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of the measurements of rhodopsin regeneration.) The relationship
between the log threshold and the percentage of rhodopsin bleached
with exponent 3.7 (time constant of rhodopsin regeneration 360 s) is
shown in
Fig. 24.1
.
Actually, the relationship found between threshold elevation and
the amount of bleached rhodopsin might also be valid for the rat and
skate retina as evidenced by the results of Dowling and Ripps (
1970
).
With these animals they obtained results corresponding to those of
the rod monochromat in the region of 20-70% bleached rhodopsin
by measuring concomitantly the b-wave of the electroretinogram and
amount of rhodopsin bleached. Thus, the available evidence indicated
that in this region a 10% regeneration of rhodopsin was coupled with a
0.5 log unit fall in threshold in humans, rat and skate, suggesting that
the long-term dark-adaptation process was determined by variations
in the amount of bleached rhodopsin, independently of interspecies
variations in neural organization.
Since it was well-known that the electrophysiological (ERG)
technique was relatively insensitive at low intensity levels, the marked
difference between the electrophysiological and psychophysical data
obtained during the last 20% of the rhodopsin regeneration period
could reasonably be explained by the suggestion that the ERG
technique used with the rat and skate did not adequately register
the sensitivity change of the receptors during the last phase of the
dark-adaptation process (Stabell
et al.
,
1989
).
24.3 D i f f e r e n c e s b e t w e e n ro d a n d c o n e
dark adaptation
To investigate whether the new formula obtained could be applied
to cone vision, a series of cone dark-adaptation curves was measured
and compared to cone photopigment regeneration curves provided by
Rushton and co-workers (see Stabell & Stabell,
1989
; Stabell
et al
.,
1990
).
The psychophysical results revealed an interesting similarity
between the rod and cone dark-adaptation curves. Thus, it was found
