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.
Given that the corpus callosum shows growth in childhood, leftward bias might take a very different form in children – for at least some ages, one might expect diminished leftward bias. To investigate this possibility, Hausmann, Waldie & Corballis asked ninety-eight 5th, 8th & 13th-grade German students to mark the middle point on each of 17 lines, using each hand. The students covered their previous bisections as they continued to bisect more lines, all of which appeared on the same page in pseudorandom locations.
Just like adults, the results showed that in general there was a strong leftward bias, particularly when students used their left hands, and particularly when the lines themselves appeared on the left side of the page (and thus processed primarily by the right hemisphere).
However, the youngest age group (10-12 year olds) showed a different pattern when using their right hand – now they actually showed a right bias, particularly when bisecting lines on the right half of the page. In other words, they were more likely to bisect line just right-of-center when using the right hand (again, under left-hemispheric control).
So Hausmann et al. demonstrate that the normal adult bias to bisect lines a little bit left-of-center is also present in children, but only in the youngest age groups when they are using their left hand. These children may not have a fully developed corpus callosum, meaning that the right hemisphere has a much stronger effect on the left hand. Conversely, the left hemisphere of these children has a much stronger effect on their right hands, and as a result they show a right bias that is not observable in those with a more fully developed corpus callosum.
At some point between 10 and 13, there might be a point where children don’t show a left or right bias – presumably the time when the corpus callosum is developed just enough to allow a counterbalancing of these spatial biases through inter-hemispheric communication. The idea that this age may be somehow “balanced” is certainly interesting from other perspectives as well.
For example, 10 year olds also show less temporal distortion effects than any other age group: whereas adults are more likely to misremember a previously-presented tone as longer, and whereas younger children are more likely to misremember a previously-presented tone as shorter, 10-year-old children do not show biases in these temporal estimation error patterns (more on children’s perception of time). So 10-year olds may show more numerically “correct” performance both in terms of line bisection and temporal estimation.
Future work might reveal similar effects at even younger ages. Some evidence suggests that cortical reorganization may be cyclical, in that short-range left-hemispheric connections may be reorganized, followed by longer-range left-hemispheric reorganization, then followed by short-range right hemispheric reorganization, followed by longer-range right hemispheric reorganization (all ascertained through time-frequency analysis of spontaneous EEG between 2 months and 18 years of age). If this were true, then one might expect multiple ages to show numerically correct performance in these tasks, or cyclical changes in individual performance over time.
Whatever the reason, it’s remarkable that 10 year olds seem to perform numerically better than adults in these two tasks. This is in some ways reminiscent of other findings showing that children, or even monkeys, may outperform human adults on at least some memory tasks and even one task involving basic reasoning.