Role of supplementary eye field in saccade initiation: executive, not direct, control

Performance monitoring local field potentials in the medial frontal cortex of primates: supplementary eye field

We describe intracranial local field potentials (LFPs) recorded in the supplementary eye field (SEF) of macaque monkeys performing a saccade countermanding task. The most prominent feature at 90% of the sites was a negative-going polarization evoked by a contralateral visual target. At roughly 50% of sites a negative-going polarization was observed preceding saccades, but in stop signal trials this polarization was not modulated in a manner sufficient to control saccade initiation. When saccades were canceled in stop signal trials, LFP modulation increased with the inferred magnitude of response conflict derived from the coactivation of gaze-shifting and gaze-holding neurons. At 30% of sites, a pronounced negative-going polarization occurred after errors. This negative polarity did not appear in unrewarded correct trials. Variations of response time with trial history were not related to any features of the LFP. The results provide new evidence that error-related and conflict-related but not feedback-related signals are conveyed by the LFP in the macaque SEF and are important for identifying the generator of the error-related negativity.

Repetition suppression dissociates spatial frames of reference in human saccade generation

The path from perception to action involves the transfer of information across various reference frames. Here we applied a functional magnetic resonance imaging (fMRI) repetition suppression paradigm to determine the reference frame(s) in which the cortical activity is coded at several phases of the sensorimotor transformation for a saccade, including sensory processing, saccade planning, and saccade execution. We distinguished between retinal (eye-centered) and nonretinal (e.g., head-centered) coding frames in three key regions: the intraparietal sulcus (IPS), frontal eye field (FEF), and supplementary eye field (SEF). Subjects (n = 18) made delayed saccades to one of five possible peripheral targets, separated at intervals of 9° visual angle. Target locations were chosen pseudorandomly, based on a 2 × 2 factorial design, with factors retinal and nonretinal coordinates and levels novel and repeated. In all three regions, analysis of the blood oxygenation level dependent dynamics revealed an attenuation of the fMRI signal in trials repeating the location of the target in retinal coordinates. The amount of retinal suppression varied across the three phases of the trial, with the strongest suppression during saccade planning. The paradigm revealed only weak traces of nonretinal coding in these regions. Further analyses showed an orderly representation of the retinal target location, as expressed by a contralateral bias of activation, in the IPS and FEF, but not in the SEF. These results provide evidence that the sensorimotor processing in these centers reflects saccade generation in eye-centered coordinates, irrespective of their topographic organization.

Medial frontal cortex motivates but does not control movement initiation in the countermanding task

Voluntary control of behavior implies the ability to select what action is performed. The supplementary motor area (SMA) and pre-SMA are widely considered to be of central importance for this ability because of their role in movement initiation and inhibition. To test this hypothesis, we recorded from neurons in SMA and pre-SMA of monkeys performing an arm countermanding task. Temporal analysis of neural activity and behavior in this task allowed us to test whether neural activity is sufficient to control movement initiation or inhibition. Surprisingly, 99% (242 of 243) of movement-related neurons in SMA and pre-SMA failed to exhibit time-locked activity changes predictive of movement initiation in this task. We also found a second group of neurons that was more active during successful response cancelation. Of these putative inhibitory cells, 18% (7 of 40) responded early enough to be able to influence the cancelation of the movement. Thus, when tested with the countermanding task, the SMA/pre-SMA region may play a role in movement inhibition but does not appear to control movement initiation. However, the activity of 76% (202 of 267) of movement-related neurons was contingent on the expectation of reward and 42% of them reflected the amount of expected reward. These findings suggest that the movement-related activity in pre-SMA and SMA might represent the motivation for a specific action but does not determine whether or not that action is performed. This motivational signal in pre-SMA and SMA could provide an essential link between reward expectation and motor execution.