Biology Reference
In-Depth Information
Polyak's arguments prevailed, but his conclusion raised the
important question of how rod impulses that mediated achromatic
sensation, irrespective of wavelength, and cone impulses that
mediated more than a hundred different hue sensations dependent on
wavelength, could be transmitted through the same neural pathway.
Polyak presented several solutions to this problem:
1.
Since rods and cones may function under different conditions, the two
receptor systems might use the common pathway at different times.
Accordingly, Polyak suggested that, for example, the mop bipolar cells
might mediate achromatic colour sensation in scotopic vision, while in
photopic vision they might mediate bluish colours.
2.
Rod impulses in photopic vision might be excluded from the common
pathway by being inhibited either at the rod-bipolar synapse or in the
bipolar cells themselves by the passage of the cone impulses.
3.
Under conditions where the two receptor systems were activated
simultaneously and both transmitted impulses to the brain, for
example during the transition from photopic to scotopic vision or
vice versa, there would be no serious problem as long as the impulses
from the rods and cones both subserved achromatic colours and
hence could be assumed to represent the same biophysical process.
For conditions where the rod and cone impulses subserved different
colour qualities, the rods achromatic, the cones chromatic sensation,
Polyak offered several alternative solutions: (a) the different qualities
might fuse into one entity, (b) the rod and cone impulses might
alternate in time, (c) the cone impulses could be partly identical to
the rod impulses and partly made up of other components different
from and superimposed upon the rod activity, and (d) the mop
bipolar cells alone might 'pick' the impulses common to both the
rods and cones, whereas the other diffuse bipolar cells (brush and
flat) and the midget bipolar cells might receive impulses predom-
inantly or exclusively from the cones, each variety of bipolar cell
'picking' its own peculiar quality.
Polyak held that these and similar possibilities should be tested
out by experiments before any firm and valid functional interpret-
ation of the common pathways could be decided on.
