Friday, December 11, 2015

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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