This post is also available in Dutch.
You see what you see, right? Well, it’s not that simple. It turns out that what you are seeing right now is partly determined by what you have just seen before.
A busy world
Sight is one of our most important senses. Every second of our waking life we ??rely on our ability to perceive the world. Look around you: you should notice that you can observe everything around you both well and sharply. But the information your eyes send to your brain is much less rich and detailed than you might think. For example, from our visual field, there’s actually only a thumbnail-sized part that we can perceive sharply and in colour from an arm’s length. In addition, the information that our brain receives is often vague and constantly changing because, for example, we move our head and eyes. How can our brain form such an incredibly rich, stable and accurate picture of our environment?
The world is constantly in motion and if you, yourself are also in motion, you can imagine that the image on your retina is rather chaotic. But, secretly, much also remains the same. Even when you look around you, the objects you are looking at change slowly or don’t change at all; the screen on which you are reading this text will not suddenly disappear completely. Thus, much remains constant over time. This stability or regularity over time is called a temporal context. A prominent idea is that our brain uses these regularities to make visual perception more efficient.
Effect of time on vision
There’s a lot of evidence that visual processing in our brain is indeed influenced by what has been seen before. Thus, visual perception is adapted to what was seen in the recent past. For example, if you look at a waterfall for a while and you then look at a stationary object, that object will appear to move upwards. Try it yourself:
This is yet one example of many “motion aftereffects” that shows how visual perception shifts to the opposite of what has been seen recently; the falling waterfall provides the illusion of a movement upwards: a repelling effect.
The reverse can also be the case; visual perception can shift in the direction of the recent past, which is an attracting effect. An example of this is when participants in a computer task judge the orientation of a line as more similar to a previous line than it actually is. Perhaps this is how the brain helps us to deal with unstable information: by making what we see look more similar over time.
Matthias Fritsche, who works at the Donders Institute, devoted his PhD research to this particular phenomenon. His research has proven that the specific point in time at which you examine the effect of the past, affects the way how you perceive something. The effect is different at the time when you are actually seeing something compared to when you have to say what you have just seen a little while later. That sounds weird, but seeing something and making a decision about what you saw are not necessarily the same. How do you research that??
To look at the effect of time on visual perception, Fritsche asked participants to compare 2 lines located next to each other. In order to research the effect of time on perceptual decisions, a participant had to reproduce one of the lines immediately after seeing it. For such a task, a type of memory called working memory is required. And guess what? The way something looks shifts in the opposite direction of what you have just seen, much like the repelling aftereffect mentioned earlier. But if there’s some time between what you actually saw and the decision you have to make about it, an attracting effect on that decision can be noted. It also matters whether you pay attention to what you are looking at; if you pay attention to anything other than the orientation of the line, such as the colour, the effect will be significantly less. How long ago you actually saw something also matters: strangely enough, a decision about something you see is drawn to something we have just seen, but repelled by things we saw a lwhile, up to a few minutes, ago. So what you see is more like what you have seen recently and less like what you saw earlier.
Two effects, two purposes
Why these opposite effects? Fritsche says that the brain actually has two purposes. On the one hand, it aims at stable visual perception. You don’t want what you see to be completely distorted by small deformations, for example, by an eye movement. On the other hand, the brain aims at visual perception that is sensitive to small but important changes, such as a snake that you see in the corner of your eye.
By using a computational model, Fritsche shows how the brain achieves this. It is important to divide visual processing into two phases: firstly, visual information is stored (encoding) and, next, that information is used when you need it to make a decision (decoding), for example, about what you see. In these two phases, the brain uses regularities over time in different ways, leading to the combination of repelling and attracting tendencies that is found while making decisions about what we see.
Thus, you don’t just simply see what you see; your brain is constantly trying to see visual information in the context of what has been perceived previously. This is a nice example of how the brain adapts to the world around us in an intriguing way in order to perceive it as well as possible.
Fritsche obtained his doctorate with a Cum Laude disctinction this past Monday, a rare feat!