The Organization and Connections of Second Somatosensory Cortex in the Agouti

Abstract

In order to understand how the mammalian sensory cortex has been structured during evolution, it is necessary to compare data from different species across distinct mammalian lineages. Here, we investigated the organization of the secondary somatosensory area (S2) in the agouti (Dasyprocta aguti), a medium-sized Amazonian rodent, using microelectrode mapping techniques and neurotracer injections. The topographic map obtained from multiunit electrophysiological recordings were correlated with both cytochrome oxidase (CO) histochemistry and with patterns of corticocortical connections in tangential sections. The electrophysiological mapping of the lateral strip of parietal cortex adjacent to the primary somatosensory area (S1) revealed that S2 displays a mirror-reversed topographical representation of S1, but with a smaller cortical magnification factor. The caudal border of S2 is surrounded by sensory fields which also respond to auditory stimulation. BDA injections into the forelimb representation of S2 revealed a dense homotopic ipsilateral projection to S1, supplemented by a less dense projection to the caudolateral cortex located near the rhinal sulcus (parietal rhinal area) and to a frontal region probably associated with the motor cortex. Our findings were similar to those described in other mammalian species, reinforcing the existence of a common plan of organization for S2 in the mammalian parietal cortex.

Spatiotemporal properties of whisker-evoked tactile responses in the mouse secondary somatosensory cortex

“…In agreement with reported electrophysiological data 22 , the overall body map in the mouse S2 lies laterally from S1 with a mirrored topographic organization, global features that seem to be shared with other rodent species 6,45,46 . Stimulation of up to 24 individual whiskers in 9 mice revealed that although the cortical territory devoted to whiskers in S2 is about an order of magnitude smaller than in S1, it is characterized by a clear somatotopic organization with a discernable topographic arrangement of individual whiskers.…”

Orientation Preference Maps in Microcebus murinus Reveal Size-Invariant Design Principles in Primate Visual Cortex

“…Similarly, the squirrel neocortex volume 80 and the ratio 79 sourced from previous literature. Neocortex volume of the agouti was calculated by multiplying the cortical flat-mount area 81 with cortical thickness. 21 The agouti V1/Neocortex ratio was measured from a published cortical flat-mount.…”

“…21 The agouti V1/Neocortex ratio was measured from a published cortical flat-mount. 81 The capybara neocortex volume was taken from a study by Campos and Welker 82 and the ratio was measured from a schematic diagram found in the same study. Note that the diagram was not of a flat-mount, but more akin to the dorsal view and does not take into account the gyrification found in the capybara cortex.…”

The functional characterization of callosal connections

“…These results can be reconciled with the “midline rule”, according to which callosal connections preferentially link “midline” sensory representations ( Berlucchi et al, 1967 ; Hubel and Wiesel, 1967 ; see also Section 5.2 ), if one only considers that skilled bimanual actions, such as object manipulations, mostly occur thanks to the simultaneous use of the thumb, index and middle fingers of each hand under foveal control and generally close to the body midline. The convergence of callosal axons at the SI/SII borders observed in rodents ( Fenlon et al, 2017 ; Suárez et al, 2014b ; Zhou et al, 2021 ) is related to the underlying somatotopy ( Olavarria and Van Sluyters, 1995 ), and conforms to the “midline rule”: the somatosensory maps in SI and SII are very similar in rat, mouse, hamster, and in agouti, evidencing that body regions close to the midline (head, upper vibrissae, trunk and proximal limbs) are represented in the lateralmost region of SI and in the bordering medialmost region of SII, in a mirror-like fashion ( Krubitzer et al, 2011 ; Santiago et al, 2019 ). In both somatosensory and visual areas of mice, bilateral correlations, as measured with calcium imaging tend to be strongest close to the midline representations ( Shimaoka et al, 2019 ).…”