A new study suggests that the human brain processes images in two very distinct ways.
Tzvi Ganel, a psychologist at Ben-Gurion University of the Negev, and his colleagues presented research participants with the "Ponzo" illusion, an image common in psychological research that makes two objects that are similar in length appear drastically different.
The researchers then hooked participants' index finger and thumb to computerized position tracking equipment and asked them to grasp the objects with their fingers.
They observed that, even though the object appeared to be larger or smaller than its original size, the participant's grasp reflected the object's real rather than apparent size.
For good measure, the research team arranged the illusion so that the object that appeared to be the smaller of the two was actually the larger of the two.
Ganel says that the findings of his study provide compelling support for the "two visual systems hypothesis", which was put forward more than a decade ago by Mel Goodale and David Milner.
According to this view, one system called 'vision-for-perception' gives people their conscious visual experience of the world, and allows them to see objects in the rich context of the scenes in which they are embedded. It is also the one that is fooled by optical illusions.
The other system called 'vision-for-action' provides the visual control that is needed to move about and interact with objects. This system does not have to be conscious, but does have to be quick, goal-directed, and accurate.
As a consequence, say the researchers, this system is much less likely to be fooled by illusions such as the one used by them.
"The idea of two visual systems in a single brain might seem initially counter-intuitive. After all, it seems obvious that it is the same subjective image that allows us both to recognize the coffee cup on our desk and to pick it up," writes Ganel.
However, this belief is an illusion, as the new research demonstrates. When there is a conflict between what we perceive and what is really out there in the world, it seems that it is our fingers have an advantage.
The study has been published in the journal Psychological Science, a journal of the Association for Psychological Science.