Timing function of the frontal cortex in sequential motor and learning tasks.

Abstract

The timing aspects of human frontal lobe function are discussed in the light of the results of three experiments on movement-related cerebral potentials. Experiment I is based on use of a sequential tracking task and experiment II a motor learning task; experiment III deals with frontal hemispheric specialisation by comparing self-initiated writing and drawing. The Bereitschaftspotential (BP) preceding voluntary movement is maximum over the supplementary motor area (SMA) for all movements, including finger, toe, speech, and eye movements, regardless of each movement's different localisation in the brain, e.g., motor cortex, temporal lobe, or midbrain. The assumption that all motor events are governed by the primary (rolandic) motor cortex is erroneous. The motor system is widely decentralised. It is only when this decentralization is recognized that the close temporal association between the onset of all movements and the preceding Bereitschaftspotential in the SMA can be understood. A plausible explanation would be that the SMA decides on the starting time of all the various movements. The frontal function of motivation is not a single entity but has several subfunctions. It has to decide what to do, how to do and when to do. The latter is probably the task of the SMA. A comparison of different motivational situations makes this clear. In the usual BP paradigm, such as self-initiated simple finger or eye movements, only the SMA becomes active among all the frontal areas. If, however, motivation is required to modify motor programs in motor learning, as it is in experiment II, the entire convexity of the frontal lobe shows a large surface-negative potential, the amplitude of which reveals a significant positive correlation with the success in learning. On the other hand, in experiment I, which uses a manual pursuit-movement task requiring attention to unpredictable changes in stimulus direction but providing a fixed time for these changes (so that their timing is foreseeable), the SMA shows anticipatory behaviour; it takes the form of a large negative potential which ceases 0.5 s prior to the end of the directed-attention potential over parietooccipital areas. In other words, in this special situation, where the SMA can anticipate the onset of movement, it seems to delegate the final execution of the movement to the cortical area most specialised for it, in this case the parietooccipital cortex. The supervision of the tasks concerning what to do and how to do may be provided mainly by the orbital cortex and the frontolateral cortex, respectively (Kleist 1934).(ABSTRACT TRUNCATED AT 400 WORDS)