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Research
Summary
Phototransduction
The experience of sight begins when particles of light (photons) bounce
off objects, stream into the eye and encounter photoreceptors (rods
and cones). If one of these photons is absorbed by a photoreceptor,
it sets off a cascade of chemical events within the cell, ultimately
leading to a tiny electrical signal. Thus photoreceptors have evolved
into the ultimate light sensors, requiring just a single particle
of light for detection. In my laboratory we are interested in the
molecular events within the cell that generate these minute electrical
signals. We also want to discover how these signals are combined to
create an electrical code for the color, shape and brightness of the
objects we perceive. |
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Recording
Electrical Response.
To record the electrical response, we use electrodes that are constructed
from fine glass tubes drawn to a microscopic point and applied to
single photoreceptors dissected from monkey or human eyes. These measurements
can then be compared to related aspects of human visual perception.
For example, by determining the manner in which the electrical responses
of cones vary with the wavelength of light, we could explain in a
very precise way the appearance of mixtures of lights of different
colors (why red plus green makes yellow). Another example is related
to the visual phenomenon of light adaptation. It is well known that
as light levels increase, we become progressively less sensitive:
that is why birthday candles appear dimmer with the room lights turned
back on. We found that in the case of rods, this reduction of sensitivity
is accompanied not by a reduction in the size of rod signals, but
rather by an increase in electrical noise (like static on your radio).
Ultimately, the signals from single photoreceptors must be used to
reconstruct the visual scene. In order to do this, neurons in the
eye gather information from many photoreceptors by way of specialized
contacts. Surprisingly, this convergence of inputs happens even within
the photoreceptors themselves. We found that in the monkey eye, rods
feed their signals into neighboring cones. We speculate that this
rod/cone connection might be responsible for the secondary loss of
cones in patients with a molecular defect in their rods.
Photoreceptor Studies. Photoreceptors are
the only nerve cells in the eye that are capable of responding directly
to light. The other retinal neurons get light information only by
way of their connections to photoreceptors. Consequently, even if
all the other neurons in the retina are normal, the loss of photoreceptors
in diseases like retinitus pigmentosa or macular degeneration, lead
to blindness. We are now conducting experiments to try to transform
neurons into photoreceptors so that retinas that lack photoreceptors
might regain their light-sensing abilities. |
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