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Reza Shadmehr

 

Tuesday 28th April 2015

Time: 4.00pm

 

Basement Seminar Room

Alexandra House, 17 Queen Square, London, WC1N 3AR

 

Encoding of action by Purkinje cells of the cerebellum

 

Execution of accurate eye movements and other movements depends critically on the cerebellum, suggesting that Purkinje cells (P-cells) may predict the state of the moving eye. Yet, this encoding has remained a long-standing puzzle: firing of P-cells show little modulation with respect to saccade speed or direction, and critically, lasts longer than duration of a saccade. Here, we analyzed P-cell discharge in the oculomotor vermis of behaving monkeys during saccades. We found neurons that increased their activity during saccades, as well as neurons that decreased their activity. When we estimated the synaptic inhibition that these two populations produced via their projections to the caudal fastigial nucleus (cFN), we uncovered a signal that precisely predicted the real-time motion of the eye, an encoding that was not present in either population alone. When we aligned the simple spike activity of each P-cell to a coordinate system that depended on that cell’s complex spike (CS) tuning, the result unmasked a pattern of inhibition at cFN that encoded saccade speed and direction via a multiplicative gain-field. Therefore, our results suggested two ideas: reliable encoding of movement metrics does not occur in the firing of individual P-cells, but via synchronized inputs of bursting and pausing cells onto cFN; in this encoding, speed and direction are multiplicatively represented via a gain-field. These functional observations make a novel prediction regarding the anatomy of the cerebellum: projections of P-cells to nucleus neurons cannot be random, but organized by the complex-spike tuning properties of the P-cells.