Because of the intrinsic PLR, the overall pupil constriction in an illuminated eye should be stronger than the consensual constriction in the contralateral, unilluminated eye. Accordingly, we simultaneously monitored both pupils of a mouse while subjecting one eye to Ganzfeld illumination (2-min light step; Methods
). For WT animals, the intensity-response (I-R) relations for the ipsilateral and contralateral PLRs were identical at the dimmest intensities but diverged thereafter, with the ipsilateral PLR indeed being always stronger within a given animal (, top panel). For Opn4−/−
animals, some bilateral asymmetry persisted but it was noticeably smaller especially at high intensities, with both I-R relations being broadly similar to WT except for a lower maximal PLR as found previously18
(, second panel from top). For mice lacking rod and cone signals15
), the I-R relation was shifted to much higher light-step intensities owing to exclusive signaling by melanopsin18
, but the bilateral asymmetry at high intensities again became more obvious than Opn4−/−
(, third panel from top). The residual asymmetry in Opn4−/−
may suggest slightly stronger rod/cone signals to the ipsilateral PLR as well, perhaps explaining some of the WT ipsi/contralateral disparity in PLR especially at lower intensities. Finally, Gnat1−/−
mice had practically no steady PLR (Ref. 31
, but see Ref. 32
) (, bottom panel).
Figure 6 Simultaneous direct (ipsilateral) and consensual (contralateral) PLRs to unilateral illumination for different mouse genotypes in situ. 2-min of 505-nm LED light in a–c. PLR in ipsilateral, illuminated eye was measured at peak during this 2-min (more ...)
The above difference in bilateral asymmetry between WT and Opn4−/−
PLRs cannot distinguish between an intrinsic iridic PLR and a bilateral asymmetry in ipRGC signaling to the PLR because both mechanisms involve melanopsin. To isolate the intrinsic PLR, we eliminated retinal signaling from one eye in the WT mouse by transecting its optic nerve (Methods
). When the denervated eye was illuminated (at >7 days postsurgery), the intact contralateral eye failed to respond as expected, whereas the PLR persisted in the denervated eye (, right panel), with the action spectrum of melanopsin (Supplementary Fig. S8
). This residual component is the isolated intrinsic PLR. Its I-R relation on the intensity axis (, left panel) relative to that for the ipsilateral PLR of non-operated WT animals (, top panel) indicates that the intrinsic PLR begins to contribute when the normal overall PLR is ~90% complete. Nonetheless, the intrinsic PLR even by itself would have been able to drive the pupillary constriction 80–90% to completion over ~2.5 log units of light-step intensities. Furthermore, the intrinsic PLR has, in reality, an even lower light threshold (thus contributing even more to the overall PLR) because its I-R relation shifted by ~1 log unit to lower intensities after topical application of TTX to the cornea (, left panel), which blocked any tonic autonomic inputs to the dilator and sphincter muscles. In short, the intrinsic component participates in the highest ~3.5 log units of the overall ~9-log-unit dynamic range of light intensities spanned by the normal PLR in mouse, at least during 2 min of steady illumination.
To translate into natural conditions, the yellow-shaded region in , left panel, spans approximately from laboratory light to outdoor daylight (Methods). We also directly simulated ambient light with white xenon-arc light (400–650 nm) of matched power (Methods). For example, in room light, an ipsilateral (i.e., intrinsic) PLR of 0.47±0.10 fractional constriction was elicited from the denervated eye of WT mice (, 5 animals), versus 0.89±0.02 when ipRGCs were also active (Gnat1−/−
cl mice without transected optic nerve) (, 6 animals). Thus, the intrinsic PLR contributes substantially to the melanopsin component of the overall PLR even in room light. Finally, exposing the bilaterally-denervated eyes of a dark-adapted mouse to genuine room or outdoor light indeed led to intrinsic PLRs of extents expected from above (not shown).
Similar denervation experiments on rhesus and owl monkeys revealed no intrinsic PLR (2 animals each; Supplementary Text
), consistent with the above negative findings from their sphincter muscles and with clinical observations from human patients presenting complete unilateral optic neuropathy (resulting in loss of photosensitivity in the affected eye) (Supplementary Text