Multifrequency VEP
program

Reference: PVM-ST

The principle of this exam is to perform several simultaneous stimulations of the visual system with a different temporal frequency.

The visual evoked response is analyzed with a Fast Fourier transform which allows the separation of the responses to each stimulation.

This new approach offers several advantages:

Multi frequency VEP exams provide objective responses
responses are obtained very rapidly: 10 to 40 seconds
as the two stimulations are simultaneous, their responses can easily be compared

Field test VEP

In this exam, two areas of the visual field are stimulated simultaneously at different frequencies.

For example, the macula area can be stimulated with a pattern alternating at 10 Hz whereas the right peripheral visual field is stimulated with a larger pattern alternating at 15 Hz. This test can be used for the follow-up of patients with glaucoma and patients with Sabril medication.

Another example would be the simultaneous stimulation of the right and left hemifields for the evaluation of patients with hemianopsia.

example of field test VEP stimulation:
the macula area is stimulated with a pattern alternating at 10 Hz whereas the right peripheral visual field is stimulated with a larger pattern alternating at 15 Hz

The examination result shows the amplitude of the VEP response versus frequency. The example hereby shows a 10 Hz peak corresponding to the response from the macula and a 15 Hz peak to the response from the periphery.

Several cases may occur:

if both responses are present: the patient is fixating and the peripheral field is responding
if the macular response is not present: the patient is not fixating or has a macular dysfunction. The response from the periphery should be interpreted with caution.
if the macular response is present but there is no response from the periphery: a visual field problem may be suspected

VEP response to field test stimulation

Generation of dichoptic stimulations

Dichoptic stimulations are stimulations which are different for each eye. They are used for binocular beats, correlograms and stereograms VEP exams.

The subject wears a stereo mask with very high speed ferroelectric shutters whose switching frequence is extremely rapid (switching time of less than 1 millisecond). The fast shutters are synchronized with the stimulation monitor (120 Hz) so that odd number images are seen by the right eye whereas even number images are seen by the left eye.

Binocular beats VEP

The binocular beats stimulation consists in inverting the checkboard pattern. The inverting frequency is different for both eyes: 10 Hz for the right eye and 15 Hz for the left eye and vice versa.

The examination result is displayed as the amplitude of the VEP response versus frequency and shows:

two peaks corresponding to the stimulation of each eye (respectively 10 Hz for the right eye and 15 Hz for the left eye)
peaks corresponding to a combination of these two frequencies representing interactions on the cortical level between information coming from the right eye and information coming from the left eye (binocular beats)

This test can be used to evaluate the relative response of the 2 eyes (ocular balance) and to detect the presence of binocular cortical activity.

VEP response to binocular beat stimulations

Correlograms VEP

These stimulations are constituted of dots presented randomly on the display and rapidly renewed (15 to 30 times per second). These stimualtions appear as snow when viewed monocularly.

For both eyes, 2 configurations of dots are alternatively presented. For the first configuration, said to be "correlated", the position of dots are identical for both eyes. For the second configuration, said to be "anticorrelated", white dots on the image of the right eye correspond to black dots for the left eye and vice versa.

The result obtained from a "normal" subject shows a first response peak at 30 Hz corresponding to the frequency of the random dots images. Peaks appearing at 5 Hz and its harmonics 10, 15 and 20 Hz are the responses corresponding to the alternance between correlated and anti-correlated configurations. These responses can only be obtained if there is fusion of the stimuli of the 2 eyes.

VEP response to correlogram stimulations

Stereograms VEP

Like in correlograms, stereogram stimulations are made of dots presented randomly on the display and rapidly renewed (15 to 30 times per second). The result in monocular vision is a snow like display.

For both eyes, 2 configurations of dots are alternatively displayed. For the first configuration with "no disparity", all the dots appear at the same distance. For the second configuration, with "disparity", some dots positions are modified according to a checkboard pattern to create a disparity between images seen by both eyes. This disparity creates a relief effect, the checkboard appearing in a plan situated in front of the display.
The alternance between "disparity" and "no disparity" configuration is typically performed 2.5 times per second.

The response of a "normal" subject to a stimulation constituted with a 2 degrees side and a 10 arc minutes disparity shows a first 15 Hz peak corresponding to the frequence of renewing of random dots images. Peaks appearing at 2,5 Hz and its 7,5 Hz harmonic correspond to the alternance between "no disparity" and "disparity" dots configuration. They are present only if patient has stereopsis.

VEP response to stereogram stimulations

Synthesis

Each of these 3 tests provides complementary information.

Bibliography

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Vision Monitor systems compatible with this program:

Multifrequency VEP program