Rhodopsin is an extremely sensitive molecule.  As long as the light is of low intensity, rods quickly regenerate rhodopsin and the retina continues to respond to light stimuli.  In high light intensity rhodopsin is bleached as quickly as rhodopsin can be produced making some of the rods nonfunctional.  If this happens the cones take over.  There is an automatic adjustment of retinal sensitivity to the amount of light present.  This automatic adjustment is not only explained by the breakdown of photoreceptor pigments, other retinal neurons are involved


Light adaptation

This occurs when we move from the dark into bright light.   The bright light momentarily dazzles us and all we see is white light because the sensitivity of the receptors is set to dim light.  Rods and cones are both stimulated and large amounts of the photopigment are broken down instantaneously, producing a flood of signals resulting in the glare.


Adaption occurs in two ways:

  • The sensitivity of the retina decreases dramatically.

  • Retinal neurons undergo rapid adaptation inhibiting rod function and favouring the cone system.


Within about one minute the cones are sufficiently excited by the bright light to take over.  Visual accuracy and colour vision continue to improve over the next ten minutes.  During light adaptation retinal sensitivity is lost.


Dark adaptation

Dark adaptation is essentially the reverse of light adaptation.  It occurs when going from a well light area to a dark area.  Initially blackness is seen because our cones cease functioning in low intensity light.  Also, all the rod pigments have been bleached out due to the bright light and the rods are initially nonfunctional.  Once in the dark, rhodopsin regenerates and the sensitivity of the retina increases over time (this can take approximately one hour).  During these adaptation process reflexive changes occur in the pupil size.

Dark adaptation curve.  The shaded area represents 80% of the group of subjects. Hecht and Mandelbaum's data from From Pirenne M. H., Dark Adaptation and Night Vision. Chapter 5. In: Davson, H. (ed), The Eye, vol 2. London, Academic Press, 1962.(6 K jpeg image)

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