Why You Can't Correctly Divide a Line In Half (unless you're 10 years old)

Hemispatial neglect might be the most striking example of brain trauma's cognitive effects: patients with damage to right parietal regions appear unaware of the left half of space. For example, they'll often shave only the right side of their face, will only eat food from the right half of their plate, and when asked to copy a variety of drawings will include only their right half.

As you can tell from these examples, right parietal cortex is particularly important for our understanding of space. Although left parietal cortex may be involved in similar computations, the right-sided region is particularly crucial. For example, damage to the left parietal cortex generally doesn't result in a similar pattern of spatial neglect -- instead, the right-hemisphere can even compensate for the damage in left hemispheric regions.

This right-hemispheric "dominance" in spatial tasks can be demonstrated even in healthy adults: if I ask you to mark the half-way point along a line, you'll tend to make the mark a little left of center. This phenomenon of "leftward bias" (also known as "pseudoneglect") is even easier to see if you make the mark with your left hand (which is under primarily right-hemispheric control). However, it's also present if you make the mark with your right hand, suggesting that the influence of the right hemisphere must cross between the two hemispheres, probably via the bundle of white matter known as the corpus callosum.

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