Biology Reference
In-Depth Information
6.6 E x p l a n at i of n s of f ac u i t y a n d s e n s i t i v i t y
differences between rods and cones
Schultze (
1866
) in his formulation of the duplicity theory could give no
adequate explanation of the difference in acuity performance obtained
psychophysically between the rod and cone receptor systems. Since
the rod receptors were closely packed, more slender and also much
more numerous than the cones, Schultze had expected, contrary to
the facts, that rods would generally show better acuity performance
than the cones.
A solution to this paradox was found when the Golgi method
was introduced to reveal fine structural relationships in the retina.
Thus, the histological investigations of both Cajal and Polyak revealed
that the rod system was arranged in spatially larger functional units
than the cone system. Polyak's investigation was the most important
one, since he investigated retinas of primates and also discovered
the so-called midget cone system, i.e. a monosynaptic relationship
between cones and bipolar cells and between bipolar and ganglion
cells, affording a 'private' conducting cone system in the central
fovea of the primate. Polyak held that such a system would be best
suited to serve as a mechanism for the most delicate space resolving
and localizing functions. Its degree of refinement would depend on
the cone gradient only and, hence, peak in the centre of the fovea.
A complication was introduced in that the midget system was
strictly monosynaptic only in the central foveal region becoming
gradually coarser out towards the orea serrata, since polysynaptic
connections increased collecting impulses from several cones into the
functional units. Hence, acuity would no longer depend on cone gradient
alone, but also upon the synaptic relations of the midget system.
The reason for the relatively low acuity performance of the rod
system was clearly revealed when Polyak showed that in the central
part of the retina several rods were connected to a functional unit and
that the rod system, even more than the midget cone system, displayed
an increasing coarsening of the functional unit from the foveal region
