a painting of a tadpole surrounded by neurons

Khakhalin lab: Intrinsic temporal tuning in the tectum

A new paper from the Khakhalin lab:

Intrinsic temporal tuning of neurons in the optic tectum is shaped by multisensory experience
Silas E. Busch and Arseny S. Khakhalin
5 SEP 2019  https://doi.org/10.1152/jn.00099.2019

(The published version is behind a paywall, but you can find a free version here: https://www.biorxiv.org/content/10.1101/540898v2)

In his senior project, which eventually became the foundation for the paper, Silas Busch ’16 asked whether different neurons in the optic tectum of Xenopus tadpoles are tuned to inputs of different duration. Now, there’s lots to unpack in this sentence! So let’s talk about it bit by bit. Silas worked with tadpoles: the larvae of Xenopus frogs (you might have seen these frogs: they look a bit like underwater rubber toys, and are popular as pets). Even though tadpoles are small, they still have a brain, and in this brain, they have a part called “the optic tectum”. This brain area helps tadpoles to navigate in the water. As everything else in the body, the brain is made of cells, and these cells, called neurons, are connected to each other in some meaningful fashion that, frankly, we still don’t completely understand. Neurons send electrochemical impulses to each other, and it is this dance of activation in the tectum that allows tadpoles to swim without running head-first into walls or other tadpoles.

The question that Silas asked in his paper, is whether tectal neurons are different from each other in one very specific way. He looked at whether they all respond similarly to fast and slow patterns of activation, arriving from other neurons, or whether some of them have a preference for either fast or slow activation profiles. Say, if a neuron receives signals from 3 other neurons at the same time, will it respond to them in the same way as it would respond if these signals were a tiny bit staggered? To answer this question, Silas used a fancy technique called the “Dynamic clamp” that allowed him to connect to neurons one by one with a tiny glass electrode, and then control electrical currents in each neuron with a computer, simulating different patterns of activation.

What Silas found is that most tectal neurons do have a preference for either short or long (synchronous or asynchronous) patterns of activation, and that this preference changes depending on what tadpoles see and hear. It means that the tadpole brain as a whole, and each individual neuron in this brain on its own, adjust to changes in the world around the animal; presumably, to give the tadpole an ability to better navigate and survive. This particular type of neuron-by-neuron temporal tuning was not described in the tectum before, and also nobody yet used the dynamic clamp technique to look for tuning of this type. We don’t yet know exactly what these findings could mean for our understanding of the brain, but it is very exciting to learn that there is one more aspect to brain develpoment, that was so far somewhat overlooked!

It is also curious to think that this paper would not have happened if Silas hadn’t returned to the lab to work for 4 extra days immediately after graduation, on a Sunday after his commencement ceremony! In April 2016, as he was writing his senior project, Silas realized that some of the data he recorded could not be used. Even though he was obviously tired, and excited to graduate and leave Bard, Silas still decided to spend four more (!) full days in a lab without windows (our lab just happens to not have windows), to finish the work. It is not that often that you can point at one seemingly minor decision and realize that it was a key for success, but for this paper, it is really true. Without these four extra days of work, we would not have had enough data, and this paper would have never happened.