Each of your eyes transfers information to your brain at about the same speed as a fast Ethernet connection, US researchers have calculated.

That might sound impressive, but the scientists say that our neurons could move data a lot faster than that. The fact that they do not suggests that our nervous system is trading off speed against energy-efficiency.

The researchers, based at the University of Pennsylvania in Philadelphia, US, attached a guinea pig retina to an array of electrodes, while sustaining it in a nutrient-rich soup. They projected a number of images onto the retina, and recorded the patterns of electrical firing in the retina’s ganglion cells. These cells feed information down the optic nerve, towards the brain’s visual cortex.

From these recordings, the researchers calculated that a guinea pig retina transfers data at about 875 kilobits per second. Human retinas have about ten times as many ganglion cells, giving a “bandwidth” of 8.75 megabits per second.


But it could be faster, says physicist Vijay Balasubramanian, one of the study’s authors. “Each neuron is capable of firing close to once a millisecond, but average activity is only four times a second,” he says. “You have to ask: Why is this?”

Brisk and sluggish

The answer is energy consumption. The human brain makes up only 2% of our total body mass, but requires 20% of our metabolism to keep it running. Neural pathways eat up a lot of energy, and the guzzling gets worse at high speeds.

To compensate for this, nature has divided retinal ganglion cells into two general categories – brisk and sluggish. The most important visual information you receive is brought to you by brisk cells. Moving data at up to 13 bits per second per cell, they detect rapidly moving objects like predators or prey – where a split second could mean the difference between life or death.

Only about 30,000 of the 100,000 cells in the guinea pig retina are brisk, so sluggish cells end up doing most of the work. Each of them operates at a bandwidth of 6 to 8 bits per second. They mostly perform unglamorous duties such as detecting an object’s edges or stabilising your eyes while you walk.

In the end, Balasubramanian thinks that the lopsided distribution of ganglion cells in the retina is the result of an evolutionary cost-benefit analysis. Brisk cells seem less efficient: the researchers found that each electrical spike in sluggish cells carried about 2.1 bits of information. Although they fired more frequently, the brisk cells could only manage about 1.8 bits per spike.

“You don’t get much more information for a lot more energy consumption,” says Balasubramanian, who believes the same cost-benefit equation could be in play throughout the brain. “We could all think a lot faster if it wasn’t so expensive.”

Journal reference: Current Biology (vol 16, p 1428)