"Yeah, you're so good at it. A systematic habit" --GarbageBefore we answer why the second task was harder, let's give the phenomenon a name. In the psychological literature, it is called the Stroop Effect, named after the scientist who made the discovery famous .
Remember my claim that we are serial processors, meaning that we can only process one stream of information at a time? Well, that is mostly true. If we get really, really good at a skill, then it can become automatic. Once a skill becomes automatic, we can carry it out without devoting much attention to it. The process of executing a skill without requiring attention is called Automaticity.
Since you elected to click on a link directing you to a highly verbal resource (i.e., this blog), I assume you are an expert reader. Expertise, obviously, is a matter of degree, but if you've been practicing the skill of reading for more than 10 years, you are most likely an expert reader . Being an expert, you have automatized the process of reading. When symbols that resemble letters are arranged in what looks like words, your brain will automatically try to read what the letters say. I also assume that, if you are older than a toddler, you have probably automatized the skill of recognizing primary colors.
Learning How to Walk, Talk, and Chew GumHow does one automate a skill? The answer is pretty straightforward. There's no magic formula. You have to practice the skill. A lot. And preferably with implicit and/or explicit feedback. By "explicit" feedback, I am referring to a teacher or tutor who tells you that you are doing well or that you made a mistake. By "implicit" feedback, I mean the external environment signals that your performance was flawed in some way. For example, if you swing a bat at a ball and miss, the fact that you missed will become obvious to you when the ball continues past you instead of connecting with your bat.
The outcome of automatizing a skill is glorious. It means that you can carry out the task without much conscious awareness. You can suddenly (by "suddenly", I mean after 10 years of deliberate practice) do two things at once. You can finally multitask!
The downside, however, is when two automatic skills conflict. The Stroop Effect is a powerful example of this collision. The automatic skill of reading comes straight into conflict with the automatic skill of recognizing colors. When the output from each of those processes are consistent (e.g., RED), then performance is fast and error free. However, when they are in conflict (e.g., RED), then performance slows down and is fraught with errors.
The STEM ConnectionAutomaticity and education have a very long history together. Where did the idea of "flashcards" come from if not from the application of automaticity to education? Some skills just need to be automatized (e.g., addition, subtraction, multiplication, and division for integers up to 12). The advantage of automatizing certain skills is that it frees up the student's attention and working memory so she can devote additional cognitive resources to acquiring more complex skills.
The problem with undergoing the process of automaticity is that it can be extremely boring. Trial after trial. Flashcard after flashcard. When will it end?! The instructional designer's job is to help motivate the learner so she can reap the benefits afforded by automaticity. Here is where I think educational games can make a positive contribution. The video game industry has figured out what motivates us. How can we take those lessons and apply them to the development of instructional experiences that help reduce the monotony of working towards automaticity?
One of my favorite examples of an educational game is Akira, which is a game that helps build fluency in decimal and fractional comparisons . The goal of this game is to make number-magnitude estimates as quickly as possible. One of the important elements of these types of games is "time pressure" so that students have to work as quickly as possible, all the while avoiding errors. I also like Battleship Numberline, another educational game that attempts to build number fluency.
With the rise of mobile devices and the availability of inexpensive development tools, the potential for creating educational games has never been better.
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For More Information
 Ericsson, K. A., Krampe, R. T., & Tesch Roemer, C. (1993). The role of deliberate practice in the acquisition of expert performance. Psychological Review, 100(3), 363–406.
 Just to be on the up-and-up, Akira is currently being developed by the company I work for; however, I am not a member of that particular development team nor do I derive financial benefit from its success.