Human cone light sensitivity and melatonin rhythms following 24-hour continuous illumination

Area of Science:

• Ophthalmology
• Chronobiology
• Neuroscience

Background:

• The human retina possesses complex photoreceptor systems, including cones responsible for color vision and daytime acuity.
• Circadian rhythms, internal biological clocks, regulate numerous physiological processes, but their direct influence on human cone function remains under investigation.
• Understanding retinal circadian rhythms is crucial for diagnosing and managing visual disorders.

Purpose of the Study:

• To investigate the existence of an endogenous circadian rhythm influencing human retinal cone function.
• To assess cone electroretinogram (ERG) parameters over a 24-hour period under controlled light conditions.
• To explore the relationship between cone function rhythms and melatonin secretion.

Main Methods:

• Full-field cone electroretinograms (ERGs) were recorded every 2 hours for 24 hours in nine healthy subjects.
• Recordings were conducted under continuous, rod-saturating white light (53 ± 30 lux) with pupils dilated.
• Salivary melatonin levels were measured, and data were mathematically corrected for linear trends.

Main Results:

• Distinct circadian variations were observed in cone ERG parameters, particularly in b-wave and oscillatory potential implicit times.
• Cone responsiveness was greatest around 20:00 h and least around 06:00 h after correcting for linear trends.
• The ERG circadian rhythm's phase was not synchronized with the salivary melatonin rhythm, and melatonin levels were suppressed under constant light.

Conclusions:

• An endogenous circadian rhythm significantly influences human retinal cone function.
• This cone response rhythm, distinct from melatonin rhythms, may serve as a novel index for central or retinal circadian clock time.
• The observed decline in cone responsiveness under constant light suggests an adaptive mechanism within the cone-driven retinal system.