Organization of sensory cortex in a Madagascan insectivore, the tenrec (Echinops telfairi)

Abstract

We identified subdivisions of somatosensory cortex, and the borders and extents of auditory and visual cortex in Madagascan tenrecs (Echinops telfairi) by using microelectrode recording techniques and cortical myeloarchitecture. There was evidence for three distinct somatosensory fields. The primary somatosensory area (S1) contained an orderly representation of the contralateral body surface that stained darkly for myelin. Neurons were activated by light touch, and receptive fields were often small, especially for the snout. Immediately rostral to S1, a lightly myelinated rostral field (R) also contained a representation of the contralateral body, although the internal topography was not fully determined. Neurons in R responded to manipulations of body parts and tissue displacements. A small, moderately myelinated area lateral to S1 was termed PV/S2 because it possessed features that were similar to both the parietal ventral area (PV) and the second somatosensory area (S2) in other mammals. Neurons in PV/S2 responded to light tactile stimulation. A densely myelinated oval of cortex caudal to PV/S2, the auditory area (A), contained neurons that responded to clicks, and the densely myelinated caudomedial visual area (V) contained neurons that were activated by stimulation of one or both eyes. Some characteristics of V were similar to the primary visual area (V1) described in other mammals. A visual area located in rostromedial cortex (RV) contained neurons that were highly responsive to visual stimulation. Area RV may be a specialization of tenrecs or an elaboration of a visuomotor field that has been retained in most extant mammals. The results support the view that most of the neocortex of primitive mammals was composed of a few sensory areas. J. Comp. Neurol. 379:399-414, 1997.

Principles underlying mammalian neocortical scaling

“…Best-®t line via linear regression is y 0:4032x À 0:4988 (R 2 0:8197, n 11, p < 0:001), the slope of which is not signi®cantly dierent from 1/3 (p > 0:2). Counts of cortical areas taken from Kaas (1987), Krubitzer (1995), and Krubitzer et al (1997). There are disagreements on how to distinguish and count areas, but while dierent methodologies are likely to lead to dierent constants in the scaling law, they are unlikely to lead to dierent scaling exponents.…”

Neocortical neuron types in Xenarthra and Afrotheria: implications for brain evolution in mammals

“…We identified cortical regions for examination based on location and architecture observed in sections stained for Nissl, myelin, NPNFP, and PV. In addition, we referred to published atlases and cortical maps of echidna (Hassiotis et al 2004), nine-banded armadillo (Royce et al 1975), tammar wallaby , Madagascan lesser hedgehog tenrec (Krubitzer et al 1997), Cape elephant shrew (Dengler-Crish et al 2006), and rat (Paxinos and Watson 2005). For this study, we did not perform a detailed areal parcellation of the cerebral cortex in these species.…”

“…Previous investigations of neocortical organization in xenarthrans and afrotherians include electrophysiological studies of functional localization in the nine-banded armadillo (Dom et al 1971;Royce et al 1975), two-toed sloth (Meulders et al 1966), three-toed sloth (De Moraes et al 1963;Saraiva and Magalhães-Castro 1975), Madagascan lesser hedgehog tenrec (Krubitzer et al 1997), and Cape elephant shrew (Dengler-Crish et al 2006). In addition, there are reports of cortical architecture and parcellation in armadillos (Dom et al 1971;Ferrari et al 1998;Royce et al 1975), the two-toed sloth , Madagascan lesser hedgehog tenrec (Schmolke and Künzle 1997), manatee (Marshall and Reep 1995;Reep et al 1989;Sarko and Reep 2007), and elephant (Cozzi et al 2001).…”

Multiarchitectonic characterization of insular, perirhinal and related regions in a basal mammal, Echinops telfairi

“…These data, however, have only been obtained in a few species and insular and perirhinal regions have never been analysed experimentally in mammals with poorly differentiated brains. Such information is needed to reveal basic mammalian circuits and supplement our knowledge of the areal evolution of the cerebral cortex (Northcutt and Kaas 1995;Krubitzer et al 1997;Rosa 1999) with data about meso-or periallocortical regions.…”