"Errors are ubiquitous," notes a paper that appears in this week's edition of Science. So, it's rather important that we have the ability to catch and correct them. Researchers have come up with a number of models for how this process works but, according to a paper published in this week's edition of Science, these models have typically been tested in overly simplistic systems. So, they devised a way of studying error recognition in a more complex task: set a typist loose on a computer, and randomly introduce errors or correct misspellings on them. The typists' responses suggest that at least two different error recognition processes are at work.

The authors hypothesize that there are two distinct systems at work when a person starts to type. The first plays an executive function, figuring out what words need to be typed and comparing the final output with the overall goal (they term that the "outer loop"). That process feeds its intentions to a second one that the authors term an "inner loop." This translates the general intentions to the muscle movements that actually get the goal accomplished. By changing words on the typists, the researchers managed to separate the error recognition processes involved in these two loops.

As is usual for academic research, the subjects of the study were undergrads, a population that now includes many skilled typists who can maintain word counts similar to those of professionals. They were asked to type a large collection of words at a computer. About six percent of the time, the computer would introduce an error to a word the subject typed. For the remainder of the words, the computer kept track of whether the subject committed a typo; about half the time, it would automatically correct them.

The inner loop, which handles the actual process of punching the keys, was not fooled by any of this. The authors could measure the amount of time between keystrokes as the subjects plowed through words. As with past research, they could detect a slight delay immediately after a typo was committed. This delay persisted even if the word on screen was corrected to look like nothing had gone wrong. Just as significantly, there was no delay when the computer slipped in an error. In short, whatever part of the brain was working the fingers knew exactly what they were doing.

The outer loop, which keeps track of the overall goals, was much more readily fooled. In most cases, the changes to the words done by the computer created what the authors termed an "illusion of authorship." Basically, if it appeared on the screen, the typists were likely to believe they had done it. Even after 600 words, three quarters of the typists didn't comment on anything unusual happening unless prompted.

Only when specifically asked about inserted errors or corrections were people likely to recognize them. Even then, at about 50 words, well over half the typists seemed unaware that changes had been made (that number dropped to 20 percent after a 600-word passage).

In a second experiment, the authors asked the typists to stop after every word and note whether they'd typed it correctly. Again, the illusion of authorship kicked in, with subjects identifying on-screen errors as their own more often than not, even if the error were computer generated. The same was true for corrected errors.

To get the illusion to break down, the authors had to explicitly ask the subjects to identify inserted errors or automatic corrections after every single word was typed. Here, the subjects were actually able to identify when the computer had created an error in their otherwise flawless typing. But, apparently, the illusion of authorship remained strong when the end result was good, as the subjects were evenly split between calling these correct and calling them corrections.

The authors conclude that there is a hierarchical control system at work when it comes to typing, and argue that it may apply to other systems that have a similar structure of an overall goal and a number of discrete steps—they suggest examples like playing a musical instrument or performing navigation. But, more generally, they warn that other studies need to be interpreted cautiously, since it would be easy to design a test that only stressed one or the other of these two systems.

Science, 2010. DOI: 10.1126/science.1190483 (About DOIs).