What Your Brain Decides in a Fifth of a Second

Before you finish reading this sentence, your brain will already have completed, several times over, a process scientists have spent more than a century and a half timing down to the millisecond.

You don’t need to solve a puzzle.

You don’t even need to think, in the usual sense of the word.

Reacting to a light turning on is enough. And that act, so simple it barely seems “mental,” hides something psychology has spent a long time examining closely.

A button, a light, and a stopwatch from 1868

Psychologist Arthur Jensen devotes a good part of The g Factor (1998) to a type of experiment that sounds almost too simple to matter: reaction time.

We owe the original method to Dutch physiologist Franciscus Donders, who in the 19th century devised an ingenious way to measure mental processes that can’t be observed directly.

• In simple reaction time (SRT), the person holds down a key and releases it as soon as a stimulus appears (a light, a sound). There’s nothing to choose, just react.
• In discrimination reaction time, one of two possible stimuli appears (a yellow light or a blue one, say) and only one of them calls for a response.
• In choice reaction time, each stimulus is tied to a different response: yellow light, left button; blue light, right button.

Donders’ idea, known as the subtraction method, was simple: subtract the time for the simpler task from the time for the more complex one, and what’s left is the time the brain specifically needed for that additional step (discriminating, choosing).

A fifth of a second, and what gets added afterward

In young adults, simple reaction time averages around 200 milliseconds: a fifth of a second.

From there, each extra layer of processing adds a bit more:

1. Discriminating between two stimuli adds, on average, between 30 and 100 milliseconds more, depending on how hard they are to tell apart.
2. Choosing between two different responses adds additional time on top of that.
3. The more complex the task, the bigger the difference between one person and another.

And here’s the detail that connects all of this to the g factor: that extra time, so small it’s measured in fractions of a second, correlates with IQ. According to the data Jensen documents, the correlation grows as task complexity increases: around -0.10 for simple reaction, -0.20 for discrimination, -0.30 for choice (negative because less time goes with higher IQ).

Why the simplest task tells you the least

There’s an important nuance that often goes unnoticed: the simpler the task, the less it varies from one person to another.

• In simple reaction time, almost everyone looks alike: there’s barely any room for the “processing capacity” variable to show up.
• In choice tasks, there are more mental steps involved (perceiving, discriminating, deciding, executing), and therefore more room for some people to clearly take less time than others on that specific part.

Jensen sums it up with an idea that runs through much of his book: the “active ingredient” in these correlations isn’t muscle speed or sensory sharpness, it’s g, the general factor that also comes into play in far more complex tasks, like solving an abstract reasoning problem.

What it does measure, and what it definitely doesn’t

It’s worth being precise about what this finding says and doesn’t say.

• It does show that even the most elementary cognitive tasks, measured with millisecond precision, hold a statistical relationship with the g factor.
• It doesn’t mean that a person who’s slow reacting to a light therefore has less mental capacity: the correlation is moderate and only holds up in large samples, not in an individual case.
• It isn’t a diagnostic test or a substitute for any serious psychometric evaluation.

What this now century-old experiment does leave behind is a striking idea: it doesn’t take a complex problem for the brain to start telling people apart. A light, a button, and a stopwatch precise enough to measure milliseconds will do.

If you’re curious how the brain itself reacts, with no conscious response involved at all, to an equally simple stimulus, your brain responds to a click before you know what it is explains the equivalent experiment using brain waves.

Sources

• Jensen, A. R. (1998). The g Factor: The Science of Mental Ability. Praeger.
• Donders, F. C. (1868). Method described and analyzed in detail by Jensen (1998, pp. 207-211) as the historical origin of mental chronometry (chronometrics) applied to the study of the g factor.