Variation of Inter-stimulus-Intervals and Luminance in multifocal ERG
Mayser H. M.
University Eye Hospital Tuebingen, Germany, Dept. of Neuroophthalmology and Pathology of Vision
Purpose: Multifocal ERG (mfERG) recordings are often done without dark frames between stimuli, i.e. no inter-stimulus-interval (ISI), to keep recording sessions short and coping easy for the patient. This lack of ISI leads to a superposition of responses originating from separate stimuli and the obtained amplitudes are smaller than in recordings with an ISI.
Method: We did vary the standard parameters used in clinical routine mfERG testing for luminosity and stimulus frequency (inter-stimulus-intervals) in 8 healthy subjects between 20 and 40 years of age. Multifocal ERGs were obtained with a Macintosh G3 ESI VERIS 4.03 System and a small white phosphor CRT Tube attached to a Heidelberg Engineering HRA Scanning Laser Ophthalmoscope (SLO), using the IR laser image of the SLO to monitor fixation. DTL electrodes and mydriatics were used. Luminance was set to 5 discrete grey values corresponding to 4, 35, 109, 272 and 440 cd/sqm with the monitor we used, ISI was changed between 0 frames (of 13,3ms at 75 Hz) and 18 frames.
Results: Amplitudes were typically around 60 nV/deg*2 in the center and 20 nV/deg*2 in the periphery when no dark frames were used and around 140nV/deg*2 in the center and 60-70nV/deg*2 in the periphery at 18 dark frames (= ISI of approx. 240msec.). Latency for N1 was relatively constant, whereas latency for P1 grew with longer ISIs.
Conclusions: Responses reache
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