Sweep VEP
program

Reference: PVM-SS

The sweep VEP technique uses a pattern stimulus which is alternated at a high temporal frequency rate (in the range of 5 to15 Hz), producing a steady state response. The example hereby shows the plot of the amplitude of the fundamental response versus time. After the subject closes his eyes, the amplitude of the response decreases rapidly to the noise level.

The shape of the pattern stimulation is rapidly decreased (about 10 seconds) to determine the smallest checkerboard size generating an evoked response. The VEP signals are processed by Fast Fourier Transform (FFT) to extract the magnitude and phase of the response. One major advantage of frequency analysis is that it can detect a response extremely rapidly, typically less than 2 seconds

Visual acuity is estimated from the smallest size of the pattern stimulus that produces a response.

The program starts by generating a cartoon in order to attract the attention of the child.

The near infra red camera placed on the stimulator allows the monitoring of the patient fixation.

The cartoon is followed by the presentation of a checkerboard with large dimensions. The traces displayed in real time on the control screen correspond to the frequency spectrum of the recorded signal showing almost immediatly a response to the stimulation.
This response is characterized by the appearance of a peak (indicated by yellow bars) whose stimulation frequency is significantly higher than neighbour frequencies.

The green curve on top represents the amplitude of the response as a function of time. The sweep beginning is indicated by the vertical line and its duration (12 seconds) by the thick horizontal line.

When the size of the pattern decreases, the amplitude of the response first increases, reaching a maximum and then decreases rapidly.

Visual acuity is estimated from the smallest size of pattern that produces a response. In this example, 3 sweeps (only 2 are seen on the screen) have been realized and visual acuity is automatically estimated as 0.47 (between 4/10 and 5/10).

The dashed curve represents the level of noise: it is much inferior to the measured response.

The green curve at bottom represents the phase of the response (or delay with respect to stimulation). It increases slowly as the pattern size decreases.

Vision Monitor systems compatible with this program:

• MonPack3: standard visual electrophysiology
• MonColor: advanced visual electrophysiology