Biology Reference
In-Depth Information
27
Modern conceptions of
sensitivity regulation
During the last two decades the presumption that sensitivity regula-
tion is basically controlled by photochemical processes located in
the outer segment of the receptors has been replaced by much more
complex theoretical conceptions. A recent review paper on gain
controls in the retina by Dunn and Rieke ( 2006 ) gives an excellent
illustration of this development. The authors argue that there are
multiple retinal gain controls (i.e. adaptation mechanisms) that
adjust sensitivity to different aspects of the light stimulus such as
changes in mean intensity, variability about the mean (i.e. contrast
variability) and spectral composition, and that the gain controls have
diverse temporal and spatial properties, serve different functional
roles and are located at different sites in the retina. Indeed, an
additional dimension of complexity is introduced in that the gain
controls are assumed to interact with other computations carried out
in the retinal pathways.
In support of their suggestion that gain controls of the cone
system may operate at different sites in the retina, they present
evidence that both small and large steps in mean intensity and
contrast may alter the gain adaptation level of ganglion cells, while
only large steps change the gain of the receptors. Strong support in
favour of the suggestion that gain controls may operate at different
sites had previously been provided by Ahn and MacLeod ( 1993 ) and
Yeh et al . ( 1996 ).
Dunn and Rieke ( 2006 ) argued that gain controls of the cone
and rod systems must be fundamentally different, since the intrinsic
noise of rods is several log units less than that of cones. This, of course,
gives the rods the advantage of being able to operate at extremely
low light levels, but the scarcity of photons absorbed by the rods
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