# Illumination by short-wavelength light inside the blind spot decreases light detectability

## TL;DR

Blue light in the blind spot, where there are no photoreceptors, reduces the ability to detect light in the surrounding area, suggesting melanopsin's role in light detection.

## Contribution

Demonstrates that melanopsin in the blind spot affects light detectability through wavelength-specific mechanisms.

## Key findings

- Blue light in the blind spot decreases detectability of light in the dark.
- Red light in the blind spot does not affect light detectability.
- Melanopsin may modulate baseline noise for weak light detection.

## Abstract

Although the optic disk corresponding to the blind spot contains no classical photoreceptors, it contains photopigment melanopsin. To clarify whether melanopsin is involved in light detection, we conducted detection tasks for light stimuli presented in the normal visual field, with and without another illumination inside the blind spot. We found that a blue blind-spot illumination decreased the light detectability on a dark background. This effect was replicable when it was determined immediately after the blind-spot illumination was turned off, suggesting the contribution of a sluggish system rather than scattering. Moreover, the aforementioned effect was not observed when the blind-spot illumination was in red, indicating wavelength specificity in favor of melanopsin’s sensitivity profile. These findings suggest that melanopsin is activated by the blind-spot illumination and thereby interferes with light detection near the absolute threshold. Light detection originating from conventional photoreceptors is modulated by melanopsin-based computation presumably estimating a baseline noise level.

•Light inside the blind spot is unseen but can be received by photopigment melanopsin•Blue light within the blind spot lowered light sensitivity elsewhere in the darkness•Sensitivity did not change if instead we used red light to which melanopsin is blind•Melanopsin may be monitoring the ambient light levels in the detection of weak light

Light inside the blind spot is unseen but can be received by photopigment melanopsin

Blue light within the blind spot lowered light sensitivity elsewhere in the darkness

Sensitivity did not change if instead we used red light to which melanopsin is blind

Melanopsin may be monitoring the ambient light levels in the detection of weak light

Sensory neuroscience; Biophysics