Abstract
Abstract: :
Purpose: To report on the utilization of a software based stimulus presentation system and its applicability for sweep pattern stimulation and its correlation to visual acuity. Methods: Checkerboard onset and reversal sweeps appeared on a 21’’ planar computer monitor and the timing was controlled with a software for stimulus presentation (Presentation, Neurobehavioral Systems Vers. 0.53). The software allows for sending a TTL pulse via the parallel port (LPT1) at the start of the appearance of each pattern. This pulse was used for triggering the signal acquisition system running on a second computer (System Plus, Micromed).VEPs were recorded with surface electrodes from O1, O2, and Pz. Sweep checkerboards were presented at a viewing distance of 174 cm and a viewing angle of 16°. The size of each checkerboard element in min of arc were 174, 87, 43.5, 21.8, 10.9, 5.9, respectively with a Michelson contrast of 90 %. The study was performed in seven healthy subjects with a visual acuity of 20/20. By using convex glasses for blurring and Bangerter occlusives the visual acuity was artificially set to 20/30, 20/60, 20/200, 20/400. Results: With the software based presentation of sweep checkerboards reproducible and stable waveforms could be recorded in all subjects. Tuning curves were derived from the VEP amplitudes with respect to spatial frequency and visual acuity. The maximum for the amplitude of the VEP could be found at a spatial frequency of 21.8’. Higher or lower spatial frequencies showed significant smaller responses. The tuning curves for pattern onset sweeps showed the VEP peak more obvious than the checkerboard reversal sweep. After artificial reduction of the visual acuity the maximum of the tuning curves was shifted to larger patterns and the maximal amplitude was found to be decreased. A paradoxic effect was found with very low acuities when the EP amplitude increased with small patterns. This effect was more obvious when convex glasses were used for acuity reduction than Bangerter foils. Conclusions: The software based stimulus presentation system proved to be applicable in a routine setting for VEP recordings. Sweep VEP techniques as objective tools for visual acuity determination could be utilized. The results were comparable to previously published data obtained with hardware based settings. The paradoxic effect of an increase of the EP amplitude with small patterns in the low acuity range is maybe due to a luminance component in the response. This should be excluded by further technical measurements.
Keywords: electrophysiology: clinical • visual acuity • visual cortex