Studies Yield Insight Into The Numerical Brain
Two studies in the January 18, 2007, issue of the journal Neuron,
published by Cell Press, shed significant light on how the brain
processes numerical information--both abstract quantities and their
concrete representations as symbols. The researches said their findings
will contribute to understanding how the brain processes quantitative
information as well as lead to studies of how numerical representation
in the brain develops in children. Such studies could aid in
rehabilitating people who suffer from dyscalculia--an inability to
understand, remember, and manipulate numbers. The researchers also said
their findings offer insight into the mystery of how the brain learns
to associate abstract symbols precisely with quantities.
Both studies reveal in unprecedented detail how structures in the
parietal cortex--the region of higher cognitive processing just above
the forehead--activates during perception of both abstract quantities
and numerical symbols.
In one paper, Manuela Piazza and colleagues showed that regions of the
parietal lobe activate in response to numbers, either when they are
presented as patterns of dots or as Arabic numerals.
In their experiments, the researchers asked human volunteers to pay
attention to the quantities conveyed by groups of dots or numeric
digits presented to them. During the process, the subjects' brains were
scanned using functional magnetic resonance imaging (fMRI), in which
harmless magnetic fields and radio waves are used to measure blood flow
in brain regions, which reflects activity.
The researchers found that the initial presentation of the numeric
stimuli activated the parietal region of the subjects' brains, which
subsided as they adapted to the stimulus. However, the activation
rebounded when the subjects were presented with an abrupt change in the
quantity, whether it was represented in the same (dots versus dots) or
different (dots versus Arabic numerals) notation as the original. This
rebound indicated that the region was processing numerical information.
However, to unambiguously establish that the subjects' brains were
really reacting to numerical quantity, the researcher occasionally
injected a "deviant stimulus" into the second presentation of a
quantity as the brain was adapting to it. This deviant stimulus
consisted of a different number that was either close to, or far from,
the number being presented. The researchers found that this deviant
quantity interrupted adaptation more if it was distant from the
adaptation quantity than if it was closer--conclusive evidence that the
subjects were processing numerical quantities.
The researchers concluded that their findings "indicate an important
role for parietal cortex in the coding of symbolic and nonsymbolic
quantities."
They also concluded that "crucially, we observed crossnotation
adaptation and recovery, particularly in the right parietal cortex,
supporting the idea that shared neural populations encode nonsymbolic
quantities and symbolic stimuli." Piazza and colleagues also concluded
that their findings shed light on how the brain learns to associate
symbols with numbers.
"Our results show that, at least in the adult brain, numerical symbols
and nonnumerical numerosities converge onto shared neural
representations," they wrote. "Perhaps we attach meaning to symbols by
physically linking populations of neurons sensitive to symbol shapes to
preexisting neural populations holding a nonsymbolic representation of
the corresponding preverbal domain (e.g., numerosity)."
In the other paper in Neuron,
Roi Cohen Kadosh and colleagues conducted experiments demonstrating
that the two hemispheres of the parietal lobe function differently in
processing numbers. While the left lobe harbors abstract numerical
representations, the right shows a dependence on the notation used for
a number, they found. The researchers concluded that "results challenge
the commonly held belief that numbers are represented solely in an
abstract way in the human brain." The authors also concluded that their
results "advocate the existence of distinct neuronal populations for
numbers, which are notation dependent in the right parietal lobe."
In their experiments, the researchers also used the adaptation
phenomenon, that the brain adapts to stimuli by reducing its initial
activity--and that repeating the same quantity leads to reduced
activation compared to changing the quantity. They asked subjects whose
brains were being scanned using fMRI to view consecutive numbers
presented on a screen that represented either the same or different
quantities. Crucially, the numbers were also presented either as two
words (e.g., two or eight), two digits (e.g., 2 or 8), or a mixed
notation (two and 8).
They hypothesized "that if the assumption of an abstract representation
of numbers in the [parietal cortex] held true, the adaptation effect
would be observed within and across notations. In contrast, in the case
of nonabstract numerical representation, we expected that the
adaptation effect would be modulated by the notation type. This result
would suggest that distinct neuronal populations for notation exist."
This meant that if the brain region was purely representing an
abstraction of a number (e.g., 8) then any notational representation of
this number (e.g., 8 or eight) would cause an adaptation effect.
Alternatively, if a brain region processed a specific nonabstract
number (e.g., 8) then adaptation would only be seen for the same
notation (e.g., 8 but not eight).
Their analysis revealed an effect of notation in the right parietal
lobe, showing that this region appears to harbor neurons that process
nonabstract numerical representations, in addition to neurons that code
for abstract representations of numeric quantities.
The researchers said that exploring how the processing of numerical
symbols develops could have clinical implications. "Developmental
studies should focus on tracing the emergence of numerical
representation in the brain, investigating in particular at which stage
such a representational divergence appears. Such findings could
contribute significantly both to the field of numerical cognition
research and rehabilitation of people suffering from developmental
dyscalculia," they wrote.
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Cohen Kadosh et al.
The researchers include Roi Cohen Kadosh of Ben-Gurion University of
the Negev in Beer-Sheva, Israel and University College London in
London, UK; Kathrin Cohen Kadosh of Ben-Gurion University of the Negev
in Beer-Sheva, Israel and Birkbeck College in London, UK; Amanda Kaas
of Maastricht University in Maastricht, The Netherlands and Max Planck
Institute for Brain Research in Frankfurt am Main, Germany; Avishai
Henik of Ben-Gurion University of the Negev in Beer-Sheva, Israel;
Rainer Goebel of Maastricht University in Maastricht, The Netherlands.
This work was supported by grants to R.C.K. from the Boehringer
Ingelheim Fonds, the Zlotowski Center for Neuroscience, and the
Kreitman Foundation.
Piazza et al.
The researchers include Manuela Piazza, Philippe Pinel of INSERM,
Service Hospitalier Frédéric Joliot, CEA, DRM, DSV, and IFR49 in Orsay,
France; Denis LeBihan of Service Hospitalier Frédéric Joliot, CEA, DRM,
DSV and IFR49 in Orsay, France; Stanislas Dehaene of INSERM, Service
Hospitalier Frédéric Joliot, CEA, DRM, DSV and IFR49 in Orsay, France
and Collège de France in Paris, France.
This work was supported by INSERM, CEA, a Marie Curie fellowship of the
European Community QLK6-CT-2002-51635 (M.P.), and a McDonnell
Foundation centennial fellowship (S.D.).
Cohen Kadosh et al.: "Notation-Dependent and -Independent Representations of Numbers in the Parietal Lobes." Publishing in Neuron 53, 307-314, January 18, 2007. DOI 10.1016/j.neuron.2006.12.025. http://www.neuron.org/
Piazza et al.: "A Magnitude Code Common to Numerosities and Number Symbols in Human Intraparietal Cortex." Publishing in Neuron 53, 293-305, January 18, 2007. DOI 10.1016/j.neuron.2006.11.022. http://www.neuron.org/
Related preview by Ansari et al.: "Does the Parietal Cortex Distinguish Between "10", "Ten", and Ten Dots?"
Contact: Erin Doonan
Cell Press
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