The two bright spots swimming around in the video below are both fluorescent paramecia — single-celled organisms, similar to amoeba. One has free range and swims rapidly about, while the other one is confined to the brain of a transparent zebrafish packed in gelatin hungrily measuring its dinner. Only the first paramecium is actually real — the second is pure muse, the ethereal scintillation of the first thought ever captured on video. Watching this video we can not help but wonder what treasures will be revealed when even greater imaging resolutions are achieved.
The zebrafish is a just a millimeter or two in length and it’s brain contains a mere 300,000 neurons. While still a larva, it is transparent and therefore a favorite specimen of neuroscientists. A group in Japan has genetically modified this fish to express fluorescent molecules only in its optic tectum, the visual integration center of the fish brain. When neurons in the tectum fire off signals by rapidly changing their voltage, the fluorescent molecules shoot off photons that can be imaged. This doppelganger to the paramecium has an uncanny coherence within the zebrafish brain. Since the optic nerve of the fish crosses to the opposite hemisphere when it enters the brain, the image of the paramecium is clearly seen in the opposite tectum.
Should we be surprised to find that, at least for a visual stimulus, its correlate in the fish brain so precisely mirrors the external world? Probably not, at least for an area like the tectum that is just a single synapse away from the retina. The information the fish needs to identify and target a single paramecium for capture by muscular action would appear to be most fluidly encoded as a simple copy in space and time of the actual stimulus. The tectum does in fact project to muscle control centers, though for the fish’s simple axial propulsion system, the visual space might be expected to be a more universal metric to use, as opposed to a more abstract space defined in terms of the musculature. In other words, the paramecium does not show up in the tectum as a map of the tail muscle. In the video, we see that when the paramecium stops swimming and ceases to present new information to the retina, its image in the tectum fades. What should we expect to see then when the experimenters get around to doing the same experiment with two or more paramecia, or a predator and a paramecium? More importantly for us, would our human brain show the same economy of representation?
Unfortunately for us, our brains are not transparent. Scientists have developed solutions that match the index of refraction of brain tissue, rendering it fairly transparent, but it is not suitable for introduction to living tissue due to toxicity. We do possess the optic tectum of the fish, but in evolving into the mammalian brain its relative importance has shrunk. It remains deep in our brainstem and is interlinked with low-level balance and eye movement control — features you really don’t want to expose to massive restructuring. When brains get bigger, the tectum does not. The other brainstem pathways which thread their way around it during development are just too critical to parse on the evolutionary chopping block. Expanding the tectum would only push back the cerebellum, increasing signal latencies to it and decreasing the time available to escape the enclosing jaws of a predator. New visual processing hardware in more complex animals has simply bypassed the tectum, created in parallel a new relay station next door in the thalamus, and inflated a huge cortex to receive the incoming information.
The kicker is that of all the synapses in the visual part of relay station, the thalamus, 90% of the them are return lines from the cortex, while just 10% represent the actual visual stimulus from the retina. What this means is that even in the absence of any visual input, the brain can produce all manner of interesting figments that would be nearly indistinguishable from reality. Think about dreams, hallucinations, decoding ambiguous images, attention, and salience.
It is possible or perhaps even likely that a human brain would react to the stimulus of a paramecium in the same way as a zebrafish — and indeed that is the presumed basis of cortical prosthetics for the blind — but for more complex stimuli, all bets are off. We should remember we are talking about a fish many times smaller than our entire retina observing a stimulus much smaller than we can resolve making movements faster than we can track. This is a system much closer in scale to the building blocks of nature. Considering that the paramecium is small enough to be buffeted about by the thermal motion of water molecules themselves, we might similarly expect it to have fewer levels of abstraction between stimulus and its subsequent encoding.
What might more complex stimuli look like when globally encoded by a more complex brain?
If you flipped the zebrafish experiment on its head and instead of looking for the external world in the brain, put the brain in the external world so to speak, what do you think you would find? In other words, the visual stimulus would itself be a computer generated representation of the firing neurons. The pixels comprising the visual field would be set according to the activity of the individual or small clusters of neurons in the cortex, after suitable transformation to account for retinal and thalamic processing. In trying to harness all the details generated by experiments like just mapping the zebrafish tectum alone, neuroscientists suddenly have a huge data problem on their hands. By comparison, mapping a connectome would seem trivial.
The best extractor of meaning from data like this is no doubt the brain itself. When performing the actual experiment just described, a little speculation as to the outcome might be forgiven at this point. Activity might be presumed to gravitate towards energy-minimizing, damped or quiescent stable states, intermediate oscillatory states, or more energetic chaotic states where randomness prevails as the brain searches for new experience. Perhaps the question of how the brain would optimally map a stimulus when the stimulus is the brain itself is bit of stretch for today. Rephrased, we could ask, with what efficiency could the brain become a representation of itself? Nonetheless we may soon be able to probe questions like this and achieve greater insight into consciousness itself. In brains more advanced than the zebrafish, for example, stationary stimuli are quite readily perceived. How they are represented and maintain persistence over time in the brain will be a key pursuit for this kind of research.
These new windows into the inner life of brains will raise more questions than they immediately answer. Before videos like these, we really had no idea just how much activity there is in a defined volume of neural tissue. In additional to raw electrical transgressions of ions there are larger movements of organelles along with other pulsatile flows, pressure gradients, and temperature gradients. Each will provide unique pictures and be indispensable in our efforts to understand this marvelous machine.
Now read: How to create a mind, or die trying
Reasearch paper: 10.1016/j.cub.2012.12.040 – ”Real-Time Visualization of Neuronal Activity during Perception”