G. W. Van Marle

Variation in latency times of visually evoked cortical potentials.

Latency times of visually evoked cortical potentials stimulated by reversal of a slow checkerboard pattern are highly dependent on the time needed to accomplish the reversal movement. If, owing to the method, the pattern reversal time is not kept stable, variability of the latency times is unnecessarily high for clinical purposes. This may be the case when television equipment is used.

Electrophysiological Equipment for Total and Local Retinal Stimulation

A xenon flash is generally used to evoke electric responses of the visual system. When built in the ordinary lamphouse with a diffuser in front, the light mostly subtends a visual angle of about 30°. Such equipment has its disadvantages, the most important being, that the retina is only partly illuminated directly by the image of the diffuser. This part will have a high illumination. The rest of the retina will have a lower illumination, as it is illuminated indirectly by stray light. This stray light illumination is lower the farther away it is from the image of the lamp. As a result the response obtained is a mixture of high illuminated and low illuminated retinal fields (Berson et al., 1968). Under such circumstances measurement of latency time and peak time makes no sense.

Cortical Potentials Evoked by a TV Pattern Reversal Stimulus Varying Check Sizes and Position of the Stimulus Field

Using pattern reversal stimuli, produced by a TV system, visually evoked cortical potentials, obtained by half field and quadrant stimuli, are compared with those of full field stimulation. The lower

Two disadvantages of a television system as pattern stimulator for evoked potentials

The use of television systems to generate patterned stimuli for the examination of EPs has disadvantages due to the frame frequency of 50Hz of television sets. Because of this, changes in the pattern last 20 msec, with the result that the variability in EP latencies is larger than in the results obtained by projector systems. Furthermore, 50 Hz noise on the recordings, transmitted through the visual system, may be seen.

Interference of 50 Hz electrical cortical potentials evoked by TV systems.

Application of TV systems for generating patterned stimuli for the examination of the electrical cortical potentials has disadvantages caused by the frame frequency of 50 Hz of TV sets. Apart from a larger variability in latency times of the electrical potentials as compared to the results obtained by projector systems, a 50-Hz noise on the recordings mediated through the visual system may be seen. These disadvantages can be ameliorated by increasing the frame frequency of the TV set.

Digitalized Echo Oculometry

An ultrasound echograph was expanded with three electronic counters to measure the time intervals between the echoes of cornea and retina, cornea and anterior lens surface, and between both lens surfaces. The thickness of a cataractous lens, often causing multiple echos, can be measured too. The echogram is photographed together with marks indicating which time intervals are measured, furthermore with the numerical values which are obtained. The equipment has already been applied for more than two years, making oculometry more convenient and reliable.

A Microcomputer – Based Averager

A disadvantage of commercially available averagers is the time needed for the output routine. This paper describes a microcomputer, programmed as an averager, with additional features to obtain an examination time as short as possible, which indeed is important for clinical application. Through this apparatus, the time needed for a standard ERG and VECP examination could be reduced from 50 to 25 min

Influence of Superimposing Cartoons on TV Pattern-evoked Cortical Potentials

Superimposing cartoons causes an alteration of the brightness of a pattern stimulus. Therefore, the influence of luminance changes on pattern-evoked cortical potentials was investigated and compared t