(Philadelphia, PA) - Researchers at the University
of Pennsylvania School of Medicine estimate that the human
retina can transmit visual input at about the same rate as an Ethernet
connection, one of the most common local area network systems used
today. They present their findings in the July issue of Current
Biology. This line of scientific questioning points to ways
in which neural systems compare to artificial ones, and can ultimately
inform the design of artificial visual systems.
research on the basic science of vision asks what types of information
the brain receives; this study instead asked how much. Using an
intact retina from a guinea pig, the researchers recorded spikes
of electrical impulses from ganglion cells using a miniature multi-electrode
array. The investigators calculate that the human retina can transmit
data at roughly 10 million bits per second. By comparison, an Ethernet
can transmit information between computers at speeds of 10 to 100
million bits per second.
The retina is actually a piece of the brain that has grown into
the eye and processes neural signals when it detects light. Ganglion
cells carry information from the retina to the higher brain centers;
other nerve cells within the retina perform the first stages of
analysis of the visual world. The axons of the retinal ganglion
cells, with the support of other types of cells, form the optic
nerve and carry these signals to the brain.
Investigators have known for decades that there are 10 to 15 ganglion
cell types in the retina that are adapted for picking up different
movements and then work together to send a full picture to the brain.
The study estimated the amount of information that is carried to
the brain by seven of these ganglion cell types.
The guinea pig retina was placed in a dish and then presented with
movies containing four types of biological motion, for example a
salamander swimming in a tank to represent an object-motion stimulus.
After recording electrical spikes on an array of electrodes, the
researchers classified each cell into one of two broad classes:
“brisk” or “sluggish,” so named because
of their speed.
The researchers found that the electrical spike patterns differed
between cell types. For example, the larger, brisk cells fired many
spikes per second and their response was highly reproducible. In
contrast, the smaller, sluggish cells fired fewer spikes per second
and their responses were less reproducible.
But, what’s the relationship between these spikes and information
being sent? “It’s the combinations and patterns of spikes
that are sending the information. The patterns have various meanings,”
says co-author Vijay Balasubramanian, PhD, Professor
of Physics at Penn. “We quantify the patterns and work out
how much information they convey, measured in bits per second.”
Calculating the proportions of each cell type in the retina, the
team estimated that about 100,000 guinea pig ganglion cells transmit
about 875,000 bits of information per second. Because sluggish cells
are more numerous, they account for most of the information. With
about 1,000,000 ganglion cells, the human retina would transmit
data at roughly the rate of an Ethernet connection, or 10 million
bits per second.
“Spikes are metabolically expensive to produce,” says
lead author Kristin Koch, a PhD student in the
lab of senior author Peter Sterling, PhD, Professor
of Neuroscience. “Our findings hint that sluggish cells might
be ‘cheaper,’ metabolically speaking, because they send
more information per spike. If a message must be sent at a high
rate, the brain uses the brisk channels. But if a message can afford
to be sent more slowly, the brain uses the sluggish channels and
pays a lower metabolic cost.”
“In terms of sending visual information to the brain, these
brisk cells are the Fedex of the optic system, versus the sluggish
cells, which are the equivalent of the U.S. mail,” notes Sterling.
“Sluggish cells have not been studied that closely until now.
The amazing thing is that when it’s all said and done, the
sluggish cells turned out to be the most important in terms of the
amount of information sent.”
Study co-authors are Judith McLean and Michael A. Freed, from Penn,
and Ronen Segev and Michael J. Berry III, from Princeton University.
The research was supported by grants from the National Institutes
of Health and the National Science Foundation.
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