Vol. 1: Methodology and Controls.

! Cambridge, Mass.: Addison - Wesley. p. 56.

! 3) Pampiglione, G. (1966). Proceedings of the

! Electrophysiological and Technology Association, 13, 166.

Additional References: (electrode caps, amps)

www.gtec.at www.brainproducts.com/docs/Products/braincap.htmwww.psylab.com/html/default_electrodes_transducers.htmlwww.biosemi.com/products.htmwww.EGI.com/

APPLICATION OF ELECTRODE NET TO ADULT