Supplementary eye field as defined by intracortical microstimulation: connections in macaques

In macaques, the frontal eye field and the recently defined supplementary eye field play a role in the production of eye movements. Whereas the structure and function of the frontal eye field are well understood, little is known about the supplementary eye field. The goal of this study was to determine the connections of the physiologically defined supplementary eye field. In each case, the location of the supplementary eye field was determined by using intracortical microstimulation, the borders were marked with small electrolytic lesions, and horseradish peroxidase conjugated to wheat germ agglutinin was injected into the supplementary eye field. After the tissue was incubated with tetramethyl benzidine, it was determined that in three cases the injection site was confined to the physiologically defined supplementary eye field. The present results indicate that the supplementary eye field is reciprocally connected with the claustrum, ventral anterior nucleus, including pars magnocellularis, nucleus X, posterior subdivision of the ventral lateral nucleus, multiform, parvocellular, magnocellular, and densocellular subdivisions of the medial dorsal nucleus, central lateral nucleus, parafascicular nucleus, and suprageniculate-limitans complex. The supplementary eye field projects to the putamen, caudate, reticular nucleus of the thalamus, central densocellular nucleus, zona incerta, subthalamic nucleus, rostral interstitial nucleus of the medial longitudinal fasciculus, parvocellular part of the red nucleus, intermediate and deep layers of the superior colliculus, central gray, cuneiform nucleus, mesencephalic reticular formation, pontine gray, nucleus reticularis tegmenti pontis, and nucleus reticularis pontis oralis. The supplementary eye field is reciprocally and bilaterally connected with periprincipal and inferior prefrontal cortex, with periarcuate cortex, including the frontal eye field, the frontal ventral region, and with postarcuate premotor cortex, and cortex surrounding the supplementary eye field, including the supplementary motor area. The supplementary eye field is also reciprocally connected ipsilaterally with cortex in and around the cingulate sulcus and the intraparietal sulcus, whereas cortex within the superior temporal sulcus projects to the supplementary eye field. The connections of the physiologically defined supplementary eye field are compared to previously demonstrated connections of the supplementary motor region and of the physiologically defined frontal eye field. Comparisons between the connections of the frontal and supplementary eye fields reveal that both regions are connected with structures related to visuomotor functions, but the frontal eye field has more extensive connections with vision-related structures, and the supplementary eye field has more extensive connections with structures related to prefrontal and skeletomotor functions. Such connectional differences suggest functional differences between these two sensorimotor regions of the frontal lobe.