6.6.1.3 Part 3
You should find that your friend’s spoken estimate of the second line is less than her estimate of the first, even though both lines were the same length. That’s the visual illusion. (If you used different length lines, this difference will be in relative terms.) And yet her walked-out estimates should be pretty much the same (i.e., not tricked by the illusion), or at least you should find she underestimates the second line’s length far less when walking. That is, her conscious judgment should be tricked more by this illusion (a version of a famous illusion called the Muller-Lyer illusion), than her walked-out estimate, controlled by her dorsal stream.
6.6.2. How It Works
How it works depends upon whom you ask. Advocates of the dual-stream theory of visual processing argue that these demonstrations, of the immunity of our actions to visual illusions, are evidence for the separateness of the dorsal (action) and ventral (perception) streams. The ventral stream is susceptible, they argue, because it processes objects relative to their surroundings, assessing the current context in order that we might recognize things. The dorsal stream, by contrast, is invulnerable to such illusions because it processes objects of interest in egocentric coordinates, relative to the observer, so that we might accurately interact with them.
Doubters of the dual-stream theory take a different view. One reason we are sometimes duped by illusions, and sometimes not, they argue, is all to do with the type of task, far less to do with there being separate processing pathways in our brain. For instance, when we view the Ebbinghaus illusion (Figure 6-6), we are typically asked to compare the two central disks. Yet, when we reach for one of the disks, we are focused on only one disk at a time. Perceptual tasks tend to involve taking context and nearby objects into account, whereas motor tasks tend to involve focusing on one object at a time and, by necessity, using egocentric coordinates to interact accurately. When changing the task conditions reverses these tendencies, the visuomotor system can be found to be susceptible to illusion or the perceptual system invulnerable.
Which argument is right? Well, there’s evidence both ways and the debate will probably roll on for some time yet.2,3 What is clear, is that this phenomenon provides yet another example [Hack #62] of how our illusory sense of a unified self keeps all these conflicting processes conveniently out of mind.
Does the world really appear as you’re seeing it? Who cares? Just sit back and enjoy the view, accurate or not, while your neurons fight things out.
6.6.3. End Notes
Aglioti, S. et al. (1995). Size contrast illusions deceive the eye but not the hand. Current Biology, 5, 679-685.
Franz, V. H. (2001). Action does not resist visual illusions. Trends in Cognitive Sciences, 5, 457-459.
Milner, D., & Dyde, R. (2003). Why do some perceptual illusions affect visually guided action, when others don’t? Trends in Cognitive Sciences, 7, 10-11.
Christian Jarrett
Taken from : Mind Hacks
