We all have our own set of burning questions—those inquiries
that sit in the back of our minds and refuse to leave us alone. They vary
widely from person to person; Alessandro Volta was apparently tormented by the
mystery, “What would happen if I completed the circuit of a 50-volt battery by
sticking two metal rods into my ears?” while Paul Broca wondered (somewhat more
prudently), “What does the brain’s left frontal lobe have to do with our ability
to speak?”
A question that bothers me on occasion (while perhaps not quite
as creative as Volta’s) is one regarding optical illusions. Specifically, “Why
does my brain tell me that the picture below is a spiral?”
The image consists of concentric circles (you can trace one
with your finger for proof). I know that
the image consists of concentric circles, but no matter how I try to convince
myself of the fact, all I can see is a spiral. The illusion, known a Fraser
spiral or a false spiral, was first studied by British psychologist James
Fraser in 1908. It combines a regular pattern of circles with misaligned,
differently colored strands, which create visual distortion. And as if the
tilted strands weren’t hard enough on our unsuspecting brains, the checkered
background also contains spiral components to heighten the illusion.
The deception happens through a combination of simple image
processing in the retina and more complex processing in the brain’s striate
cortex, a primary visual receptive area. Orientation-sensitive cells in the
cortex make horizontal connections with each other that change depending on
context. In the case of this irritating not-spiral, the cells interpret the
message of diagonal bands—that is, the misaligned black and white strands—that
they receive from the retina as an unbroken line, creating the appearance of a
spiral.
Maybe not the most dramatic answer, but at least it didn’t
require closing an electrical circuit with my ears.
Sources
<http://www.psychologie.tu-dresden.de/
i1/kaw/diverses%20Material/www.illusionworks.com/html/fraser_spiral.html>.
<http://mathworld.wolfram.com/FrasersSpiral>.
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