What you see is not always what you do

Imagine: during lunch your colleague throws an apple to you. You catch it (of course) without difficulty. No problem. But what actually happened? Did you consciously decide to catch the apple with two hands? And how did your hands know where they had to be to catch the apple? According to Dutch researcher Hemke van Doorn you can catch an apple like this thanks to the close cooperation between two separate visual systems. He has now established for the first time how these areas cooperate.

Van Doorn allowed a large number of study subjects to carry out different tests. He wanted to know if we do indeed work with two systems: one system that ensures conscious observation and one visual system that takes care of the movement. In order to demonstrate that the two visual areas are clearly separated, Van Doorn showed the study subjects rods with arrows at the ends. These arrows 'trick' our observation by making the rods seem longer or shorter.

When the study subjects had to say how long the rod was, they were tricked by the arrows. They estimated the length incorrectly. However, as soon as the study subjects were asked to pick up the rod, the size of their hand opening was found to be accurately adjusted to the actual length of the rod. The system for conscious observation and the visual system that guides movement are therefore clearly separated from each other. After all, one system made an error and the other did not.

Besides demonstrating that the two systems are separated, Van Doorn also revealed how the two systems cooperate. A lot of thinking and doing goes into catching an apple: an apple is flying towards me, that's a big apple, it's coming from the left, it's flying quite high, I must turn to catch it and I need two raised hands for that, I now catch the apple with two hands before it flies into my face. Up until now, however, it was not known which of the two systems was responsible for each aspect of catching the apple and other actions and responses. Van Doorn discovered that the system of conscious observation effectively does everything except for initiating the movement.

Furthermore, other tests revealed that determining the direction of a movement - 'I must turn to catch the apple' and selecting the movement - 'I need two hands' - are the tasks of the visual system that consciously observes information, and not of the system that guides the movement. The two systems use other information as well. By measuring the gaze direction of the study subjects, Van Doorn could establish that the eyes are focused at different points for different tasks. With his research he demonstrated how our brains guide movement and the influence of our observations on this.

Hemke van Doorn carried out his research within the Open Competition of the NWO Division for the Social Sciences. With the Open Competition, NWO seeks to stimulate innovative and high-quality scientific research.