PMID-17360898[] Relationship between Unconstrained Arm Movements and Single-Neuron Firing in the Macaque Motor Cortex
- the best explanation of neuronal firing was the final mulijoint configuration of the arm - it accounted for 36% of the SUA variance.
- the search for the 'correct' motor parameter (that neurons are tuned to) is an ill-posed experimental question because motor parameters are very intercorrelated.
- they knock experiments in which the animals are overtrained & the movements limited - and they are right!
- single electrode recording with cronically implanted steel chamber - e.g. it took a damn long time!
- imaged the central sulcus through the dura.
- verified location with single unit responses to palpation of the contralateral hand/arm (in S1) & microstimulation-evoked movements in M1.
- used optotrak to measure the position of the monkey.
- occasionally, the monkey attemptted to scratch the experimenter with fast semi-ballistic arm movement. heh. :)
- movements were seprarated based on speed analysis - that is, all the data were analyzed as discrete segments.
- neurons were inactive during periods of hand stasis between movements.
- tested the diversity of their training set in a clever way: they simulated neurons tuned to various parameters of the motion, and tested to see if their analysis could recover the tuning. it could.
- however, they still used unvalidated regression analysis to test their hypothesis. regression analysis estimates how much variance is estimated by the cosine-tuning model - it returns an R^2.
- either averaged the neuronal tuning over an entire movement or smoothed the firing rate using a 10hz upper cutoff.
- Moran & Schwartz' old result seems to be as much a consequence of averaging across trials as it is a consequence of actual tuning...
- whithout the averaging, only 3% of the variance could be attributed to speed tuning.
- i think that they have a good point in all of this: when you eliminate sources of variance (e.g. starting position) from the behavior, either by mechanical restraint or simple omission of segments or even better averaging over trials, you will get a higher R^2. but it may be false, a compression of the space along an axis where they are not well correlated!
- a model in which the final position matters little, but the velocity used to get there does, has been found to account for little of the neuronal variance.
- instead, neurons are tuned to any of a number of movements that terminate near a preferred direction.
- observational studies of of the normal psontaneous behavior of monkeys indicate that a high proportion of time is spent using the arm as a postural device.
- therefore, they expect that neurons are tuned to endpoint posture.
- modeled the neuronal firing as a gaussian surface in the 8-dimensional space of the arm posture.
- in comparison to other studies, the offset between neural activity and behavior was not significantly different, over the entire population of recorded neurons, from zero. This may be due to the nature of the task, which was spontaneous and ongoing, not cue and reaction based, as in many other studies.
- quote: This result suggests that the neuronal tuning to posture reflects reatively more and anticipation of the future state of the limb rather than a feedback signal about a recent state of the limb.
____References____
|