Not Knowing: A
Variety of Agnosias
Brains are
great. They’re pretty terrific for a variety of reasons, of course, one of
which being that when they’re running smoothly, it’s easy to forget how
impressively multi-functional they are. Only when something goes wrong do we
realize just how much there is to go
wrong—which, naturally, also leads to some quite interesting stories of things
going wrong.
Take
disorientation. Orientation and navigation aren’t so bad for the average
person—maybe you take a wrong turn on your way to the dentist and have to go
around the block. Worst case scenario, you pull out your phone and map your
location. For some people it’s harder; they’re at the low end of the
navigational norm (and they include myself). Then, however, there’s a leap to
those far outside the normal spectrum. The full scope of what your brain does
to ensure that you recognize your own hand, your neighbor’s face, or the shop
on the corner really comes into view with the startlingly broad range of possible
orientational disorders.
For
starters, there exists a seemingly endless array of subtleties to being lost.
Maybe you have an impaired ability to identify landmarks. Maybe you can identify landmarks, but can’t gain
useful directional information from them. Or maybe you have no trouble with
landmarks at all, but can’t form a mental map of your surroundings. That final
disorder has long been known to result from brain injuries or lesions, as in
the case of damage to the posterior parietal cortex, fusiform and lingual
gyrus, or hippocampus. However, it has also been more recently discovered as a
developmental disorder—developmental topographical disorientation, or DTD—in
which otherwise healthy, uninjured individuals experience a highly specific
deficit in the creation of mental maps. DTD patients may make up a heterogeneous
group in which other orientational deficits are present as well. The Radiolab
podcast series does a wonderful segment on the disorder, called “You Are Here,”
in their episode Lost & Found. It features Sharon
Roseman, a landmark DTD patient, and neuroscientist Dr. Giuseppe Iaria, who
diagnosed Roseman’s condition.
Roseman’s
disorder, also known as developmental topographagnosia, joins a host of other agnosias
that become even more intriguing when they leave the realm of the topographical
and start popping up in areas of life where it would be almost impossible for
an unaffected person to become disoriented. Prosopagnosia, or face blindness,
is just one example: the inability to recognize faces. (Which Radiolab actually
has some excellent segments on also, like “Strangers
in the Mirror.”) As is the case with DTD, deficits in the fusiform gyrus
have been linked to the disorder. Those affected by prosopagnosia must depend
upon ways other than facial recognition, such as recalling hair color, outfits,
or characteristics such as one’s walking gait, to identify friends and even
family members.
Autotopagnosia,
an inability to correctly orient body parts, is another example. Individuals
with autotopagnosia lose the ability to tell left from right, and as a result are
unable to correctly perform a task such as touching their left ear with their
right forefinger. Like the many nuances to being lost, there are a variety of
hypotheses on the cause of the disorder. Two main ones implicate lesions in the
parietal lobe, the first proposing a visuospatial element, and the second
suggesting a disruption of the patient’s body image. Yet a third hypothesis
postulates that the illness is language-related, and connected to lesions in
the posterior left hemisphere.
Undoubtedly,
we’ve come a long way from ideas like Franz Joseph Gall’s orderly phrenological
interpretation that each bump and crevice on the human skull indicates a neatly
packaged area dedicated to a single function. The compelling and frustrating
part of orientational disorders (and most neurological disorders, really) is
just how interconnected and complex the systems in question are—how much
unconscious control occurs to keep us running smoothly. And here I thought a
person was simply lost or found.
—Kate Oksas
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