title: A central source of movement variability.
abstract: Movements are universally, sometimes frustratingly, variable. When such variability causes error, we typically assume that something went wrong during the movement. The same assumption is made by recent and influential models of motor control. These posit that the principal limit on repeatable performance is neuromuscular noise that corrupts movement as it occurs. An alternative hypothesis is that movement variability arises before movements begin, during motor preparation. We examined this possibility directly by recording the preparatory activity of single cortical neurons during a highly practiced reach task. Small variations in preparatory neural activity were predictive of small variations in the upcoming reach. Effect magnitudes were such that at least half of the observed movement variability likely had its source during motor preparation. Thus, even for a highly practiced task, the ability to repeatedly plan the same movement limits our ability to repeatedly execute the same movement.
authors.affiliation: Neurosciences Program, Stanford University, Stanford, California 94305, USA.
authors.forename: Mark M
authors.initials: MM
authors.lastname: Churchland
authors.forename: Afsheen
authors.initials: A
authors.lastname: Afshar
authors.forename: Krishna V
authors.initials: KV
authors.lastname: Shenoy
mesh_terms: D000818:Animals; D001522:Behavior, Animal; D001696:Biomechanical Phenomena; D002452:Cell Count / Q000379:methods; D004576:Electromyography / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology; D008253:Macaca mulatta; D008297:Male; D009044:Motor Cortex / Q000166:cytology*; D009068:Movement / Q000502:physiology*; D009474:Neurons / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D012043:Regression, Psychology