Past literature following the discovery of melanopsin has largely investigated direct or consensual pupillary responses as markers of photoreceptor activity.
11–14,17–32 Accordingly, monocular photoreceptor summation of the intrinsic pathway has been found to exhibit the following properties. First, melanopsin-mediated pupillary response amplitude is a function of retinal area stimulated.
11,14 As the stimulation area increases from hemifield to central-field to full-field stimulation, an increasingly larger PIPR is induced.
14 Second, Park and McAnany
27 found that melanopsin-mediated intraretinal responses are well accounted for by corneal flux density (CFD = stimulus luminance × retinal area stimulated), showing that greater PIPR can be induced by increasing retinal area or increasing the stimulus luminance. Consistent with these reports, our results demonstrated that additional retinal recruitment under binocular viewing conditions accounts for the reported amplitude increase in PIPR, a finding consistent with the “photon-counting” properties of the melanopsin system.
9 Similarly, in a recent study, Tsika and colleagues
48 investigated the influence of anterior ischemic optic neuropathy (AION) on intrinsic ipRGC activity. Interestingly, despite diminished monocular pupillary responses to bright blue (high melanopic lux) light relative to controls, no impairment was evident under binocular viewing conditions. The authors suggest that in this clinical population, summation of intrinsic ipRGC signaling from the two eyes may mediate this effect. However, it is important to consider the methodological paradigm used in their study when interpreting these results. First, the high melanopic lux monocular protocol they used was different from that of the high melanopic lux binocular protocol in terms of intensity, recording time, and number of flashes presented. Second, the PIPR was measured at 6 seconds post stimulus offset, at which point the PIPR may not have reached its full magnitude and residual cone influence may still exist.
11 Third, no direct binocular–monocular comparison was made within each group to quantify the magnitude of summation present. Nevertheless, Tsika and colleagues
48 offer intriguing circumstantial evidence suggesting ipRGC binocular summation takes place within the context of a three-part photoreceptor pathway in mediating pupillary constriction, a finding we now demonstrate explicitly in visually normal observers.