Theories of motor control postulate that the brain uses internal models of the body to control movements accurately. Internal models are neural representations of how, for instance, the arm would respond to a neural command, given its current position and velocity. Previous studies have shown that the cerebellar cortex can acquire internal models through motor learning. Because the human cerebellum is involved in higher cognitive function as well as in motor control, we propose a coherent computational theory in which the phylogenetically newer part of the cerebellum similarly acquires internal models of objects in the external world. While human subjects learned to use a new tool (a computer mouse with a novel rotational transformation), cerebellar activity was measured by functional magnetic resonance imaging. As predicted by our theory, two types of activity were observed. One was spread over wide areas of the cerebellum and was precisely proportional to the error signal that guides the acquisition of internal models during learning. The other was confined to the area near the posterior superior fissure and remained even after learning, when the error levels had been equalized, thus probably reflecting an acquired internal model of the new tool.

Rhythm is determined solely by the relationship between the time intervals of a series of events. Psychological studies have proposed two types of rhythm representation depending on the interval ratio of the rhythm: metrical and nonmetrical representation for rhythms formed with small integer ratios and noninteger ratios, respectively. We used functional magnetic resonance imaging to test whether there are two neural representations of rhythm depending on the interval ratio. The subjects performed a short-term memory task for a seven-tone rhythm sequence, which was formed with 1:2:4, 1:2:3, or 1:2.5:3.5 ratios. The brain activities during the memory delay period were measured and compared with those during the retention of a control tone sequence, which had constant intertone intervals. The results showed two patterns of brain activations; the left premotor and parietal areas and right cerebellar anterior lobe were active for 1:2:4 and 1:2:3 rhythms, whereas the right prefrontal, premotor, and parietal areas together with the bilateral cerebellar posterior lobe were active for 1:2.5:3.5 rhythm. Analysis on individual subjects revealed that these activation patterns depended on the ratio of the rhythms that were produced by the subjects rather than the ratio of the presented rhythms, suggesting that the observed activations reflected the internal representation of rhythm. These results suggested that there are two neural representations for rhythm depending on the interval ratio, which correspond to metrical and nonmetrical representations.

Successful motor behavior requires making appropriate response (response selection) at the right time (timing adjustment). Earlier psychological studies have suggested that the response selection and timing adjustment processes are performed serially in separate stages. We tested this hypothesis using functional magnetic resonance imaging. The subjects performed a choice reaction time task in four conditions: two (on-line response selection required or not) by two (on-line timing adjustment required or not). We found that the neural correlates for the two processes were indeed separate: the anterior medial premotor cortex (presupplementary motor area) was selectively active in response selection, whereas the cerebellar posterior lobe was selectively active in timing adjustment. However, the functional separation was only partial in that the lateral premotor cortex and the intraparietal sulcus were active equally for response selection and timing adjustment. The lateral premotor cortex was most active when both processes were required, suggesting that it integrates the information on response selection and the information on timing adjustment; alternatively, it might contribute to the allocation of attentional resources during dual information processing. The intraparietal sulcus was equally active when either response selection or timing adjustment was required, suggesting that it modifies, rather than integrates, these processes. Furthermore, our results suggest that these activations related to response selection and timing adjustment were distinct from sensory or motor processes.

Functional magnetic resonance imaging was used to investigate neural activity during the judgment of visual stimuli in two groups of experiments using seven and five normal subjects. The subjects were given tasks designed differentially to involve orthographic (more generally, visual form), phonological, and lexico-semantic processes. These tasks included the judgments of whether a line was horizontal, whether a pseudocharacter or pseudocharacter string included a horizontal line, whether a Japanese katakana (phonogram) character or character string included a certain vowel, or whether a character string was meaningful (noun or verb) or meaningless. Neural activity related to the visual form process was commonly observed during judgments of both single real-characters and single pseudocharacters in lateral extrastriate visual cortex, the posterior ventral or medial occipito-temporal area, and the posterior inferior temporal area of both hemispheres. In contrast, left-lateralized activation was observed in the latter two areas during judgments of real- and pseudo-character strings. These results show that there is no katakana "word form center" whose activity is specific to real words. Activation related to the phonological process was observed, in Broca's area, the insula, the supramarginal gyrus, and the posterior superior temporal area, with greater activation in the left hemisphere. These activation foci for visual form and phonological processes of katakana also were reported for the English alphabet in previous studies. The present activation showed no additional areas for contrasts of noun judgment with other conditions and was similar between noun and verb judgment tasks, suggesting two possibilities: no strong semantic activation was produced, or the semantic process shared activation foci with the phonological process.

The function of the lateral part of the human cerebellum was investigated through cerebro-cerebellar functional connectivity. We propose a laterality index method to reveal a functional and possibly anatomical pathway between the cerebral cortex and the cerebellum. The brain activity involved in learning a visually-guided tracking skill using a novel computer mouse was measured by functional magnetic resonance imaging. The imaging data analyzed using the method suggest that the simple lobule and semilunar lobule of the lateral cerebellum have connections with the pars opercularis and pars triangularis in the inferior frontal gyrus. A possible function of this cerebro-cerebellar communication loop is tool usage, which is in-between the cognitive and motor functions of the human cerebellum.