98th Annual Meeting DOG 2000

P 145

Influence of stimulus geometry on multifocal ERG responses

U. Kretschmann1,2,3, R. Gockeln2, S. Scholz1

Introduction: To record multifocal ERGs different stimulus geometries are used leading to different spatial resolution. The purpose of this study is to find whether summed responses of a certain area also differ for different stimulus geometries.

Methods: Multifocal ERGs were recorded with the VERIS-System (EDI, San Francisco). The stimulus was presented on a 75Hz CRT with a mean luminance of 51.8 cd/sq.m and 93 % contrast. It contained 7 - 509 scaled hexagonal elements in a field of about 60° diameter. The screen was patched with a cartboard with a central opening of 25° diameter leading to a stimulation of an equal retinal area regardless the stimulus used. For quantative analysis, a global averaged response of the first and second order kernel was calculated as the difference between the first trough to the first major peak.

Results: The response density of the first order kernel in the central 25° was highest for the coarse stimulus of 7 elements (8.2 +/- 1.31 nV/sqdeg (n=5)) and was lower for the stimulus with 509 elements (6,6 +/- 1.7 nV/sqdeg). As for the amplitude the implicit time decreased with increasing the number of stimulus elements: 31.1 +/- 1.0 ms for the 7 element stimulus and 29.5 +/- 1.1 ms for the 509 element stimulus. No major differences were found in the amplitudes of the second order in respect to the numbers of stimulus elements while the implicit time decreased when element number increased.

Conclusions: The summed responses of the multifocal ERG of a certain area differed from each other when stimuli with different element numbers were used. Not only the amount of light stimulating the retinal area in a given time span influences the response but there are also effects of etches across neighboring stimulus elements. The selection of stimuli of different geometry may not only lead to different resolution and signal-to-noise ratio in a single area but also to changes of other response properties such as amplitude and implicit time.

Supported by DFG, Bonn (grant Zr 1/10-1)
Dept. of Pathophysiology of Vision and Neuroophthalmology, Tübingen. 2Eye Hospital of the Medical School Hannover. 3Dept. of Pediatric Ophthalmology, Regensburg