Differential roles of the frontal cortex, basal ganglia, and cerebellum in visuomotor sequence learning.

Memory can be divided into two categories: memory of events or facts, and memory for skills and rules. These are often called declarative memory and procedural memory (Squire, 1986; Tulving, 1985), or they are often characterized as ‘‘what’’ memory and ‘‘how’’ memory. An accumulation of declarative memories will comprise knowledge; an accumulation of procedural memories will comprise intelligence. This classification is based largely on the fact that amnesic patients, who cannot remember daily episodes, can still learn new behavioral procedures (Cohen and Squire, 1980; Mishkin and Appenzeller, 1987). Because we know that the medial temporal areas including the hippocampus are crucial for declarative memory, we naturally assume that neural systems for procedural memory should be somewhere outside the medial temporal areas. Which brain areas are responsible for holding procedural memory, then? The answer is still far out of reach, partly because procedural memory is difficult to examine experimentally. Procedural memory cannot be acquired just by showing something to the subject; the subject must perform the procedure repeatedly. The presence of procedural memory cannot be indicated by just a yes-or-no answer; it can be expressed only by performing the procedure. Our research on procedural learning and memory is an attempt to break through such difficulties. For this purpose, we devised a sequential button press task in which the subjects learn new visuomotor sequences and perform well-learned sequences (Hikosaka et al., 1995).