Fig. 7A–H Functional activa- tion during ocular tracking (experiment 2). Strong activa- tion is seen in visual cortical areas (A–C), including extra- striate motion-sensitive areas MT/V5 (A). The most pro- nounced cerebellar-activation cluster was in the vermis (D), although there were small acti- vation sites in bilateral areas of intermediate cortex (E) and paramedian and/or biventer lobules (F). These consecutive axial slices correspond to those of Fig. 4, spatially normalised onto the brain of a different subject
no hand-tracking component to this task, and the only target displayed on the screen was the white square, which either followed the same trajectory as used in the other experiments, or remained stationary at the screen centre. The ocular tracking condition most strongly acti- vated the primary visual cortex (Fig. 7), and the two ex- tra-striate areas weakly activated in the other tracking conditions (compare Fig. 7A with Fig. 4A). There was also significant activation at the same site in the oculo- motor vermis (Fig. 7D, E) as previously seen at a low significance level. Restricted parts of the sites in the ansiform, paramedian and biventer lobules of the cere- bellum that had been activated by hand movement with- out eye movement (Table 2 ) were also significantly acti- vated (Fig. 7D, F, G).
There are three main points for discussion from these re- sults. First, there were two activated areas in the lateral ansiform and paramedian lobules of the cerebellum con- cerned with visually guided hand movements. Second, there was also significant vermal activation seen with ocular tracking. Third, the lateral site in the ansiform lobule and the vermal site were strongly activated by hand and eye movement, respectively, but were each sig- nificantly more activated in co-ordinated eye and hand tracking than the summed activity in hand-alone or eye- alone tracking.
Together, these results support the hypothesis that the cerebellum makes a particularly important contribution to movements under direct visual control. We suggest that the last result provides support for a role of the cere- bellum in the co-ordination of eye and hand movements.
First, the two main areas in cerebellar cortex con- cerned with visually guided hand movements, in the lat-
eral ansiform lobule and paramedian lobule, have now been documented several times in different movement tasks (Ellerman et al. 1998; Flament et al. 1996; Jueptner et al. 1997a, 1997b). The ansiform activation areas are also consistent with sites explored electro-physiologically in monkeys performing visuo-motor tracking tasks (Ebner and Fu 1997; Marple-Horvat and Stein 1987, 1990; Miall 1998; Ojakangas and Ebner 1992; Thach et al. 1992), while inactivation of this area in monkeys leads to ataxic movement of the ipsilateral arm and wrist (Miall et al. 1987). The paramedian site has not often been explored electro-physiologically. However, it is not clear from the current experiments whether this activation site lies only within the paramedian lobule or if it in- cludes activation of the adjacent biventer lobule. This is the human analogue of the dorsal paraflocculus in the monkey and is known to have many cells responsive to visual motion (Glickstein et al. 1989; Stein and Glick- stein 1992) or both arm movement and visual motion (Marple-Horvat and Stein 1990).
Second, it seems that there is a selective activation of the cerebellar vermis in these visuomotor tracking tasks. Inoue et al. (1998) found in a comparison of reaching movements to visual targets with and without visual feedback that the vermis was the only cerebellar site as- sociated with the visual-feedback condition. Ellerman et al. (1998) then contrasted a similar step-tracking task with non-visually guided eye or hand movements (move- ment in complete darkness) and showed that the vermal activation in their visually guided condition was not seen in the non-visual arm movement condition. Jueptner et al. (1996) found lateral cerebellar cortical and posterior lobe vermal activation in a PET study comparing a line- re-tracing task with a new line-drawing task, indicating again that the vermis is particularly activated in the visu- al feedback controlled condition. Interestingly, Jueptner et al. (1997a) suggest that the majority of cerebellar acti-