Functional imaging of the cone photoreceptor using adaptive optics

Ravi Jonnal, UCD

In vertebrate eyes, vision begins when the photoreceptor's outer
segment absorbs photons and generates a signal destined for the
brain. As the locus of the origin of vision, the outer segment's
structure and function are of great interest, to expand our
understanding of both normal vision and visual dysfunction. The past
fifteen years have seen astounding growth in our ability to observe
this fundamental component of the living human visual system, largely
due to the application of adaptive optics (AO) in the field of retinal
imaging. AO has allowed unprecedented resolution of retinal
structures; cones in particular have enjoyed hundreds of studies, in
vivo, of their structure, size, topography, alignment, optical
density, spectral properties, and sampling of the ocular image. Cone
function, by contrast, has received comparatively little attention,
for a few key reasons: the optical correlates of cone function are
small and difficult to discriminate from the noise intrinsic to
retinal imaging systems; physiological processes may be rapid and
microscopic, requiring correspondingly fast and sensitive imaging
systems; and imaging cellular function requires tracking of eye
movements with sub-cellular precision. The main objective of my
research has been to use AO to observe and measure functional changes
in living human cone photoreceptors. This has required the combined
application of AO, imaging systems capable of acquiring images of the
retina with speed and signal sufficient to measure functional changes,
and post-processing algorithms capable of tracking and monitoring the
cells’ optical properties. In this talk I will describe these tools
and their application to the investigation of two aspects of cone
function: phototransduction--the cascade of biochemical and structural
changes evoked by visible stimulation of the cell--and cellular
renewal--the process by which the cone maintains its structural and
biochemical integrity.