ed the anterior parts (the anterior portion of the quadran- gular lobule), the same area strongly activated in the eye-hand task in our study.
We also took care to balance the visual inputs within the four conditions of experiment 1, and sensory inputs associated with eye or hand movement would not be ex- pected to differ in these tasks. The spatial separation be- tween ocular and manual targets on the screen (Fig. 1) is not expected to affect the cerebellum. Clower et al. (1996) reported only activity in area PEG of the posteri- or parietal lobe, and not cerebellum, due to the re- integration and alignment of visual and proprioceptive representations of the hand distorted by prism lenses.
The cerebellum has been proposed to be concerned with error detection or error processing, and in motor learning (see Clower et al. 1996; Cordo et al. 1997; Kitazawa et al. 1998). However, our subject’s perfor- mance in these tasks was stable over time and similar across conditions (Fig. 3). Hence, there is no reason to suppose differential cerebellar activation because of tracking errors in the different conditions, nor significant motor learning during scanning. The equivalent perfor- mance across the tasks also suggests that the contrast be- tween the conditions is not likely to be confounded by the increase in difficulty moving from single tasks to du- al tasks. The final aspects of the task that would be ex- pected to sensitively activate the cerebellum are tracking speed and velocity errors. We can estimate these from the total mouse movement and tracking errors (cumula- tively measured every 4.4 s), which were not significant- ly different in the two tasks. In fact, the total mouse mo- tion was smaller in the eye-hand condition than in the hand-only condition, and thus the difference in average speed is of the wrong sign to explain the increased cere- bellar activity observed.
Thus, we argue that the cerebellum is particularly concerned with inter-communication between different motor effectors, to allow co-ordinated control. Visually guided control of the arm involves communication from oculomotor centres carrying information about the posi- tion and velocity of the eye motion, as these define the target’s motion. Likewise, signals from the arm-control centres provide information that allows the eyes to accu- rately track the hand through visual space. So one might expect that the oculomotor site, active in eye tracking, would be more active in eye and hand tracking, if it then receives and processes information from the hand con- trol site. The same would be expected for the hand- control areas. However, there are no direct connections between the cerebellar cortical regions shown to be acti- vated by these experiments. Hence, the communication must involve extra-cerebellar relays, perhaps including areas in the premotor cortices or the cortical eye fields. It is our aim to now test cerebral areas in the same tasks to challenge this point.
Acknowledgements This work was funded by a Wellcome Trust Senior Research Fellowship. We very gratefully acknowledge the use of the imaging facilities of the CRL Laboratory, Tokyo, and also the facilities of the FMRIB Centre, Oxford. We thank Edwin
Robertson for comments on the manuscript and also the research teams at CRL, Tokyo and ERATO, ATR, Kyoto. In particular, we would like to thank Professor Mitsuo Kawato for his help and sup- port.
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