Changes in cortical field potentials associated with learning processes of audio-initiated hand movements in monkeys

Summary

Field potentials on the surface and at a depth of 2.0–3.0 mm in the cerebral cortex were recorded with chronically implanted electrodes in various areas of a monkey. The potentials associated with movements in response to auditory stimuli (audio-initiated hand movement) were observed during the learning process of the movement. The monkey had to lift a lever by wrist extension within the duration of the stimulus (tone of 1000 Hz) lasting for about 900, 700 or 500 ms depending on the stage of the learning process. On the first training day, potentials appeared in the primary auditory, auditory association, prefrontal and premotor cortices of a naive monkey. The potentials of the auditory association, prefrontal and premotor cortices became less marked on the next and following days. After a few weeks of training, the potential of the auditory association cortex started to increase again, while the monkey still lifted the lever randomly. When the potential of the auditory association cortex grew to a certain extent, the monkey began to respond to the stimulus with the movement, and potentials appeared in the motor cortex in response to the auditory stimulus. After this process, the potentials in the auditory association and motor cortices gradually increased with further training, and the movement became shorter and less variable in reaction time. The potential in the motor cortex was shown to be mediated by the neocerebellum and superficial thalamo-cortical projection. The changes of potentials associated with the learning processes of audio-initiated movements were respectively compared with those of the learning processes of the visuo-initiated movement, reported previously (Sasaki and Gemba 1982), and cortical and subcortical mechanisms related to the learning process were discussed.