I bet you know this tune:
And even if you don't, if you listen to it, chances are you can sing back bits of the tune (unless you are tone deaf, all bets are off there). This is because many of us depend upon hearing songs and speech to be able to learn words and tunes and repeat them correctly. But if someone becomes deaf at some point in their lives, their speech in particular begins to degrade. They become hard to understand and lose things like tonal inflections that make words sound natural when we hear them.
In this way, humans are a lot like birds.
A bird depends on hearing song to learn it, and then will repeat it back. Below we have a young male zebrafinch, reach maturity, and singing a stereotypical song:
However, if you deafen that bird, by two weeks later, its song will degrade and lose complexity.
You can see this happen when you look at the graphical representation of the songs.
(Click to embiggen)
So what is happening when these birds lose their hearing?
Tschida and Mooney. "Deafening drives cell type-specific changes to dendritic spines in a sensorimotor nucleus important to learned vocalizations" Neuron, March 7, 2012.
The key brain area involved appears to be the High vocal center, a brain area in songbirds which is used in both the learning and the production of birdsong. This area has what we call a sensorimotor function. It can process sensory information (in this case, sound), and also play a role in controlling the subsequent sounds that the bird makes. In this case, the neurons of the high vocal center connect to motor systems in areas like the basal ganglia to control the sounds that the birds produce.
The authors of this study wanted to look at the high vocal center in particular to see how it responds to deafening specifically, and what the responses in this area meant for how the bird would sing after deafening. In order to see how the brain responded to deafening in real time, they used a technique called two-photon microscopy, a technique which uses light to excite fluorescent dyes. This can be done in the living brain and go in to a distance of about a millimeter. It isn't much, but it works well enough for the high vocal center of the zebra finch. They injected flourescent dyes called retrograde tracers into the downstream targets of the high vocal center. The high vocal center projects to the striatum area X, and the song prenucleus RA. Retrograde tracers are tracers which will get taken up and transported retrogradely, back to where the projections of the neurons originally came from. Since the projects all come from the high vocal nucleus, the dyes should all end up at the high vocal nucleus, with one color coming from the striatum area X, and the other coming from the song prenucleus RA. Then you can use light to excite the dyes, and you get this:
(Gorgeous, huh. Photo caption and credit: Neurons (nerve cells) are labeled with green fluorescent protein, and other neurons in the brain are labeled in the background with either red or blue tracers. The small bulbs (i.e., dendritic spines) on the spidery dendrites show places where nerve cells connect and communicate, called synapses, and when these spines shrank over time, this predicted vocal degradation in the songbirds. CREDIT: Katie Tschida, Duke Department of Neurobiology)
This technique is so specific that you can see the tiny dendritic spines and actual synapses where neurons connect with each other. So you can see how the connections in the high vocal center change, and with the different color dyes, you can see which pathway is being affected, with the striatal X pathway in one color, and the song prenucleus in another. And this is key for seeing what happens when a zebrafinch goes deaf.
They injected all of these tracers (with flourescent protein to label the neurons), and took images of the neurons in normal birds. Then they removed the cochlea (the inner ear area) of the birds, rendering them deaf. Then they continued to take images of the neurons in the high vocal area, and were able to correlate them with how badly the birds' songs degraded.
What they found was that degradation happened very quickly, within 24 hours of the bird losing its hearing. But only ONE type of synapse began to degrade, those synapses leading to the striatal X pathway, not those leading to the song prenucleus.
But it takes 2-3 days for a bird's song to degrade, and only 24 hours for the synapses to degrade. It turns out that the degradation of the synapses can predict how badly a bird's song will degrade, preceding the breakdown of the song by about 12 hours.
So the neurons in the high vocal nucleus have synapses that degrade in response to deafening, meaning they can't receive auditory input. The synapses that are degrades are specifically those leading to the striatal pathway, part of the basal ganglia which controls motor output and the production of song in birds. This could mean that after deafening, the inability to process sounds means that zebrafinches can't successfully act to change the motor output making song, causing the bird's song to break apart after deafening.
A finding like this is really cool, and not only because they could use two photon microscopy to image the cells over time in living birds. It's also a really interesting study of what happened in a deafened brain over time. We know that deafening in humans results in anatomical changes like those that occur in birds, but we're not sure yet how similar those changes are. But findings like these could be used to understand how humans might respond to changes in sound processing, in particular how much speech areas and speech production might degrade after deafening.
Katherine A. Tschida and Richard Mooney (2012). Deafening drives cell type-specific changes to dendritic spines in a sensorimotor nucleus important to learned vocalizations Neuron