What Do You See Just Before You Are Born?

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Think back to your earliest memory. It probably comes from early childhood, perhaps around age three. Maybe it’s a hazy image or a vague feeling tied to a particular location such as your backyard or kitchen.

Now go even further back. What images might have been running through your head just before you were born? Of course, you were in the dark. Not only were you enclosed in your mother’s womb, your eyes were closed. Your visual system was also still under construction.

But if there was activity in the neurons in your eyes, could you have had some kind of visual experience?

We now know that even in the darkness of the womb, patterns of activity do occur in the light-sensitive retina at the back of our eyes just before birth.

It goes without saying that researchers haven’t been doing invasive neurophysiology experiments on human perinates (babies about to be or just born) to find out what they are “seeing.” But neuroscientists have shown since the 1980s that patterns of excitation wash across the retina before eye-opening in other animals.

By studying numerous other species over the past 30 years, neuroscientists have developed a good understanding of the patterns of neural activity going on in the eye before the animal's eyes open. We can extrapolate to ourselves because the basic developmental processes that organize the light-sensitive retina are quite similar in humans and non-human primates, and indeed in mammals and vertebrates more generally. So the retinal patterns in other species are probably very similar to those in us.

The patterns are called retinal waves and, in a physiological sense, they are the same kind of activity generated by the normal process of vision with light striking your retina.

Spontaneous retinal activity before eye-opening was discovered by Italian neuroscientists in the late 1980s but was brought to spectacular heights in the following decades by U.S. researchers led by Rachel Wong, now of the University of Washington; Carla Shatz, now of Stanford University; and Marla Feller, now of the University of California at Berkeley. What Wong, Shatz, Feller, and their colleagues brought was cutting-edge imaging set-ups that allowed visualization of the patterns as they happen—not so simple when the object of study is living tissue inside the eye of a creature that is itself inside another creature.

In every vertebrate species tested, including monkeys, chickens, turtles, mice, rabbits, ferrets, cats, and even fish, there is a similar pattern of visual activity. It usually starts a few days or weeks before an animal’s eyes open. The exact timing varies: In some animals, the eyes open at birth, and in others, this occurs a few days after birth, but the basic pattern is quite similar across species.

What does this activity look like? Since they open their eyes a few weeks after birth, mice and ferrets have provided the most extensive data on retinal waves. To see a high-resolution example from the mouse retina (shown at ten times normal speed), scroll down to Movie 2 in this article from Feller’s lab. See also the latest imaging work from Feller's lab that achieves even higher resolution.

Being found in all vertebrates tested so far, retinal waves are clearly a useful (and indeed necessary) evolved trait. What purpose do they serve?

The primary purpose is organizing brain development. Retinal waves are essential for building the right connections between the eye and a part of the brain called the thalamus, which serves as a network backbone for the brain. In the thalamus, axons originating from the two eyes stimulate thalamic neurons that form alternating stripes fed by one eye or the other eye. When retinal waves are chemically blocked from one eye, the stripes that correspond to that eye become much thinner, while the stripes from the other eye develop normally. Essentially, developmental processes rely on a balance of spontaneous activity between the two eyes to build the intricate and essential connections in our early visual system. If the retina were quiet, the system would not build the right connections.

But retinal waves may do more than provide a basic input to help the brain develop properly. Simulation studies by computational neuroscientist Mark Albert and colleagues showed that retinal waves could give us a preview of the general shapes and motions that we will experience in the visual world once we open our eyes.

Retinal waves are spontaneous patterns. They are organized by complex chemical interactions, yet, through evolution, these interactions are able to self-organize into relatively simple patterns. One could imagine other scenarios in which the chemical reactions lead to totally random activity, like snow on a TV screen, or a single flashing region. This might not be so useful. Instead, retinal waves erupt, move in a more or less coherent way across part of the retina, then die off. The process repeats, producing a pleasantly drifting wave traveling across part of the retina. Over time, waves cover all of the retina.

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Retinal waves don't look exactly like the visual world, but they have properties like those of the world. For example, retinal waves have regions bounded by edges, much as objects in the natural visual world do. As Albert and colleagues showed, the basic statistical properties of the edges in a retinal wave movie are similar to those in nature. More recent work from researchers at Yale suggests that retinal waves also provide patterns of motion that match basic aspects of optic flow, which is how the visual world changes as we move through it.

Albert calls retinal waves an example of "innate learning," a pre-programmed training system that gives us a kind of learned experience, one that can help train later steps in visual processing.

So do babies about to be born “see” retinal waves? We will never know. As I noted, perinates’ visual systems are still under construction, something that retinal waves contribute to. And retinal waves are generated independently in the two eyes, so they would be tough to combine into a single percept. Beyond this, we can’t scrutinize other people’s conscious experience because it is totally private and happening only in their head. This is true for adults also, although at least adults can try to describe what they experience.

But to the extent that we are aware of any visual experience just before birth, it might well "look" like retinal waves. And regardless of whether perinates “see” them, visualizations of retinal waves can be captivating to adult humans, especially once we understand what we are looking at. To me, the patterns feel soothing and meditative and—more than anything—full of wonder.

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