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ref: Turner-2010.12 tags: STN DBS basal ganglia motor learning vigor scaling review date: 02-16-2012 21:27 gmt revision:3 [2] [1] [0] [head]

PMID-20850966[0] Basal ganglia contributions to motor control: a vigorous tutor.

  • Using single-cell recording and inactivation protocols these studies provide consistent support for two hypotheses: the BG modulates movement performance ('vigor') according to motivational factors (i.e. context-specific cost/reward functions) and the BG contributes to motor learning.
  • Most BG associated clinical conditions involve some form of striatal dysfunction -- clincal sings occur when the prinicpal input nucleus of the BG network is affected.
    • Lesions of the output nuclei are typically subtle, consistent that pallidotomy is an effective treatment for PD and dystonia.
    • It is better to block BG output completely than pervert the normal operations of motor areas that receive BG output.
    • Pathological firing patters degrade the ability of thalamic neurons to transmit information reliably.
      • Bad BG activity may block cortico-thalamic-cortico communication.
      • Hence BG treatment does not reflect negative images of normal function.
  • Years of debate have been resolved by a confirmation that the direct and indirect pathways originate from biochamically distinct and morphologically disctinct types of projection neurons [97, 105].
    • Direct: D1; indirect = D2, GPe.
  • CMPf projects back to the striatuim.
  • Movement representation in the BG: ref [36]
  • Results of GPi inactivation:
    • RT are not lengthened. These results are not consistent with the idea that the BG contributes to the selection or initiation of movement.
    • GPi inactivation does not perturb on-line error correction process or the generation of discrete corrective submovements.
      • Rapid and-path corrections are preserved in PD.
      • Challenges the idea that the BG mediates on-line correction of motor error.
    • GPi inactivation does not affect the execution of overlearned or externally cued sequences of movements.
      • contradicts claims, based on neuroimaging and clinical evidence, that the BG is involved in the long term storage of overlearned motor sequences or the ability to string together successive motor acts.
    • GPi inactivation reduces movement velocity and acceleration.
      • Very consistent finding.
      • Mirrors the bradykinesia observed in PD.
      • Common side-effect of DBS of the GPi for dystonia.
    • GPI inactivation produces marked hypometria -- unsershooting of the desired movement extent.
      • Un accompanied by changes in movement linearity or directional accuracy.
  • Conclusion: impaired gain.
    • Movement: bradykinesia and hypometria
    • hand-writing: micrographia
    • speech: hyophonia [65].
    • There is a line of evidence suggesting that movement gain is controlled independently of movement direction.
    • Motor cost terms, which scale with velocity, may link and animals' previous experience with the cost/benefit contingencies of a task [75] to its current allocation of energy to meet the demands of a specific task.
      • This is consistent with monkey rapid fatiguing following BG lesion.
      • Schmidt et al [5] showed that patients with lilateral esions of the putamen or pallidum are able to control grip forces normally in response to explicit sensory instructions, but do not increase grip force spontaneously despite full understanding that higher forces will earn more money.
    • Sensory cuse and curgent conditions increase movement speed equally in healthy subjects and PD patients.
  • BG and learning:
    • role in dopamine-mediated learning is uncontroversial and supported by a vast literature [10,14,87].
    • Seems to be involved in reward-driven acquisition, but not long-term retention or recall of well-learned motor skills.
    • Single unit recording studies have demonstrated major changes in the BG of animals as they learn procedural tasks. [88-90]
      • Learning occurs earlier in the striatum than cortex [89,90].
    • One of the sequelae associated with pallidotomy is an impaired ability to learn new motor sequences [22 92] and arbitrary stimulus-response associations [93].
    • BG is the tutor, cortex is the storage.

____References____

[0] Turner RS, Desmurget M, Basal ganglia contributions to motor control: a vigorous tutor.Curr Opin Neurobiol 20:6, 704-16 (2010 Dec)