visual evoked potentials
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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