ARE MULTIFOCAL OPs (mfOPs) EQUIVALENT TO FLASH OPs (FOPs)?

Abstract

Abstract: : Purpose:The number, relative amplitude and timing of mfOPs vary depending on the protocol used. Can we use FOP knowledge to predict (explain) mfOPs behaviour? Methods: Photopic (flash: 0.9 log cd.m^–2.sec; background: 30 cd.m^–2) OPs were evoked from normal subjects (N=10) and compared to mfOPs evoked to stimuli (white: 800 cd.m^–2; black: 0 cd.m^–2; background: 200 or 400 cd.m^–2) composed of an array of 19, 37, and 61 hexagons. The m–sequence was progressively slowed down with the addition of 2, 4, 8 and 16 dark frames between consecutive stimulus frames. Data analysis was limited to the first order kernel of OP waveforms summed over concentric rings as well as the entire stimulated field. Results: Irrespective of the number of hexagons, there is a progressive reduction in the number of mfOPs with gradually faster m–sequences. Slow sequences (16 or 8 blanks) elicited 3 mfOPs almost identical in morphology, relative OP amplitude (approximately 19%, 32% and 48% of summed OPs) and timing (approximately 17, 23 and 28 msec) to that observed in suprathreshold photopic flash OP responses (relative amplitude of the 3 FOPs: 26%, 26%, 48% of summed OPs; timing: 15, 22, 28 msec.). Faster sequences gradually reduced the number of mfOPs to 2 (4 blanks) and 1 (2 blanks). In responses gathered against the highest background, the middle OP was abolished first, then the short latency OP while the long latency OP is most resistant (Sequence 1). The reverse sequence is observed with the dimmer background (Sequence 2). The exact same pattern is obtained with the FOP if rate of presentation of the flash stimulus is increased from 1/s to 15/s and the flashes presented against a photopic background (Sequence 1) or in its absence (Sequence 2). Conclusions:Our results provide an explanation for the previously reported enhancement of mfOPs with slow m–sequences. Because of the shorter time interval between consecutive stimuli, the fast m–sequences acts like a flicker and therefore has the same impact on OP genesis as that exerted by a full–field flicker on FOPs. Consequently we feel confident to state that OPs recorded with the mfERG are governed by similar retinal mechanisms as those obtained with full field ERGs and thus most probably represent one and the same thing. Funded by CIHR, FRSQ Réseau–Vision.

Keywords: electroretinography: non–clinical