The retina receives signals from all over the brain, and that is kind of weird

As a neuroscientist, when I think of the retina I am trained to think of a precise set of neurons that functions like a machine, grinding out the visual basis of the world and sending it on to the brain. It operates independently of the rest of the system with the only feedback coming from muscles that move the eye around and dilate the pupils. So when someone [Philipp Berens] casually mentioned to me that yes, the retina does in fact receive signals from the brain? Well, I was floored.

I suppose I should not have been surprised. In fruit flies, there has been a steady accumulation of evidence that the brain sends signals to the eye to get it ready to compensate for any movement the animal will make. Intuitively, that makes a lot of sense. If you are trying to make sense of the visual world, of course you would want to be able to compensate for any sudden changes that you already know about.

It turns out that there is a huge mass of feedback connections from the brain to the retina in birds and mammals, something termed the centrifugal visual system. And inputs are sent via this system from both visual areas and non-visual areas (olfactory, frontal, limbic, and so on). So imagine – your eye knows about what you are smelling. Why? In order to do what?

The answer, it turns out, is that we don’t know. It sends all sorts of neurotransmitters and neuromodulators. The list of peptides it sends are long (GnRH, NPY, FMRF, VIP, etc) as is the list of regions that send feedback to the retina. It seems as if which regions send feedback to the retina is very species-specific, suggesting something about the environment each animal is in. But why?

This is a post long on questions and short on answers. It is more a reminder that the nice, feedforward systems that we have simple explanations for are really complex, multimodal systems designed to create appropriate behaviors in appropriate circumstances. Also it is a reminder to myself about how little I know about the brain, and how mistaken I am about even the simplest things…

I would love someone more knowledgable than me to pipe up and tell me something functional about what these connections do?

References

Repérant J, Médina M, Ward R, Miceli D, Kenigfest NB, Rio JP, & Vesselkin NP (2007). The evolution of the centrifugal visual system of vertebrates. A cladistic analysis and new hypotheses. Brain research reviews, 53 (1), 161-97 PMID: 17059846

Vereczki, V. The centrifugal visual system of rat. Doctoral Thesis. PDF.

The eye(s)

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(Photo by artofdreaming)

Ed Yong has an absolutely brilliant article in National Geographic about the receptical for neuroscience’s favorite sensory system, vision:

But simple eyes should not be seen as just stepping-stones along a path toward greater complexity. Those that exist today are tailored to the needs of their users. A sea star’s eyes—one on the tip of each arm—can’t see color, fine detail, or fast-moving objects; they would send an eagle crashing into a tree. Then again, a sea star isn’t trying to spot and snag a running rabbit. It merely needs to spot coral reefs—huge, immobile chunks of landscape—so it can slowly amble home. Its eyes can do that; it has no need to evolve anything better. To stick an eagle’s eye on a sea star would be an exercise in ludicrous excess…

“Insects and crustaceans have become so successful despite their compound eyes, not because of them,” says Nilsson. “They would have done so much better with camera-type eyes. But evolution didn’t find that. Evolution isn’t clever.”

Eric Warrant, Nilsson’s next-door neighbor at Lund University, takes a more lenient view. “Insect eyes have a much faster temporal resolution,” he says. “Two flies will chase each other at enormous speed and see up to 300 flashes of light a second. We’re lucky to see 50.” A dragonfly’s eye gives it almost complete wraparound vision; our eyes do not. And the elephant hawk moth, which Warrant has studied intensely, has eyes so sensitive that it can still see colors by starlight. “In some ways we’re better, but in many ways, we’re worse,” Warrant says. “There’s no eye that does it all better.” 
Our camera eyes have their own problems. For example, our retinas are bizarrely built back to front. The photoreceptors sit behind a tangled web of neurons, which is like sticking a camera’s wires in front of its lens. The bundled nerve fibers also need to pass through a hole in the photoreceptor layer to reach the brain. That’s why we have a blind spot. There’s no benefit to these flaws; they’re just quirks of our evolutionary history.

It does make you wonder what it is like to be a bat, so to speak. Consider the scallop:

The mantle of the bay scallop (Argopecten irradians) is festooned with up to 100 brilliant blue eyes. Each contains a mirrored layer that acts as a focusing lens while doubling the chance of capturing incoming light.

What is it like to have access to 100 eyes? It is not so simple as just imagining that you could ‘see more’. Our eyes sense more than what we just see (so to speak). In human retinas, melanopsin isn’t used to form images but to help entrain circadian rhythms. These neurons send information to a different part of the brain (the suprachiasmatic nucleus) in a way that we can fundamentally feel in a different way. Now imagine those 100 eyes: are they all there for the same thing? Is the feeling of one the same as the feeling of another?

 

How many smells can you smell, audience-friendly edition

If you have a keen memory, you might recall that there was a little kerfuffle a couple months back concerning ‘how many smells can you discriminate‘. Here’s a layman’s introduction to the basis of the question, by It’s Okay To Be Smart.

And yes, I’m mostly posting this because it refers to this blog in the ‘sources’ section 😉

How words make color

Go check out this great interactive explanation of how words represent colors in English versus in Chinese.

Color words in Mandarin Color words in English

Interestingly, the most common color words in Chinese are for red, green, and blue while in English they are blue, green and pink!

[via FlowingData]

The sound of silence

Sensory neurons receive input from the outside world and send these signals on to the rest of the nervous system. This makes the concept of ‘silence’ fairly intriguing: what happens when there is very little sensory signal for the rest of the brain to process? It is well known that, after a while, no sensory stimulation means massive hallucinations. But quiet – brief periods of weak or no relevant stimulus – is different:

In the mid 20th century, epidemiologists discovered correlations between high blood pressure and chronic noise sources like highways and airports. Later research seemed to link noise to increased rates of sleep loss, heart disease, and tinnitus. (It’s this line of research that hatched the 1960s-era notion of “noise pollution,” a name that implicitly refashions transitory noises as toxic and long-lasting.)

Sound waves vibrate the bones of the ear, which transmit movement to the snail-shaped cochlea. The cochlea converts physical vibrations into electrical signals that the brain receives. The body reacts immediately and powerfully to these signals, even in the middle of deep sleep. Neurophysiological research suggests that noises first activate the amygdalae, clusters of neurons located in the temporal lobes of the brain, associated with memory formation and emotion. The activation prompts an immediate release of stress hormones like cortisol. [neuroecology: really? all noises?]

…He found that the impacts of music could be read directly in the bloodstream, via changes in blood pressure, carbon dioxide, and circulation in the brain. (Bernardi and his son are both amateur musicians, and they wanted to explore a shared interest.) “During almost all sorts of music, there was a physiological change compatible with a condition of arousal,” he explains…But the more striking finding appeared between musical tracks. Bernardi and his colleagues discovered that randomly inserted stretches of silence also had a drastic effect, but in the opposite direction. In fact, two-minute silent pauses proved far more relaxing than either “relaxing” music or a longer silence played before the experiment started.

In light of this, I found the experiences of a hermit who has been living alone since the 1980s fascinating:

He explained about the lack of eye contact. “I’m not used to seeing people’s faces,” he said. “There’s too much information there. Aren’t you aware of it? Too much, too fast.” (Note: he may have asperger’s)

“But you must have thought about things,” I said. “About your life, about the human condition.”

Chris became surprisingly introspective. “I did examine myself,” he said. “Solitude did increase my perception. But here’s the tricky thing—when I applied my increased perception to myself, I lost my identity. With no audience, no one to perform for, I was just there. There was no need to define myself; I became irrelevant. The moon was the minute hand, the seasons the hour hand. I didn’t even have a name. I never felt lonely. To put it romantically: I was completely free.”

…”What I miss most,” he eventually continued, “is somewhere between quiet and solitude. What I miss most is stillness.” He said he’d watched for years as a shelf mushroom grew on the trunk of a Douglas fir in his camp. I’d noticed the mushroom when I visited—it was enormous—and he asked me with evident concern if anyone had knocked it down. I assured him it was still there. In the height of summer, he said, he’d sometimes sneak down to the lake at night. “I’d stretch out in the water, float on my back, and look at the stars.”