BASIC TECHNOLOGY

Electrodes

Skin electrodes such as sintered silver-silver chloride, standard silver-silver chloride, or gold disc electrodes are recommended for recording VEPs. The skin should be prepared by cleaning and a suitable paste or gel used to ensure good, stable electrical connection. The electrode impedances should be below 5 kΩ measured between 10 and 100 Hz and, to reduce electrical interference, they should not differ by more than 20% between electrode sites.

Electrode placement. The scalp electrodes should be placed relative to bony landmarks, in proportion to the size of the head, according to the International 10/20 system [7] (See Fig. 1A). The anterior/posterior midline measurements are based on the distance between the nasion and the inion over the vertex. The active electrode is placed on the scalp over the visual cortex at Oz with the reference electrode at Fz. A separate electrode should be attached to a relatively indifferent point and connected to the ground; commonly used ground electrode positions include the forehead, vertex (Cz), mastoid, earlobe (A1 or A2) or linked earlobes.

• Figure 1A. Location of Active and Reference Electrodes for Standard Responses. The active electrode is located along the midline at Oz. The reference electrode is located at location Fz. The subscript z indicates a midline position.
• Figure 1B. The locations of the lateral active electrodes, O1, O2, PO7, and PO8 are indicated along with the midline active electrode location, Oz.

Stimulus parameters

There are two major classes of standard VEP stimulation, flash and pattern. The reader may refer to the ISCEV Calibration Guidelines [5] for guidance regarding the measurement and definition of stimulus parameters. Standard stimulus and recording conditions are described below and are summarized in Table 2. All stimulus parameters should be calibrated either locally or by the manufacturer and regular recalibration is advised [5].

Pattern Stimuli: The standard pattern stimulus is a high contrast black and white checkerboard. The viewing distance, typically between 50 cm and 150 cm, can be adjusted to obtain a suitable field size and the required check sizes for any physical size of display screen.

Field and check size: Patterned stimuli are defined by the visual angle subtended by the side of a single check in degrees (°) or minutes of arc (min) subtended at the eye. For standard pattern VEPs, two check element sizes should be used: 1° ± 20% and 0.25° ± 20% of arc per side. All checks should be square and there should be an equal number of light and dark checks. It is not necessary to use a square field but the aspect ratio between width and height should not exceed 4:3 and the field size should at least 15 deg in its narrowest dimension. The stimulus field size should be specified in degrees of visual angle, with an indication of field shape (e.g., rectangular field a deg x b deg, or a circular field of c deg diameter or radius). A fixation point, when used, should be positioned at the corner of four checks and located at the center of the field.

TABLE 2A: ISCEV STANDARD FOR VEP ASSESSMENT: Standard Stimuli

Luminance and contrast: The luminance of the white checks should be 100 ± 20 candelas per meter squared (cd·m^-2). The luminance of the black checks should be low enough in the black checks to achieve a Michelson contrast^[1] of greater than 80%. Thus, the mean luminance of the checkerboard will be between 40 and 67 cd·m^-2. The luminance and contrast of the stimulus should be uniform between the center and the periphery of the field. However, we recognize that many optical and electronic systems do not provide truly uniform fields. Therefore, variation from center to periphery of up to 30% is acceptable. We encourage those following the standards to use stimulus displays that are as uniform as possible. The luminance of the background beyond the checkerboard stimulus field is not critical to the results. Lighting should be homogenous, with an average luminance approximately equal to the average stimulus luminance. Practically, this can be achieved by subdued room lighting with no bright sources visible to the subject.

Pattern-reversal stimuli: For the pattern-reversal protocol, the black and white checks change phase abruptly (i.e., black to white and white to black) and repeatedly at a specified number of reversals per second. There must be no overall change in the luminance of the screen, which requires equal numbers of light and dark elements in the display, and no transient luminance change during pattern reversal. The large check (1°) and small check (0.25°) stimuli are specified by the check width (visual angle), the stimulus rate (in reversals per second), the number of reversals, the mean luminance, the pattern contrast and the field size. A reversal rate of 2 reversals per second (+/- 10%) should be used to elicit the standard pattern reversal VEP. (Each full cycle consists of two reversals so this equates to frequencies of 1.0 Hz.)

Pattern onset/offset stimuli: For pattern onset/offset the checkerboard pattern is abruptly exchanged with a diffuse gray background. The mean luminance of the diffuse background and the checkerboard must be identical with no change of luminance during the transition from pattern to diffuse blank screen. This may be difficult to achieve. Pattern onset duration should be 200 ms separated by 400 ms of diffuse background. The ISCEV standard onset/offset response is the onset response. This temporal pattern ensures that the pattern onset response is not contaminated by the pattern offset response. The data acquisition system must indicate the appearance of the stimulus. At least two pattern element sizes should be used: checks of 60 min and 15 min per side.

Flash stimulus: The flash VEP should be elicited by a brief flash that subtends a visual field of at least 20 deg, presented in a dimly illuminated room. The strength (time-integrated luminance) of the flash stimulus should be 3 (2.7-3.3) photopic candelas seconds per meter squared (cd·s·m^-2). This can be achieved using a flashing screen, a hand held stroboscopic light or by positioning an integrating bowl (ganzfeld) such as that used for ERG tests in front of the patient [3]. The flash rate should be 1 per second (1.0 Hz ±20%).

Recording parameters

Amplification and filtering: Amplification of the input signal by 20,000-50,000 times is usually appropriate for recording the VEP. The input impedance of the pre-amplifiers must be at least 100 MΩ and the common mode rejection ratio should exceed 120dB. The amplifiers must be electrically isolated from the patient and must meet the current standards for safety for clinical biologic recording equipment in the user's country. The analogue signal should be digitized at a minimum sample rate of 500 samples per second per channel with a minimum resolution of 12 bits. Automatic artifact rejection based on signal amplitude should be used to exclude signals exceeding + 50-100 μV in amplitude. The amplifiers must return to baseline rapidly following artifactual signals.

Analogue high pass and low pass filters [-3dB points] should be set at ≤1Hz (corresponding to a time constant 0.16 s or more) and at ≥100 Hz, respectively. Analogue filter roll-off slopes should not exceed 12dB per octave for low frequencies and 24 dB per octave for the high frequencies. While other filter settings may be required in particular circumstances, it must be realized that all analogue filters produce an apparent change in the timing or peak time of the components of the VEP particularly if low pass filters below 100 Hz are used. The use of notch or comb line frequency filters is strongly discouraged. Users should adhere to the current ISCEV Guidelines [5] that include details on the measurement of electrode impedance as well as amplifier filtering and gain.

Averaging and signal analysis: The number of sweeps per average depends upon the signal to noise ratio between the VEP and the background noise. In most clinical settings, the minimum number of sweeps per average should be 64. At least two averages should be performed to verify the reproducibility of each VEP. For infants and young children, a smaller number of sweeps per average may sometimes produce a clearer response. The longer recording time required to increase sample size introduces the possibility of increased variability due to loss of attention and/or increased movement.

Analysis Time: The minimum analysis time (sweep duration) for all adult transient flash and pattern reversal VEPs is 250 ms post stimulus plus at least 20 ms of pre-stimulus baseline. To analyze both the pattern onset and offset responses elicited by onset-offset stimuli, the analysis time (sweep duration) must be extended to 500 ms. The VEP in infants has longer peak latencies and longer sweep times will be required to adequately visualize the response.

^[1]Michaelson contrast ={[Lmax - Lmin]/[Lmax + Lmin]} x 100%, where L = luminance, max = maximum of the white squares and min = minimum of the black squares

-- TWikiAdminUser - 22 May 2009

Comments on Basic Technology

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Suggest consideration of added language along the lines below:

Note: It has become evident during 2009 that the phasing out of CRT (cathode-ray tube) display technology in favor of flat screen displays (such as LCD and Plasma screens) may pose new problems in the calibration of VEPstimuli. Specifically,

(1) these display technologies may frequently introduce additional delays between the electrical control signal (such as VGA computer output) and the consequent changes in the screen light output, as compared with traditional CRT displays. Spurious delays of 15-20 msec have been reported.

(2) Flat-screen technologies may introduce asymmetries between light-on and light-off events in the time-course of the light output, possibly causing transient fluctuations in overal screen luminance with each pattern reversal.

Users are cautioned against casual substitution of dispaly devices, particularly against the casual subsitution of a flat-panel display for a traditional CRT display.

Users are encouraged to communicate with equipment manufacturers to address this problem and verify steps which have been taken to ensure test results comparable to those traditionally obtained with CRT display devices.

-- ScottBrodie - 13 Jul 2009