Brain Bits, 9/12/15

lightning brain2

credit: A. Ajifo

Welcome to Brain Bits, where I highlight important or interesting recent news in the world of neuroscience. Today’s Brain Bits are going to be chunkier than usual because there are some really cool recent papers that I want to talk about and actually explain properly. I know, I’ve been away too long! (I’ve turned one of the chunks into a full post for next week, so stay tuned.)

 

How fruit flies get horny

Male fruit flies display a complicated courtship ritual toward females, consisting of steps like following the female around, tapping her, licking her butt, and singing to her. (Who says flies are so different from humans?)

fly courtship

The fruit fly courtship ritual. (from Sokolowski, Nature Reviews Genetics, 2001)

Many of these courtship behaviors are triggered by a single cluster of neurons in the fly brain called P1 neurons. A new study from Vanessa Ruta’s lab at Rockefeller traces the pathways by which these super-important P1 neurons get activated. Her lab found that pheromones from female flies activate taste neurons on the male’s legs; these taste neurons activate another neuron called vAB3, which directly activates P1 neurons. Ta-dah! But wait—the researchers also found that vAB3 has a second branch that indirectly inhibits P1 neurons.

Ruta summary

Simplified diagram showing inputs to P1 neurons

You should be wondering, why on earth would vAB3 want to both excite and inhibit the same set of cells? Wouldn’t that just cancel things out?? Turns out, having parallel excitation and inhibition is a common motif in the brain. It can be a way to control the precise timing of neuronal activation: if the excitation starts just before the inhibition, then the cell can only fire during that small time window when it’s receiving pure excitation. In the case of P1 cells, that ensures that males only display courtship behavior immediately after they’ve detected a female, so they don’t waste their time singing and dancing if she’s already run away.

Parallel excitation and inhibition can also help control the strength of neuronal activation, ensuring that neurons respond consistently to both weak and strong stimuli. In this case, that’s useful because both brief and sustained contact with female pheromones still mean the same thing—there’s a girl out there to woo! Overall, this study not only elucidates the neural pathways involved in courtship behavior, but also shows (as I’ve discussed before) how certain circuit motifs are repeatedly employed from invertebrate to mammalian brains.

Where bottom-up and top-down meet

Much of the information in your brain can be divided into two categories: “simple” information coming from your senses (called “bottom-up”), and more abstract, “cognitive” information coming from higher brain areas (called “top-down”). It’s not well understood how the brain integrates these different streams of information. A recent study from Takaki Komiyama’s lab at UCSD now sheds light on how bottom-up and top-down information are integrated in the visual cortex (abbreviated V1), a key brain region for processing visual information.

The bottom-up inputs to V1, which carry simple visual information from your eyeballs, are fairly well-known. The top-down inputs are less clear, so Komiyama’s group used neural tracing techniques to identify a brain region called the retrosplenial cortex that transmits top-down information to V1.

The researchers then trained mice to run on a treadmill whenever they saw certain visual stimuli; mice that didn’t run received an electric shock. Interestingly, as mice learned to perform the task, the top-down pathway got stronger while the bottom-up pathway got weaker. As this balance shifted, corresponding changes could be observed in the V1 neurons that integrate bottom-up and top-down information: the neurons started caring less about the visual stimulus and more about when the potential shock was going to happen. There’s a lot more in the paper about how the strength of these two pathways is controlled, but the main takeaway is that shifting the balance between bottom-up and top-down information is likely an important part of learning.

komiyama summary

Makino and Komiyama (2015) show that learning shifts the balance of bottom-up and top-down inputs, which are integrated by specific neurons in V1 (layer 2/3 pyramidal cells).

 

Forget about walking and chewing gum, how do we walk and see at the same time?

We often think of the brain as something that gathers information about the world through our senses and uses that information to dictate our behavior. However, our behavior can actually change the sensory information we receive. For example, turning your head will make your eyes perceive the world as moving. How can your brain tell whether it’s you or the outside world that’s in motion?

Since your movements are controlled by the motor parts of your brain, one solution is for those motor centers to send a signal to the visual area of your brain whenever they’re telling you to move. Basically like a, “Heads up dude, we’re about to turn left so don’t worry if everything looks like it’s moving to the right”. While this idea has been around for decades, few studies have found direct evidence of this hypothetical motor to sensory signal, called an “efference copy”.

A recent study from Gaby Maimon’s lab at Rockefeller addressed this question in fruit flies. The flies were tethered to a string and could voluntarily turn left or right while viewing a blank screen. The researchers simultaneously recorded the activity of visual neurons that are normally activated when flies see something moving horizontally. They found that those neurons received either an excitatory or inhibitory signal whenever the fly turned—an efference copy. The type of movement-induced signal (excitatory or inhibitory) was matched to counteract the normal visual response of the neuron, thereby canceling out the fly’s perception of horizontal motion and making sure it doesn’t get confused when it’s turning. This study thus provides insight into how animals can accurately perceive the world while moving through it.

Maimon summary

Schematic of how the brain filters out visual signals generated by self-motion. Turning left normally makes your visual system think the world is moving to the right, but this information is cancelled out by a second signal, the efference copy, transmitted from the same motor area that caused the left turn in the first place.

 

Did you see any recent neuroscience news that you’d like to share? Leave a comment below!

 


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