RE: Sensory modalities and the possibility of semi-arbitrary additions

From: Phil Goetz (philgoetz@yahoo.com)
Date: Thu Nov 03 2005 - 15:46:49 MST


--- "Stuart, Ian" <Ian.Stuart@woolpert.com> wrote:

> But I do not believe that elephants, or in fact any other creature as
> far as my Googling has found, has 6 full layers of cortical tissue,
> and
> it is this, rather than total area or volume of tissue which allows
> us
> to think on a more abstract level than that of other creatures and
> gives
> us our "creative" edge.

Whoa - I was seriously startled by the idea that other mammals might
have brains with a different number of layers than ours. So I Googled.
 And what do you know? Some mammals don't have 6 layers.

They have 7.

Cortical Layer VII and Persistent Subplate Cells in Mammalian Brains
Roger L. Reep

Department of Physiological Sciences, College of Veterinary Medicine
and Brain Institute, University of Florida, Gainesville, Fla., USA

Brain, Behavior and Evolution 2000;56:212-234 (DOI: 10.1159/000047206)

Abstract

Layer VII is the deepest cortical layer in rats, and consists of a thin
layer of persistent subplate cells overlain by a cell-sparse,
myelin-rich stratum through which many corticocortical axons travel.
Layer VII neurons participate in local and long-distance
corticocortical connections. The present study was undertaken to
determine whether layer VII is a typical feature in rodent brains, and
to determine which other mammalian taxa exhibit a layer VII. The adult
brains of 144 species from 22 orders were examined. Of these, 43
species in 6 orders exhibit a layer VII. These include the sciurognath
Rodentia, Insectivora, Paucituberculata, Paramelemorphia, some
Xenarthra, and some Chiroptera. In all taxa interstitial cells were
observed scattered throughout the white matter. The observed
distribution of layer VII in this sample of mammalian taxa suggests
that layer VII is a typical feature in some orders, but is not present
in most orders. The heterogeneous distribution of layer VII in the
Rodentia and Chiroptera suggests that species-level developmental
dynamics are involved. It is hypothesized that the timing of subplate
apoptosis in relation to the establishment of corticocortical
connections is the major factor that determines whether layer VII is
present in the adult stage.

Copyright 2000 S. Karger AG, Basel

        
                
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