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The concept of a motor program has been used to interpret a diverse range of empirical findings related to preparation and initiation of voluntary movement. In the absence of an underlying mechanism, its exploratory power has been limited to that of an analogy with running a stored computer program. We argue that the theory of cortical cell assemblies suggests a possible neural mechanism for motor programming. According to this view, a motor program may be conceptualized as a cell assembly, which is stored in the form of strengthened synaptic connections between cortical pyramidal neurons. These connections determine which combinations of corticospinal neurons are activated when the cell assembly is ignited. The dynamics of cell assembly ignition are considered in relation to the problem of serial order. These considerations lead to a plausible neural mechanism for the programming of movements and movement sequences that is compatible with the effects of precue information and sequence length on reaction times. Anatomical and physiological guidelines for future quantitative models of cortical cell assemblies are suggested. By taking into account the parallel re-entrant loops between the cerebral cortex and basal ganglia, the theory of cortical cell assemblies suggests a mechanism for motor plans that involve longer sequences. The suggested model is compared with other existing neural network models for motor programming.

Until recently the basal ganglia of the mammalian brain have attracted little attention from theoretical neurobiologists. Traditional views of the functioning of the basal ganglia are based on their biomedical importance in disorders such as Parkinson's disease. Their contribution to normal brain functions has remained poorly understood. Experimental investigations over the past few decades have produced a wealth of detailed information about the structure of the basal ganglia and the physiological properties of their component neurones. It has become evident that the basal ganglia play a role in the selection and performance of learnt behaviours, and also in the effects of reinforcement on acquisition and maintenance of new behaviours. At present it is difficult to link the symptoms of basal ganglia disorders to these basic facts, in part because very few theoretical models attempt to incorporate the information that is now available. Computational modelling can help to advance theoretical understanding in this area by establishing explicit links between different levels of organization: from the effects of neurotransmitters such as dopamine on synaptic plasticity, through the dynamic interactions within subpopulations of neurons, to system-level interactions between the basal ganglia and cerebral cortex. The aim of this review is to outline existing knowledge of the basal ganglia in relation to previous computer modelling work, and to suggest ways of making use of the new experimental findings in the next generation of models.

A movement task was used to investigate the effects of precued variables on reaction time. The task involved rapid rotation of a hand-held manipulandum to target locations and required either pronation or supination of the forearm through short or long extent. The effects on reaction time of precues signalling target direction, extent, or a combination of direction and extent, were measured. The longest reaction times occurred when no information about direction or extent was provided in the precue (all parameters uncertain). Complete prior specification of target position produced the shortest reaction times. Specification of direction when extent was uncertain produced a significantly larger reduction in reaction time than specification of extent when direction was uncertain. Prior specification of extent also produced a small but significant reduction in reaction time relative to the condition in which direction and extent were specified in a mutually conditional manner. The results are discussed in relation to parameter precuing and motor programming, in which the direction is programmed by the pre-selection of neurons representing the muscles to be used in the task while programming of extent is represented by their level of activity during task performance.

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