In 1965 H.H. Kornhuber and L. Deeke found that brain activity precedes a conscious choice (voluntarily pressing a button) by 500 to 1,000 milliseconds. But in 1983 a team led by B. Libet found that when people were asked when they consciously decided to press a button, they said their decision came about 200 milliseconds before pressing it — after their brain had started to process the task.

So did you “decide” to read this post after your brain had already committed to clicking on the link? It’s possible, but it’s also possible that there’s simply a lag between when you were aware of having made a decision and when you actually decided.

Or it may be that you’re only certain of having made a decision after you see evidence that that’s what you chose to do: Hearing the click of the mouse and seeing the page load might be what actually makes you aware of deciding to read it. William Banks and Eve Isham have come up with a clever way of discerning whether evidence we see and hear affects our awareness of having made a decision.

They asked eight Pomona College undergraduates to watch a representation of a clock on a computer screen. While they watched, their hand was on a button that was hidden from their own view. A cursor moved around the clock’s dial once every 2.6 seconds. The students were told to press the button whenever they wished, and then report exactly where the cursor was at the moment they made the decision to press the button. This was repeated 160 times for each student.

The trick was that as they pressed the button, the computer made a short beep. Unknown to the students, there was a slight delay between when the button was pressed and when the beep sounded. This delay varied randomly between between 5 and 60 milliseconds. Did the timing of the beep affect when the students believed they had decided to press the button? Here are the results:

This graph charts the delay of the “beep” from the actual button-pushing against the time viewers said they decided to push the button (measured in milliseconds before the button was pushed). The shorter the delay, the earlier the decision was made. In other words, the timing of the beep, which was randomly selected, had a significant effect on when the students said they made the decision to press the button.

Here’s another way of looking at the same results:

This graph compares the time from when students said they decided to push the button to the timing of the beep. As you can see, the time from the reported decision and the beep was nearly constant: students said they decided to push the button around 130 milliseconds before they heard the beep, regardless of the actual timing of the beep.

So a randomly-timed beep has a larger effect on when we think we decided to press a button than the actual time we pressed the button.

In a second experiment, the researchers showed volunteers a video feed of their hand pressing the button superimposed on the clock. Like the beep in the first experiment, the video was randomly delayed by 5 to 60 milliseconds. The results were the same.

Does this mean we have no free will?

Not necessarily. It’s still possible that the students did freely choose when to press the button, but that their awareness of that decision is affected by external factors. The awareness of the decision is never actually later than the time the button was pushed, so the cognition of making a decision is never completely divorced from the action itself.

Perhaps we’re not ever conscious of having made a decision. Rather, we infer making a decision from other evidence: the sensation of pressing a button, the sound the computer makes to register our choice. That choice may be our own, but the process by which we make it remains a mystery.

Banks WP, & Isham EA (2009). We infer rather than perceive the moment we decided to act. Psychological science : a journal of the American Psychological Society / APS, 20 (1), 17-21 PMID: 19152537