Sulcus

Abstract: The projections to the frontal cortex from the various subdivisions of the posterior parietal region in the rhesus monkey were studied by means of autoradiographic technique. The rostral superior parietal lobule (area PE) projects to the dorsal areas 4 and 6 on the lateral surface of the frontal lobe as well as to the supplementary motor area (MII) on its medial surface. The caudal area PE sends its connections to dorsal area 6 and MII. The projections from the medial parietal cortex (areas PEc and PGm) are similar to those of the superior parietal lobule but they tend to concentrate in the more rostral part of dorsal area 6, MII, and in the cingulate gyrus (area 24). The most caudal part of the medial parietal cortex also projects to area 8. The anteriormost part of the inferior parietal lobule (area PF) projects to the ventral area 6, including the caudal bank of the lower branch of the arcuate sulcus, to the ventral area 46 below the sulcus principalis, and to the frontal and pericentral opercular cortex. The middle inferior parietal lobule (areas PFG and PG) projects to the ventral part of area 46 and area 8, whilst the posteriormost inferior parietal lobule (caudal PG and area Opt) is connected with both dorsal and ventral area 46, dorsal area 8, as well as the anteriormost dorsal area 6, and the cingulate gyrus (area 24).

Abstract: We have examined the circuitry connecting the posterior parietal cortex with the frontal lobe of rhesus monkeys. HRP-WGA and tritiated amino acids were injected into subdivisions 7m, 7a, 7b, and 7ip of the posterior parietal cortex, and anterograde and retrograde label was recorded within the frontal motor and association cortices. Our main finding is that each subdivision of parietal cortex is connected with a unique set of frontal areas. Thus, area 7m, on the medial parietal surface, is interconnected with the dorsal premotor cortex and the supplementary motor area, including the supplementary eye field. Within the prefrontal cortex, area 7m's connections are with the rostral sector of the frontal eye field (FEF), the dorsal bank of the principal sulcus, and the anterior bank of the inferior arcuate sulcus (Walker's area 45). In contrast, area 7a, on the posterior parietal convexity, is not linked with premotor regions but is heavily interconnected with the rostral FEF in the anterior bank of the superior arcuate sulcus, the dorsolateral prefrontal convexity, the rostral orbitofrontal cortex, area 45, and the fundus and adjacent cortex of the dorsal and ventral banks of the principal sulcus. Area 7b, in the anterior part of the posterior parietal lobule, is interconnected with still a different set of frontal areas, which include the ventral premotor cortex and supplementary motor area, area 45, and the external part of the ventral bank of the principal sulcus. The prominent connections of area 7ip, in the posterior bank of the intraparietal sulcus, are with the supplementary eye field and restricted portions of the ventral premotor cortex, with a wide area of the FEF that includes both its rostral and caudal sectors, and with area 45. All frontoparietal connections are reciprocal, and although they are most prominent within a hemisphere, notable interhemispheric connections are also present. These findings provide a basis for a parcellation of the classically considered association cortex of the frontal lobe, particularly the cortex of the principal sulcus, into sectors defined by their specific connections with the posterior parietal subdivisions. Moreover, the present findings, together with those of a companion study (Cavada and Goldman-Rakic: J. Comp. Neurol. this issue) have allowed us to establish multiple linkages between frontal areas and specific limbic and sensory cortices through the posterior parietal cortex. The networks thus defined may form part of the neural substrate of parallel distributed processing in the cerebral cortex.

Abstract: 'What' and 'where' visual streams define ventrolateral object and dorsolateral spatial processing domains in the prefrontal cortex of nonhuman primates. We looked for similar streams for auditory‐prefrontal connections in rhesus macaques by combining microelectrode recording with anatomical tract-tracing. Injection of multiple tracers into physiologically mapped regions AL, ML and CL of the auditory belt cortex revealed that anterior belt cortex was reciprocally connected with the frontal pole (area 10), rostral principal sulcus (area 46) and ventral prefrontal regions (areas 12 and 45), whereas the caudal belt was mainly connected with the caudal principal sulcus (area 46) and frontal eye fields (area 8a). Thus separate auditory streams originate in caudal and rostral auditory cortex and target spatial and non-spatial domains of the frontal lobe, respectively.

Abstract: Working memory is the process of maintaining an active representation of information so that it is available for use. In monkeys, a prefrontal cortical region important for spatial working memory lies in and around the principal sulcus, but in humans the location, and even the existence, of a region for spatial working memory is in dispute. By using functional magnetic resonance imaging in humans, an area in the superior frontal sulcus was identified that is specialized for spatial working memory. This area is located more superiorly and posteriorly in the human than in the monkey brain, which may explain why it was not recognized previously.