There are some people who cannot see even with their eyes open. Although these people are blind, there is nothing wrong with the eyes themselves. Instead, there has been an injury to the part of the brain that helps them see: the visual cortex (this and all other words printed in bold font are explained in the Section “Glossary” at the end of this text). What is fascinating is that even though they cannot see, some of these people can reach out and grasp for an object correctly more often than if they were just guessing where the object was. Even more fascinating is an experiment showing that a person with this type of blindness can find his way down a long corridor and correctly avoid the obstacles placed along the path without using any help like a cane [1]. It seems as if these people are able to sense the objects that are invisible to them. Medically, this phenomenon is called “blindsight” [2].

This means that some people with an impaired sense of vision can sometimes “act” as if they were actually seeing the things around them, like a sighted person. How is that possible? The accepted theory is that the brain of these blind people has found ways to process information about objects even though these objects are invisible to the person. This remarkable ability of the human brain has led scientists to question whether healthy people with normal vision can also react to and “interact” with objects that they cannot see, i.e., that they are not aware of.

The important question is how do we make things invisible to people who have normal eyesight? Normally, our left and right eyes see almost the same thing (as long as we have both eyes open of course). Our two eyes take in the same image, send this information to the brain, and the brain processes the information so that we can see the image. But we can also separate what the two eyes see and make them see different things, for example, by using special glasses which are shown in Figure 1. What do you think would happen if you saw a dog with your left eye and an ice cream with your right eye? Well, you may think that we see a mixture of the two, i.e., half ice cream and half dog. Surprisingly, we do not! What we do see is the image of the ice cream and dog switching back and forth. This is called binocular rivalry because the two (“bi”) different inputs to the eyes (“ocular”) compete (or, “rival”) for dominance (to be the image we finally see).

Using a method called continuous flash suppression (CFS), which is based on the same principle as binocular rivalry, we can make objects invisible to people who have normal vision. In the CFS method, one eye is presented with the image of interest, also called the target, while at the same time the other eye is made to see colorful, flickering patterns. So using the same example as above, one eye is shown the image of an ice cream and the other eye is now shown colorful, flickering patterns instead of the dog. In such a situation, because the colorful patterns are flickering compared to the static ice cream, what we finally see (most of the time) are the colorful, changing patterns (i.e., the dominant image).

Which image is dominant (the flickering masks or the ice cream) also depends on our eye dominance. But remember; the target image (the ice cream in our example) is actually shown to one eye and this eye and also parts of the brain are still receiving the information of the ice cream. It is just rendered invisible by the masks so that we are not consciously aware of it. This technique can be used by scientists to make things invisible for up to many seconds [3] and help them study the processing of information in healthy subjects, who can otherwise see normally, before they become aware of the information itself.

How is CFS related to the phenomenon of “blindsight” that we mentioned above? Remember that CFS is a technique and blindsight is a phenomenon. Blindsight was originally observed in patients with damage to the visual cortex. In order to study whether blindsight also occurs in people without brain damage, techniques are needed to make stimuli invisible for them. One such technique is CFS. Most importantly, however, objects made invisible by CFS can still be processed by the intact visual cortex and can at some point become visible. Objects presented to a patient with blindsight will remain invisible because the visual cortex is damaged.

You can try the method of CFS out for yourself using Figure 2 and a pair of red-green glasses. You can make your very own red-green glasses using cardstock paper, a pair of scissors, and red and green transparent paper. All you have to do is draw the picture of your glasses (big enough to fit on your face) onto the cardstock paper, cut it out, and make two holes for the eyes. Stick the red transparent paper onto one eye hole and the green transparent paper on the other eye hole like in Figure 1 and you have your special glasses.

Now that we have a method to make things invisible to people with normal, healthy vision; we can study whether they can still process and react to this invisible image that is hidden behind the flickering masks. In one experiment, we used a video camera to record volunteers’ eye movements while they searched for an invisible pattern. An interesting fact about our visual system is that our eyes make several scanning movements (also called saccades) each second. Can you guess what we observed when our volunteers had to search for the invisible pattern?

Amazingly, we found that although the volunteers said that they could not see the pattern and could not even guess where it was, their eye movements were more often directed to the place where the pattern was [4]! Somehow, even though the pattern was invisible to the volunteers, they seem to have been able to use the information. So from this experiment, there is evidence that healthy humans can process some information without complete awareness of it. But can we also grasp for an object that is invisible to us like people with blindsight can sometimes do, as we have learned above?

In another set of experiments, CFS was used to make objects invisible and volunteers were asked to grasp these invisible objects. While one group of scientists could show that people were able to reach out for invisible objects (although they were not nearly as good as when they saw them) [5], we found that healthy people could not correctly grasp invisible objects [6].

What do these findings tell us? Well, first of all, that not all experiments in science give you the same result. The way in which an experiment is carried out plays a huge role. Sometimes even small changes in experimental setups can have large impacts on your results, and part of what scientists do is to try and explain these differences. It also tells us that science is a field where people talk about their new findings, and that even if the outcome of the experiment is different from what you expected, you have learned something new. The field of studying the processing of invisible information in healthy people is vast and there is so much to be discovered. Thus, although we have come a long way and know quite a bit about the processing of invisible information in healthy people, we do not know (yet) if we can grasp for that ice cream we cannot see. Finally, in order to establish a concept in science so that you can read about it in your textbook, it has to be supported over and over again by the work of several different research groups. And who knows, even after all that, you can come up with a new theory that you can test with your own experiment.