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.

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.

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any parent can tell you that from approximately the age of 3 yrs children can outperform adults on memory tasks (3yrold: "you promised i could have a cookie", dad: "you mean three days ago?" 3yrold:"yeah, i never got it") and basic reasoning (anytime bedtime, TV or videogames are involved anyway)

Thanks for the comment (and nice blog! hadn't seen it before).

I know you're being facetious but your comment strikes a nerve.

I hate the fact that basic psychology research often gets a response like "duh!" The fact is that folk psychological theories always make a variety of contradictory predictions: basic research always verifies one of them, the others are conveniently forgotten, and then people are free to say "well duh."

Just venting - but not at you or your comment in particular; you just made me think of it :)

Are results like these typically dependent on handedness? E.g., do left-handed adults tend to mark the line a little right of center? Also, what happens if you perform such tasks with either the right or left eye covered?

Agreed CHCH. Also trained in exp psych. Also grapple with this issue and attempt to explain "yeah but we didn't really *know* that until demonstrated and oftimes results were counterintuitive". so sorry for the smart-alec'ing.

it's a common human failing though. across disciplines. think about the grant and paper peer review process.

Step 1: get a little data on a new model/organism/drug/dose or whatever and propose a grant: grant reviewers always know what's going to happen in advance, often feel like the 'programmatic' studies proposed are incremental advances, "we already knew that", etc.

Step 2: in the multiple proposal revision process, generate enough "preliminary" data to publish a paper showing the potential of this new approach: paper reviewers- well, okay but you really need to do X, y, Z studies (amounting to full R01 length project). much skepticism that the new approach is really finding anything novel. find a sympathetic editor eventually and get it published anyway.

Step 3: review of revised grant: "but the PI is already publishing on this!"

Step 4: review of next series of papers on the topic. "oh, but of course the new approach is providing different results, we know it all along. ..and wah, the discussion shouldn't be so strong, *both* models are important'

Step 5: , okay, time for new proposal changed enough to not trigger the A3 filter at CSR....

Qetzal - those are all interesting questions. As for handedness, it doesn't seem to make much of a difference (at least according to this study) but that may be because handedness and hemispheric dominance don't have a one-to-one relationship either.

As for closing one eye or the other, that probably wouldn't make a difference, because it's the left hemifield of each eye that gets routed to the right hemisphere, rather than the left eye as a whole.

Interestingly some people with unilateral neglect can learn to rotate their plates 180 degrees so as to eat all their food. But that's neither here nor there :)

This is actually one of those phenomena that surprised me a great deal when I heard it -- I wouldn't have guessed that people would be so bad at bisecting lines. I didn't believe it to the extent that I actually sat down and did it myself, and lo and behold, there it is. Interestingly, I found that the bias was noticeably stronger for me when I used my right hand than my left. If it's not measurement error or something, what would that mean in terms of the corpus colossum? (I'm mostly right-handed, but weirdly ambidextrous for certain things, if that matters).

Hey Amy - That's weird! I have no idea. One thing to note is that women tend to show this effect less than men, supposedly.


Is there still a bias if the lines are vertical, or at some (significant) angle to the horizontal?

I know that I seem to make more accurate architectural sketches if I frequently rotate my drawing..... (maybe eliminating the bias, by averaging?)


The model proposed by the author is what we have inherited from Sir Isaac Newton (Query # 15, Opticks, 1704).
The reason for the "pseudoneglect" when deviding a line, according to a new understanding on the subject, is the fact that moving the eyes to the left takes more time than that taken for moving them to the right. This exess time is shown as under-estimation of the left half of the space that (as right handers) we intended to divide. Phylosophical consequences of this exchage between time and space are breath-taking (to say the least).
More on neurological underpinning of the observations made by the authore is avaiable at (www.mimickingman.com).
I. Derakhshan, MD, Neurologist

By iraj derakhshan (not verified) on 02 Jun 2007 #permalink

There are quite a few theories on why "normal" people can't do line bisection tasks. One is a motor theory involving a strange desire not to enter the left side of space with your body. The other is actual perception; do we just not percieve that the line is longer on the right than the left? In my studies as an undergrad psych major, I've found that the most evidence supports the perception theory but there are no causal studies that show any significant results for where or how the tasks are performed mentally.

By jennabear (not verified) on 19 Oct 2007 #permalink