Seeing in the dusk: physiological basis and investigation

Abstract

Dark adaptation and seeing in the dusk require a complex interaction of the cone and rod system. Whereas the former provides high temporal resolution and colour vision in daylight, temporal resolution of the latter is smaller but sensitivity higher by a factor of 100 to 1,000. The two operational ranges overlap by several decades. Characteristic symptoms of disease may be derived from the systems' physiological function and should be enquired about specifically. Problems due to opacity of the optic media or reduced visual acuity should be differentiated from night vision disorders in the more specific sense. Apart from a routine ophthalmological examination a set of other tests should be used: When rods function normally the second limb of the dark adaptation curve initiates at 5 - 12 min und reaches a normal absolute threshold after 30 - 40 min; dark adapted visual fields are unrestricted apart from a physiological central scotoma, and the dark adapted electroretinogram (ERG) shows normal amplitudes and latencies for low intensity flashes. The influence of glare on mesopic contrast sensitivity may be investigated with a mesoptometer. Night vision disorders may arise from cone system dysfunction, too. Intact cone vision provides high visual acuity, normal colour vision, normal photopic visual fields, a quick regeneration within minutes during the first limb of the dark adaptation curve and normal single flash and oscillatory potentials in the light-adapted ERG. Localised defects may be detected using the multifocal ERG. Interpretation of the results must account for the age-related decay of contrast sensitivity and speed of adaptation.