Sunday, August 06, 2017

Systems Neuroscience Highlights: June and July 2017

There were lots of great articles the last couple of months, in particular a series of articles on the fly's representation of it's position in space that seem to be coming together nicely into a really beautiful narrative.

Sensory Coding

Singla et al.. A cerebellum-like circuit in the auditory system cancels responses to self-generated sounds -- Nature Neuroscience [Pubmed] The authors found that cells in the dorsal cochlear nucleus (DCN) of the brain stem response robustly to externally generated auditory cues, but not to self-generated auditory cues in mice (such as the sounds generated when they lick). Amazingly, when they artificially correlated sounds with licking behavior, such DCN neurons began to filter out responses to such sounds, as if the brain were starting to treat them as being generated by the animal. 
     It is a ubiquitous problem in the animal kingdom to separate out which sensory inputs are generated by your own behavior, and which are generated by the world. The authors seem to have found a beautiful model for such sensory cancellation effects, a beautiful demonstration of effect efference copy, very much like that seen in the mormyrid electric fish (as discussed by Abbott's group recently here). It will be interesting to see how similar the principles are in these different systems as folks dig under the hood.

Guo et al.. A Corticothalamic Circuit for Dynamic Switching between Feature Detection and Discrimination -- Neuron [Pubmed]. A thought-provoking paper in which they activated layer six corticothalamic (CT) neurons in primary auditory cortex (A1) of the mouse when tones were presented, and when the mice were either performing a discrimination or detection task. There are many results here, but the most thought provoking is that activation of the neurons had different effects on discrimination and detection, and this depended on teh relative timing of the stimulation relative to stimulus onset. 
     This paper is something of an experimental tour-de-force, and will provide a great deal of food for thought for the next few years: there is a lot of great data here. The big question, obviously, is whether these temporal codes are something that the actual brain exploits in vivo, or whether they are epiphenomena that emerge when experimenters artificially manipulate timing in circuits. Time will tell. This was the topic of my PhD thesis.


I have to mention the beautiful studies of how the fly keeps track of where they are going: the fly's representation of their orientation in space. This was summarized by Heinze in Current Biology. The three papers that came out in the past two months are:
  • Turner-Evans et al.. Angular velocity integration in a fly heading circuit -- Elife [Pubmed].
  •  Green et al.. A neural circuit architecture for angular integration in Drosophila -- Nature [Pubmed].
  • Kim et al.. Ring attractor dynamics in the Drosophila central brain -- Science  [Pubmed]



Motor Control
Park et al.. Moving slowly is hard for humans: limitations of dynamic primitives -- J. Neurophysiology [Pubmed]. While you will often hear of the speed-accuracy tradeoff (that is, the faster you try to do something, the more likely you are to make a mistake), it is not very often that people study what happens when you really push speeds way down. In this study they did just that. They had human subjects move their hands back and forth at different speeds, sometimes extremely slowly, so slowly that it seemed they could no longer maintain a smooth oscillatory behavior, but started to halt and stop and start again, as if they were shifting from a continuous to a novel, discrete, behavioral strategy. 
     While this paper doesn't have any neuronal data, it is significant for its attempt to infer underlying mechanisms of motor control strategies from a clever and creative extension of simple behavioral techniques. A colleague of mine pointed out that it would be interesting to see how much improvement we would see with training on this task.

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