Numerotopy: How quantities are mapped onto the brain

Different sites on the brain’s surface
respond maximally to different numbers
of visually-presented items. Image: Ben
M. Harvey, Utrectht University
Earlier this week I posted my book
chapter about topographic
mapping , or how sights, sounds and
touch are mapped onto different
parts of the brain's surface in an
orderly manner. This organization
is found in all of the brain's
sensory systems – for example,
adjacent regions of your visual
field project onto adjacent light-
sensitive cells in your retina, and
this spatial organization is
maintained in the pathway that
transmits the visual information
from the eye to the brain.
Topographic mapping, or 'otopy,' is
a key organizing principle in
neuroscience, one that is essential
for the brain's information
processing capabilities. It is found
in the primary sensory cortices,
which receive and process
information from the sense organs,
and the primary motor cortex,
which sends commands for
movements down to the nerve cells
in the spinal cord that signal to the
muscles. A study published in the
latest issue of the journal Science
now shows that quantities are also
represented topographically in the
brain.
Numerosity – or "number sense" –
is a deeper understanding of the
meaning of numbers that occurs
without the ability to count, or any
knowledge of numerical symbols.
This ability has obvious
evolutionary advantages, and is
not restricted to humans: macaque
monkeys can place images in order
according to the number of objects
they contain, and their brains
contain neurons that are tuned to
quantities of items in the visual
field. These cells appear to be
organized according to some sort of
non-linear scaling principle ,
suggesting that numbers may be
represented topographically in the
macaque brain, but until now there
has been no evidence of any such
number map in humans.
The design of the new study was
relatively simple. Benjamin Harvey
of Utrecht University in the
Netherlands and his colleagues
recruited 8 participants and
showed them a series of visual
stimuli, each containing different
numbers of dots, while scanning
their brains using state-of-the-art
high-resolution functional
magnetic resonance imaging
(fMRI). They then examined
activity in the posterior parietal
cortex, a part of the brain just
above and behind the ears, which is
known to play an important role in
numerical thinking.
The researchers found distinct
populations of neurons that are
tuned to small quantities, in a strip
of tissue approximately 2cm wide,
on both sides of the brain. The cells
were indeed organized
topographically, with those nearest
to the brain's midline firing in
response to the smallest quantities,
those furthest away firing in
response to the largest, and those in
between firing in response to
intermediate quantities.
They also found that the tuning
widths of the neuronal populations
changed systematically from one
side of the quantity map to the
other. In other words, cells nearest
the midline responded to very
specific small quantities, but as the
preferred quantity of the cells
increases, so too does the range of
quantities to which those cells are
tuned. Consequently, there is far
more tissue devoted to smaller
quantities than larger ones.
The findings suggest that abstract
features such as numerosity can be
organized topographically, just as
is basic sensory information. The
layout of the number map in the
human brain seems to correspond
with the way in numerosity-
sensitive cells are organized in the
macaque brain, and it may explain
why we find it so much harder
estimating large quantities than
smaller ones.
The monkey number map was
charted with single cell recordings,
which gives the highest possible
resolution, and far higher than
fMRI, which visualises neuronal
activity as 'voxels,' or volume
pixels, each corresponding to a tiny
cube of brain tissue containing tens
of thousands of cells or more. We
therefore know the monkey
number map in far greater detail,
but future studies, using either
more sophisticated fMRI
technology or single cell recordings
from the human brain , will
undoubtedly give us a better sense
of how our brains represent
numbers and quantities.