Trends in Neurosciences
Consolidation of motor memory
Introduction
It is clear from experience that the nervous system can form multiple long-term (>24 h) motor memories; for example, we apparently never forget how to swim, to ride a bicycle or to drive a car. However, despite the ubiquity of long-term motor memory in everyday life, experimental demonstration of how such memories are formed and retained has proven controversial. This review will focus on the specific issue of consolidation of motor memory, with consolidation defined as a set of processes whereby a long-term memory becomes more stable with the passage of time [1]. This definition implies that consolidation is proven if the memory is susceptible to disruption by a competing memory or a focal lesion during a limited time-window.
Motor memory differs from declarative memory in that it is demonstrated through savings in performance over several trials, rather than through recall of a single item (single-trial memory). ‘Savings’ refers to a more rapid rate of relearning compared with the rate of original learning. Thus, a motor memory has consolidated when there is a limited time-window within which savings can be disrupted. Here, we first describe studies of simple associative motor learning for two well-characterized reflexes, where there is good evidence for consolidation and insight into its underlying neural circuitry. We then address consolidation of skilled sequential finger movements. Finally, we examine adaptation of arm movements to novel dynamics and to new visuospatial mappings, two types of motor learning for which evidence of consolidation is more elusive.
Section snippets
Consolidation of associative motor learning
Experimental investigation of Pavlovian eyelid conditioning (EC) adaptation and vestibulo-ocular reflex (VOR) adaptation in several species has provided valuable insights into the neural circuitry, specifically in the cerebellum, involved in savings [2] and consolidation of motor memory 3, 4, 5. EC and VOR adaptation are similar in that they both involve associative learning in a reflex circuit [6], in which a conditioned stimulus (CS; e.g. tone or head motion) is paired with an unconditioned
Consolidation of skilled finger movements
Repetitive 1 Hz transcranial magnetic stimulation (rTMS) has been used to investigate the role of primary motor cortex (M1) in retention of a simple finger opposition task [36]. Subjects performed a metronome-paced (0.5 Hz) ballistic pinch between the index finger and thumb of the non-dominant left hand. The performance measure was acceleration of the pinching movement, which was assessed across two practice sessions. Control subjects showed increases in peak pinch acceleration within a first
Consolidation of internal models for reaching
In the past 20 years, a computational framework has been developed to characterize the behavioral and neural basis for planning and execution of reaching (reviewed extensively in [41]). It appears that the CNS plans reaching movements in extrinsic space 42, 43, 44, with target and hand location initially coded as vectors with respect to fixation that are then subtracted to produce an intended movement vector in a hand-centered coordinate system. The transformation of this vector into motor
Consolidation of internal models in bimanual skills
In bimanual movements, one hand often serves a supportive role by holding an object that is manipulated by the other hand. For example, if one hand holds an electronic notepad and the other hand writes on the device, the supporting hand shows compensatory adjustments to counteract the anticipated forces arising from the writing hand. A similar situation occurs when one hand holds a book while the other hand lifts it: the postural hand reduces the upward forces precisely at the moment when the
Concluding remarks
There is evidence for motor consolidation in several experimental systems – that is, the behavioral correlates of the memory suggest that under some conditions the neural substrate becomes resistant to disruption within hours or days. In specific tasks, the cerebellar cortex appears to be crucial in the initial storage of the memory but, with time, the memory can be represented elsewhere. Because motor learning comprises several distinct processes (planning of sequences of action, adaptation of
Acknowledgements
This work was supported by National Institute of Neurological Disorders and Stroke Grants K23-NS02138 (to J.W.K.), and R01-NS037422 (to R.S.). We thank Javier Medina and Michael Mauk for insightful discussions, and Toni Pearson for critical comments on the manuscript.
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