The name is absent

73

5.2 Measurement of coupling resistance using light stimuli

We estimated the space constant, the distance for the voltage to decay e fold, in the sala-
mander retina using a bar shaped light stimulus, which is non-invasive and physiologic. We
recorded the voltage response of rods and cones in response to a flahsed bar of light stepped
across the retina in 5 μm increments. With a bar shaped stimulus, the voltage distribution in
the rod network is reduced from a two dimensional distribution to one dimension, simplify-
ing our analysis. Light was projected onto the whole retina using a custom made projection
system (see Methods), and voltage responses over two orthogonal directions were averaged.
Figure 5. IA shows an image of the bar projected onto the retina. First, to assess the effect
of light scattering by the retina on the stimulus, we measured the response of a cone to the
bar of light. Because cones are only weakly coupled to rods, and because the background
light of the projector largely saturated the rod response, this cone response represents the
response of a single photoreceptor to the profile of the scattered light. Figure 5.1 B shows
the example traces of the voltage response of a rod and cone to the bar stimulus.

With a -1 log neutral density filter installed to attenuate the light intensity, we performed
the same experiment on rods. The average maximum response for 6 rods measured several
times over two directions was 3.5 mV. Without the -1 log filter, the average peak cone re-
sponse for several replications of two directions in 3 cells was 6.4 mV. Figure 5. IC shows
the spatial profiles of the rod and cone response peaks centered around their maximal re-
sponse. As expected, rods showed a slower falloff in their response than cones as the bar
moved farther from the cell center. This effect is due to both coupling between rods, and

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