Tag Archives: Motion perception

Measuring what the brain thinks it is perceiving rather than the stimulus itelf

It’s usually not hard to do neuropsychology experiments.  The hard part is being smart enough to think of a good one.  I found a recent one absolutely brilliant, as the authors were able to measure a signal which had to be coming from the conscious perception of motion in a particular direction [ Proc. Natl. Acad. Sci. vol. 116 pp. 5096 – 5101 ’19 ].

Throw any stimulus at a living human and you’ll get some sort of measurable electrical response or a measurable change in blood flow in a particular brain area (you can use functional MRI — fMRI to measure the latter).  But how do you know whether the response has anything to do with conscious perception.  You don’t.

Here’s where the cleverness of the authors comes in.  Probably most people reading this post know about Cartesian coordinates, but to not leave the nonMathematically inclined behind, I’ll use baseball to describe the experimental set up.

We talk about a baseball diamond, and that’s the way it looks to people sitting in the stands behind home plate.  But actually the 4 bases form a perfect square 90 feet on a side.

So turn the ‘diamond’ on its side so the path between home plate and first base is horizontal, as is the path between 2nd and third while the paths between first and second and between third base and home are vertical.

Now that you’re oriented, imagine this on a computer screen. What the authors did was to light up first and third for .15 seconds, turn things off for .067 seconds and then light up home plate and second base for .15 seconds.  So the dot pairs alternate about 4 times a second.

But what does this look like to a human being?  For about 10 seconds the dots actually appear to actually be moving horizontally, then they appear to be moving vertically.  Remember the dots themselves  aren’t moving at all, just blinking.

The brilliance of the setup is that with exactly the same stimulus (alternately lit pairs of dots) the same person will have two different perceptions of the way the dots are moving at different times.

What do you think they did next?

They put the same people in an MRI machine and then showed the dots actually moving across the screen horizontally and then vertically.  Different parts of the brain responded to vertical motion than responded to horizontal motion.  The response was increased blood flow to that area, which is what fMRI actually measures.

So then back to the original set up with alternate pairs of dots on and off about 4 times a second.  Then they asked people which way the dots appeared to be moving, and the area of the brain which lit up (showed increased flow) was the same one which lit up to actual motion in that direction.

So they were actually measuring conscious perception of motion, rather than some nonspecific response to the visual stimulus, because the stimulus didn’t change regardless of the way it was perceived.

One of things this means is that the brain is producing the same neural response when it perceives motion in one direction (even though none is present) that real motion produces.

I think this is just brilliant.  Bravo. Something for the philosophers among you to chew on.

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