SEQUENTIAL MOTOR BEHAVIOR AND THE BASAL GANGLIA Evidence from a serial reaction time task in monkeys

An important aspect of motor skill lies in the ability to acquire and utilize representations of ordered sequences of motor acts, thereby allowing one to select and execute a sequence of movements as a whole. The importance of this ability becomes evident when one considers alternatives; for instance, having to tie one’s shoes every morning based on the conscious selection of each constituent movement of the tying sequence. Understanding the neural control of motor sequences is also important because of the prevalence of human neuropathologies in which movement sequences are impaired differentially [e.g., Parkinson's disease (Benecke et al., 1987)] and because of the widening use of surgical therapies for those disorders (e.g., deep brain stimulation). The importance of basal ganglia (BG) networks for sequential motor control remains a topic of debate (Marsden, 1984; Mink, 1996). Several lines of research suggest that the motor circuit of the BG plays an important role in sequential behaviors (reviewed recently by Keele et al., 2003; Graybiel, 2004). One of the most prevalent hypotheses is that the BG contributes to the "proceduralization" of action sequences (Eichenbaum and Cohen, 2001). Procedural learning provides the capacity for series of events or actions to be recalled as a whole [i.e., as "chunks" (Graybiel, 1998)] with minimal attentional load. It is possible that the BG contributes both to the laying down of new procedural memories (e.g., as a "teacher") and to the recall or expression of already-learned procedures (Cromwell and Berridge, 1996; Aldridge and Berridge, 1998; Hikosaka et al., 2002). Several investigators have made a strong case for a role for the BG in switching between components of a familiar sequence (i.e., between the constituent movements of "natural units of action" or "chunks")(Brotchie et al., 1991; Graybiel, 1998; Keele et al., 2003; Aldridge et al., 2004). Other investigators have challenged the importance of the BG in this function (Wenger et al., 1999; Smith et al., 2001; Exner et al., 2002; Penhune and Doyon, 2002; Rhodes et al., 2004), especially for the production of short familiar sequences (Verwey et al., 2002).