I believe that there is a right number and it is based upon critical
stages when capacities come to be able to process particular data;

If at particular critical stages of a cats vision development it is
shown only vertical lines the nerves required for recognizing
horizontal lines die and so that capacity is not as readily available.

----------------------------------

Experiments on visual development in cats have shown that the feline's
mature visual system is affected by the kind of light stimulation it
receives early in life. Kittens that were allowed to experience only
horizontal lines, for example, could not detect vertical barriers in a
test maze and repeatedly blundered into them. Evidently the brain cells
that normally detect vertical lines had ceased functioning.
Neuroscientists have articulated a "use it or lose it" principle to
account for such findings. This would clearly apply to the sensory
problems of feral children. A similar explanation may apply to the
language problems of socially isolated children, most of whom can
master only a few words even after years of dedicated instruction. This
also explains the problems of socially deprived institutional children.

Sensory problems that are rooted in developmental brain anomalies are
likely to be irreversible. Orphanages in which there is extreme social
and sensory deprivation may produce sensory deficits resembling those
found among feral children. Children adopted out of extremely
neglectful Romanian orphanages during the 1990s had a persistently
impaired capacity to respond to most types of sensory information,
probably because their brains had not received enough stimulation early
in life. They lacked the normal capacity to feel pain, hear sounds, and
guide movements using visual feedback. Institutional children generally
score about ten points lower in IQ tests than others, which is possibly
due to the effects of early social and sensory deprivation on brain
development.

Enriched environments promote the development of brain cells leading to
increased ability to solve problems. Social stimulation is a critical
component of environmental enrichment.

The Science of Romance - by Nigel Barber
http://www.amazon.com/exec/obidos/tg/detail/-/1573929700/

-----------------------------

Notice the layers of interpretation of various features:
http://www.owlnet.rice.edu/~psyc351/Images/visualcortex.jpg
http://www.owlnet.rice.edu/~psyc351/imagelist.htm
http://www.bruker-biospin.de/MRI/applications/images/biov2a.jpg
http://image.korea.ac.kr/korean/Research/Retina/retina2.jpg

Taking Apart the Visual Image
http://williamcalvin.com/bk7/bk7ch13.htm
http://williamcalvin.com/bk7/img/bk7p209.jpg

http://www.normanallan.com/Sci/vision.htm
http://www.normanallan.com/Sci/images/hubelsimp2.jpg

http://www.brain.riken.go.jp/bsi-news/bsinews5/no5/issue1e.html

http://camelot.mssm.edu/~andrei/elastic_net_tutorial/elastic_net_tutorial.htm
http://camelot.mssm.edu/~andrei/elastic_net_tutorial/od_or_maps_hubener.jpg

http://www.bbsonline.org/Preprints/OldArchive/bbs.quartz.html

http://www.ifi.uio.no/infneuro/Gamle/H2002/hafliger/coding/
http://www.ifi.uio.no/infneuro/Gamle/H2002/hafliger/coding/hubel_wiesel.gif

http://www.cs.iastate.edu/~baojie/acad/current/hnn/hypercolumn.gif
http://www.cs.iastate.edu/~baojie/acad/current/hnn/hnn.htm

Vulnerable periods during the development of the nervous system are
sensitive to environmental insults because they are dependent on the
temporal and regional emergence of critical developmental processes
(i.e., proliferation, migration, differentiation, synaptogenesis,
myelination, and apoptosis). Evidence from numerous sources
demonstrates that neural development extends from the embryonic period
through adolescence. In general, the sequence of events is comparable
among species, although the time scales are considerably different.
Developmental exposure of animals or humans to numerous agents (e.g.,
X-ray irradiation, methylazoxymethanol, ethanol, lead, methyl mercury,
or chlorpyrifos) demonstrates that interference with one or more of
these developmental processes can lead to developmental neurotoxicity.
Different behavioral domains (e.g., sensory, motor, and various
cognitive functions) are subserved by different brain areas. Although
there are important differences between the rodent and human brain,
analogous structures can be identified. Moreover, the ontogeny of
specific behaviors can be used to draw inferences regarding the
maturation of specific brain structures or neural circuits in rodents
and primates, including humans. Furthermore, various clinical disorders
in humans (e.g., schizophrenia, dyslexia, epilepsy, and autism) may
also be the result of interference with normal ontogeny of
developmental processes in the nervous system. Of critical concern is
the possibility that developmental exposure to neurotoxicants may
result in an acceleration of age-related decline in function. This
concern is compounded by the fact that developmental neurotoxicity that
results in small effects can have a profound societal impact when
amortized across the entire population and across the life span of
humans. Key words: abnormal neurological development, apoptosis,
behavioral testing methodology, delayed neurotoxicity, differentiation,
migration, myelination, neurobiological substrates of function,
neuronal plasticity, neurotrophic factor, primate, rodent. -- Environ
Health Perspect 108(suppl 3):511-533 (2000).

http://ehpnet1.niehs.nih.gov/docs/2000/suppl-3/511-533rice/abstract.html


The vulnerability of the developing brain is dependent on two main
exposure issues. The first factor relates to whether an agent or its
active metabolite(s) reaches the developing nervous system, and the
second factor relates to the period of exposure. Exposure to
environmental toxicants coincident with the ontogeny of developmental
processes is more likely to cause adverse effects if they interfere
with the cascade of these developmental processes. In general, if
exposure occurs before or after an organ develops, it is less
vulnerable to perturbation than if exposure occurs during development
of that organ.

It is critical for the risk assessment of environmental chemicals that
we have a good understanding of the time lines of normal neural
development in humans and other species used in toxicological testing.
This understanding should include both pharmacokinetic and
pharmacodynamic factors that may differ across species. Both species-
and age-related differences in pharmacokinetic parameters are out of
the scope of this review paper, but other papers have attempted to
describe these parameters in a limited way with pesticides (1-5),
metals (6-10), and a solvent (11).

There is general recognition that the developing nervous system is
qualitatively different from the adult nervous system. The normal
ontogeny of neural development in rodents is different from humans
because rodents have considerable postnatal development and humans have
considerably more prenatal maturation of their nervous systems. These
differences may be confounded with differences in the route of exposure
during critical periods of nervous system development and thus
differences in vulnerability between developing animals and humans
(e.g., lactational transfer during the first postnatal week in rodents
and transplacental transfer during the third trimester in humans).

Key aims of this article are to characterize the profiles of
developmental processes, describe similarities and differences in
windows of vulnerability between rodents and humans, and describe
similarities and differences in effects after perturbations of
developmental processes in humans and animal models, including rodents
and monkeys. The brain areas subserving various behavioral functions
are described for human infants and the ontogeny of these specific
structures is discussed in the context of the effects of lesions in
nonhuman primates with altered performance on these tasks. This will
further our understanding of developmental neurotoxicology, improve our
ability to predict adverse outcomes in animals, and assist in the
extrapolation of these adverse outcomes to risks to human health.


Development of the Brain in Utero

The cell precursors of the brain and spinal cord, which compose the
central nervous system (CNS), begin to develop early in embryogenesis
through the process called neurulation. The notochord, which is a
cellular rod that defines the primitive axis of the embryo, will be
incorporated into the vertebral system. The notochord induces the
overlying ectodermal tissue to form the neural plate at approximately 2
weeks of gestation in humans. This overlying ectodermal tissue gives
rise to the CNS. In humans on approximately gestational day (GD) 18,
the neural plate invaginates along its central axis to form the neural
groove with neural folds on each side. By the end of the third week of
gestation in humans, the neural folds have begun to move together and
fuse, forming the neural tube near the anterior end of the notochord;
this fusion progresses both cranially and caudally in a zipperlike
manner. The neural tube then separates from the overlying ectoderm,
which becomes a contiguous surface over the back of the embryo and
differentiates into the epidermis of the skin. As the neural tube
forms, a population of cells separates from the surface ectoderm at the
apex of the neural folds to form the neural crest, which will give rise
to the sensory ganglia of spinal and cranial nerves, Schwann cells (the
cells covering peripheral nerves), the meningeal covering of the brain
and spinal cord, and some skeletal and muscle components of the head,
among other structures. The neural tube begins to close in the area of
the hindbrain above the origin of the notochord and proceeds anteriorly
and posteriorly, creating a caudal-to-rostral gradient in development
of the brain. Neural tube formation is complete at approximately GD
10.5-11 in rats and from GD 26 to 28 in humans; the anterior neuropore
closes first (rats GD 10.5, humans GD 24-26) and the posterior
neuropore closes later (rats GD 11.3, humans GD 25-28) [reviewed by
DeSesso (12)].

Interruption of neural development during this early period can result
in severe abnormalities of the brain and spinal cord. ...

http://ehp.niehs.nih.gov/members/2000/suppl-3/511-533rice/rice-full.html

http://www.google.com/search?hl=en&q=periods+during+the+development+of+the+nervous+system&btnG=Google+Search