Eye Sends Information at Ethernet Rates
Neuroscientists from Pennsylvania and New Jersey calculated the information rate of the eye. Using guinea pigs (real guinea pigs, not humans as guinea pigs), they came up with a number and interpolated it for humans:
In the classic “What the frog’s eye tells the frog’s brain,” Lettvin and colleagues showed that different types of retinal ganglion cell send specific kinds of information. For example, one type responds best to a dark, convex form moving centripetally (a fly). Here we consider a complementary question: how much information does the retina send and how is it apportioned among different cell types? Recording from guinea pig retina on a multi-electrode array and presenting various types of motion in natural scenes, we measured information rates for seven types of ganglion cell. Mean rates varied across cell types (6-13 bits • s-1) more than across stimuli. Sluggish cells transmitted information at lower rates than brisk cells, but because of trade-offs between noise and temporal correlation, all types had the same coding efficiency. Calculating the proportions of each cell type from receptive field size and coverage factor, we conclude (assuming independence) that the approximately 105 ganglion cells transmit on the order of 875,000 bits • s-1. Because sluggish cells are equally efficient but more numerous, they account for most of the information. With approximately 106 ganglion cells, the human retina would transmit data at roughly the rate of an Ethernet connection.
Their article, published in Current Biology,1 also discussed the difference between sight and sound processing: specifically, why is auditory information sent to the brain at much higher efficiency? Frog auditory nerve fibers, for instance, “are reported to encode naturalistic stimuli with an efficiency sometimes reaching approximately 90% of capacity,” three-fold better than optic fibers. “Naturally one wonders why an optic fiber fares so poorly in these comparisons,” they said, then proposed an answer based on the different ganglion cell types and the difference in information fields between sight and sound:
Auditory fibers apparently achieve their high coding efficiency via a “tuned” nonlinear filter that selectively amplifies the anticipated signal. A similar strategy is apparently used by the mammalian rod bipolar cell to encode single photon responses. However, this coding strategy, highly effective when the anticipated signal is sparse and well defined, may serve poorly for ganglion cells because the information of biological interest in natural scenes is so varied that highly tuned, nonlinear filters would either reject too much information or require too many cell types.
Given the ganglion cell strategy of broad tuning and equal coding efficiency, why does the retina not send all visual information over one cell type with a high information rate? This is possibly because the energetic cost of signaling increases nonlinearly with temporal frequency and information rate of individual axons.
That’s why many of the ganglion cells are of the “sluggish” variety. “Because the dominant metabolic cost in neural signaling is associated with spiking, the cables with lower firing rates would save considerable energy. Likewise, theoretical studies predict that metabolic cost is minimized when signals are distributed over many weakly active cells.” That may not be the only reason for multiple cell types. There’s a lot of processing the eye has to do. Some cells zero in on the narrow details, and others need to summarize a rapidly-changing big picture. The solution is a mixture of cell types, to optimize the benefits and trade-offs of each sensory strategy:
Spatial acuity requires narrow-field cells with a high sampling rate. Because such a type must necessarily distribute densely, its information rate should be relatively low to reduce costs. On the other hand, encoding of high stimulus velocities requires extended spatial summation and thus a broad-field cell—plus the ability to transmit at high bit rates so as not to lose the higher temporal frequencies. Such a cell type must necessarily be expensive, but given the extended dendritic field, this type can be sparse. Consequently energetic considerations probably interact with other constraints to set the number of cell types and a general information rate of roughly 10 bits • s-1 and 2 bits • spike-1.
By the way, the so-called “sluggish” ganglion cells spiked at up to 75 times per second (though averaging 4 per second over the recording time). Some of the rapid cells spike at over 300 times per second. No wonder your eyes get tired.
Incidentally, this paper did not mention anything about the evolution of these capabilities for the frog, the guinea pig, or the human.
1Koch et al., “How Much the Eye Tells the Brain,” Current Biology, Volume 16, Issue 14, 25 July 2006, pages 1428-1434, doi:10.1016/j.cub.2006.05.056.
When our body’s capabilities are compared with machinery, the comparisons are often wonderful and amazing. If 10 megabit-per-second ethernet-eyes don’t seem particularly cutting-edge in this age of gigabit-ethernet rates, consider that eyes are only one of millions of sensors across the body transmitting information on touch, taste, smell, and hearing as well as vision. In its little 3-pound CPU, the brain must process that information 24 x 7 for decades. Plus, the kind of information your brain handles is in neural-net form, not the serial data that computers process. It is sent down tiny bundles of neurons in a package that doesn’t short out when you go swimming.
Imagine yourself in a recording studio, watching an orchestra playing a score in sync with a new movie about Robinson Crusoe. Your brain is taking in the complex waveform of a hundred instruments and performing Fourier transforms on it such that you can make out each individual instrument. Simultaneously, the eyes can see the rapid motions of the violinists’ bows, the action on the screen, and the static information from the studio walls and ceiling. Millions of touch sensors are sending information on the temperature of the room, the feel of your socks, the comfort of the seat, how hungry you are, and much more. Your tongue is reporting the mint candy in your mouth. Your nose is deciphering complex chemical signals in the environment through a series of decoding maps. The brain filters and focuses on information that is important for each moment. As the intensity of the music or screen action rises, adrenaline races through your system switching on organs to be ready for action. Next, your mind is transported to an exotic island, and you become Robinson Crusoe, using all your native senses to the hilt to survive and find your next meal.
And this all runs on potatoes! (as Dr. A. E. Wilder-Smith used to say), or on lettuce! (for the guinea pig). Undoubtedly, if we were aware of all the factors involved in the transfer of information from the environment to the mind via our sensory apparatus, the comparison with ethernet transfer rates would seem foolishly simplistic. In this story we have seen again that the more detail is shared about organic workings, the less there is a tendency to discuss evolution. Scientific detail is inversely proportional to evolutionary storytelling.
Articles like this also raise interesting philosophical questions. What is it we are really seeing? Clearly, the input data is being massaged. Scientists tell us that there are not really walls and chairs and violins, but quarks separated mostly by empty space. There are not shades of blue and beige and jade, but electromagnetic waveforms. There are not sounds, but pressure waves in a gas. There are not smells, but molecules. The data points impinging on our sensors go through multiple stages of information transfer from one medium to another before arriving in our brains, with multiple rounds of filtering, processing and interpretation before and after it arrives. What you see may be what you get, but what you get may not be what really is out there. But then, also, unless you believe all our internal technology is nothing more than glorified mashed potatoes, you are more than what you eat.