Differential Roles of the Frontal Cortex, Basal Ganglia, and Cerebellum in Visuomotor Sequence Learning

TWO TYPES OF MEMORYMemory can be divided into two categories: memory of events or facts, andmemory for skills and rules. These are often called declarative memory andprocedural memory (Squire, 1986; Tulving, 1985), or they are often character-ized as ‘‘what’’ memory and ‘‘how’’ memory. An accumulation of declarativememories will comprise knowledge; an accumulation of procedural memorieswill comprise intelligence.This classification is based largely on the fact that amnesic patients, whocannot remember daily episodes, can still learn new behavioral procedures(Cohen and Squire, 1980; Mishkin and Appenzeller, 1987). Because we knowthat the medial temporal areas including the hippocampus are crucial fordeclarative memory, we naturally assume that neural systems for proceduralmemory 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 isdifficult to examine experimentally. Procedural memory cannot be acquiredjust by showing something to the subject; the subject must perform the proce-dure repeatedly. The presence of procedural memory cannot be indicated byjust 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 breakthrough such difficulties. For this purpose, we devised a sequential buttonpress task in which the subjects learn new visuomotor sequences and performwell-learned sequences (Hikosaka et al., 1995).BEHAVIORAL ANALYSIS OF VISUOMOTOR SEQUENCE LEARNINGIn front of a subject, in this case a Japanese monkey, is a panel on which16 LED buttons are arranged in a 4 14 matrix (Fig. 1A). Beneath the panelis another button called the ‘‘home key.’’ If the monkey presses the home key,