fMRI Investigation of Cortical and Subcortical Networks in the Learning of Abstract and Effector-Specific Representations of Motor Sequences



ABSTRACT

A visuomotor sequence can be learned as a series of visuo-spatial cues or as a
sequence of effector movements. Earlier imaging studies have revealed that a network
of brain areas is activated in the course of motor sequence learning. However these
studies do not address the question of the type of representation being established at
various stages of visuomotor sequence learning. In an earlier behavioral study, we
demonstrated that acquisition of visuo-spatial sequence representation enables rapid
learning in the early stage and progressive establishment of somato-motor
representation helps speedier execution by the late stage. We conducted functional
magnetic resonance imaging (fMRI) experiments wherein subjects learned and
practiced the same sequence alternately in
normal and rotated settings. In one rotated
setting (visual), subjects learned a new motor sequence in response to an identical
sequence of visual cues as in
normal. In another rotated setting (motor), the display
sequence was altered as compared to
normal, but the same sequence of effector
movements were used to perform the sequence. Comparison of different
rotated
settings revealed analogous transitions both in the cortical and subcortical sites during
visuomotor sequence learning — a transition of activity from parietal to parietal-
premotor and then to premotor cortex and a concomitant shift was observed from
anterior putamen to a combined activity in both anterior and posterior putamen and
finally to posterior putamen. These results suggest a putative role for engagement of
different cortical and subcortical networks at various stages of learning in supporting
distinct sequence representations.

Key Words: Sequence representation, Anterior striatum, Posterior striatum, DLPFC,
pre-SMA, SMA.

INTRODUCTION

It is commonly observed that when a skill is being acquired subjects are circumspect
and deliberate in the initial attentive phase but later on as the skill is acquired they
move into an automatic phase when attention can be engaged in other tasks
simultaneously (Fitts, 1964). When performing well-mastered skills, it appears as if
the body parts know what to do and no overt attention is necessary. Further, the
memory of over-learned skills seems robust and lasts for long time. In this scenario it
is interesting to find out if the representation of skill memory and the associated
neural bases are different at various stages of learning. Previous studies on sequence
learning addressed
where and when activity is found in various cortical and
subcortical areas using implicit learning (Grafton et al., 1995) and explicit learning by
trial and error (Sakai et al., 1998; Toni et al., 1998). This paper addresses the question
of
what is actually learned in different areas at different stages of explicit sequence
learning.

Earlier studies that investigated representational changes during motor sequence
learning emphasized either implicit sequence learning in the serial reaction time
(SRT) paradigm (Grafton et al., 1998), explicit sequencing but without learning
(Harrington et al., 2000), or the recall of motor sequences at various stages of learning
(Karni et al., 1995; Penhune and Doyon, 2002). Grafton et al. (1998) found learning-
related increases in regional cerebral blood flow (rCBF) in the sensorimotor cortex
reflecting effector-specific representation and in the inferior parietal cortex reflecting
abstract representation of motor sequences. Sakai et al. (1998) and Toni et al. (1998)
used trial and error learning paradigm to study the time course of changes during
explicit visuo-motor sequence learning.



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