Rushton's paradox: rod dark adaptation after flash photolysis

Area of Science:

• Vision Science
• Photobiology
• Retinal Physiology

Background:

• Rushton's paradox highlights discrepancies between rhodopsin regeneration and rod dark adaptation.
• The Dowling-Rushton equation fails to fully explain human rod dark adaptation after flash photolysis.

Purpose of the Study:

• To resolve Rushton's paradox in human rod dark adaptation.
• To investigate the relationship between quantum absorptions and bleaching adaptation.
• To clarify the underlying mechanisms of scotopic visual adaptation.

Main Methods:

• Comparison of rod dark adaptation curves after photoregenerating flash and quantum-equivalent bleach.
• Experimental rejection of the coincidence hypothesis for adaptation curve agreement.
• Analysis of bleaching adaptation across a broad scotopic energy range.

Main Results:

• Rod dark adaptation is identical after a photoregenerating flash and a 30-second bleach, despite different rhodopsin regeneration levels.
• Bleaching adaptation is determined by the number of rhodopsin molecules absorbing quanta within a specific time frame (600 μs to 30 s).
• This holds true across the studied scotopic energy range.

Conclusions:

• Rod bleaching adaptation is governed by a by-product of scotopic excitation, not photoproducts.
• This by-product's production is monotonic with initial quantum absorptions and precedes the metarhodopsin I to II transition.