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J Neurophysiol (July 19, 2006). doi:10.1152/jn.00307.2006
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Submitted on March 21, 2006
Accepted on July 10, 2006

Preparatory activity in premotor and motor cortex reflects the speed of the upcoming reach

Mark M Churchland1*, Gopal Santhanam2, and Krishna V Shenoy1

1 Neurosciences Program and Dept. of Electrical Engineering, Stanford University, Stanford, California, United States
2 Dept. of Electrical Engineering, Stanford University, Stanford, California, United States

* To whom correspondence should be addressed. E-mail: church{at}stanford.edu.

Neurons in premotor and motor cortex show preparatory activity during an instructed-delay task. It has been suggested that such activity primarily reflects visuo-spatial aspects of the movement, such as target location, or reach direction and extent. We asked whether a more dynamic feature, movement speed, is also reflected. Two monkeys were trained to reach at different speeds ('slow' or 'fast', peak speed being ~50-100% higher for the latter) depending on target color. Targets were presented in seven directions and at two distances. Of 95 neurons with tuned delay-period activity, 95%, 78% and 94% showed a significant influence of direction, distance and instructed-speed respectively. Average peak modulations with respect to direction, distance and speed were 18, 10 and 11 spikes/s. Although robust, modulations of firing rate with target direction were not necessarily invariant: for 45% of neurons the preferred direction depended significantly on target distance and/or instructed-speed. We collected an additional dataset, examining in more detail the effect of target distance (5 distances from 3-12 cm in two directions). Of 41 neurons with tuned delay-period activity, 85%, 83% and 98% showed a significant impact of direction, distance, and instructed-speed. Statistical interactions between the effects of distance and instructed-speed were common, but it was nevertheless clear that distance 'tuning' was not in general a simple consequence of speed 'tuning'. We conclude that delay-period preparatory activity robustly reflects a non-spatial aspect of the upcoming reach. However, it is unclear whether the recorded neural responses conform to any simple reference frame, intrinsic or extrinsic.




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