Penn Researchers Calculate How
Much the Eye Tells the Brain
(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.
Much 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
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
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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