How Processing Neural Data Works: A Blowfly Perspective

Blowflies. They are nearly impossible to swat dead, because they are so good at getting out of the way, and they are very very fast. For this reason, the blowfly, while an annoying creature, is an excelent model for research into rapid sensory information processing.

A team of scientists from Indiana University, Princeton University and the Los Alamos National Laboratory recently gained new insight into how blowflies process visual information. The findings, published in an article in the Public Library of Science Journals, show that the precise, sub-millisecond timing of "spikes" from visual motion-sensitive nerve cells encodes complex, detailed information of what the fly is seeing.

"There's a long-standing debate over whether precise, millisecond-scale timing is important to encode information in the nervous system," said Robert de Ruyter van Steveninck, a biophysics professor at IU who conducted many of the experiments. "Depending on the nature of the information, in some cases it might not be. But for motion sensitive neurons in the blowfly visual system, we show that timing is obviously important, especially in the context of natural visual stimulation." [press release]

The central question is this: Neural communication involves discrete electrical pulses (a.k.a. "spikes"). The frequency and timing of the spikes is clearly important, but there has been controversy over just how important precision is (or, perhaps "could be" ... as this could vary in different systems).

In other words, imagine the following two sequences of spikes transmitted over a very short ime interval, where each dot is a moment in time, and each bar is a spike:


The same number of spikes is transmitted in each case, and in fact, each spike is transmitted at almost the same time.

So, these two signals differ, but not much. In a given neural system, do these two signals signify the same thing, or different things?

Putting it yet another way,

Should we think of each spike arrival time as having meaning down to millisecond precision, or does the brain only keep track of the number of spikes occurring in much larger windows of time? Is precise timing relevant only in response to rapidly varying sensory stimuli, as in the auditory system, or can the brain construct specific patterns of spikes with a time resolution much smaller than the time scales of the sensory and motor signals that these patterns represent

This blowfly research addresses this question by setting up a very near natural environment, wiring up the blowflies, and making stuff happen very quickly, to see what kinds of signals are obtained.

The result:

...the timing of spikes is important with a precision roughly two orders of magnitude greater than the temporal dynamics of the stimulus. Second, the fly goes a long way to utilize the redundancy in the stimulus in order to optimize the neural code and encode more refined features than would be possible otherwise.

But how do you do that? This must be a very tricky experimental procedure!

Well, first you get a blow fly, and you strap it into this baby:


The fly is stuck in some wax and placed in a tube with only the head sticking out. An electrode is inserted into the brain region via the back of the head to pick up the signals from a certain neuron.

Then you rig this diabolic device up to some fancy electronic circuitry like this:


The signal from the above mentioned neuron is amplified through this circuity.

Meanwhile, a complex timing device in this circuitry operates the step motor that moves the fly around.

After that, it starts to get complicated, but this rig spits out a bunch of data that are used to test hypotheses that are summarized in the findings mentioned above.

The key discovery here is that the neurons are able to produce spikes that are timed very precisely. Referring to the analogy of the dots and bars (above): Yes, these two signals would be very different in what they "mean." This precision in timing of spikes allows a given neuron to have a very large "vocabulary" of what these researchers are calling "words."

At the timing resolutions being seen in this particular example, there is enough room in the units of time being used to produce close to a billion different words.

Apparently there is a lot of redundancy in the signal that the blowfly uses ... the amount of encoded visual information it is capable of passing is way more than is used, and the information passing along in connection with a certain response (by the fly to what it sees) is more than would be needed given the specificity of the words. Redundancy, which by the way is also a feature of human language and other communication systems ... may be an important part of this system.

No flies were harmed during the conduct of this experiment. Except the ones that were waxed up and stuffed into the tube with electrodes in their brains. Unfortunately, all the other blowflies got away, free to lay their eggs in whatever dung they could find, with the hopes of raising a nice family of maggots....

Nemenman, I., Lewen, G.D., Bialek, W., de Ruyter van Steveninck, R.R., Friston, K.J. (2008). Neural Coding of Natural Stimuli: Information at Sub-Millisecond Resolution. PLoS Computational Biology, 4(3), e1000025. DOI: 10.1371/journal.pcbi.1000025

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