January 26, 2012

Hemispatial neglect: A one-sided world

Check out the dog, Barley, in this video. Notice how he doesn't seem interested in the food on the left side of his bowl. Perhaps he's blind in his left eye?

But that can't be the case. Close your left eye. You can still see much of your left visual field, and you'd certainly smell any food placed under your nose. And what dog is one to walk away without finishing their food?

In fact, Barley is displaying signs of hemispatial neglect, a strange condition in which brain damage, despite normal vision, results in complete neglect of the left side of one's world. Barley had, in fact, suffered a stroke.

The cortex of the brain (the squiggly outer layer we stereotypically associate with brain tissue) consists of four regions, called lobes. Shown in yellow (above) is the parietal lobe, which functions primarily in determining our spatial sense and navigation. It's why we don't run into walls when we walk down a hallway, or why we can see a shape (a cone, for instance) and still recognize it as a cone when it's oriented in any direction.

Hemispatial neglect most commonly occurs after injury to the right parietal lobe like, in Barley's case, stroke. It is not as common with left parietal lobe damage—it is thought that the right hemisphere of the brain is generally more specialized for spatial memory, while the left side is better tuned for language.

The left side of a person's world is ignored, then—damage to the right side of the brain reduces the amount of neural activity that crosses over the left via the large fiber tract connecting the two halves, called the corpus collosum (right).

A number of strange symptoms can arise in a person suffering from hemispatial neglect:

• A man may only shave the right half of his face, and a woman may only apply make-up to their right side.
• Someone asked to draw a clock may only write the numbers 12 through 6, or if asked to draw a star will only put points on the right side.
• One will often run into walls or door frames to their left. Their existence was not processed by the brain.
• Try to address a person from their left side—it is as though you don't exist. Walk over to their right, and proceed to enjoy a perfectly normal interaction.

While I describe it as hemispatial neglect, it is true that other senses are also compromised, including smell, touch, pain, and hearing. The strangest thing to remember in all of this is that a person's visual system is completely intact—hemispatial neglect is a fascinating study in brain damage, connectivity, and how different regions are specialized for different functions.

Treatments typically consist of long-term spatial-oriented therapy designed to re-engage the right side of the brain. Success, if any, is slow.

A recent study published this month in Neurology found that 10 sessions of theta-burst stimulation in those suffering from hemispatial neglect resulted in a 16.3% improvement in the Behavioral Inattention Test compared to those experiencing placebo treatment.

Researchers Koch and colleagues from the University of Rome are not entirely certain why this two-week treatment period has shown success. They hypothesize that left hemisphere hyperexcitability—a result of right hemisphere damage—may be counteracted by the theta bursts. Regardless, the study represents an important step forward in the field of neglect rehabilitation.

...and hopefully soon, because, beyond the potential dangers that accompany this condition, I don't think the half-beard will become a fashion statement anytime soon.

Video courtesy oggie114 (YouTube). Photos courtesy Wikipedia, iBiblio, and Smashing Lists.

Koch G, Bonnì S, Giacobbe V, Bucchi G, Basile B, Lupo F, Versace V, Bozzali M, & Caltagirone C (2012). Theta-burst stimulation of the left hemisphere accelerates recovery of hemispatial neglect. Neurology, 78 (1), 24-30 PMID: 22170878

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