Rapid Assessment of the Time Course of the Monkey Rod Photoresponse with the Paired Flash Electroretiongram

Abstract

Abstract: : Purpose: The time course of the rod photoresponse may be derived in-vivo from paired flash ERGs.¹ A bright probe flash (e.g. 4.0 log Td-sec) is used in order to drive the rods into saturation before the onset of post-receptoral components. The disadvantage of using a bright probe is the time required (30-120 sec) for the rods to fully recover between flash pairs. The purpose of the present study was to determine if the time course of the rod photoresponse could be accurately determined using a lower probe flash intensity. Methods: Full-field ERGs were recorded from 5 dark-adapted, anesthetized monkeys using a Burian-Allen electrode. The time course of the rod photoresponse was determined for weak test flashes (1.4 & 6.3 Td-sec) followed by 1 of 4 probe flashes (2.3, 2.8, 3.3, 3.9 log Td-sec) at intervals from 10-1200 msec. Probe flash ERG amplitudes were measured at 6 to14 msec, before the a-wave peaks. In a separate experiment, identical probe flashes were presented at intervals from 0.5-60 sec. The time to achieve full rod recovery from a probe flash was determined from the plot of normalized amplitude (second/first response) as a function of inter-flash interval. All flashes were monochromatic (blue) and cone responses (obtained during rod saturation) were subtracted to obtain rod isolated ERGs. Results: The time required to achieve full rod recovery was a linear function of probe flash intensity (linear-log plot) and ranged from 3.6 to 20.1 secs for the 2.8 to 3.9 log Td-sec probes. As a result, the interval between flash pairs was set at 5, 10, 20 and 30 secs for the 2.3, 2.8, 3.3 and 3.9 log Td-sec probes respectively. The time courses and amplitudes of the rod photoresponses derived using the 4 probe flashes were very similar. The peaks of the rod photoresponses occurred at 110-140 msec. For the 2.8 and 3.9 log Td-sec probes, the plots of amplitude at a fixed interstimulus interval (110 msec) against test flash intensity were well described by the same saturating function. Conclusions: The time course of the dark adapted rod photoresponse may be accurately determined using moderately bright probe flashes with relatively short intervals between flash pairs. ¹Pepperberg DR et al. Vis Neuroscience 1997;14;73-82

Keywords: photoreceptors • electroretinography: non-clinical • receptors: pharmacology/physiology