Abstract
People can carry out everyday actions nearly automatically. Such habitual actions may become invalid if the environment changes, at which point we need to switch behavior under volitional control. The pre-supplementary motor area (pre-SMA) enables this sort of behavioral switching by first suppressing a habitual action and then facilitating a desired controlled action (Isoda and Hikosaka, 2007). Pathways through which the pre-SMA inhibits or facilitates individual actions remain unknown. To test our hypothesis that the inhibition of habitual action is mediated by the subthalamic nucleus (STN), we studied single-neuron activity as monkeys performed a saccade-overriding task (Isoda and Hikosaka, 2007). We found that successful switching from a habitual to an alternative saccade was associated with a significant change in presaccadic activity for 30% of STN neurons (43/145; "switch neurons"). Such switch-selective activity was direction-selective and emerged mostly when the saccade was to be switched to the contralateral side. In support of our hypothesis, a majority of switch neurons were considered to inhibit unnecessary habitual saccades, which was demonstrated using a saccade GO/NOGO task (Isoda and Hikosaka, 2007). Moreover, while the temporal evolution of switch-selective activity in the STN was early enough to initiate saccade switching, it followed that in the pre-SMA by 16 ms. These data suggest that the STN plays a critical role in inhibiting habitual saccades and the pre-SMA uses the STN to overcome habitual undesired saccades. [J Physiol Sci. 2008;58 Suppl:S149]
