Insight #6: Do we see more than we suppose? Two separate vision systems hypothesis

saunter (60)in #insight • 7 years ago (edited)

How does one raise a cup without knowing what this object is, or what size and shape it has? How do 3D illusions affect our actions? Welcome to the sixth entry of the Insight series!

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Ebbinghaus illusion

Please look at the picture below.

_{Okręgi Titchenera} _{(image source)}

Are the brown/orange circles of the same size or do they differ? Our eyesight suggests that the right circle on larger. However, this is an optical illusion! It is probably caused by the fact that human visual perception is strongly context dependent. Very young children are less subject to this illusion precisely because their perception is still developing context through experience :)

But when we try to transfer the illusion of Ebbinghaus into a three-dimensional space, really weird things will happen! Aglioti, DeSouz and Goodale were probably pretty surprised in 1995 when they decided to put this plan into practice. The scientists created three-dimensional versions of the Ebbinghaus illusion in order to see how it will affect people's actions, namely the grip.

The circles were turned into plastic wire-mounted discs and were very easy to grasp. The authors prepared two sets of these illusions: in version A, the central circles were the same, but they seemed different, while in version B the circles were of different sizes, but this size was matched in such a way that they seemed to be identical.

_{(image source)}

Versions A and B were presented to the participants in various configurations. The task of the participants was to grab a one of the discs (right or left). The choice of a disc depended on whether the disc was perceived as the same or different. In this way experimenters verified whether the participants are being subjected to the Ebbinghaus illusion. In addition, researchers measured the distance between thumb and forefinger while grasping the discs with three-dimensional cameras.

The results were rather surprising - the subjects were strongly influenced by an optical illusion, but the calibration of finger grasp was always adjusted to the real physical size of the disc! The action of capturing the discs was largely independent of how they were perceived! In other words - subjects grasped the discs as if they were not experiencing illusion at all, even though they experienced it. Why did it happen?

Two Forms of Vision

_{Visual information moves from the visual cortex (blue) in two directions - dorsal stream (green) and ventral stream (purple).}
_{(image source)}

It turns out that two paths separate paths (or streams) of visual information in our visual system are responsible for this. In other words, in our cerebral cortex separate routes are activated depending on whether we only perceive (look at) or also interact with an object. These two separate nervous trails correspond respectively to vision-for-perception and vision-for-action. The first one is a function of ventral stream, the second one is caused by dorsal stream.

Vision for Perception

Vision-for-perception corresponds to what we intuitively understand under the term "seeing". Our brain uses this stream to calculate the size, location, shape and orientation of the object we look at. But most of all it sees the object in the context of its relation to other objects and background. The information received by the ventral stream is perceived as phenomenological mental representations in our mind - in other words, it is what you experience as "seeing" an object.

While the ventral stream deals with the relationship between the observed object and its surroundings, it does not determine the precise position and real size of the object in relation to the observer. That is why we experience the Ebbinghaus illusion! The group of smaller circles around the central one enlarges it, whereas the closeness of the larger circles results in the ventral stream interpreting it as smaller than it actually is. "Since the central circle is smaller than the large blue circles, it is probably also smaller than the second central circle" - in such an exceptionally relational way, ventral stream "thinks" when it is subjected to Ebbinghaus illusion.

Vision for Action

The dorsal stream, on the other hand, is responsible for calculating the actual size of an object, its exact orientation in space and, first of all, its position in relation to the observer, especially the part of body which manipulate the object (e.g. a hand, like in the aforementioned experiment). It is said, that it establishes "egocentric coordinates" of the object. It ignores the relation between the object and its background and other objects - it is only interested in "where" the object is and how to precisely catch/grasp/manipulate it.

Thus vision-for-action estimates the true physical size of circles. Hence our brains "know" that what they see is not an illusion, but thet don't tell us that! This is proven when someone grasps the disc of 3D version of Ebbinghaus illusion. We don't experience this at all, however, one could saythat we see the true size of Tichener's circles unconsciously. While the ventral stream provides us with conscious, sensual visual experiences, the dorsal stream does not give us the any conscious sensations.

Visual agnosia

One of the most important and interesting evidence supporting the hypothesis of two distinct visual streams was the famous "DF patient". This woman suffered from visual agnosia as a result of ventral stream damage - the one responsible for vision-for-perception.

DF was able to see, but she couldn't recognize objects, define their elements, say where they end and where they begin. She couldn't copy a simple drawing, but she was able to draw objects from her memory easily. When she looked at the cup, she could not determine what it was. However, when she was asked to catch the object, she picked up the cup in the same way everyone lifts the cup! She easily matched the letter to the letterbox's hole, despite the fact that she couldn't tell how big these objects were.

I'll end with a short interview with Kevin, a man suffering from visual agnosia. He describes how he copes with his condition in everyday life and what is the only type of objects that he can visually recognize:

_{Bibliography:}

_{- Aglioti, S., DeSouza, J. F., & Goodale, M. A. (1995). Size-contrast illusions deceive the eye but not the hand. Current biology, 5(6), 679-685.}_{- Goodale, M. A., & Milner, A. D. (1992). Separate visual pathways for perception and action. Trends in neurosciences, 15(1), 20-25.}_{- Haffenden, A. M., & Goodale, M. A. (1998). The effect of pictorial illusion on prehension and perception. Journal of Cognitive Neuroscience, 10(1), 122-136.}_{- Króliczak, G. (2005). Czy iluzje zwodzą jedynie „oko”, ale już nie rękę.
Przez, R. Z. M. G. P., & Szwabe, P. P. I. J. (2011). Czy można widzieć i nie być tego świadomym?.}

Previous entries in the Insight series:

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