Acalculia is the inability to perform simple
mathematical problems, such as addition,
subtraction, multiplication, division, or stating
which of two numbers is larger. It is often
caused by damage to the left parietal lobe
(especially the angular gyrus)  or the frontal lobe.
The frontal lobe is an area of brain tissue
located in the front of the brain. The parietal lobe
is an area of brain tissue located behind the
frontal lobe. The left angular gyrus is a triangular-
shaped region at the back of the parietal lobe
that is often implicated in acalculia.
Damage to the left angular
gyrus (red) causes acalculia
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However, different areas of the parietal lobe are believed to play specific roles in math.
The left angular gyrus (see picture above) plays an important role in memorized math
facts for example (e.g., multiplication tables), largely leaving subtraction intact. However,
damage to other areas of the parietal lobe is known to cause more problems with
subtraction but leaving multiplication largely intact.

Damage to the right hemisphere (which controls visual-spatial functioning) can cause
problems in visual-spatial aspects of math for the representation or interpretation of
numbers such as borrowing and carrying, organizing columns, and placement of decimal
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Acalculia can be a sign of dementia. Dementia
is a mental disorder characterized by a
significant loss of intellectual and cognitive
abilities without impairment of perception or
consciousness. Acalculia is differentiated from
dyscalculia, with the latter referring to difficulty
(but not an inability) in performing simple math
problems. Dysclaculia is considered a
developmentally-based problem (e.g., when the
person is initially learning math) while acalculia
is considered to be an acquired deficit
sustained later in life.  
Acalculia comes from the Greek word "a" meaning "to do without" and the Latin word “calculo” meaning “to reckon.” Put the words together and you have “to do without to reckon.”