Connections of somatosensory cortex in megachiropteran bats: The evolution of cortical fields in mammals

Abstract

The cortical connections of the primary somatosensory area (SI or 3b), a caudal somatosensory field (area 1/2), the second somatosensory area (SII), the parietal ventral area (PV), the ventral somatosensory area (VS), and the lateral parietal area (LP) were investigated in grey headed flying foxes by injecting anatomical tracers into electrophysiologically identified locations in these fields. The receptive fields for clusters of neurons were mapped with sufficient density for injection sites to be related to the boundaries of fields, and to representations of specific body parts within the fields. In all cases, cortex was flattened and sectioned parallel to the cortical surface. Sections were stained for myelin and architectonic features of cortex were related to physiological mapping and connection patterns. We found patterns of topographic and montopographic connections between 3b and adjacent anterior parietal fields 3a and 1/2, and fields caudolateral to 3b (SII and PV). Area 1/2 had both topographic and nontopographic connections with 3b, PP, and SII. Connections of SII and PV with areas 3b, 3a, and 1/2 were roughly topographic, although there was clear evidence for nontopographic connections between these fields. SII was most densely connected with area 1/2, while PV was most densely connected with 3b. SII had additional connections with fields in lateral parietal cortex and with subdivisions of motor cortex. Other connections of PV were with subdivisions of motor cortex and pyriform cortex. Laminar differences in connection patterns of SII and PV with surrounding cortex were also observed. Injections in the ventral somatosensory area revealed connections with SII, PV, area 1/2, auditory cortex, entorhinal cortex, and pyriform cortex. Finally, the lateral parietal field had very dense connections with posterior parietal cortex, caudal temporal cortex, and with subdivisions of motor cortex. Our results indicate that the 3b region is not homogeneous, but is composed of myelin dense and light regions, associated with 3b proper and invaginations of area 1/2, respectively. Connections of myelin dense 3b were different from invaginating portions of myelin light area 1/2. Our findings that 3b is densely interconnected with PV and moderately to lightly interconnected with SII supports the notion that SII and PV have been confused across mammals and across studies. Our connectional evidence provides further support for our hypothesis that area 1/2 is partially incorporated in 3b and has led to theories of the evolution of cortical fields in mammals.