Anatomy of the neural circuits
Cortico-cortical connections
The different areas of the Superior parietal lobule of the macaque monkey have been injected using retrograde neuronal tracers in order to delineate the specific neuronal circuits that directly involve this part of the primate brain. Each area of SPL is connected to a set of parietal and premotor areas involved in specific functional roles (Fig.1). Compared to the nearby extrastriate visual areas, posterior areas of the SPL (as areas V6A, PGm and PEc) are connected with parietal and premotor areas involved in visuomotor coding, receiving bimodal (visual and somatosensory) inputs together with somatomotor ones. Anterior areas (like areas PE/5, and other subdivisions of area 5 as PEci in the mesial surface of the hemisphere and PEip in the depth of intraparietal sulcus) are mainly connected with the somatosensory primary area and motor and pre-motor areas, and they are involved in the somatomotor guidance of limb actions.
1) Gamberini M, Passarelli L, Fattori P, Galletti C, (2020) Structural connectivity and functional properties of the macaque superior parietal lobule. Brain Struct Funct.2020; 225:1349-1367.
2) Passarelli L, Rosa MG, Bakola S, Gamberini M, Worthy K H, Fattori P, Galletti C. (2018) Uniformity and diversity of cortical projections to precuneate areas in the macaque monkey: what defines area PGm? Cereb Cortex, May 2018;28: 1700–1717
Sub-cortical connections
Subcortical afferents involving thalamus and claustrum have been analyzed by the use of the same neuronal tracer injections focused in parietal cortical areas. The subcortical inputs from the thalamus are in agreement with cortico-cortical circuits and indeed areas of the SPL receive from lateral posterior and medial pulvinar nuclei that integrates visuo-motor information, motor functions from the ventral lateral nucleus and eye-related activity from medial dorsal nucleus (Fig.2, right). All the areas of SPL are also directly connected with the claustrum. The claustral connections well agree with the functional characteristics of SPL areas and, even if a certain level of variability is evident, they receive inputs mainly from the postero-ventral portion of this subcortical structure known to be involved in sensory encoding (visual and somatosensory, Fig.2, left).
1) Gamberini M, Passarelli L, Impieri D, Worthy KH, Burman KJ, Fattori P, Galletti C, Rosa MGP, Bakola S, (2020) Thalamic afferents emphasize the different functions of macaque precuneate areas. Brain Struct Funct.2020; 225:853-870.
2) Gamberini M, Passarelli L, Bakola S, Impieri D, Fattori P, Rosa MG, Galletti C. (2017) Claustral afferents of Superior parietal areas PEc and PE in the macaque. J Comp Neurol. 525: 1475-1488, 2017.
