Neuronal activity related to rule and conflict in macaque supplementary eye field
Introduction
The supplementary eye field (SEF) is an area on the mediodorsal surface of the frontal lobe in which neurons are active during the preparation and execution of saccadic eye movements [1], [2]. Although the SEF is widely regarded as an oculomotor area, its functions are more complex than simply to specify the metrics of saccades. This is evident in the fact that some SEF neurons represent the directions of saccades relative to an object-centered reference frame [3], [4], [5], [6], some fire most strongly when stimulus-response associations have not yet been fully automatized [7] and some are active during the anticipation or delivery of reward [8], [9]. Of particular relevance to the current study is the finding that some neurons exhibit elevated activity during the performance of tasks involving conflict.
Conflict-related activity has been reported in two contexts: the countermanding task, in which, at the last moment before executing a saccade, the monkey is instructed to suppress it [9], and the antisaccade task, in which, when a peripheral cue is presented, the monkey must suppress the tendency to glance at it and look away instead [10]. These results suggest that neuronal activity is dependent on conflict but do not prove it. In the countermanding task, there is no control that holds constant the physical parameters of the monkey's behavior but eliminates conflict. In the antisaccade task, there is a conflict-free control, the prosaccade task, which requires the monkey to execute a saccade to the cue rather than away from it. However, the antisaccade task differs from the prosaccade task not only in the presence of conflict but also in requiring that the target be selected by a more difficult rule (select the location diametrically opposite the cue with respect to fixation). Neuronal activity might thus be elevated on antisaccade trials because of the nature of the rule for target selection rather than as a result of the presence of conflict. The aim of the study described here was to resolve this issue by characterizing neuronal activity in the SEF during performance of a task allowing independent manipulation of conflict and of the difficulty of the rule for target selection.
Section snippets
Methods
The ‘color-spatial’ task used in this study involved the following sequence of events: the monkey began by fixating a central spot; then four potential targets came on at locations right, up, left and down relative to fixation; then a cue flashed in superimposition on one of the targets; then there was a delay; and, finally, offset of the fixation spot instructed the monkey to execute a saccade to the target associated with the cue (Fig. 1A). If the monkey executed a saccade directly to the
Results
Data were collected from 170 neurons in the SEF of both hemispheres of three rhesus macaque monkeys. The aim of the analysis was twofold: first, to determine whether the level of neuronal activity differed according to the rule in use on a given trial (color or spatial) and, second, to determine whether the level of neuronal activity during color trials differed according to whether the monkey had to execute a saccade toward or away from the cue's location.
To assess the impact of the
Neuronal activity dependent on the difficulty of the rule for target selection
We have found that many SEF neurons fire at different rates according to whether the monkey is selecting targets by a spatial rule or a color-location association rule. This finding is in harmony with previous reports on dorsal premotor cortex [12], dorsolateral prefrontal cortex [13], [14], [15] and the SEF [11], indicating that neurons fire at different levels depending on whether targets are selected by a spatial rule or by association with a color or pattern. The unique contribution of the
Acknowledgments
We thank Karen Rearick and Karen McCracken for excellent technical assistance. Support was provided by the Center for the Neuroscience of Mental Disorders, DBCNR/NIMH MH45156 (C.R.O.), NIH RO1 EY011831 (C.R.O.) and the McDonnell-Pew Program in Cognitive Neuroscience (S.N.G.). Technical support was provided by a NIH core grant (EY08098).
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