Tuesday, 13 May 2014

Wiring of cells in retina explains motion detection

Scientists mapping connections between neurons in a mouse eye may have discovered how the retina is able to detect movement. 
Researchers have known since the 1960s that the retina can respond to motion before any signal is passed onto the brain, and that the mechanisms were sensitive to direction. But exactly how they worked remained unclear.
Now a team in the US believe it may be determined by how three types of cell are hardwired together in ‘motion detector’ circuits, which are able to detect movement in a ‘preferred’ direction. 
Citizen neuroscientists
A team of researchers, led by H. Sebastian Seung at the Massachussets Institute of Technology, used a crowdsourcing game to map the dense network of neural connections. 
Crowdsourcing, or ‘citizen science’, is an increasingly popular ‘divide and conquer’ approach to large scale analysis and has been used previously to identify individual galaxies from maps of millions, and even to identify whale calls.
By breaking down intricate electron microscope scans of retinal tissue into smaller segments, bitesized blocks less than five microns across could be processed by volunteers using an online game they developed, called EyeWire
EyeWire has already recruited more than 120,000 members of the public to help map the connections. The volunteer ‘citizen neuroscientists’ colour coded branches of the connections between neurons, guided by artificial intelligence as they go.
So far, the result is the highest resolution of the branching connections between the different types of neurons in the eye, in particular, starburst amacrine cells (SACs) and two types of bipolar cells (BPs). 
Clusters of these neurons are arranged like circuits in the retina, with the two types of BPs connected to an SAC at the centre. The researchers say that the structure reveals an inbuilt time-delay switch which could account for motion detection, with the two BPs needed to fire to activate the central SAC. 
Time delay switch
The innermost (slow) BP cells have a slight delay when firing, whereas the outermost (fast) BP cells do not. It is only when the signals from the both BP cells reach the centre of the circuit, the SAC, at the same time that they activate it.
If the direction of motion corresponds to the orientation of the neurons (ie which direction the circuit is ‘pointing’) then the signal moves from the centre outwards. The slow BP is activated first – like a slow-burning fuse – followed shortly after by the fast BP, and both signals arrive at the same time, activating the SAC.
The team think that this mechanism could explain how the retina is able to detect motion, although experiments are now needed to confirm the theory. They are also cautious that other neuron types may be involved.
Speaking to Scientific American about the project late last year, Professor Seung said: “Neurons come in many cell types, and one of the important tasks in neuroscience today is to identify and enumerate all these different types of neurons in the brain. Nobody knows how many there are... we have to combine human intelligence and artificial intelligence in order to solve this problem.”
The research is reported in Nature.
IMAGE: Rachel Prentki/Eyewire