Visual Evoked Potentials

Electrophysiological Assessment of Visual

Cortical Functioning

E. Eugenie Hartmann, PhD

School of Optometry

Advantages of Electrophysiology

Objective (??)

Non-Invasive

Finding the Signal

EEG = On-going electrical activity

Visual Signal = Elicited Response

Principles of Electrophysiology

detection of electrical activity

signal averaging

voltage versus time two-dimensional waveforms

Generation of responses

neural activity

localized regions become depolarized or hyperpolarized

creates “sinks” or sources of current

Visual Electrodiagnostics

Retinal FunctioningERG ElectroretinogramEOG Electro-oculogram

Optic Nerve and Cortical FunctioningVEP Visual Evoked Potential

Confirmation of (Early) Disease

testing may be helpful

to confirm the diagnosis

to rule out alternative diagnoses

VEP

VEP Visual Evoked Potential

VER Visual Evoked Response

VECP Visual Evoked Cortical Potential

VEP

Assesses visual pathwayFrom optic nerve to V1

Spatial visual processing in pre-verbal and non-verbal individuals

VEP Overview

VEP Recording

cortical magnification of the representation

of the fovea

approximate cancellation of dipoles in periphery

Butler, 1987

V1 Topography Contributes to Foveal Dominance

Photic Driving is a Crude VEP

Chiappa, 1979

Recording VEPs from Colin

Number of averages

Signal to Noise is Proportional to the Square Root of the Number of Averages

Chiappa

Spehlmann, 1985

Latency and Amplitude Measurements

VEP Waves and Generators N70: standing wave, thalamocortical input P100: standing wave, intracortical inhibition

in striate cortex but also extrastriate activity. This is the most robust component.

N145 and later components: standing wave, striate and extrastriate activity

These waves are foveally-dominated, especially for small checks or fine gratings. Striate cortex dominates N70 and P100, but extrastriate cortices are active.

VEP Criteria for Abnormality

P-100 latency prolongation Absent VEP P-100 interocular latency difference P-100 interocular amplitude difference,

only if at least 4:1 Abnormal waveform (if monocular)

Types of VEP RecordingsSpatial Domain

Flash

Pattern

spatial variations

contrast variations

Maturation of FVEP Response

CheckerboardsVarying Size

Fourier Analysis and Synthesis

-15

-10

-5

0

5

10

15

0 100 200 300 400

Phase (degrees)

Am

pli

tud

e (

mV

)

data

Fundamental

Second Harmonic

Third Harmonic

Fourth Harmonic

Sum

Unfiltered Transient VEP

Fourier Analysis of Transient VEP

Filter low-passSet Filter

Filter low-pass

Filter Odd Harmonics

Fourier Synthesis

Transient VEPs from Child and Adult

Grating Stimuli

Swept-parameter VEP Pattern changes rapidly

contrastspatial dimension

Gratingsteady-state

Checkerboardtransient

Steady-state Sweep VEP

Gratings

1-second per pattern size

6 different gratings

5 - 10 sweeps averaged

Steady-state Sweep VEP OD and OS 33 Weeks

Steady-state Sweep VEP Grating Sweep, 7.5 Hz 5 runs JF991 24 weeks OD JF991 24 weeks OS Acuity = 11.03 cpd Acuity = 10.62 cpd

   

   

   

   

   

   

   

   

   

   

   

   

-5

0

5

10

15

20

25

0.1 1 10 100

2nd

Har

mon

ic A

mpl

itude

(mV

)

-5

0

5

10

15

20

25

0.1 1 10 100

2nd

Har

mon

ic A

mpl

itude

(mV

)

Spatial Frequency (cpd)

Effect of Fatty Acids on Acuity Measured with VEP

Standard FormulaAA and DHA addedHuman breast milk

Check Size Determines Effective Spatial Contrast

1/8 deg (7.5 min)

1/4 deg (15 min)

4 deg (240 min)

1 deg (60 min)

8 deg (480 min)

very large checks: few contours, C and

S act antagonistically

very small checks: below resolution of

many receptive fields

25 msec

2 mV

Cz-Oz

VEP Criteria for Abnormality

P-100 latency prolongation Absent VEP P-100 interocular latency difference P-100 interocular amplitude difference,

only if at least 4:1 Abnormal waveform (if monocular)

Factors that alter P100 waveform in normal subjects:

Visual acuity (<20/200 for P100 to be abnormal)

Pupillary size (causes interocular latency difference)

Age (latency increase with age especially after 60)

Sex (females have typically shorter latencies than males)

Subject cooperation

Normal VEP

25 msec5 mV

stim OS

stim OD

Cz-Oz1/2 deg (30 min)

32 y.o., r/o MS

20/20 OS 20/20 OD

1/4 deg (15 min)

stim OS

stim OD

Cz-Oz

P100 latencies are similar in the two eyes

P100 latency increases slightly with

smaller checks

No lens: 20/15

+1D: 20/20

+2.5D: 20/100

+2D: 20/40

25 msec

3 mV

Cz-Oz

1/4 deg (15 min)

Substantial defocus will prolong latency and reduce

amplitude due to reduction in retinal contrast.

Effect of Defocus

25 msec15 mV

stim OS

stim OD

Cz-Oz

1/2 deg (30 min)

20 y.o., r/o MS

20/20 OS 20/20 OD

P100 prolonged, but amplitude preserved

Substantial interocular latency difference

Unilateral Delay

Small Check Size Increases Sensitivity

1/4 deg (15 min)

1/2 deg (30 min)

25 msec5 mV

stim OS

stim OD

Cz-Oz

2 deg (120 min) 25 y.o., r/o MS

20/40 OS 20/20 OD

Significant interocular latency difference

Normal P100 and no interocular difference

25 msec5 mV

stim OS

stim OD

Cz-Oz1/2 deg (30 min)

30 y.o., MS 20/400 OS 20/20 OD

acute attack OS20/40 OS 20/20 OD

5 mos later20/20 OS 20/20 OD

6 yrs later

4 deg (240 min)

stim OS

stim OD

Cz-Oz

Acute Demyelination and Recovery

demyelination and recovery

OS

asymptomatic attack OD