explaining the cell layers of the retina.
Robert Sanders photo
scientists realize that the eye is not merely a camera providing
digital input to the brain, the general consensus has been
that the world projected onto the retina and detected by
cells called photoreceptors got sent to the brain after
some relatively simple processing.
and Werblin showed that retinal cells do a lot of processing
to extract only the essence of the picture to send to the
brain. The anatomy of the retina is layered to facilitate
initially impinges on the light-sensitive cells of the eye,
the photoreceptors, which fire off signals to a layer of
horizontal cells and thence to bipolar cells. Since 1969,
Werblin has been recording from all retinal cells and has
detailed how each cell type processes data from the photoreceptors.
cells funnel signals down their axons the outgoing
wires of the nerve cell and relay them to the dendrites
or input wires of ganglion cells, which send the processed
information to the brain. All these cell types are arrayed
in unique layers, stacked one atop the other.
excites the photoreceptors (top), which relay signals
through the horizontal cells (green) to the bipolar
cells, which in turn send signals through the various
layers of dendrites to the ganglion cells (bottom).
The ganglion cells bundle together into the optic nerve
and carry the output of the eye to the brain. See video
page for detailed explanation.
Frank Werblin image
noted earlier that all ganglion cells were not alike and
that they fired off different information to the brain,
though the details were hazy. Part of the reason is that
the axons from the bipolar cells synapse with or touch the
dendrites of the ganglion cells in a tangled region (the
inner plexiform layer) that made biologists despair of making
sense of the connections.
discovered, however, that this region of tangled axons and
dendrites is really laid out in orderly strata. By staining
the cells from which he recorded, he found that bipolar
cell axons converge on 12 or so well-defined layers, where
they synapse with the dendrites of the ganglion cells. Each
layer of dendrites belongs to a specific population of ganglion
interaction between layers, though, the signal emerging
from the tangle would not be much different from the original
12-channel output of the bipolar cells. The critical element
is another type of cell, the amacrine cells, which send
processes to the various layers of dendrites and allow the
layers to talk with one another. This cross-talk is what
allows the layers to process the visual data and extract
the sparse information that the ganglion cells send up to
layers actually converse with each other, they make comparisons
and subtractions and differences," Werblin said. "They
say, what is the essential feature here. Then they send
out these 12 or so moving pictures to the brain.
experimental technique allowed him not only to measure the
output of ganglion cells, but the excitations they received
from bipolar cells and the inhibitions they received from
amacrine cells. With this information he is now reconstructing
the conversations between layers that result in the final
output going to the brain.
one layer is being read out by, let's say, a hundred thousand
cells, each with its own axon sending information to the
brain. There's another layer reading out another hundred
thousand. And all of those combine in the full optic nerve,
which is carrying maybe a dozen different masses of fibers,"
Werblin explained. "Eventually many of these movies
get to the visual cortex, the entry to whatever we use to
Roska is returning to Hungary for a year, he will continue
his experiments there on the amacrine cells.
"The fundamental question now is, why do certain layer
talk to one another," said Roska, who a year from now
will embark on a prestigious three-year fellowship at Harvard
University as a junior fellow of the Society of Fellows
when people studied ganglion cells, they would look at the
cell and flash lights. One of Botond's major contributions
to this was, he thought about this not as the cell, but
as the layer of processes from which the cell is reading.
So, we began to think in terms of layers, and all of the
activity we measured corresponded to what happened in a
particular layer," Werblin explained. "Then it
became clear that these layers were actually talking to
each other. Previously no one had even thought that these
layers talked to one another, even though 100 years ago
the picture was there. No one had really looked at that
was supported by grants from the Office of Naval Research
and the National Institutes of Health.