Pupillary light reflex to light inside the natural blind spot

Abstract

When a light stimulus covers the human natural blind spot (BS), perceptual filling-in corrects for the missing information inside the BS. Here, we examined whether a filled-in surface of light perceived inside the BS affects the size of the short-latency pupillary light reflex (PLR), a pupil response mediated by a subcortical pathway for unconscious vision. The PLR was not induced by a red surface that was physically absent but perceptually filled-in inside the BS in the presence of a red ring surrounding it. However, a white large disk covering the BS unexpectedly induced a larger PLR than a white ring surrounding the BS border did, even though these two stimuli must be equivalent for the visual system, and trial-by-trial percepts did not predict PLR size. These results suggest that some physiological mechanism, presumably the retinal cells containing the photopigment melanopsin, receives the light projected inside the BS and enhances PLR.

Invisible light inside the natural blind spot alters brightness at a remote location

“…It is known that the somas of ipRGCs are located outside the BS, receive light on their own, and also process visual signals originating from the rods/cones 10 , 12 – 14 . Excitation of the ipRGC axons in the optic disk may contribute to brightness perception via neural pathways of the ‘image-forming vision,’ in addition to contribution to non-image-forming vision, as reported in our previous study related to PLR enhancement 8 . Moreover, melanopsin has peak light absorption at 482 nm 15 , indicating that it is most responsive to blue light 16 .…”

“…We propose that BS illumination uniformly influences perception within the entire visual field outside the BS. In our previous report 8 , BS illumination cannot by itself trigger the PLR but is able to enhance the amount of PLR activated by abrupt uniform increment of luminance outside the BS. Thus, BS illumination facilitates ocular ‘photometry’ of the entire visual field.…”

“…Prior to the main experiments, the blind spot (BS) of each observer’s right eye was mapped using the method reported previously 8 , 29 . First, an oval stimulus was presented to the right of a fixation point located at the centre of the screen.…”

“…Hence, we experience a seamless visual world. Recently, we found that, even though the BS has no rods/cones, the involuntary pupil reflex—known as the short-latency pupillary light reflex (PLR) 7 , 8 —in response to light increments outside the BS was enhanced by concurrent light exposure within the BS 8 . This enhancement was not related to perceptual filling-in; instead, direct light stimulation inside the BS was the key factor.…”

The Method of Silent Substitution for Examining Melanopsin Contributions to Pupil Control

“…For example, the retinal location corresponding to the blind spot does not contain rods and cones, but light might stimulate melanopsin in the axons of ipRGCs. Delivering a stimulus only in the blind spot would therefore ensure that only melanopsin would be activated ( 58 – 60 ), but there could be scatter on rod and cone photoreceptors near the blind spot, and accidental displacement of a small circumscribed stimulus field would need to be controlled for. In the temporal domain, melanopsin photoreception is much slower than cone- and rod-mediated photoreception, and thus, the temporal properties of a stimulus can be optimized to bias the measured response toward melanopsin-mediated properties, e.g., the steady-state pupil size under continuous light ( 61 ).…”