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Imaging Basal Ganglia Function J. Anat. (2000) 196, pp. 543–554, with 5 figures Printed in the United Kingdom 543 Review Imaging basal ganglia function DAVID J. BROOKS MRC Cyclotron Unit, Hammersmith Hospital, London, UK (Accepted 14 September 1999) In this review, the value of functional imaging for providing insight into the role of the basal ganglia in motor control is reviewed. Brain activation findings in normal subjects and Parkinson’s disease patients are examined and evidence supporting the existence for functionally independent distributed basal ganglia- frontal loops is presented. It is argued that the basal ganglia probably act to focus and filter cortical output, optimising the running of motor programs. Key words: Motor control; Parkinson’s disease; positron emission tomography. inhibiting unwanted movements (Penney & Young, 1983; Mink & Thach, 1991), alerting animals to the While there have been considerable advances in our presence of novel or rewarding circumstances (Brown understanding of the pathophysiology of movement & Marsden, 1990; Ljungberg et al. 1992; Schultz, disorders, based on development of animal models of 1992), and mediating selective attention (Wichman & parkinsonism and chorea and observations of the DeLong, 1999) and conditional learning and planning functional effects of stereotactic surgery in tremor and (Taylor et al. 1986; Gotham et al. 1988; Robbins et al. Parkinson’s disease (PD), rather less is known about 1994). The basal ganglia, however, clearly do not act the precise role that the basal ganglia play in motor in isolation but are part of a system of parallel control. Single unit electrical recordings have been distributed corticosubcortical loops. At least 5 sep- obtained from the basal ganglia of lesioned and intact arate loops have been described (Alexander et al. awake animals, both at rest and during performance 1990) and these loops link the basal ganglia via the of rewarded motor acts, and also from the pallidum ventral and dorsomedial thalamus to cortical pre- and subthalamus of PD patients undergoing place- motor and prefrontal areas including dorsolateral ment of electrical stimulators. Additionally, the prefrontal cortex (DLPFC), orbitofrontal cortex behavioural problems of patients with PD, Hunting- (OFC), and anterior cingulate cortex (ACA). These ton’s disease (HD), and dystonia and the nature of loops are believed to be spatially segregated, based on their involuntary movements has been extensively the findings of anterograde and retrograde tracer documented. As a result of these observations it has experiments in nonhuman primates (Middleton, been suggested that the basal ganglia may play a role 1997). It has been proposed that the basal ganglia may in determining the force and velocity of movement act to ‘bind’ the functions of these segregated loops (Hallett & Khoshbin, 1980; Anderson & Horak, (Wichman & DeLong, 1999). 1985), preparing for movement (Schultz & Romo, The striatum (caudate and putamen) receives input 1988; Alexander & Crutcher, 1990; Kimura, 1990), from all cortical areas and from the thalamus and selecting motor programs (Brotchie et al. 1991), substantia nigra pars compacta. It then sends projec- enabling movements to become automatic (Brotchie tions to the internal segment of the globus pallidus et al. 1991), facilitating sequential movement (GPi) via direct and indirect pathways and it is known (Marsden, 1987; Kimura, 1990; Brotchie et al. 1991), that there is considerable convergence present in these Correspondence to Prof. David J. Brooks, MRC Cyclotron Unit, Imperial College School of Medicine, Hammersmith Hospital, Du Cane Rd, London W12 0NN, UK. 544 D. J. Brooks (a) (b) Fig. 1a, b. For legend see opposite. Imaging basal ganglia function 545 Fig. 2. Areas of relatively increased cortical and subcortical activation in normal subjects overlayed on an MRI template (a) when learning a novel sequence of finger movements by trial and error movements is compared with performance of prelearned sequential finger movements and (b) when free selection of finger movements is compared with repetitive finger movements. Novel sequence learning activates the caudate–prefrontal loop. (Courtesy of M. Jueptner.) "& connections (Penney & Young, 1986; Parent, 1996). ganglia function have involved H# O PET so this The internal segment of the globus pallidus (GPi) and review will concentrate on this modality. The func- substantia nigra pars reticulata (SNr) are the major tional resolution of state of the art PET cameras is output relays of the basal ganglia. The posteroventral 4 mm and this is adequate to enable blood flow GPi projects to arcuate premotor cortex and primary changes in the putamen, pallidum, and head of motor cortex via the thalamus while the posterodorsal caudate to be independently monitored. GPi projects to the supplementary motor area (SMA) and anterodorsal GPi to dorsal prefrontal cortex (DPFC) (Middleton & Strick, 1997). The subthalamic nucleus (STN) receives input from both the frontal Setting the basic parameters of movement cortex and the external pallidum (GPe) and sends efferent projections back to GPe and to GPi. It also One approach for determining whether specific brain communicates with the pedunculopontine nuclei areas have a role in mediating a function is to examine which are thought to control gait and posture. whether they show correlated levels of blood flow Positron emission tomography (PET), single pho- when that function is performed at different inten- ton emission tomography (SPECT) and magnetic sities. It has been suggested that the basal ganglia have resonance imaging (MRI) all provide a means of a role in controlling the force and velocity of studying changes in regional cerebral blood flow movements based in part on the observation that (rCBF) in man in vivo under activating conditions. patients with Parkinson’s disease have bradykinesia The majority of activation studies examining basal and take longer to achieve full power during actions. Fig. 1a, b. Areas of significantly increased cortical and subcortical activation in normal subjects overlayed on an MRI template where increases in rCBF correlate with sequence complexity. Rostral SMA, putamen, pallidum, thalamus, and cerebellum show correlated activation but not sensorimotor cortex. (Courtesy of H. Boecker.) 546 D. J. Brooks Two PET activation studies have addressed this the role of the basal ganglia. One approach for question: Jenkins et al. (1997a) measured rCBF when determining loop function is to look for activation in paced joystick movements were performed in freely components which is correlated with particular dimen- chosen directions at frequencies increasing from once sions of paradigms during their performance. every 5 s to once every second. Sensorimotor cortex It has been proposed that the basal ganglia play an and cerebellar blood flow increased in parallel with important role in mediating sequential limb move- frequency of movement over this 0n2–1 Hz range ment. It is well recognised that, while patients with while dorsal prefrontal and lentiform nucleus blood Parkinson’s disease can perform single finger move- flow remained at a constant level of activation. ments efficiently, attempts to perform repetitive or Dettmers et al. (1995) demonstrated that sensorimotor sequences of finger movements result in a rapid fall in cortex rCBF rises when increasing levels of force are amplitude and often motor arrest. Boecker et al. used to press a morse key. When 50% of maximal (1998) examined brain activation when normal sub- force is exerted the rCBF response plateaus. The jects carried out varying sequences of finger–thumb putamen was also activated by finger presses, however, opposition movements. The sequences ranged from 4 levels of rCBF remained stable between 10% and to 8 moves in length and were overlearned before 50% of maximal force. The findings of these 2 studies scanning so that they could be performed auto- therefore indicate that, while sensorimotor cortex and matically. Individual finger–thumb opposition move- cerebellum play a primary role in determining basic ments were paced at a regular frequency of 1 Hz. movement parameters, the putamen has some other Performance of finger movements compared with rest modulatory role. Single unit recordings from putamen activated contralateral motor cortex, lateral and in awake nonhuman primates support this conclusion; medial premotor areas and underlying anterior cingu- neuronal firing rates were principally determined by late cortex, parietal cortex, thalamus, putamen and moving to particular targets rather than the force or pallidum, and the cerebellum. Activation-induced velocity of movement applied (Alexander & Delong, blood flow changes that correlated with sequence 1985). complexity were seen in the rostral supplementary motor area (SMA), anterior putamen and pallidum, Volitional versus cued action thalamus, and cerebellum but not in the lateral premotor cortex, caudal SMA, or primary motor Jueptner et al. (1996) examined brain activation cortex (see Fig. 1a, b). These findings therefore during performance of a task where subjects had to support the notion that a rostral SMA-putamen loop freely draw a series of connected straight lines, facilitates performance of learned patterns of move- changing direction every 3 s, on a computer screen ment while the motor cortex and caudal SMA are using a mouse. The sequence of lines was then played important for precise control of individual move- back to the subject who had to trace over the lines as ments.
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