Journal of Neuroscience, Vol 16, 2251-2260, Copyright © 1996 by Society for Neuroscience
Photopigment transmittance imaging of the primate photoreceptor mosaic
OS Packer, DR Williams and DG Bensinger
Center for Visual Science, University of Rochester, New York 14627, USA.
We introduce a new technique for classifying many photoreceptors
simultaneously in fresh, excised primate retina on the basis of their
absorptance spectra. Primate retina is removed from the pigment epithelium
and illuminated under a microscope from the same direction as in the intact
eye. To facilitate the guiding of light into the receptor outer segments,
the optical axes of the photoreceptors are oriented parallel to the optical
axis of the microscope. Photoreceptor outer-segment tips are imaged on a
charge-coupled device array, which provides radiometric measurements of the
light passing through each photoreceptor. These images are acquired
sequentially at three wavelengths chosen to maximize the absorptance
differences among the three cone photopigments. After the photopigment is
bleached, a second set of three images is acquired. The ratios of the
images before and after bleaching at each wavelength are photopigment
transmittance maps of the retina. These are combined into a single
trichromatic image showing the distribution of photopigment if the retina
could be viewed directly in white light without bleaching. We have found
patches of receptors in peripheral macaque retina where the measured
absorptance at the wavelength of maximum absorptance is consistent with the
predicted axial absorptance of th photopigment. The cones in these patches
cluster into two groups corresponding to the middle wavelength- sensitive
(n = 53, mean absorptance = 0.28) and the long wavelength- sensitive (n =
63, mean absorptance = 0.30) cones. The mean absorptances of 273 macaque
and 183 human rods were 0.51 and 0.41, respectively.
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