Brains of Early Mammals
Motivation
--The organization of the neocortex of early mammals seems to be largely
conserved in modern small-brained mammals
--Therefore, examining the brain of, say, the opossum can show us the underlying
plan that has been altered in large-brained and specialized mammals.
Features
--Primary sensory cortex in mammals is easily distinguishable from surrounding
cortex by the appearance of its layers: layer 4 has somewhat smaller and more densely
packed neurons.
--Visual cortex is located caudally, auditory cortex is located inferiorly in the
temporal lobe, and somatosensory cortex is located just caudal to motor cortex in the
parietal lobe.
--Frontal cortex of opossums does not have obvious motor areas; motor-related
information from the cerebellum is relayed to primary sematosensory cortex.
Variations
--Sensory inputs important to a particular animal receive expanded
representations in cortex
--Duck-billed platypus cortex, for example, is dominated by S1, which is mostly
composed of a large representation of the animal’s bill. Star-nosed mole S1 is taken up by
its star-shaped nose, while human somatosensory cortex contains expanded hand and
mouth representations.
--Large brains do not contain more multimodal or “abstract” cortex, but instead
contain more sensory areas.
--Corpus callosum first appears in placental mammals
--Most sensory areas organized topographically
Primate brains
--Occipital and temporal lobes expanded, indicating an emphasis on vision;
roughly one-half of human cortex is devoted to visual processing (one-eight is dedicated
to somatosensory, auditory, and motor processing). Old World monkeys have
(approximately) 30 visual areas.
--Visual cortex contains a number of subdivisions unique to primates
--In non-anthropoid mammals, the thalamus connects directly to both S1 and S2;
in primates, somatosensory cortex is organized hierarchically
--All primates have a deep calcarine fissure, and most have a Sylvian fissure. The
larger primates, such as Old World monkeys, developed more fissures, such as the central
sulcus and superior temporal sulcus.
Apes and hominids
--It was previously believed that humans possess proportionately more frontal
cortex than any other animal, bestowing on us the capacity for abstract thought
--However, recent work has showed that apes devote proportionately almost as
much of their forebrain to the frontal lobe as humans. There is speculation that the
prefrontal areas may have expanded in humans, but there is little evidence to support this
for the time being.
--Human brains are asymmetrical: the planum temporale is larger on the left than
on the right. This may be related to the left hemisphere’s dominance for language
--Brain areas specialized for language in humans may have developed from preexisting areas originally used for other purposes.
--Early hominids had brains similar in size to African apes, but hominid brains
tripled in size over the last 2 million years.
Brain size
--Larger brains tend to contain more neurons, which causes a problem: it becomes
harder to keep each neuron connected to the same percentage of other neurons
--To solve this problem, larger brains tend to be more modular, containing a
larger number of smaller areas that contain many intra-area connections but fewer
connections between areas.
--In addition, larger brains also tend to contain hemispheres with specialized
functions, as it is less efficient to send information from one hemisphere to the other if
they are overly large.
--Larger sensory areas contain neurons that receive information from a smaller
area. This can make it difficult to perceive global properties of a stimulus. Over the
course of evolution, sensory areas enlarge more slowly than the brain as a whole, leaving
room for the addition of more small cortical areas.
--Late-developing brain structures are larger in big brains than early-developing
structures.