Slow and fast pathways in the human rod visual system - CVRL main

1664 J. Opt. Soc. Am. A/Vol. 8, No. 10/October 1991rods signals both to be evident in the b wave of the ERG,the two signals must be segregated at or before the bipolarcell level.Where then does the cancellation take place? One attractivepossibility is that it occurs at the depolarizing(ON) cone bipolars, one of the points at which the twoanatomical pathways conveying rod signals in the mammalianeye converge. At the scotopic levels at which the15-Hz null is found, the cone bipolars might be driven bythe rod signals traveling through the cone photoreceptorsand the rod signals traveling through the rod bipolars andthe AII amacrine cells. The most obvious problem withthis model, however, is that the signal in the rod bipolarcell, although able to cancel the signal in the depolarizingON cone bipolar, remains itself uncanceled. And, despiteits being uncanceled, there is no trace of it in the b waveof the ERG, even though its (assumed) signal is clearly evidentbelow the null. Another problem with this type ofmodel is that the rod signal entering the cones shouldappear in both ON- and OFF-cone bipolars, so that thesignal in the OFF-cone bipolars remains uncanceled too.To some extent, we can contend with these difficulties byarguing that the null in the b wave is the result of theelectrical cancellation of signals in the separate rod andcone bipolars, whereas the perceptual null is due to neuralcancellation at a later site, say, at the ganglion cell layer.But then one must additionally assume that the electricalcancellation at the bipolar-cell level coincides exactly withthe physiological cancellation at the later stage.There is a further objection, however, to the argumentthat the faster rod signals travel over the cone gap-junctionpathway and that the locus of interaction between theslower and faster signals is the depolarizing cone bipolars.It concerns the typical, complete achromat observer;namely, how does the faster pathway originate in the achromatobserver for whom there is no psychophysical evidenceof postreceptoral cone vision and for whom themajority of the anatomical evidence 2 l- 2 4 suggests that conephotoreceptors are altogether missing or reduced to atmost 5-10% of their normal population (but see the discussionabove)?Thus attempting to correlate the psychophysically isolatedslow and fast pathways with the electrophysiologicallyrevealed rod bipolar and cone gap-junction pathwaysmay turn out to be misdirected. 5 4 It seems possible thatthe interaction leading to the cancellation is taking placeeither at the rod bipolars themselves or before the bipolarcell level, possibly within the rod spherules, and that horizontalcells are implicated somehow. 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