Recent Studies of the Basal Ganglia

A number of recent studies have investigated the range of basal ganglia motor control function. For instance, Kimura et al. [35] probed the striatal neurons of monkeys performing arm motion tasks, concluding that

The majority of the striatal neurons showed activity when a particular sensory stimulus was linked to a conditioned movement or when the movement was performed in a particular context (e.g., sensorially guided or internally guided). This activity profile of striatal neurons is in strong contrast to that of pyramidal tract cells in the primary motor cortex, which show strong dependence on parameters of movement such as torque or direction [18]. Striatal neuron activity has been demonstrated to be only poorly related to parameters of movement [49].

This supports the viewpoint that the basal ganglia are tied in with the planning or sequencing of motor activities, rather than with the details of their execution.

Another study points to the relation between basal ganglia damage and rhythmic motion. Rhythmic motions can be either voluntary or involuntary (tremor). Both have characteristic frequencies, with involuntary rythmic motion being the upper bound on the speed of voluntary rhythmic motion [21]. According to Freund and Hefter [21], basal ganglia disorders affect ``involuntary and voluntary alternating movements in different motor subsystems to the same extent.'' There is also a loss of frequency modulation for higher frequencies, known as ``hastening.'' ``At higher frequencies the patient is clamped to a particular higher frequency that actually corresponds to his/her tremor frequency [40].'' Freund and Hefter conclude that ``The data thus support the view that basal ganglia and the cerebellum are important for setting CPG [central pattern generator] characteristic frequencies.''

Brotchie et al. [6] believe that they have recorded neuronal discharges in the basal ganglia which are related to switching between components of a motor plan. They recorded pallidal neuron activity in monkeys performing sequential wrist tasks. They observed phasic pallidal neuron discharge ``which is influenced by the animal's preparation for movement and which appears to have a role in the performance of sequential movement tasks. Its role appears to be an internal cue which may signal the end of one movement in order that the next movement in the sequence may commence.''

This phasic activity does not seem to correlate with regulating the parameters of motion; Brotchie et al. suggest that it appeared to be dependent on the predictability and automaticity of the movement.

Referring to studies by Schwab et al. [70] and Talland and Schwab [77], which deal with the performance of simultaneous motor tasks, Brotchie et al. argue that

these studies demonstrated that it is not possible to perform two movements simultaneously in hypokinesia and that it is always the more subconscious movement that fails and that the more conscious movement is completed. These studies indicate that the role of the BG is important for the performance of subconscious movements and our results reveal a possible neuronal basis for this function... Our theory is that phasic activity in pallidal cells may be the internal cue used by the motor system to switch between motor programs in SMA [supplementary motor area].

The idea that the motor system is using internal cues to step between tasks is supported both by the difficulty that persons with Parkinson's disease have with sequential tasks, and (as Brotchie et al. point out) by the ability of Parkinson's patients to use external stimuli to partially overcome this difficulty. In this viewpoint, ``visual or proprioceptive stimuli can signal the arrival at each point in a sequence, thereby providing the necessary cues for switching motor programs.'' This is perhaps the anatomical basis of kinesia paradoxa.